The Elements of Geology
by
William Harmon Norton
Part 4 out of 7
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BLOCK MOUNTAINS. Dislocations take place on so grand a scale that
by the upheaval of blocks of the earth's crust or the down-
faulting of the blocks about one which is relatively stationary,
mountains known as block mountains are produced. A tilted crust
block may present a steep slope on the side upheaved and a more
gentle descent on the side depressed.
THE BASIN RANGES. The plateaus of the United States bounded by the
Rocky Mouirtains on the east, and on the west by the ranges which
front the Pacific, have been profoundly fractured and faulted. The
system of great fissures by which they are broken extends north
and south, and the long, narrow, tilted crust blocks intercepted
between the fissures give rise to the numerous north-south ranges
of the region. Some of the tilted blocks, as those of southern
Oregon, are as yet but moderately carved by erosion, and shallow
lakes lie on the waste that has been washed into the depressions
between them. We may therefore conclude that their displacement is
somewhat recent. Others, as those of Nevada, are so old that they
have been deeply dissected; their original form has been destroyed
by erosion, and the intermontane depressions are occupied by wide
plains of waste.
DISLOCATIONS AND RIVER VALLEYS. Before geologists had proved that
rivers can by their own unaided efforts cut deep canyons, it was
common to consider any narrow gorge as a gaping fissure of the
crust. This crude view has long since been set aside. A map of the
plateaus of northern Arizona shows how independent of the immense
faults of the region is the course of the Colorado River. In the
Alps the tunnels on the Saint Gotthard railway pass six times
beneath the gorge of the Reuss, but at no point do the rocks show
the slightest trace of a fault.
RATE OF DISLOCATION. So far as human experience goes, the earth
movements which we have just studied, some of which have produced
deep-sunk valleys and lofty mountain ranges, and faults whose
throw is to be measured in thousands of feet, are slow and
gradual. They are not accomplished by a single paroxysmal effort,
but by slow creep and a series of slight slips continued for vast
lengths of time.
In the Aspen mining district in Colorado faulting is now going on
at a comparatively rapid rate. Although no sudden slips take
place, the creep of the rock along certain planes of faulting
gradually bends out of shape the square-set timbers in horizontal
drifts and has closed some vertical shafts by shifting the upper
portion across the lower. Along one of the faults of this region
it is estimated that there has been a movement of at least four
hundred feet since the Glacial epoch. More conspicuous are the
instances of active faulting by means of sudden slips. In 1891
there occurred along an old fault plane in Japan a slip which
produced an earth rent traced for fifty miles (Fig. 192). The
country on one side was depressed in places twenty feet below that
on the other, and also shifted as much as thirteen feet
horizontally in the direction of the fault line.
In 1872 a slip occurred for forty miles on the great line of
dislocation which runs along the eastern base of the Sierra Nevada
Mountains. In the Owens valley, California, the throw amounted to
twenty-five feet in places, with a horizontal movement along the
fault line of as much as eighteen feet. Both this slip and that in
Japan just mentioned caused severe earthquakes.
For the sake of clearness we have described oscillations,
foldings, and fractures of the crust as separate processes, each
giving rise to its own peculiar surface features, but in nature
earth movements are by no means so simple,--they are often
implicated with one another: folds pass into faults; in a deformed
region certain rocks have bent, while others under the same
strain, but under different conditions of plasticity and load,
have broken; folded mountains have been worn to their roots, and
the peneplains to which they have been denuded have been upwarped
to mountain height and afterwards dissected,--as in the case of
the Alleghany ridges, the southern Carpathians, and other ranges,
--or, as in the case of the Sierra Nevada Mountains, have been
broken and uplifted as mountains of fracture.
Draw the following diagrams, being careful to show the direction
in which the faulted blocks have moved, by the position of the two
parts of some well-defined layer of limestone, sandstone, or
shale, which occurs on each side of the fault plane, as in Figure
184.
1. A normal fault with a hade of 15 degrees, the original fault
scarp remaining.
2. A normal fault with a hade of 50 degrees, the original fault
scarp worn away, showing cliffs caused by harder strata on the
downthrow side.
3. A thrust fault with a hade of 30 degrees, showing cliffs due to
harder strata outcropping on the downthrow.
4. A thrust fault with a hade of 80 degrees, with surface
baseleveled.
5. In a region of normal faults a coal mine is being worked along
the seam of coal AB (Fig. 193). At B it is found broken by a fault
f which hades toward A. To find the seam again, should you advise
tunneling up or down from B?
6. In a vertical shaft of a coal mine the same bed of coal is
pierced twice at different levels because of a fault. Draw a
diagram to show whether the fault is normal or a thrust.
7. Copy the diagram in Figure 194, showing how the two ridges may
be accounted for by a single resistant stratum dislocated by a
fault. Is the fault a STRIKE FAULT, i.e. one running parallel with
the strike of the strata, or a DIP FAULT, one running parallel
with the direction of the dip?
8. Draw a diagram of the block in Figure 195 as it would appear if
dislocated along the plane efg by a normal fault whose throw
equals one fourth the height of the block. Is the fault a strike
or a dip fault? Draw a second diagram showing the same block after
denudation has worn it down below the center of the upthrown side.
Note that the outcrop of the coal seam is now deceptively
repeated. This exercise may be done in blocks of wood instead of
drawings.
9. Draw diagrams showing by dotted lines the conditions both of A
and of B, Figure 196, after deformation had given the strata their
present attitude.
10. What is the attitude of the strata of this earth block, Figure
197? What has taken place along the plane bef? When did the
dislocation occur compared with the folding of the strata? With
the erosion of the valleys on the right-hand side of the mountain?
With the deposition of the sediments? Do you find any remnants of
the original surface baf produced by the dislocation? From the
left-hand side of the mountain infer what was the relief of the
region before the dislocation. Give the complete history recorded
in the diagram from the deposition of the strata to the present.
11. Which is the older fault, in Figure 198, or When did the lava
flow occur? How long a time elapsed between the formation of the
two faults as measured in the work done in the interval? How long
a time since the formation of the later fault?
12. Measure by the scale the thickness lie of the coal-bearing
strata outcropping from a to b in Figure 199. On any convenient
scale draw a similar section of strata with a dip of 30 degrees
outcropping along a horizontal line normal to the strike one
thousand feet in length, and measure the thickness of the strata
by the scale employed. The thickness may also be calculated by
trigonometry.
UNCONFORMITY
Strata deposited one upon, another in an unbroken succession are
said to be conformable. But the continuous deposition of strata is
often interrupted by movements of the earth's crust, Old sea
floors are lifted to form land and are again depressed beneath the
sea to receive a cover of sediments only after an interval during
which they were carved by subaerial erosion. An erosion surface
which thus parts older from younger strata is known as an
UNCONFORMITY, and the strata above it are said to be UNCONFORMABLE
with the rocks below, or to rest unconformably upon them. An
unconformity thus records movements of the crust and a consequent
break in the deposition of the strata. It denotes a period of land
erosion of greater or less length, which may sometimes be roughly
measured by the stage in the erosion cycle which the land surface
had attained before its burial. Unconformable strata may be
parallel, as in Figure 200, where the record includes the
deposition of strata, their emergence, the erosion of the land
surface, a submergence and the deposit of the strata, and lastly,
emergence and the erosion of the present surface.
Often the earth movements to which the uplift or depression was
due involved tilting or folding of the earlier strata, so that the
strata are now nonparallel as well as unconformable. In Figure
201, for example, the record includes deposition, uplift, and
tilting of a; erosion, depression, the deposit of b; and finally
the uplift which has brought the rocks to open air and permitted
the dissection by which the unconformity is revealed. From this
section infer that during early Silurian times the area was sea,
and thick sea muds were laid upon it. These were later altered to
hard slates by pressure and upfolded into mountains. During the
later Silurian and the Devonian the area was land and suffered
vast denudation. In the Carboniferous period it was lowered
beneath the sea and received a cover of limestone.
THE AGE OF MOUNTAINS. It is largely by means of unconformities
that we read the history of mountain making and other deformations
and movements of the crust. In Figure 203, for example, the
deformation which upfolded the range of mountains took place after
the deposit of the series of strata a of which the mountains are
composed, and before the deposit of the stratified rocks, which
rest unconformably on a and have not shared their uplift.
Most great mountain ranges, like the Sierra Nevada and the Alps,
mark lines of weakness along which the earth's crust has yielded
again and again during the long ages of geological time. The
strata deposited at various times about their flanks have been
infolded by later crumplings with the original mountain mass, and
have been repeatedly crushed, inverted, faulted, intruded with
igneous rocks, and denuded. The structure of great mountain ranges
thus becomes exceedingly complex and difficult to read. A
comparatively simple case of repeated uplift is shown in Figure
204. In the section of a portion of the Alps shown in Figure 179 a
far more complicated history may be deciphered.
UNCONFORMITIES IN THE COLORADO CANYON, ARIZONA. How geological
history may be read in unconformities is further illustrated in
Figures 207 and 208. The dark crystalline rocks a at the bottom of
the canyon are among the most ancient known, and are overlain
unconformably by a mass of tilted coarse marine sandstones b,
whose total thickness is not seen in the diagram and measures
twelve thousand feet perpendicularly to the dip. Both a and b rise
to a common level nn and upon them rest the horizontal sea-laid
strata c, in which the upper portion of the canyon has been cut.
Note that the crystalline rocks a have been crumpled and crushed.
Comparing their structure with that of folded mountains, what do
you infer as to their relief after their deformation? To which
surface were they first worn down, mm' or nm? Describe and account
for the surface mm'. How does it differ from the surface of the
crystalline rocks seen in the Torridonian Mountains, and why? This
surface mm' is one of the oldest land surfaces of which any
vestige remains.
It is a bit of fossil geography buried from view since the
earliest geological ages and recently brought to light by the
erosion of the canyon.
How did the surface mm' come to receive its cover of sandstones b?
From the thickness and coarseness of these sediments draw
inferences as to the land mass from which they were derived. Was
it rising or subsiding? high or low? Were its streams slow or
swift? Was the amount of erosion small or great?
Note the strong dip of these sandstones b. Was the surface mm'
tilted as now when the sandstones were deposited upon it? When was
it tilted? Draw a diagram showing the attitude of the rocks after
this tilting occurred, and their height relative to sea level.
The surface nn' is remarkably even, although diversified by some
low hills which rise into the bedded rocks of c, and it may be
traced for long distances up and down the canyon. Were the layers
of b and the surface mm' always thus cut short by nn' as now? What
has made the surface nn' so even? How does it come to cross the
hard crystalline rocks a and the weaker sandstones b at the same
impartial level? How did the sediments of c come to be laid upon
it? Give now the entire history recorded in the section, and in
addition that involved in the production of the platform P, shown
in Figure 130, and that of the cutting of the canyon. How does the
time involved in the cutting of the canyon compare with that
required for the production of the surfaces mm', nn', and P?
CHAPTER X
EARTHQUAKES
Any sudden movement of the rocks of the crust, as when they tear
apart when a fissure is formed or extended, or slip from time to
time along a growing fault, produces a jar called an earthquake,
which spreads in all directions from the place of disturbance.
THE CHARLESTON EARTHQUAKE. On the evening of August 31, 1886, the
city of Charleston, S.C., was shaken by one of the greatest
earthquakes which has occurred in the United States. A slight
tremor which rattled the windows was followed a few seconds later
by a roar, as of subterranean thunder, as the main shock passed
beneath the city. Houses swayed to and fro, and their heaving
floors overturned furniture and threw persons off their feet as,
dizzy and nauseated, they rushed to the doors for safety. In sixty
seconds a number of houses were completely wrecked, fourteen
thousand chimneys were toppled over, and in all the city scarcely
a building was left without serious injury. In the vicinity of
Charleston railways were twisted and trains derailed. Fissures
opened in the loose superficial deposits, and in places spouted
water mingled with sand from shallow underlying aquifers.
The point of origin, or FOCUS, of the earthquake was inferred from
subsequent investigations to be a rent in the rocks about twelve
miles beneath the surface. From the center of greatest
disturbance, which lay above the focus, a few miles northwest of
the city, the surface shock traveled outward in every direction,
with decreasing effects, at the rate of nearly two hundred miles
per minute. It was felt from Boston to Cuba, and from eastern Iowa
to the Bermudas, over a circular area whose diameter was a
thousand miles.
An earthquake is transmitted from the focus through the elastic
rocks of the crust, as a wave, or series of waves, of compression
and rarefaction, much as a sound wave is transmitted through the
elastic medium of the air. Each earth particle vibrates with
exceeding swiftness, but over a very short path. The swing of a
particle in firm rock seldom exceeds one tenth of an inch in
ordinary earthquakes, and when it reaches one half an inch and an
inch, the movement becomes dangerous and destructive.
The velocity of earthquake waves, like that of all elastic waves,
varies with the temperature and elasticity of the medium. In the
deep, hot, elastic rocks they speed faster than in the cold and
broken rocks near the surface. The deeper the point of origin and
the more violent the initial shock, the faster and farther do the
vibrations run.
Great earthquakes, caused by some sudden displacement or some
violent rending of the rocks, shake the entire planet. Their waves
run through the body of the earth at the rate of about three
hundred and fifty miles a minute, and more slowly round its
circumference, registering their arrival at opposite sides of the
globe on the exceedingly delicate instruments of modern earthquake
observatories.
GEOLOGICAL EFFECTS. Even great earthquakes seldom produce
geological effects of much importance. Landslides may be shaken
down from the sides of mountains and hills, and cracks may be
opened in the surface deposits of plains; but the transient
shiver, which may overturn cities and destroy thousands of human
lives, runs through the crust and leaves it much the same as
before.
EARTHQUAKES ATTENDING GREAT DISPLACEMENTS. Great earthquakes
frequently attend the displacement of large masses of the rocks of
the crust. In 1822 the coast of Chile was suddenly raised three or
four feet, and the rise was five or six feet a mile inland. In
1835 the same region was again upheaved from two to ten feet. In
each instance a destructive earthquake was felt for one thousand
miles along the coast.
THE GREAT CALIFORNIA EARTHQUAKE OF 1906. A sudden dislocation
occurred in 1906 along an ancient fault plane which extends for
300 miles through western California. The vertical displacement
did not exceed four feet, while the horizontal shifting reached a
maximum of twenty feet. Fences, rows of trees, and roads which
crossed the fault were broken and offset. The latitude and
longitude of all points over thousands of square miles were
changed. On each side of the fault the earth blocks moved in
opposite directions, the block on the east moving southward and
that on the west moving northward and to twice the distance. East
and west of the fault the movements lessened with increasing
distance from it.
This sudden slip set up an earthquake lasting sixty-five seconds,
followed by minor shocks recurring for many days. In places the
jar shook down the waste on steep hillsides, snapped off or
uprooted trees, and rocked houses from their foundations or threw
down their walls or chimneys. The water mains of San Francisco
were broken, and the city was thus left defenseless against a
conflagration which destroyed $500,000,000 worth of property. The
destructive effects varied with the nature of the ground.
Buildings on firm rock suffered least, while those on deep
alluvium were severely shaken by the undulations, like water
waves, into which the loose material was thrown. Well-braced steel
structures, even of the largest size, were earthquake proof, and
buildings of other materials, when honestly built and
intelligently designed to withstand earthquake shocks, usually
suffered little injury. The length of the intervals between severe
earthquakes in western California shows that a great dislocation
so relieves the stresses of the adjacent earth blocks that scores
of years may elapse before the stresses again accumulate and cause
another dislocation.
Perhaps the most violent earthquake which ever visited the United
States attended the depression, in 1812, of a region seventy-five
miles long and thirty miles wide, near New Madrid, Mo. Much of the
area was converted into swamps and some into shallow lakes, while
a region twenty miles in diameter was bulged up athwart the
channel of the Mississippi. Slight quakes are still felt in this
region from time to time, showing that the strains to which the
dislocation was due have not yet been fully relieved.
EARTHQUAKES ORIGINATING BENEATH THE SEA. Many earthquakes
originate beneath the sea, and in a number of examples they seem
to have been accompanied, as soundings indicate, by local
subsidences of the ocean bottom. There have been instances where
the displacement has been sufficient to set the entire Pacific
Ocean pulsating for many hours. In mid ocean the wave thus
produced has a height of only a few feet, while it may be two
hundred miles in width. On shores near the point of origin
destructive waves two or three score feet in height roll in, and
on coasts thousands of miles distant the expiring undulations may
be still able to record themselves on tidal gauges.
DISTRIBUTION OF EARTHQUAKES. Every half hour some considerable
area of the earth's surface is sensibly shaken by an earthquake,
but earthquakes are by no means uniformly distributed over the
globe. As we might infer from what we know as to their causes,
earthquakes are most frequent in regions now undergoing
deformation. Such are young rising mountain ranges, fault lines
where readjustments recur from time to time, and the slopes of
suboceanic depressions whose steepness suggests that subsidence
may there be in progress.
Earthquakes, often of extreme severity, frequently visit the lofty
and young ranges of the Andes, while they are little known in the
subdued old mountains of Brazil. The Highlands of Scotland are
crossed by a deep and singularly straight depression called the
Great Glen, which has been excavated along a very ancient line of
dislocation. The earthquakes which occur from time to time in this
region, such as the Inverness earthquake in 1891, are referred to
slight slips along this fault plane.
In Japan, earthquakes are very frequent. More than a thousand are
recorded every year, and twenty-nine world-shaking earthquakes
occurred in the three years ending with 1901. They originate, for
the most part, well down on the eastern flank of the earth fold
whose summit is the mountainous crest of the islands, and which
plunges steeply beneath the sea to the abyss of the Tuscarora
Deep.
MINOR CAUSES OF EARTHQUAKES. Since any concussion within the crust
sets up an earth jar, there are several minor causes of
earthquakes, such as volcanic explosions and even the collapse of
the roofs of caves. The earthquakes which attend the eruption of
volcanoes are local, even in the case of the most violent volcanic
paroxysms known. When the top of a volcano has been blown to
fragments, the accompanying earth shock has sometimes not been
felt more than twenty-five miles away.
DEPTH OF FOCUS. The focus of the Charleston earthquake, estimated
at about twelve miles below the surface, was exceptionally deep.
Volcanic earthquakes are particularly shallow, and probably no
earthquakes known have started at a greater depth than fifteen or
twenty miles. This distance is so slight compared with the earth's
radius that we may say that earthquakes are but skin-deep.
Should you expect the velocity of an earthquake to be greater in a
peneplain or in a river delta?
After an earthquake, piles on which buildings rested were found
driven into the ground, and chimneys crushed at base. From what
direction did the shock come?
Chimneys standing on the south walls of houses toppled over on the
roof. Should you infer that the shock in this case came from the
north or south?
How should you expect a shock from the east to affect pictures
hanging on the east and the west walls of a room? how the pictures
hanging on the north and the south walls?
In parts of the country, as in southwestern Wisconsin, slender
erosion pillars, or "monuments," are common. What inference could
you draw as to the occurrence in such regions of severe
earthquakes in the recent past?
CHAPTER XI
VOLCANOES
Connected with movements of the earth's crust which take place so
slowly that they can be inferred only from their effects is one of
the most rapid and impressive of all geological processes,--the
extrusion of molten rock from beneath the surface of the earth,
giving rise to all the various phenomena of volcanoes.
In a volcano, molten rock from a region deep below, which we may
call its reservoir, ascends through a pipe or fissure to the
surface. The materials erupted may be spread over vast areas, or,
as is commonly the case, may accumulate about the opening, forming
a conical pile known as the volcanic cone. It is to this cone that
popular usage refers the word VOLCANO; but the cone is simply a
conspicuous part of the volcanic mechanism whose still more
important parts, the reservoir and the pipe, are hidden from view.
Volcanic eruptions are of two types,--EFFUSIVE eruptions, in which
molten rock wells up from below and flows forth in streams of LAVA
(a comprehensive term applied to all kinds of rock emitted from
volcanoes in a molten state), and EXPLOSIVE eruptions, in which
the rock is blown out in fragments great and small by the
expansive force of steam.
ERUPTIONS OF THE EFFUSIVE TYPE
THE HAWAIIAN VOLCANOES. The Hawaiian Islands are all volcanic in
origin, and have a linear arrangement characteristic of many
volcanic groups in all parts of the world. They are strung along a
northwest-southeast line, their volcanoes standing in two parallel
rows as if reared along two adjacent lines of fracture or folding.
In the northwestern islands the volcanoes have long been extinct
and are worn low by erosion. In the southeastern island. Hawaii,
three volcanoes are still active and in process of building. Of
these Mauna Loa, the monarch of volcanoes, with a girth of two
hundred miles and a height of nearly fourteen thousand feet above
sea level, is a lava dome the slope of whose sides does not
average more than five degrees. On the summit is an elliptical
basin ten miles in circumference and several hundred feet deep.
Concentric cracks surround the rim, and from time to time the
basin is enlarged as great slices are detached from the vertical
walls and engulfed.
Such a volcanic basin, formed by the insinking of the top of the
cone, is called a CALDERA.
On the flanks of Mauna Loa, four thousand feet above sea level,
lies the caldera of Kilauea, an independent volcano whose dome has
been joined to the larger mountain by the gradual growth of the
two. In each caldera the floor, which to the eye is a plain of
black lava, is the congealed surface of a column of molten rock.
At times of an eruption lakes of boiling lava appear which may be
compared to air holes in a frozen river. Great waves surge up,
lifting tons of the fiery liquid a score of feet in air, to fall
back with a mighty plunge and roar, and occasionally the lava
rises several hundred feet in fountains of dazzling brightness.
The lava lakes may flood the floor of the basin, but in historic
times have never been known to fill it and overflow the rim.
Instead, the heavy column of lava breaks way through the sides of
the mountain and discharges in streams which flow down the
mountain slopes for a distance sometimes of as much as thirty-five
miles. With the drawing off of the lava the column in the duct of
the volcano lowers, and the floor of the caldera wholly or in part
subsides. A black and steaming abyss marks the place of the lava
lakes. After a time the lava rises in the duct, the floor is
floated higher, and the boiling lakes reappear.
The eruptions of the Hawaiian volcanoes are thus of the effusive
type. The column of lava rises, breaks through the side of the
mountain, and discharges in lava streams. There are no explosions,
and usually no earthquakes, or very slight ones, accompany the
eruptions. The lava in the calderas boils because of escaping
steam, but the vapor emitted is comparatively little, and seldom
hangs above the summits in heavy clouds. We see here in its
simplest form the most impressive and important fact in all
volcanic action, molten rock has been driven upward to the surface
from some deep-lying source.
LAVA FLOWS. As lava issues from the side of a volcano or overflows
from the summit, it flows away in a glowing stream resembling
molten iron drawn white-hot from an iron furnace. The surface of
the stream soon cools and blackens, and the hard crust of
nonconducting rock may grow thick and firm enough to form a
tunnel, within which the fluid lava may flow far before it loses
its heat to any marked degree. Such tunnels may at last be left as
caves by the draining away of the lava, and are sometimes several
miles in length.
PAHOEHOE AND AA. When the crust of highly fluid lava remains
unbroken after its first freezing, it presents a smooth, hummocky,
and ropy surface known by the Hawaiian term PAHOEHOE. On the other
hand, the crust of a viscid flow may be broken and splintered as
it is dragged along by the slowly moving mass beneath. The stream
then appears as a field of stones clanking and grinding on, with
here and there from some chink a dull red glow or a wisp of steam.
It sets to a surface called AA, of broken, sharp-edged blocks,
which is often both difficult and dangerous to traverse.
FISSURE ERUPTIONS. Some of the largest and most important outflows
of lava have not been connected with volcanic cones, but have been
discharged from fissures, flooding the country far and wide with
molten rock. Sheet after sheet of molten rock has been
successively outpoured, and there have been built up, layer upon
layer, plateaus of lava thousands of feet in thickness and many
thousands of square miles in area.
ICELAND. This island plateau has been rent from time to time by
fissures from which floods of lava have outpoured. In some
instances the lava discharges along the whole length of the
fissure, but more often only at certain points upon it. The Laki
fissure, twenty miles long, was in eruption in 1783 for seven
months. The inundation of fluid rock which poured from it is the
largest of historic record, reaching a distance of forty-seven
miles and covering two hundred and twenty square miles to an
average depth of a hundred feet. At the present time the fissure
is traced by a line of several hundred insignificant mounds of
fragmental materials which mark where the lava issued.
The distance to which the fissure eruptions of Iceland flow on
slopes extremely gentle is noteworthy. One such stream is ninety
miles in length, and another seventy miles long has a slope of
little more than one half a degree.
Where lava is emitted at one point and flows to a less distance
there is gradually built up a dome of the shape of an inverted
saucer with an immense base but comparatively low. Many LAVA DOMES
have been discovered in Iceland, although from their exceedingly
gentle slopes, often but two or three degrees, they long escaped
the notice of explorers.
The entire plateau of Iceland, a region as large as Ohio, is
composed of volcanic products,--for the most part of successive
sheets of lava whose total thickness falls little short of two
miles. The lava sheets exposed to view were outpoured in open air
and not beneath the sea; for peat bogs and old forest grounds are
interbedded with them, and the fossil plants of these vegetable
deposits prove that the plateau has long been building and is very
ancient. On the steep sea cliffs of the island, where its
structure is exhibited, the sheets of lava are seen to be cut with
many DIKES,--fissures which have been filled by molten rock,--and
there is little doubt that it was through these fissures that the
lava outwelled in successive flows which spread far and wide over
the country and gradually reared the enormous pile of the plateau.
ERUPTIONS OF THE EXPLOSIVE TYPE
In the majority of volcanoes the lava which rises in the pipe is
at least in part blown into fragments with violent explosions and
shot into the air together with vast quantities of water vapor and
various gases. The finer particles into--which the lava is
exploded are called VOLCANIC DUST or VOLCANIC ASHES, and are often
carried long distances by the wind before they settle to the
earth. The coarser fragments fall about the vent and there
accumulate in a steep, conical, volcanic mountain. As successive
explosions keep open the throat of the pipe, there remains on the
summit a cup-shaped depression called the CRATER.
STROMBOLI. To study the nature of these explosions we may visit
Stromboli, a low volcano built chiefly of fragmental materials,
which rises from the sea off the north coast of Sicily and is in
constant though moderate action.
Over the summit hangs a cloud of vapor which strikingly resembles
the column of smoke puffed from the smokestack of a locomotive, in
that it consists of globular masses, each the product of a
distinct explosion. At night the cloud of vapor is lighted with a
red glow at intervals of a few minutes, like the glow on the trail
of smoke behind the locomotive when from time to time the fire bos
is opened. Because of this intermittent light flashing thousands
of feet above the sea, Stromboli has been given the name of the
Lighthouse of the Mediterranean.
Looking down into the crater of the volcano, one sees a viscid
lava slowly seething. The agitation gradually increases. A great
bubble forms. It bursts with an explosion which causes the walls
of the crater to quiver with a miniature earthquake, and an
outrush of steam carries the fragments of the bubble aloft for a
thousand feet to fall into the crater or on the mountain side
about it. With the explosion the cooled and darkened crust of the
lava is removed, and the light of the incandescent liquid beneath
is reflected from the cloud of vapor which overhangs the cone.
At Stromboli we learn the lesson that the explosive force in
volcanoes is that of steam. The lava in the pipe is permeated with
it much as is a thick boiling porridge. The steam in boiling
porridge is unable to escape freely and gathers into bubbles which
in breaking spurt out drops of the pasty substance; in the same
way the explosion of great bubbles of steam in the viscid lava
shoots clots and fragments of it into the air.
KRAKATOA. The most violent eruption of history, that of Krakatoa,
a small volcanic island in the strait between Sumatra and Java,
occurred in the last week of August, 1883. Continuous explosions
shot a column of steam and ashes. seventeen miles in air. A black
cloud, beneath which was midnight darkness and from which fell a
rain of ashes and stones, overspread the surrounding region to a
distance of one hundred and fifty miles. Launched on the currents
of the upper air, the dust was swiftly carried westward to long
distances. Three days after the eruption it fell on the deck of a
ship sixteen hundred miles away, and in thirteen days the finest
impalpable powder from the volcano had floated round the globe.
For many months the dust hung over Europe and America as a faint
lofty haze illuminated at sunrise and sunset with brilliant
crimson. In countries nearer the eruption, as in India and Africa,
the haze for some time was so thick that it colored sun and moon
with blue, green, and copper-red tints and encircled them with
coronas.
At a distance of even a thousand miles the detonations of the
eruption sounded like the booming of heavy guns a few miles away.
In one direction they were audible for a distance as great as that
from San Francisco to Cleveland. The entire atmosphere was thrown
into undulations under which all barometers rose and fell as the
air waves thrice encircled the earth. The shock of the explosions
raised sea waves which swept round the adjacent shores at a height
of more than fifty feet, and which were perceptible halfway around
the globe.
At the close of the eruption it was found that half the mountain
had been blown away, and that where the central part of the island
had been the sea was a thousand feet deep.
MARTINIQUE AND ST. VINCENT. In 1902 two dormant volcanoes of the
West Indies, Mt. Pelee in Martinique and Soufriere in St. Vincent,
broke into eruption simultaneously. No lava was emitted, but there
were blown into the air great quantities of ashes, which mantled
the adjacent parts of the islands with a pall as of gray snow. In
early stages of the eruption lakes which occupied old craters were
discharged and swept down the ash-covered mountain valleys in
torrents of boiling mud.
On several occasions there was shot from the crater of each
volcano a thick and heavy cloud of incandescent ashes and steam,
which rushed down the mountain side like an avalanche, red with
glowing stones and scintillating with lightning flashes. Forests
and buildings in its path were leveled as by a tornado, wood was
charred and set on fire by the incandescent fragments, all
vegetation was destroyed, and to breathe the steam and hot,
suffocating dust of the cloud was death to every living creature.
On the morning of the 8th of May, 1902, the first of these
peculiar avalanches from Mt. Pelee fell on the city of St. Pierre
and instantly destroyed the lives of its thirty thousand
inhabitants.
The eruptions of many volcanoes partake of both the effusive and
the explosive types: the molten rock in the pipe is in part blown
into the air with explosions of steam, and in part is discharged
in streams of lava over the lip of the crater and from fissures in
the sides of the cone. Such are the eruptions of Vesuvius, one of
which is illustrated in Figure 219.
SUBMARINE ERUPTIONS. The many volcanic islands of the ocean and
the coral islands resting on submerged volcanic peaks prove that
eruptions have often taken place upon the ocean floor and have
there built up enormous piles of volcanic fragments and lava. The
Hawaiian volcanoes rise from a depth of eighteen thousand feet of
water and lift their heads to about thirty thousand feet above the
ocean bed. Christmas Island (see p. 194), built wholly beneath the
ocean, is a coral-capped volcanic peak, whose total height, as
measured from the bottom of the sea, is more than fifteen thousand
feet. Deep-sea soundings have revealed the presence of numerous
peaks which fail to reach sea level and which no doubt are
submarine volcanoes. A number of volcanoes on the land were
submarine in their early stages, as, for example, the vast pile of
Etna, the celebrated Sicilian volcano, which rests on stratified
volcanic fragments containing marine shells now uplifted from the
sea.
Submarine outflows of lava and deposits of volcanic fragments
become covered with sediments during the long intervals between
eruptions. Such volcanic deposits are said to be CONTEMPORANEOUS,
because they are formed during the same period as the strata among
which they are imbedded. Contemporaneous lava sheets may be
expected to bake the surface of the stratum on which they rest,
while the sediments deposited upon them are unaltered by their
heat. They are among the most permanent records of volcanic
action, far outlasting the greatest volcanic mountains built in
open air.
From upraised submarine volcanoes, such as Christmas Island, it is
learned that lava flows which are poured out upon the bottom of
the sea do not differ materially either in composition or texture
from those of the land.
VOLCANIC PRODUCTS
Vast amounts of steam are, as we have seen, emitted from
volcanoes, and comparatively small quantities of other vapors,
such as various acid and sulphurous gases. The rocks erupted from
volcanoes differ widely in chemical composition and in texture.
ACIDIC AND BASIC LAVAS. Two classes of volcanic rocks may be
distinguished,--those containing a large proportion of silica
(silicic acid, SiO2) and therefore called ACIDIC, and those
containing less silica and a larger proportion of the bases (lime,
magnesia, soda, etc.) and therefore called BASIC. The acidic
lavas, of which RHYOLITE and THRACHYTE are examples, are
comparatively light in color and weight, and are difficult to
melt. The basic lavas, of which BASALT is a type, are dark and
heavy and melt at a lower temperature.
SCORIA AND PUMICE. The texture of volcanic rocks depends in part
on the degree to which they were distended by the steam which
permeated them when in a molten state. They harden into compact
rock where the steam cannot expand. Where the steam is released
from pressure, as on the surface of a lava stream, it forms
bubbles (steam blebs) of various sizes, which give the hardened
rock a cellular structure (Fig. 220), In this way are formed the
rough slags and clinkers called SCORIA, which are found on the
surface of flows and which are also thrown out as clots of lava in
explosive eruptions.
On the surface of the seething lava in the throat of the volcano
there gathers a rock foam, which, when hurled into the air, is
cooled and falls as PUMICE,--a spongy gray rock so light that it
floats on water.
AMYGDULES. The steam blebs of lava flows are often drawn out from
a spherical to an elliptical form resembling that of an almond,
and after the rock has cooled these cavities are gradually filled
with minerals deposited from solution by underground water. From
their shape such casts are called amygdules (Greek, amygdalon, an
almond). Amygdules are commonly composed of silica. Lavas contain
both silica and the alkalies, potash and soda, and after
dissolving the alkalies, percolating water is able to take silica
also into solution. Most AGATES are banded amygdules in which the
silica has been laid in varicolored, concentric layers.
GLASSY AND STONY LAVAS. Volcanic rocks differ in texture according
also to the rate at which they have solidified. When rapidly
cooled, as on the surface of a lava flow, molten rock chills to a
glass, because the minerals of which it is composed have not had
time to separate themselves from the fused mixture and form
crystals. Under slow cooling, as in the interior of the flow, it
becomes a stony mass composed of crystals set in a glassy paste.
In thin slices of volcanic glass one may see under the microscope
the beginnings of crystal growth in filaments and needles and
feathery forms, which are the rudiments of the crystals of various
minerals.
Spherulites, which also mark the first changes of glassy lavas
toward a stony condition, are little balls within the rock,
varying from microscopic size to several inches in diameter, and
made up of radiating fibers.
Perlitic structure, common among glassy lavas, consists of
microscopic curving and interlacing cracks, due to contraction.
FLOW LINES are exhibited by volcanic rocks both to the naked eye
and under the microscope. Steam blebs, together with crystals and
their embryonic forms, are left arranged in lines and streaks by
the currents of the flowing lava as it stiffened into rock.
PORPHYRITIC STRUCTURE. Rocks whose ground mass has scattered
through it large conspicuous crystals are said to be PORPHYRITIC,
and it is especially among volcanic rocks that this structure
occurs. The ground mass of porphyries either may be glassy or may
consist in part of a felt of minute crystals; in either case it
represents the consolidation of the rock after its outpouring upon
the surface. On the other hand, the large crystals of porphyry
have slowly formed deep below the ground at an earlier date.
COLUMNAR STRUCTURE. Just as wet starch contracts on drying to
prismatic forms, so lava often contracts on cooling to a mass of
close-set, prismatic, and commonly six-sided columns, which stand
at right angles to the cooling surface. The upper portion of a
flow, on rapid cooling from the surface exposed to the air, may
contract to a confused mass of small and irregular prisms; while
the remainder forms large and beautifully regular columns, which
have grown upward by slow cooling from beneath.
FRAGMENTAL MATERIALS
Rocks weighing many tons are often thrown from a volcano at the
beginning of an outburst by the breaking up of the solidofied
floor of the crater; and during the progress of an eruption large
blocks may be torn from the throat of the volcano by the outrush
of steam. But the most important fragmental materials are those
derived from the lava itself. As lava rises in the pipe, the steam
which permeates it is released from pressure and explodes, hurling
the lava into the air in fragments of all sizes,--large pieces of
scoria, LAPILLI (fragments the size of a pea or walnut), volcanic
"sand" and volcanic "ashes." The latter resemble in appearance the
ashes of wood or coal, but they are not in any sense, like them, a
residue after combustion.
Volcanic ashes are produced in several ways: lava rising in the
volcanic duct is exploded into fine dust by the steam which
permeates it; glassy lava, hurled into the air and cooled
suddenly, is brought into a state of high strain and tension, and,
like Prince Rupert's drops, flies to pieces at the least
provocation. The clash of rising and falling projectiles also
produces some dust, a fair sample of which may be made by grating
together two pieces of pumice.
Beds of volcanic ash occur widely among recent deposits in the
western United States. In Nebraska ash beds are found in twenty
counties, and are often as white as powdered pumice. The beds grow
thicker and coarser toward the southwestern part of the state,
where their thickness sometimes reaches fifty feet. In what
direction would you look for the now extinct volcano whose
explosive eruptions are thus recorded?
TUFF. This is a convenient term designating any rock composed of
volcanic fragments. Coarse tuffs of angular fragments are called
VOLCANIC BRECIA, and when the fragments have been rounded and
sorted by water the rock is termed a VOLCANIC CONGLOMERATE. Even
when deposited in the open air, as on the slopes of a volcano,
tuffs may be rudely bedded and their fragments more or less
rounded, and unless marine shells or the remains of land plants
and animals are found as fossils in them, there is often
considerable difficulty in telling whether they were laid in water
or in air. In either case they soon become consolidated. Chemical
deposits from percolating waters fill the interstices, and the bed
of loose fragments is cemented to hard rock.
The materials of which tuffs are composed are easily recognized as
volcanic in their origin. The fragments are more or less cellular,
according to the degree to which they were distended with steam
when in a molten state, and even in the finest dust one may see
the glass or the crystals of lava from which it was derived. Tuffs
often contain VOCLANIC BOMBS,--balls of lava which took shape
while whirling in the air, and solidified before falling to the
ground.
ANCIENT VOLCANIC ROCKS. It is in these materials and structures
which we have described that volcanoes leave some of their most
enduring records. Even the volcanic rocks of the earliest geological
ages, uplifted after long burial beneath the sea and exposed to view
by deep erosion, are recognized and their history read despite the
many changes which they may have undergone. A sheet of ancient lava
may be distinguished by its composition from the sediments among
which it is imbedded. The direction of its flow lines may be noted.
The cellular and slaggy surface where the pasty lava was distended
by escaping steam is recognized by the amygdules which now fill the
ancient steam blebs. In a pile of successive sheets of lava each
flow may be distinguished and its thickness measured; for the
surface of each sheet is glassy and scoriaceous, while beneath its
upper portions the lava of each flow is more dense and stony. The
length of time which elapsed before a sheet was buried beneath the
materials of succeeding eruptions may be told by the amount of
weathering which it had undergone, the depth of ancient soil--now
baked to solid rock--upon it, and the erosion which it had suffered
in the interval.
If the flow occurred from some submarine volcano, we may recognize
the fact by the sea-laid sediments which cover it, filling the
cracks and crevices of its upper surface and containing pieces of
lava washed from it in their basal layers.
Long-buried glassy lavas devitrify, or pass to a stony condition,
under the unceasing action of underground waters; but their flow
lines and perlitic and spherulitic structures remain to tell of
their original state.
Ancient tuffs are known by the fragmental character of their
volcanic material, even though they have been altered to firm
rock. Some remains of land animals and plants may be found
imbedded to tell that the beds were laid in open air; while the
remains of marine organisms would prove as surely that the tuffs
were deposited in the sea.
In these ways ancient volcanoes have been recognized near Boston,
in southeastern Pennsylvania, about Lake Superior, and in other
regions of the United States.
THE LIFE HISTORY OF A VOLCANO
The invasion of a region by volcanic forces is attended by
movements of the crust heralded by earthquakes. A fissure or a
pipe is opened and the building of the cone or the spreading of
wide lava sheets is begun.
VOLCANIC CONES. The shape of a volcanic cone depends chiefly on
the materials erupted. Cones made of fragments may have sides as
steep as the angle of repose, which in the case of coarse scoria
is sometimes as high as thirty or forty degrees. About the base of
the mountain the finer materials erupted are spread in more gentle
slopes, and are also washed forward by rains and streams. The
normal profile is thus a symmetric cone with a flaring base.
Cones built of lava vary in form according to the liquidity of the
lava. Domes of gentle slope, as those of Hawaii, for example, are
formed of basalt, which flows to long distances before it
congeals. When superheated and emitted from many vents, this
easily melted lava builds great plateaus, such as that of Iceland.
On the other hand, lavas less fusible, or poured out at a lower
temperature, stiffen when they have flowed but a short distance,
and accumulate in a steep cone. Trachyte has been extruded in a
state so viscid that it has formed steepsided domes like that of
Sarcoui.
Most volcanoes are built, like Vesuvius, both of lava flows and of
tuffs, and sections show that the structure of the cone consists
of outward-dipping, alternating layers of lava, scoria, and ashes.
From time to time the cone is rent by the violence of explosions
and by the weight of the column of lava in the pipe. The fissures
are filled with lava and some discharge on the sides of the
mountain, building parasitic cones, while all form dikes, which
strengthen the pile with ribs of hard rock and make it more
difficult to rend.
Great catastrophes are recorded in the shape of some volcanoes
which consist of a circular rim perhaps miles in diameter,
inclosing a vast crater or a caldera within which small cones may
rise. We may infer that at some time the top of the mountain has
been blown off, or has collapsed and been engulfed because some
reservoir beneath had been emptied by long-continued eruptions.
The cone-building stage may be said to continue until eruptions of
lava and fragmental materials cease altogether. Sooner or later
the volcanic forces shift or die away, and no further eruptions
add to the pile or replace its losses by erosion during periods of
repose. Gases however are still emitted, and, as sulphur vapors
are conspicuous among them, such vents are called SOLFATARAS.
Mount Hood, in Oregon, is an example of a volcano sunk to this
stage. From a steaming rift on its side there rise sulphurous
fumes which, half a mile down the wind, will tarnish a silver
coin.
GEYSERS AND HOT SPRINGS. The hot springs of volcanic regions are
among the last vestiges of volcanic heat. Periodically eruptive
boiling springs are termed geysers. In each of the geyser regions
of the earth--the Yellowstone National Park, Iceland, and New
Zealand--the ground water of the locality is supposed to be heated
by ancient lavas that, because of the poor conductivity of the
rock, still remain hot beneath the surface.
OLD FAITHFUL, one of the many geysers of the Yellowstone National
Park, plays a fountain of boiling water a hundred feet in air;
while clouds of vapor from the escaping steam ascend to several
times that height. The eruptions take place at intervals of from
seventy to ninety minutes. In repose the geyser is a quiet pool,
occupying a craterlike depression in a conical mound some twelve
feet high. The conduit of the spring is too irregular to be
sounded. The mound is composed of porous silica deposited by the
waters of the geyser.
Geysers erupt at intervals instead of continuously boiling,
because their long, narrow, and often tortuous conduits do not
permit a free circulation of the water. After an eruption the tube
is refilled and the water again gradually becomes heated. Deep in
the tube where it is in contact with hot lavas the water sooner or
later reaches the boiling point, and bursting into steam shoots
the water above it high in air.
CARBONATED SPRINGS. After all the other signs of life have gone,
the ancient volcano may emit carbon dioxide as its dying breath.
The springs of the region may long be charged with carbon dioxide,
or carbonated, and where they rise through limestone may be
expected to deposit large quantities of travertine. We should
remember, however, that many carbonated springs, and many hot
springs, are wholly independent of volcanoes.
THE DESTRUCTION OF THE CONE. As soon as the volcanic cone ceases
to grow by eruptions the agents of erosion begin to wear it down,
and the length of time that has elapsed since the period of active
growth may be roughly measured by the degree to which the cone has
been dissected. We infer that Mount Shasta, whose conical shape is
still preserved despite the gullies one thousand feet deep which
trench its sides, is younger than Mount Hood, which erosive
agencies have carved to a pyramidal form. The pile of materials
accumulated about a volcanic vent, no matter how vast in bulk, is
at last swept entirely away. The cone of the volcano, active or
extinct, is not old as the earth counts time; volcanoes are short-
lived geological phenomena.
CRANDALL VOLCANO. This name is given to a dissected ancient
volcano in the Yellowstone National Park, which once, it is
estimated, reared its head thousands of feet above the surrounding
country and greatly exceeded in bulk either Mount Shasta or Mount
Etna. Not a line of the original mountain remains; all has been
swept away by erosion except some four thousand feet of the base
of the pile. This basal wreck now appears as a rugged region about
thirty miles in diameter, trenched by deep valleys and cut into
sharp peaks and precipitous ridges. In the center of the area is
found the nucleus (N, Fig. 237),--a mass of coarsely crystalline
rock that congealed deep in the old volcanic pipe. From it there
radiate in all directions, like the spokes of a wheel, long dikes
whose rock grows rapidly finer of grain as it leaves the vicinity
of the once heated core. The remainder of the base of the ancient
mountain is made of rudely bedded tuffs and volcanic breccia, with
occasional flows of lava, some of the fragments of the breccia
measuring as much as twenty feet in diameter. On the sides of
canyons the breccia is carved by rain erosion to fantastic
pinnacles. At different levels in the midst of these beds of tuff
and lava are many old forest grounds. The stumps and trunks of the
trees, now turned to stone, still in many cases stand upright
where once they grew on the slopes of the mountain as it was
building (Fig. 238). The great size and age of some of these trees
indicate, the lapse of time between the eruption whose lavas or
tuffs weathered to the soil on which they grew and the subsequent
eruption which buried them beneath showers of stones and ashes.
Near the edge of the area lies Death Gulch, in which carbon
dioxide is given off in such quantities that in quiet weather it
accumulates in a heavy layer along the ground and suffocates the
animals which may enter it.
CHAPTER XII
UNDERGROUND STRUCTURES OF IGNEOUS ORIGIN
It is because long-continued erosion lays bare the innermost
anatomy of an extinct volcano, and even sweeps away the entire
pile with much of the underlying strata, thus leaving the very
roots of the volcano open to view, that we are able to study
underground volcanic structures. With these we include, for
convenience, intrusions of molten rock which have been driven
upward into the crust, but which may not have succeeded in
breaking way to the surface and establishing a volcano. All these
structures are built of rock forced when in a fluid or pasty state
into some cavity which it has found or made, and we may classify
them therefore, according to the shape of the molds in which the
molten rock has congealed, as (1) dikes, (2) volcanic necks, (3)
intrusive sheets, and (4) intrusive masses.
DIKES. The sheet of once molten rock with which a fissure has been
filled is known as a dike. Dikes are formed when volcanic cones
are rent by explosions or by the weight of the lava column in the
duct, and on the dissection of the pile they appear as radiating
vertical ribs cutting across the layers of lava and tuff of which
the cone is built. In regions undergoing deformation rocks lying
deep below the ground are often broken and the fissures are filled
with molten rock from beneath, which finds no outlet to the
surface. Such dikes are common in areas of the most ancient rocks,
which have been brought to light by long erosion.
In exceptional cases dikes may reach the length of fifty or one
hundred miles. They vary in width from a fraction of a foot to
even as much as three hundred feet.
Dikes are commonly more fine of grain on the sides than in the
center, and may have a glassy and crackled surface where they meet
the inclosing rock. Can you account for this on any principle
which you have learned?
VOLCANIC NECKS. The pipe of a volcano rises from far below the
base of the cone,--from the deep reservoir from which its
eruptions are supplied. When the volcano has become extinct this
great tube remains filled with hardened lava. It forms a
cylindrical core of solid rock, except for some distance below the
ancient crater, where it may contain a mass of fragments which had
fallen back into the chimney after being hurled into the air.
As the mountain is worn down, this central column known as the
VOLCANIC NECK is left standing as a conical hill (Fig. 240). Even
when every other trace of the volcano has been swept away, erosion
will not have passed below this great stalk on which the volcano
was borne as a fiery flower whose site it remains to mark. In
volcanic regions of deep denudation volcanic necks rise solitary
and abrupt from the surrounding country as dome-shaped hills. They
are marked features in the landscape in parts of Scotland and in
the St. Lawrence valley about Montreal (Fig. 241).
INTRUSIVE SHEETS. Sheets of igneous rocks are sometimes found
interleaved with sedimentary strata, especially in regions where
the rocks have been deformed and have suffered from volcanic
action. In some instances such a sheet is seen to be
CONTEMPORANEOUS (p. 248). In other instances the sheet must be
INTRUSIVE. The overlying stratum, as well as that beneath, has
been affected by the heat of the once molten rock. We infer that
the igneous rock when in a molten state was forced between the
strata, much as a card may be pushed between the leaves of a
closed book. The liquid wedged its way between the layers, lifting
those above to make room for itself. The source of the intrusive
sheet may often be traced to some dike (known therefore as the
FEEDING DIKE), or to some mass of igneous rock.
Intrusive sheets may extend a score and more of miles, and, like
the longest surface flows, the most extensive sheets consist of
the more fusible and fluid lavas,--those of the basic class of
which basalt is an example. Intrusive sheets are usually harder
than the strata in which they lie and are therefore often left in
relief after long denudation of the region (Fig. 315).
On the west bank of the Hudson there extends from New York Bay
north for thirty miles a bold cliff several hundred feet high,--
the PALISADES OF THE HUDSON. It is the outcropping edge of a sheet
of ancient igneous rock, which rests on stratified sandstones and
is overlain by strata of the same series. Sandstones and lava
sheet together dip gently to the west arid the latter disappears
from view two miles back from the river.
It is an interesting question whether the Palisades sheet is
CONTEMPORANEOUS or INTRUSIVE. Was it outpoured on the sandstones
beneath it when they formed the floor of the sea, and covered
forthwith by the sediments of the strata above, or was it intruded
among these beds at a later date?
The latter is the case: for the overlying stratum is intensely
baked along the zone of contact. At the west edge of the sheet is
found the dike in which the lava rose to force its way far and
wide between the strata.
ELECTRIC PEAK, one of the prominent mountains of the Yellowstone
National Park, is carved out of a mass of strata into which many
sheets of molten rock have been intruded. The western summit
consists of such a sheet several hundred feet thick. Studying the
section of Figure 244, what inference do you draw as to the source
of these intrusive sheets?
INTRUSIVE MASSES
BOSSES. This name is generally applied to huge irregular masses of
coarsely crystalline igneous rock lying in the midst of other
formations. Bosses vary greatly in size and may reach scores of
miles in extent. Seldom are there any evidences found that bosses
ever had connection with the surface. On the other hand, it is
often proved that they have been driven, or have melted their way,
upward into the formations in which they lie; for they give off
dikes and intrusive sheets, and have profoundly altered the rocks
about them by their heat.
The texture of the rock of bosses proves that consolidation
proceeded slowly and at great depths, and it is only because of
vast denudation that they are now exposed to view. Bosses are
commonly harder than the rocks about them, and stand up,
therefore, as rounded hills and mountainous ridges long after the
surrounding country has worn to a low plain.
The base of bosses is indefinite or undetermined, and in this
respect they differ from laccoliths. Some bosses have broken and
faulted the overlying beds; some have forced the rocks aside and
melted them away.
The SPANISH PEAKS of southeastern Colorado were formed by the
upthrust of immense masses of igneous rock, bulging and breaking
the overlying strata. On one side of the mountains the throw of
the fault is nearly a mile, and fragments of deep-lying beds were
dragged upward by the rising masses. The adjacent rocks were
altered by heat to a distance of several thousand feet. No
evidence appears that the molten rock ever reached the surface,
and if volcanic eruptions ever took place either in lava flows or
fragmental materials, all traces of them have been effaced. The
rock of the intrusive masses is coarsely crystalline, and no doubt
solidified slowly under the pressure of vast thicknesses of
overlying rock, now mostly removed by erosion.
A magnificent system of dikes radiates from the Peaks to a
distance of fifteen miles, some now being left by long erosion as
walls a hundred feet in height (Fig. 239). Intrusive sheets fed by
the dikes penetrate the surrounding strata, and their edges are
cut by canyons as much as twenty-five miles from the mountain. In
these strata are valuable beds of lignite, an imperfect coal,
which the heat of dikes and sheets has changed to coke.
LACCOLITHS. The laccolith (Greek laccos, cistern; lithos, stone)
is a variety of intrusive masses in which molten rock has spread
between the strata, and, lifting the strata above it to a dome-
shaped form, has collected beneath them in a lens-shaped body with
a flat base.
The HENRY MOUNTAINS, a small group of detached peaks in southern
Utah, rise from a plateau of horizontal rocks. Some of the peaks
are carved wholly in separate domelike uplifts of the strata of
the plateau. In others, as Mount Hillers, the largest of the
group, there is exposed on the summit a core of igneous rock from
which the sedimentary rocks of the flanks dip steeply outward in
all directions. In still others erosion has stripped off the
covering strata and has laid bare the core to its base; and its
shape is here seen to be that of a plano-convex lens or a baker's
bun, its flat base resting on the undisturbed bedded rocks
beneath. The structure of Mount Hillers is shown in Figure 248.
The nucleus of igneous rock is four miles in diameter and more
than a mile in depth.
REGIONAL INTRUSIONS. These vast bodies of igneous rock, which may
reach hundreds of miles in diameter, differ little from bosses
except in their immense bulk. Like bosses, regional intrusions
give off dikes and sheets and greatly change the rocks about them
by their heat. They are now exposed to view only because of the
profound denudation which has removed the upheaved dome of rocks
beneath which they slowly cooled. Such intrusions are accompanied
--whether as cause or as effect is still hardly known--by
deformations, and their masses of igneous rock are thus found as
the core of many great mountain ranges. The granitic masses of
which the Bitter Root Mountains and the Sierra Nevadas have been
largely carved are each more than three hundred miles in length.
Immense regional intrusions, the cores of once lofty mountain
ranges, are found upon the Laurentian peneplain.
PHYSIOGRAPHIC EFFECTS OF INTRUSIVE MASSES. We have already seen
examples of the topographic effects of intrusive masses in Mount
Hillers, the Spanish Peaks, and in the great mountain ranges
mentioned in the paragraph on regional intrusions, although in the
latter instances these effects are entangled with the effects of
other processes. Masses of igneous rock cannot be intruded within
the crust without an accompanying deformation on a scale
corresponding to the bulk of the intruded mass. The overlying
strata are arched into hills or mountains, or, if the molten
material is of great extent, the strata may conceivably be floated
upward to the height of a plateau. We may suppose that the
transference of molten matter from one region to another may be
among the causes of slow subsidences and elevations. Intrusions
give rise to fissures, dikes, and intrusive sheets, and these
dislocations cannot fail to produce earthquakes. Where intrusive
masses open communication with the surface, volcanoes are
established or fissure eruptions occur such as those of Iceland.
THE INTRUSIVE ROCKS
The igneous rocks are divided into two general classes,--the
VOLCANIC or ERUPTIVE rocks, which have been outpoured in open air
or on the floor of the sea, and the INTRUSIVE rocks, which have
been intruded within the rocks of the crust and have solidified
below the surface. The two classes are alike in chemical
composition and may be divided into acidic and basic groups. In
texture the intrusive rocks differ from the volcanic rocks because
of the different conditions under which they have solidified. They
cooled far more slowly beneath the cover of the rocks into which
they were pressed than is permitted to lava flows in open air.
Their constituent minerals had ample opportunity to sort
themselves and crystallize from the fluid mixture, and none of
that mixture was left to congeal as a glassy paste.
They consolidated also under pressure. They are never scoriaceous,
for the steam with which they were charged was not allowed to
expand and distend them with steam blebs. In the rocks of the
larger intrusive masses one may see with a powerful microscope
exceedingly minute cavities, to be counted by many millions to the
cubic inch, in which the gaseous water which the mass contained
was held imprisoned under the immense pressure of the overlying
rocks.
Naturally these characteristics are best developed in the
intrusives which cooled most slowly, i.e. in the deepest-seated
and largest masses; while in those which cooled more rapidly, as
in dikes and sheets, we find gradations approaching the texture of
surface flows.
VARIETIES OF THE INTRUSIVE ROCKS. We will now describe a few of
the varieties of rocks of deep-seated intrusions. All are even
grained, consisting of a mass of crystalline grains formed during
one continuous stage of solidification, and no porphyritic
crystals appear as in lavas.
GRANITE, as we have learned already, is composed of three
minerals,--quartz, feldspar, and mica. According to the color of
the feldspar the rock may be red, or pink, or gray. Hornblende--a
black or dark green mineral, an iron-magnesian silicate, about as
hard as feldspar--is sometimes found as a fourth constituent, and
the rock is then known as HORNBLENDIC GRANITE. Granite is an
acidic rock corresponding to rhyolite in chemical composition. We
may believe that the same molten mass which supplies this acidic
lava in surface flows solidifies as granite deep below ground in
the volcanic reservoir.
SYENITE, composed of feldspar and mica, has consolidated from a
less siliceous mixture than has granite.
DIORITE, still less siliceous, is composed of hornblende and
feldspar,--the latter mineral being of different variety from the
feldspar of granite and syenite.
GABBRO, a typical basic rock, corresponds to basalt in chemical
composition. It is a dark, heavy, coarsely crystalline aggregate
of feldspar and AUGITE (a dark mineral allied to hornblende). It
often contains MAGNETITE (the magnetic black oxide of iron) and
OLIVINE (a greenish magnesian silicate).
In the northern states all these types, and many others also of
the vast number of varieties of intrusive rocks, can be found
among the rocks of the drift brought from the areas of igneous
rock in Canada and the states of our northern border.
SUMMARY. The records of geology prove that since the earliest of
their annals tremendous forces have been active in the earth. In
all the past, under pressures inconceivably great, molten rock has
been driven upward into the rocks of the crust. It has squeezed
into fissures forming dikes; it has burrowed among the strata as
intrusive sheets; it has melted the rocks away or lifted the
overlying strata, filling the chambers which it has made with
intrusive masses. During all geological ages molten rock has found
way to the surface, and volcanoes have darkened the sky with
clouds of ashes and poured streams of glowing lava down their
sides. The older strata,--the strata which have been most deeply
buried,--and especially those which have suffered most from
folding and from fracture, show the largest amount of igneous
intrusions. The molten rock which has been driven from the earth's
interior to within the crust or to the surface during geologic
time must be reckoned in millions of cubic miles.
THE INTERIOR CONDITION OF THE EARTH AND CAUSES OF VULCANISM AND
DEFORMATION
The problems of volcanoes and of deformation are so closely
connected with that of the earth's interior that we may consider
them together. Few of these problems are solved, and we may only
state some known facts and the probable conclusions which may be
drawn as inferences from them.
THE INTERIOR OF THE EARTH IS HOT. Volcanoes prove that in many
parts of the earth there exist within reach of the surface regions
of such intense heat that the rock is in a molten condition. Deep
wells and mines show everywhere an increase in temperature below
the surface shell affected by the heat of summer and the cold of
winter,--a shell in temperate latitudes sixty or seventy feet
thick. Thus in a boring more than a mile deep at Schladebach,
Germany, the earth grows warmer at the rate of 1 degrees F. for
every sixty-seven feet as we descend. Taking the average rate of
increase at one degree for every sixty feet of descent, and
assuming that this rate, observed at the moderate distances open
to observation, continues to at least thirty-five miles, the
temperature at that depth must be more than three thousand
degrees,--a temperature at which all ordinary rocks would melt at
the earth's surface. The rate of increase in temperature probably
lessens as we go downward, and it may not be appreciable below a
few hundred miles. But there is no reason to doubt that THE
INTERIOR OF THE EARTH IS INTENSELY HOT. Below a depth of one or
two score miles we may imagine the rocks everywhere glowing with
heat.
Although the heat of the interior is great enough to melt all
rocks at atmospheric pressure, it does not follow that the
interior is fluid. Pressure raises the fusing point of rocks, and
the weight of the crust may keep the interior in what may be
called a solid state, although so hot as to be a liquid or a gas
were the pressure to be removed.
THE INTERIOR OF THE EARTH IS RIGID AND HEAVY. The earth behaves as
a globe more rigid than glass under the attractions of the sun and
moon. It is not deformed by these stresses as is the ocean in the
tides, proving that it is not a fluid ball covered with a yielding
crust a few miles thick. Earthquakes pass through the earth faster
than they would were it of solid steel. Hence the rocks of the
interior are highly elastic, being brought by pressure to a
compact, continuous condition unbroken by the cracks and vesicles
of surface rocks. THE INTERIOR OF THE EARTH IS RIGID
The common rocks of the crust are about two and a half times
heavier than water, while the earth as a whole weighs five and
six-tenths times as much as a globe of water of the same size. THE
INTERIOR IS THEREFORE MUCH MORE HEAVY THAN THE CRUST. This may be
caused in part by compression of the interior under the enormous
weight of the crust, and in part also by an assortment of
material, the heavier substances, such as the heavy metals, having
gravitated towards the center.
Between the crust, which is solid because it is cool, and the
interior, which is hot enough to melt were it not for the pressure
which keeps it dense and rigid, there may be an intermediate zone
in which heat and pressure are so evenly balanced that here rock
liquefies whenever and wherever the pressure upon it may be
relieved by movements of the crust. It is perhaps from such a
subcrustal layer that the lava of volcanoes is supplied.
THE CAUSES OF VOLCANIC ACTION. It is now generally believed that
the HEAT of volcanoes is that of the earth's interior. Other
causes, such as friction and crushing in the making of mountains
and the chemical reactions between oxidizing agents of the crust
and the unoxidized interior, have been suggested, but to most
geologists they seem inadequate.
There is much difference of opinion as to the FORCE which causes
molten rock to rise to the surface in the ducts of volcanoes.
Steam is so evidently concerned in explosive eruptions that many
believe that lava is driven upward by the expansive force of the
steam with which it is charged, much as a viscid liquid rises and
boils over in a test tube or kettle.
But in quiet eruptions, and still more in the irruption of
intrusive sheets and masses, there is little if any evidence that
steam is the driving force. It is therefore believed by many
geologists that it is PRESSURE DUE TO CRUSTAL MOVEMENTS AND
INTERNAL STRESSES which squeezes molten rock from below into
fissures and ducts in the crust. It is held by some that where
considerable water is supplied to the rising column of lava, as
from the ground water of the surrounding region, and where the
lava is viscid so that steam does not readily escape, the eruption
is of the explosive type; when these conditions do not obtain, the
lava outwells quietly, as in the Hawaiian volcanoes. It is held by
others not only that volcanoes are due to the outflow of the
earth's deep-seated heat, but also that the steam and other
emitted gases are for the most part native to the earth's interior
and never have had place in the circulation of atmospheric and
ground waters.
VOLCANIC ACTION AND DEFORMATION. Volcanoes do not occur on wide
plains or among ancient mountains. On the other hand, where
movements of the earth's crust are in progress in the uplift of
high plateaus, and still more in mountain making, molten rock may
reach the surface, or may be driven upward toward it forming great
intrusive masses. Thus extensive lava flows accompanied the
upheaval of the block mountains of western North America and the
uplift of the Colorado plateau. A line of recent volcanoes may be
traced along the system of rift valleys which extends from the
Jordan and Dead Sea through eastern Africa to Lake Nyassa. The
volcanoes of the Andes show how conspicuous volcanic action may be
in young rising ranges. Folded mountains often show a core of
igneous rock, which by long erosion has come to form the axis and
the highest peaks of the range, as if the molten rock had been
squeezed up under the rising upfolds. As we decipher the records
of the rocks in historical geology we shall see more fully how, in
all the past, volcanic action has characterized the periods of
great crustal movements, and how it has been absent when and where
the earth's crust has remained comparatively at rest.
THE CAUSES OF DEFORMATION. As the earth's interior, or nucleus, is
highly heated it must be constantly though slowly losing its heat
by conduction through the crust and into space; and since the
nucleus is cooling it must also be contracting. The nucleus has
contracted also because of the extrusion of molten matter, the
loss of constituent gases given off in volcanic eruptions, and
(still more important) the compression and consolidation of its
material under gravity. As the nucleus contracts, it tends to draw
away from the cooled and solid crust, and the latter settles,
adapting itself to the shrinking nucleus much as the skin of a
withering apple wrinkles down upon the shrunken fruit. The
unsupported weight of the spherical crust develops enormous
tangential pressures, similar to the stresses of an arch or dome,
and when these lateral thrusts accumulate beyond the power of
resistance the solid rock is warped and folded and broken.
Since the planet attained its present mass it has thus been
lessening in volume. Notwithstanding local and relative upheavals
the earth's surface on the whole has drawn nearer and nearer to
the center. The portions of the lithosphere which have been
carried down the farthest have received the waters of the oceans,
while those portions which have been carried down the least have
emerged as continents.
Although it serves our convenience to refer the movements of the
crust to the sea level as datum plane, it is understood that this
level is by no means fixed. Changes in the ocean basins increase
or reduce their capacity and thus lower or raise the level of the
sea. But since these basins are connected, the effect of any
change upon the water level is so distributed that it is far less
noticeable than a corresponding change would be upon the land.
CHAPTER XIII
METAMORPHISM AND MINERAL VEINS
Under the action of internal agencies rocks of all kinds may be
rendered harder, more firmly cemented, and more crystalline. These
processes are known as METAMORPHISM, and the rocks affected,
whether originally sedimentary or igneous, are called METAMORPHIC
ROCKS. We may contrast with metamorphism the action of external
agencies in weathering, which render rocks less coherent by
dissolving their soluble parts and breaking down their crystalline
grains.
CONTACT METAMORPHISM. Rocks beneath a lava flow or in contact with
igneous intrusions are found to be metamorphosed to various
degrees by the heat of the cooling mass. The adjacent strata may
be changed only in color, hardness, and texture. Thus, next to a
dike, bituminous coal may be baked to coke or anthracite, and
chalk and limestone to crystalline marble. Sandstone may be
converted into quartzite, and shale into ARGILLITE, a compact,
massive clay rock. New minerals may also be developed. In
sedimentary rocks there may be produced crystals of mica and of
GARNET (a mineral as hard as quartz, commonly occurring in red,
twelve-sided crystals). Where the changes are most profound, rocks
may be wholly made over in structure and mineral composition.
In contact metamorphism, thin sheets of molten rock produce less
effect than thicker ones. The strongest heat effects are naturally
caused by bosses and regional intrusions, and the zone of change
about them may be several miles in width. In these changes heated
waters and vapors from the masses of igneous rocks undoubtedly
play a very important part.
Which will be more strongly altered, the rocks about a closed dike
in which lava began to cool as soon as it filled the fissure, or
the rocks about a dike which opened on the surface and through
which the molten rock flowed for some time?
Taking into consideration the part played by heated waters, which
will produce the most far-reaching metamorphism, dikes which cut
across the bedding planes or intrusive sheets which are thrust
between the strata?
REGIONAL METAMORPHISM. Metamorphic rocks occur wide-spread in many
regions, often hundreds of square miles in area, where such
extensive changes cannot be accounted for by igneous intrusions.
Such are the dissected cores of lofty mountains, as the Alps, and
the worn-down bases of ancient ranges, as in New England, large
areas in the Piedmont Belt, and the Laurentian peneplain.
In these regions the rocks have yielded to immense pressure. They
have been folded, crumpled, and mashed, and even their minute
grains, as one may see with a microscope, have often been
puckered, broken, and crushed to powder. It is to these mechanical
movements and strains which the rocks have suffered in every part
that we may attribute their metamorphism, and the degree to which
they have been changed is in direct proportion to the degree to
which they have been deformed and mashed.
Other factors, however, have played important parts. Rock crushing
develops heat, and allows a freer circulation of heated waters and
vapors. Thus chemical reactions are greatly quickened; minerals
are dissolved and redeposited in new positions, or their chemical
constituents may recombine in new minerals, entirely changing the
nature of the rock, as when, for example, feldspar recrystallizes
as quartz and mica.
Early stages of metamorphism are seen in SLATE. Pressure has
hardened the marine muds, the arkose, or the volcanic ash from
which slates are derived, and has caused them to cleave by the
rearrangement of their particles.
Under somewhat greater pressure, slate becomes PHYLLITE, a clay
slate whose cleavage surfaces are lustrous with flat-lying mica
flakes. The same pressure which has caused the rock to cleave has
set free some of its mineral constituents along the cleavage
planes to crystallize there as mica.
FOLIATION. Under still stronger pressure the whole structure of
the rock is altered. The minerals of which it is composed, and the
new minerals which develop by heat and pressure, arrange
themselves along planes of cleavage or of shear in rudely parallel
leaves, or FOLIA. Of this structure, called FOLIATION, we may
distinguish two types,--a coarser feldspathic type, and a fine
type in which other minerals than feldspar predominate.
GNEISS is the general name under which are comprised coarsely
foliated rocks banded with irregular layers of feldspar and other
minerals. The gneisses appear to be due in many cases to the
crushing and shearing of deep-seated igneous rocks, such as
granite and gabbro.
THE CRYSTALLINE SCHISTS, representing the finer types of
foliation, consist of thin, parallel, crystalline leaves, which
are often remarkably crumpled. These folia can be distinguished
from the laminae of sedimentary rocks by their lenticular form and
lack of continuity, and especially by the fact that they consist
of platy, crystalline grains, and not of particles rounded by
wear.
MICA SCHIST, the most common of schists, and in fact of all
metamorphic rocks, is composed of mica and quartz in alternating
wavy folia. All gradations between it and phyllite may be traced,
and in many cases we may prove it due to the metamorphism of
slates and shales. It is widespread in New England and along the
eastern side of the Appalachians. TALC SCHIST consists of quartz
and TALC, a light-colored magnesian mineral of greasy feel, and so
soft that it can be scratched with the thumb nail.
HORNBLENDE SCHIST, resulting in many cases from the foliation of
basic igneous rocks, is made of folia of hornblende alternating
with bands of quartz and feldspar. Hornblende schist is common
over large areas in the Lake Superior region.
QUARTZ SCHIST is produced from quartzite by the development of
fine folia of mica along planes of shear. All gradations may be
found between it and unfoliated quartzite on the one hand and mica
schist on the other.
Under the resistless pressure of crustal movements almost any
rocks, sandstones, shales, lavas of all kinds, granites, diorites,
and gabbros may be metamorphosed into schists by crushing and
shearing. Limestones, however, are metamorphosed by pressure into
marble, the grains of carbonate of lime recrystallizing freely to
interlocking crystals of calcite.
These few examples must suffice of the great class of metamorphic
rocks. As we have seen, they owe their origin to the alteration of
both of the other classes of rocks--the sedimentary and the
igneous--by heat and pressure, assisted usually by the presence of
water. The fact of change is seen in their hardness arid
cementation, their more or less complete recrystallization, and
their foliation; but the change is often so complete that no trace
of their original structure and mineral composition remains to
tell whether the rocks from which they were derived were
sedimentary or igneous, or to what variety of either of these
classes they belonged.
In many cases, however, the early history of a metamorphic rock
can be deciphered. Fossils not wholly obliterated may prove it
originally water-laid. Schists may contain rolled-out pebbles,
showing their derivation from a conglomerate. Dikes of igneous
rocks may be followed into a region where they have been foliated
by pressure. The most thoroughly metamorphosed rocks may sometimes
be traced out into unaltered sedimentary or igneous rocks, or
among them may be found patches of little change where their
history maybe read.
Metamorphism is most common among rocks of the earlier geological
ages, and most rare among rocks of recent formation. No doubt it
is now in progress where deep-buried sediments are invaded
by heat either from intrusive igneous masses or from the earth's
interior, or are suffering slow deformation under the thrust of
mountain-making forces.
Suggest how rocks now in process of metamorphism may sometimes be
exposed to view. Why do metamorphic rocks appear on the surface
to-day?
MINERAL VEINS
In regions of folded and broken rocks fissures are frequently
found to be filled with sheets of crystalline minerals deposited
from solution by underground water, and fissures thus filled are
known as mineral veins. Much of the importance of mineral veins is
due to the fact that they are often metalliferous, carrying
valuable native metals and metallic ores disseminated in fine
particles, in strings, and sometimes in large masses in the midst
of the valueless nonmetallic minerals which make up what is known
as the VEIN STONE.
The most common vein stones are QUARTZ and CALCITE. FLUORITE
(calcium fluoride), a mineral harder than calcite and
crystallizing in cubes of various colors, and BARITE (barium
sulphate), a heavy white mineral, are abundant in many veins.
The gold-bearing quartz veins of California traverse the
metamorphic slates of the Sierra Nevada Mountains. Below the zone
of solution (p. 45) these veins consist of a vein stone of quartz
mingled with pyrite (p. 13), the latter containing threads and
grains of native gold. But to the depth of about fifty feet from
the surface the pyrite of the vein has been dissolved, leaving a
rusty, cellular quartz with grains of the insoluble gold scattered
through it.
The PLACER DEPOSITS of California and other regions are gold-
bearing deposits of gravel and sand in river beds. The heavy gold
is apt to be found mostly near or upon the solid rock, and its
grains, like those of the sand, are always rounded. How the gold
came in the placers we may leave the pupil to suggest.
Copper is found in a number of ores, and also in the native metal.
Below the zone of surface changes the ore of a copper vein is
often a double sulphide of iron and copper called CHALCOPYRITE, a
mineral softer than pyrite--it can easily be scratched with a
knife--and deeper yellow in color. For several score of feet below
the ground the vein may consist of rusty quartz from which the
metallic ores have been dissolved; but at the base of the zone of
solution we may find exceedingly rich deposits of copper ores,--
copper sulphides, red and black copper oxides, and green and blue
copper carbonates, which have clearly been brought down in
solution from the leached upper portion of the vein.
ORIGIN OF MINERAL VEINS. Both vein stones and ores have been
deposited slowly from solution in water, much as crystals of salt
are deposited on the sides of a jar of saturated brine. In our
study of underground water we learned that it is everywhere
circulating through the permeable rocks of the crust, descending
to profound depths under the action of gravity and again driven to
the surface by hydrostatic pressure. Now fissures, wherever they
occur, form the trunk channels of the underground circulation.
Water descends from the surface along these rifts; it moves
laterally from either side to the fissure plane, just as ground
water seeps through the surrounding rocks from every direction to
a well; and it ascends through these natural water ways as in an
artesian well, whenever they intersect an aquifer in which water
is under hydrostatic pressure.
The waters which deposit vein stones and ores are commonly hot,
and in many cases they have derived their heat from intrusions of
igneous rock still uncooled within the crust. The solvent power of
the water is thus greatly increased, and it takes up into solution
various substances from the igneous and sedimentary rocks which it
traverses. For various reasons these substances stances are
deposited in the vein as ores and vein stones. On rising through
the fissure the water cools and loses pressure, and its capacity
to hold minerals in solution is therefore lessened. Besides, as
different currents meet in the fissure, some ascending, some
descending, and some coming in from the sides, the chemical
reaction of these various weak solutions upon one another and upon
the walls of the vein precipitates the minerals of vein stuffs and
ores.
As an illustration of the method of vein deposits we may cite the
case of a wooden box pipe used in the Comstock mines, Nevada, to
carry the hot water of the mine from one level to another, which
in ten years was lined with calcium carbonate more than half an
inch thick.
The Steamboat Springs, Nevada, furnish examples of mineral veins
in process of formation. The steaming water rises through fissures
in volcanic rocks and is now depositing in the rifts a vein stone
of quartz, with metallic ores of iron, mercury, lead, and other
metals.
RECONCENTRATION. Near the base of the zone of solution veins are
often stored with exceptionally large and valuable ore deposits.
This local enrichment of the vein is due to the reconcentration of
its metalliferous ores. As the surface of the land is slowly
lowered by weathering and running water, the zone of solution is
lowered at an equal rate and encroaches constantly on the zone of
cementation. The minerals of veins are therefore constantly being
dissolved along their upper portions and carried down the fissures
by ground water to lower levels, where they are redeposited.
Many of the richest ore deposits are thus due to successive
concentrations: the ores were leached originally from the rocks to
a large extent by laterally seeping waters; they were concentrated
in the ore deposits of the vein chiefly by ascending currents;
they have been reconcentrated by descending waters in the way just
mentioned.
THE ORIGINAL SOURCE OF THE METALS. It is to the igneous rocks that
we may look for the original source of the metals of veins. Lavas
contain minute percentages of various metallic compounds, and no
doubt this was the case also with the igneous rocks which formed
the original earth crust. By the erosion of the igneous rocks the
metals have been distributed among sedimentary strata, and even
the sea has taken into solution an appreciable amount of gold and
other metals, but in this widely diffused condition they are
wholly useless to man. The concentration which has made them
available is due to the interaction of many agencies. Earth
movements fracturing deeply the rocks of the crust, the intrusion
of heated masses, the circulation of underground waters, have all
cooperated in the concentration of the metals of mineral veins.
While fissure veins are the most important of mineral veins, the
latter term is applied also to any water way which has been filled
by similar deposits from solution. Thus in soluble rocks, such as
limestones, joints enlarged by percolating water are sometimes
filled with metalliferous deposits, as, for example, the lead and
zinc deposits of the upper Mississippi valley. Even a porous
aquifer may be made the seat of mineral deposits, as in the case
of some copper-bearing and silver-bearing sandstones of New
Mexico.
PART III
HISTORICAL GEOLOGY
CHAPTER XIV
THE GEOLOGICAL RECORD
WHAT A FORMATION RECORDS. We have already learned that each
individual body of stratified rock, or formation, constitutes a
record of the time when it was laid. The structure and the
character of the sediments of each formation tell whether the area
was land or sea at the time when they were spread; and if the
former, whether the land was river plain, or lake bed, or was
covered with wind-blown sands, or by the deposits of an ice sheet.
If the sediments are marine, we may know also whether they were
laid in shoal water near the shore or in deeper water out at sea,
and whether during a period of emergence, or during a period of
subsidence when the sea transgressed the land. By the same means
each formation records the stage in the cycle of erosion of the
land mass from which its sediments were derived. An unconformity
between two marine formations records the fact that between the
periods when they were deposited in the sea the area emerged as
land and suffered erosion. The attitude and structure of the
strata tell also of the foldings and fractures, the deformation
and the metamorphism, which they have suffered; and the igneous
rocks associated with them as lava flows and igneous intrusions
add other details to the story. Each formation is thus a separate
local chapter in the geological history of the earth, and its
strata are its leaves. It contains an authentic record of the
physical conditions--the geography--of the time and place when and
where its sediments were laid.
PAST CYCLES OF EROSION. These chapters in the history of the
planet are very numerous, although much of the record has been
destroyed in various ways. A succession of different formations is
usually seen in any considerable section of the crust, such as a
deep canyon or where the edges of upturned strata are exposed to
view on the flanks of mountain ranges; and in any extensive area,
such as a state of the Union or a province of Canada, the number
of formations outcropping on the surface is large.
It is thus learned that our present continent is made up for. the
most part of old continental deltas. Some, recently emerged as the
strata of young coastal plains, are the records of recent cycles
of erosion; while others were deposited in the early history of
the earth, and in many instances have been crumpled into
mountains, which afterwards were leveled to their bases and
lowered beneath the sea to receive a cover of later sediments
before they were again uplifted to form land.
The cycle of erosion now in progress and recorded in the layers of
stratified rock being spread beneath the sea in continental deltas
has therefore been preceded by many similar cycles. Again and
again movements of the crust have brought to an end one cycle--
sometimes when only well under way, and sometimes when drawing
toward its close--and have begun another. Again and again they
have added to the land areas which before were sea, with all their
deposition records of earlier cycles, or have lowered areas of
land beneath the sea to receive new sediments.
THE AGE OF THE EARTH. The thickness of the stratified rocks now
exposed upon the eroded surface of the continents is very great.
In the Appalachian region the strata are seven or eight miles
thick, and still greater thicknesses have been measured in several
other mountain ranges. The aggregate thickness of all the
formations of the stratified rocks of the earth's crust, giving to
each formation its maximum thickness wherever found, amounts to
not less than forty miles. Knowing how slowly sediments accumulate
upon the sea floor, we must believe that the successive cycles
which the earth has seen stretch back into a past almost
inconceivably remote, and measure tens of millions and perhaps
even hundreds of millions of years.
HOW THE FORMATIONS ARE CORRELATED AND THE GEOLOGICAL RECORD MADE
UP. Arranged in the order of their succession, the formations of
the earth's crust would constitute a connected record in which the
geological history of the planet may be read, and therefore known
as the GEOLOGICAL RECORD. But to arrange the formations in their
natural order is not an easy task. A complete set of the volumes
of the record is to be found in no single region. Their leaves and
chapters are scattered over the land surface of the globe. In one
area certain chapters may be found, though perhaps with many
missing leaves, and with intervening chapters wanting, and these
absent parts perhaps can be supplied only after long search
through many other regions.
Adjacent strata in any region are arranged according to the LAW OF
SUPERPOSITION, i.e. any stratum is younger than that on which it
was deposited, just as in a pile of paper, any sheet was laid
later than that on which it rests. Where rocks have been
disturbed, their original attitude must be determined before the
law can be applied. Nor can the law of superposition be used in
identifying and comparing the strata of different regions where
the formations cannot be traced continuously from one region to
the other.
The formations of different regions are arranged in their true
order by the LAW OF INCLUDED ORGANISMS; i.e. formations, however
widely separated, which contain a similar assemblage of fossils
are equivalent and belong to the same division of geological time.
The correlation of formations by means of fossils may be explained
by the formations now being deposited about the north Atlantic.
Lithologically they are extremely various. On the continental
shelf of North America limestones of different kinds are forming
off Florida, and sandstones and shales from Georgia northward.
Separated from them by the deep Atlantic oozes are other
sedimentary deposits now accumulating along the west coast of
Europe. If now all these offshore formations were raised to open
air, how could they be correlated? Surely not by lithological
likeness, for in this respect they would be quite diverse. All
would be similar, however, in the fossils which they contain. Some
fossil species would be identical in all these formations and
others would be closely allied. Making all due allowance for
differences in species due to local differences in climate and
other physical causes, it would still be plain that plants and
animals so similar lived at the same period of time, and that the
formations in which their remains were imbedded were
contemporaneous in a broad way. The presence of the bones of
whales and other marine mammals would prove that the strata were
laid after the appearance of mammals upon earth, and imbedded
relics of man would give a still closer approximation to their
age. In the same way we correlate the earlier geological
formations.
For example, in 1902 there were collected the first fossils ever
found on the antarctic continent. Among the dozen specimens
obtained were some fossil ammonites (a family of chambered shells)
of genera which are found on other continents in certain
formations classified as the Cretaceous system, and which occur
neither above these formations nor below them. On the basis of
these few fossils we may be confident that the strata in which
they were found in the antarctic region were laid in the same
period of geologic time as were the Cretaceous rocks of the United
States and Canada.
THE RECORD AS A TIME SCALE. By means of the law of included
organisms and the law of superposition the formations of different
countries and continents are correlated and arranged in their
natural order. When the geological record is thus obtained it may
be used as a universal time scale for geological history.
Geological time is separated into divisions corresponding to the
times during which the successive formations were laid. The
largest assemblages of formations are known as groups, while the
corresponding divisions of time are known as eras. Groups are
subdivided into systems, and systems into series. Series are
divided into stages and substages,--subdivisions which do not
concern us in this brief treatise. The corresponding divisions of
time are given in the following table.
STRATA TIME
Group Era
System Period
Series Epoch
The geologist is now prepared to read the physical history--the
geographical development--of any country or of any continent by
means of its formations, when he has given each formation its true
place in the geological record as a time scale.
The following chart exhibits the main divisions of the record, the
name given to each being given also to the corresponding time
division. Thus we speak of the CAMBRIAN SYSTEM, meaning a certain
succession of formations which are classified together because of
broad resemblances in their included organisms; and of the
CAMBRIAN PERIOD, meaning the time during which these rocks were
deposited.
Group and Era System and Period Series and Epoch
|Quaternary-----|Recent
Cenozoic------| |Pleistocene
|
|Tertiary-------|Pliocene
|Miocene
|Eocene
|Cretaceous
Mesozoic------|Jurassic
|Triassic
|Permian
|Carboniferous--|Pennsylvanian
| |Mississippian
Paleozoic-----|Devonian
|Silurian
|Ordovician
|Cambrian
Algonkian
Archean
FOSSILS AND WHAT THEY TEACH
The geological formations contain a record still more important
than that of the geographical development of the continents; the
fossils imbedded in the rocks of each formation tell of the kinds
of animals and plants which inhabited the earth at that time, and
from these fossils we are therefore able to construct the history
of life upon the earth.
FOSSILS. These remains of organisms are found in the strata in all
degrees of perfection, from trails and tracks and fragmentary
impressions, to perfectly preserved shells, wood, bones, and
complete skeletons. As a rule, it is only the hard parts of
animals and plants which have left any traces in the rocks.
Sometimes the original hard substance is preserved, but more often
it has been replaced by some less soluble material. Petrifaction,
as this process of slow replacement is called, is often carried on
in the most exquisite detail. When wood, for example, is
undergoing petrifaction, the woody tissue may be replaced,
particle by particle, by silica in solution through the action of
underground waters, even the microscopic structures of the wood
being perfectly reproduced. In shells originally made of
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