INSTABILITY OF THE EARTH'S CRUST
The crust of the earth is unstable. To the modern student of
geology the old notion of a "terra firma" is outworn. The idea of
an unshakable, immovable earth could never have emanated from the
inhabitants of an earthquake country. In general we may recognize
two types of crustal movements--slow and sudden. To most people the
sudden movements accompanied by earthquakes are more significant and
impressive because they are more localized and evident, and often
accompanied by destruction of property, or quick, though minor, changes
in the landscape. But movements which take place slowly and quietly are
often of far greater significance in the interpretation of the profound
physical changes which have affected the earth during its millions of
years of known history.
[Illustration: Fig. 7.--Structure section across the Hudson River
Valley near West Point, New York. The shafts and tunnel, 1,200 feet
below sea level, in solid rock, show the position of the New York City
aqueduct from the Catskills. The Preglacial valley has been submerged
and filled with Postglacial sediment to a depth of nearly 800 feet.
(Redrawn by the author after Berkey, from New York State Museum
Bulletin.)]
A few well-known examples will serve to prove that upward, downward,
and differential movements of the earth's crust have actually taken
place not only in the remote ages of geologic time, but also that
such movements have geologically recently taken place, and that
similar movements are still going on. It is very important that the
reader thoroughly appreciate the fact that crustal disturbances,
often profound ones, do take place, because this is one of the most
fundamental tenets of geologic science. Let us consider the case of the
Hudson-Champlain-St. Lawrence Valley region. That the whole region was
once notably higher (at least 1,000 feet) than at present is proved by
the drowned character of the Hudson Valley, in which tidewater extends
northward for 150 miles to near Troy. Where the New York City Aqueduct
passes under the Hudson River near Newburgh, the bedrock bottom of the
old river channel is now about 800 feet below sea level as determined
by drilling. This old channel is there filled up nearly to sea level
with glacial and postglacial rock débris, which shows that the old
channel must have been cut before the oncoming of the ice of the great
Ice Age. Before the Ice Age, then, the lower Hudson Valley must have
been considerably more than 800 feet higher than at present, because it
then contained a river with sufficient current to be an active agent
of erosion, carving out the canyonlike valley in the vicinity of West
Point. This conclusion is strongly reenforced by the fact that the old
valley of the Hudson River has been definitely traced as a distinct
trench across the shallow sea bottom for about 100 miles eastward from
the entrance to New York harbor. Toward the eastern end of this trench
the depth of water is now considerably over 1,000 feet, and thus it is
obvious that, preceding the Ice Age, the earth's crust in the vicinity
of New York City must have been much higher than at present, so that
the Hudson River was able to erode its now completely drowned channel.
Somewhat similar evidence has also established the fact that the lower
St. Lawrence Valley region was much higher before the Ice Age. It is
evident, therefore, that the general Hudson-St. Lawrence Valley region
is now notably lower with reference to sea level than it was before the
Ice Age. That this was caused by actual sinking of the earth's crust
rather than by a rise of sea level is proved by the fact that similar
changes of level between land and sea did not take place at the same
time even along the Atlantic and Gulf coast of our Southern States.
We shall now proceed to the next step in the geologically recent
history of earth-crust movements in the Hudson-Champlain-St. Lawrence
Valley region by asserting that, since the Ice Age, the land was
actually notably lower than at present. In fact, the land was enough
lower to allow tidewater to extend up the St. Lawrence Valley into
the Ontario basin, and all through the Champlain-Hudson Valley. Many
beaches, bars, and delta deposits formed in these arms of the sea are
still plainly preserved, in some cases with shells and bones of marine
animals in them, now hundreds of feet above sea level. These marine
deposits are highest above sea level in the northern portion of the
Champlain Valley, where they lie at an altitude of 700 feet or more and
their altitude steadily diminishes southward to about 300 to 400 feet
in the general vicinity of Albany, and to near sea level in the general
vicinity of New York City. Obviously, then, the land stood lower during
part or all of the interval of not more than a few tens of thousands
of years since the Ice Age than at present. This leads us to the third
important conclusion regarding earth movements in this region, namely,
that still later the land has undergone a differential uplift, the rate
having steadily increased toward the north where the total uplift is
many hundreds of feet. We have discussed this region somewhat in detail
because the principles of slow up and down movements of the earth's
crust are there so plainly recorded.
Among many other regions where earth movements similar to those above
described have taken place, brief mention may be made of Norway. The
great fjords of Norway were, just before the Ice Age, stream-cut
valleys which were then more or less modified by glacial erosion,
and after the Ice Age the rivers in them were drowned due to land
subsidence. The kind of evidence is like that above given for the lower
Hudson River. Since the subsidence there has been partial reelevation,
as proved by the fact that along the sides of the larger fjords marine
terraces and beaches may be traced with gradually increasing altitude
for many miles (150 or more) back into the country where they are
hundreds of feet above tidewater.
Scandinavia is of still further special interest because very
appreciable earth movements have there come under human observation.
Marks carefully placed along the shores of Sweden by the government
have proved that during the last 150 years the southern end of the
country has actually subsided several feet, while from Stockholm north
the land has risen in increasing amount, reaching a maximum of seven or
eight feet. In southern Sweden, at Malmo, a certain street now at times
becomes covered by wind-driven high water, and during excavations made
some years ago an older street eight feet below the present one was
found.
A theory which appears to be in perfect harmony with the facts to
account for the subsidence and partial reelevation of central eastern
North America and Scandinavia since the beginning of the Ice Age is
that the great weight of ice during the Ice Age pressed the land down,
and that since the removal of the ice there has been an appreciable
tendency for the land to spring back.
Certain crustal movements which have occurred about the Bay of Naples
are of very special interest because actual human history dates can be
placed upon them. Most remarkable are the records in connection with
the temple of Jupiter Serapis which was built near the shore before
the Christian era. The land sank about five feet and a new pavement
had to be constructed; then, by the middle of the third century A. D.,
the temple rose to well above sea level. By about the ninth century
the land had subsided fully thirty feet, so that marble columns of
the temple were bored full of holes as high as twenty-one feet above
their bases by marine-shelled animals, species of which still live in
the bay. Then a slow uplift of twenty-three feet began, bringing the
bases of the columns two feet above sea level by 1749. Since that time
a slight sinking has taken place and this seems to be still going on.
Three of the marble columns with the borings still stand in upright
position.
While the movements just described were taking place, the island of
Capri, twenty miles across the Bay of Naples, has slowly sunk to an
amount estimated at thirty or forty feet as proved by evidence from the
famous Blue Grotto. About the beginning of the Christian era a large
ancient wave-cut cave, part of which is now called the Blue Grotto, had
its floor above sea level, and it was used by certain Romans as a cool
place to retire to from the heat. In order to obtain better light an
opening was cut through its upper portion. The land has sunk so much
that at the present time even part of the artificial opening (through
which tourists pass) is now under water.
By way of illustrating remarkable contrasts in direction of crustal
movements on very considerable scales in a given region, we shall
briefly mention some facts regarding part of the coast of southern
California and the neighboring islands of Santa Catalina and San
Clemente, respectively twenty-five and fifty miles offshore. Those
movements were not, however, checked up by human history records.
The mainland at San Pedro has clearly risen 1,240 feet, as proved by
the presence of unusually perfect coast terraces (so-called "raised
beaches"), while San Clemente has risen 1,500 feet as proved by the
raised beaches into which deep, youthful V-shaped stream-cut valleys
have been sunk, and a shore line characteristic of recent notable
uplift. It is a remarkable fact that at the same time the intervening
island (Santa Catalina) has notably sunk, as proved by the nature
of its shore line, and the distinctly more mature character of its
topography.
We are, however, by no means dependent upon lands along sea shores
for evidences of slow rising and sinking of land. Thus, by careful
measurements it has been shown that the general region of the Great
Lakes is now differentially rising toward the northeast at the rate
of about five inches per 100 miles per century. At Chicago the rise
of water is estimated at about nine inches per century, which means
increase of flowage through the Chicago Canal. At this rate the upper
lakes would, in some thousands of years, drain through this canal to
the Mississippi. A well-preserved shore line of the large ancestor
of Lake Ontario shows a steady increase in altitude at the rate of
several feet per mile toward the northeast from near Niagara to the St.
Lawrence Valley, thus proving a tilting of the land since the shore
line was formed.
Shore lines of the great ancestor of Great Salt Lake also show warping
of the earth's crust, some parts of a definite shore line being several
hundred feet higher than others.
Very significant evidence pointing to profound crustal movements
consist in the finding of fossil remains of marine animals in the
strata high above sea level, very commonly from one to three miles, in
many parts of the world, especially in the high mountains. In Wyoming,
nearly horizontal strata of the Mesozoic Age carrying marine fossils
lie two miles or more above sea level. The fact that given formations,
carrying marine fossils representing certain definite portions of
geologic time, are found at various altitudes up to several miles in
many parts of the world, shows that the land in those places has
really risen relative to sea level.
It should not be presumed from the above discussion that the sea level
itself has never changed. Thus, the vast areas of thick ice sheets in
both North America and Europe during the Great Ice Age represented
sufficient water withdrawn from the sea to very appreciably lower its
level. All land-derived materials, carried into the sea mainly by
rivers, displace sea water, with consequent rise of its level. If all
existing lands were worn down and carried into the sea, its level would
be raised some hundreds of feet. Subsidence of any part of the ocean
bottom would cause a lowering of sea level. There is a strong reason
to believe that some such shiftings of sea level have occurred during
the vast lapse of geologic time. During certain periods erosion of the
land predominated, and during other periods building up of the land
predominated, as pointed out in the chapters on geologic history. It is
not thought that shifting of sea level has ever amounted to more than a
few hundred feet, at least not during the millions of years of the more
clearly recorded earth history.
We have thus far considered slow upward and downward movements of the
earth's crust without notable structural changes in the rocks. Another
type of crustal disturbance causes more or less profound changes in
the structures of the rocks themselves. Just how the earth originated
is a matter of uncertainty, but we can be sure that for many millions
of years it has been a shrinking body. The outer, or crustal, portion
of the earth, in adjusting itself to the contracting interior, has had
many pressures, stresses, and strains set up within it. As results of
such forces the rocks at and near the earth's surface have in various
places, and at various times, been broken (faulted) and subjected to
sudden movements (see discussion beyond), while those well within the
crustal portion, that is to say a few miles or more down, have, in
many cases, been bent (folded), or even crumpled. For these reasons
the surface and near-surface crustal portions are called the "zone of
fracture," while the more deeply buried portions comprise the "zone
of flowage." In the zone of flowage the rocks, where subjected to
great lateral pressure, act like plastic materials and therefore bend
rather than break, because of the great weight of overlying materials.
Laboratory experiments have confirmed the findings of geologists in
this regard. Small masses of rocks properly inclosed in nickel-steel
cylinders have been subjected to slow differential pressures equivalent
to those which obtain twenty to forty miles within the earth. Under
such conditions rocks have been made to change shape very notably
without fracturing. Both geological observations and experiments have
led us to conclude that not even small fractures or crevices can remain
open at a depth greater than ten or twelve miles even in the hardest
rocks.
From time to time, during the long history of the earth, forces of
lateral pressure have been slowly exerted along more or less localized
zones or belts within the earth's crust, and the rocks have been
deformed chiefly by bending or folding, especially in those regions
where mountains of the folded type have developed. Movements of this
type are considered beyond in the chapter on mountains. Rock folds vary
in size from microscopic to miles across, and they exhibit many shapes.
Plate 7 will give the reader a good idea of actual rock folds of
common sizes and shapes in various places. Folded structures are most
clearly discernible in sedimentary rocks, because of their stratified
(layered) arrangement. Since folds in hard rocks rarely, if ever,
develop except at a depth of some miles within the earth, they show at
the surface only where great thicknesses of overlying materials have
been stripped off by erosion.
[Illustration: Fig. 8.--An outcrop of stratified crystalline limestone
(or marble) exhibiting two small sharp folds--a syncline on the
left and an anticline on the right--near Lenox, Mass, These folds
developed during the great mountain-making disturbance at the end of
the Ordovician period fully 20,000,000 years ago. (After Dale, U. S.
Geological Survey.)]
From the standpoint of our consideration of slow earth-crust movements,
it is important to bear in mind that lateral pressure in the zone of
flowage has not only notably deformed rocks, but that, as a result
of the buckling forces, given rock masses have, in many cases, been
notably shifted downward or upward--mainly upward--from their original
positions. Folded strata carrying shells of sea animals are commonly
found thousands of feet above sea level in many of the great mountain
ranges of the world. During the process of folding on a large scale the
crust of the earth is very appreciably shortened at right angles to
the direction of applied pressure, due to squeezing or bending of the
strata. In the case of the Appalachian mountains of Pennsylvania it has
been estimated that such shortening amounts to about twenty-six miles
or, in other words, that the strata originally spread out horizontally
across an area whose width was about 100 miles have been squeezed or
folded into an area whose width is twenty-six miles less.
[Illustration: Fig. 9.--Structure section showing the profile of the
mountains and relations of rocks below the surface near Livingston,
Montana. The strata were crowded together until they bent into great
sharply defined folds at the time of the Rocky Mountain Revolution
several million years ago. Then the rocks broke along the fault
fracture and the mass on the right was shoved over upon the mass on the
left. (After U. S. Geological Survey.)]
We shall now turn to a consideration of sudden earth movements and some
of their effects, including earthquakes. Mention has already been made
of the fact that, when pressures and strains are set up in the outer
portion ("zone of fracture") of the earth's crust, the rocks yield
mainly by breaking or fracturing because the rocks not being under a
great load of overlying material are there brittle. The earth's crust
has been fractured on small and large scales in many places during
the long space of geologic time. Where one block of earth's crust
has slipped or moved past another along a fracture we have what is
called a "fault." Such displacements of rock masses vary in amount
from less than an inch to some miles, and they constitute one of the
most important features of the architecture of the outer portion of
the earth. There are two types of faults fundamentally different as
to cause. In one type (so-called "normal fault") the rocks suddenly
yield to a force of tension; a fracture develops and the earth block on
one side of the fracture or fault drops with reference to that on the
other. In the other type (so-called "thrust faults") the rocks yield
suddenly to a force of compression or lateral thrust, and one block
of earth is pushed or thrust partly over another along the surface of
fracture or fault. (See Plate 8.)
Faults range in length up to hundreds of miles, those from one to
twenty miles in length being very common. Where an earth block has been
displaced thousands of feet along a fault surface, it is not to be
understood that the whole displacement resulted from a single movement,
but rather from a series of sudden movements separated by greater or
less intervals of time. Each sudden movement along a fault surface
produces a vibration of the earth near by. Many such sudden movements
are known to have caused violent earthquakes. Displacements of twenty
to fifty feet, as a result of single movements, are definitely known
to have taken place in various regions during the last fifty years;
and rarely, if ever, has any sudden displacement of as much as several
hundred feet occurred. Cliffs and steep slopes very commonly result
from faulting, but, because of the long lapse of time required for the
repeated movements in the case of great faults, the cliffs or steep
slopes begin to wear back and become more or less subdued long before
the last of the movements take place. In regions where movements along
great faults have long since ceased, the original steep slopes may be
completely obliterated by erosion.
[Illustration: Fig. 10.--Vertical sections through strata illustrating
common kinds of faults: a, "normal faults" where one mass simply sinks
below another; b, a "thrust fault" where one mass is shoved over
another. (After U. S. Geological Survey.)]
How does the geologist determine the actual amount of displacement,
especially in the case of a large fault in stratified rocks? First, the
various formations of the region, where unaffected by faulting, are
carefully studied, especially in regard to the character and thickness
of each, and their relative geologic ages or normal order as they
were deposited one layer above the other. Then, in the simple case of
a normal-fault surface at right angles to horizontal strata, it is
only necessary to find out what two formations or parts of formations
come together along the fault fracture, and the actual amount of
displacement is readily determined. Where strata and normal fault
surfaces lie at various angles, and also in thrust faults, those angles
must be determined in addition to the data above named. In many mining
regions, where valuable deposits are affected by faulting, accurate
knowledge of the direction and amount of displacements of faults is of
great economic importance.
A few examples of normal faults from well-known districts will now be
briefly described. The whole eastern front of the central and southern
Sierra Nevada Range of California is a great, steep fault slope, from
a few thousand to ten thousand or more feet high and hundreds of miles
long, of such recent geologic age that it has been only moderately
affected by erosion. In fact, it is well known that the southern
two-thirds of the range is a great tilted fault block, the total
displacement having resulted from repeated sudden movements since about
the middle of the present geologic era. A great fault also extends
along the eastern base of the great Wasatch Range of Utah and the steep
slope thousands of feet high is a fault scarp only slightly modified
by erosion. Renewed movements along this profound fault have very
recently taken place as proved by the presence of fresh fault scarps
in loose deposits which have accumulated across the mouths of some of
the canyons, as, for example, near Ogden. In fact, practically all of
the north-south ranges of the Great Basin from Utah to California are
essentially a series of tilted fault blocks. Another great fault, less
conspicuous from the topographic standpoint, is hundreds of miles long
in the Coast Range Mountains of California. At the time of the San
Francisco earthquake of 1906 there was a renewed sudden movement along
this great fracture. The eastern one-half of the Adirondack Mountains
of New York is literally a mosaic of hundreds of fault blocks. Many
of these faults are from two to thirty miles long and they commonly
show displacements of from a few hundred to 2,000 or more feet. A
glance at the geological map (in colors) of the vicinity of the great
copper mines at Bisbee, Arizona, shows most of that region to contain
a network of normal faults which separate it into a mosaic of fault
blocks. In each of the examples of faults just given a block of earth
has sunk relative to the other, or in other words, each is a "normal
fault."
We shall now turn to some large scale cases of faults in which great
masses of earth have been pushed one over another--so-called "thrust
faults." In the southern Appalachian Range, and especially well
exhibited in the vicinity of Rome, Georgia, one portion of the mountain
mass has literally been shoved over another, at a low angle over a
fault surface many miles long, for fully seven miles westward. Both the
tremendous weight of rock material actually translated and the number
of sudden movements required in the operation stagger the imagination.
It is safe to say that during the long time of this great operation
violent earthquakes were not uncommon. In the Rocky Mountains of the
northern United States and southern Canada there is the greatest known
thrust fault on the continent. It is hundreds of miles long, and the
actual displacement is commonly at least several miles. In the Glacier
National Park of Montana it has been established that the front range
portion of the Rockies has actually been pushed at least seven miles,
and possibly as much as twenty miles, eastward over a fault surface,
and out upon the Great Plains. In some cases rocks of the Prepaleozoic
Age have there been pushed upon rocks of the late Mesozoic Age, thus
locally upsetting the geologic column.
[Illustration: Fig. 11.--East-west profile and vertical structure
sections fifty-two miles long in the Mohawk Valley region of New York,
showing numerous tilted fault blocks which notably influence the
topography. Vertical scale exaggerated. The rocks are Prepaleozoic and
early Paleozoic in age. (Modified by the author after Darton, New York
State Museum.)]
The Wasatch Range of Utah, in addition to the great normal fault along
its western base, contains a remarkable system of thrust faults. In
the region now occupied by the Wasatch Mountains a number of parallel
(thrust) faults were developed close together and the broken pieces of
the earth's crust between them were pushed up, the rocks on one side
of each crack riding up over those on the other side until a great
mountain range was formed where once lay a plain. In the Ogden Canyon
one great earth block of Prepaleozoic (Algonkian) Age has been shoved
thousands of feet over late Paleozoic (Carboniferous) rock, which
latter has in turn been thrust over early Paleozoic (Cambrian) rock.
This thrust faulting was accomplished before the development of the
geologically recent normal fault along the western base of the range.
[Illustration: Fig. 12.--Vertical (structure) section through a part
of the earth's crust several miles long in Ogden Canyon, Utah, showing
the system of great thrust faults. Prepaleozoic (Algonkian) rocks have
been pushed far over upon late Paleozoic (Carboniferous) strata, which
latter have in turn been shoved over early Paleozoic (Cambrian) strata,
etc. (After U. S. Geological Survey.)]
Any sudden movement of part of the crust of the earth, due to a natural
cause, produces a trembling or shaking called an earthquake. Though
earthquakes are generally classed among the most terrifying of all
natural phenomena, those which have occurred during human historic
times have had scarcely any geological or topographical effects of
real consequence on the face of the earth. Locally, the effects may
be notable and the destruction of life and property may be great. The
earth may be locally cracked and rent, small fault scarps may develop,
landslides and avalanches may result from the shaking of the earth,
buildings may be demolished, and sea waves may be rolled upon the
land. On the other hand, many earthquakes, called "tremors," are too
slight to be noticed by people, though they are recorded by specially
constructed instruments called "seismographs." We have already stated
that actual sudden displacements causing earthquakes have amounted to
twenty or even fifty feet right along fault fractures, but during the
vibrations or quakings, which are often so destructively sent out into
the neighboring country, the earth's surface rarely actually moves
more than a small fraction of an inch. Because of the suddenness of
the movement objects on the surface may be moved inches or even feet.
Violent shocks may last one or two minutes and cause the whole earth
to tremble, though at distant points only seismographs record the
movement. It is probably true that some part of the earth is shaking
all the time.
Studies during the last fifty years have made it certain that the main
cause of earthquakes is the sudden slipping of earth blocks past each
other along fault fractures, the sudden slipping furnishing the impulse
which sends out the vibrations into the surrounding more or less
elastic crust of the earth. The low rumbling to roaring sound, which
sometimes immediately precedes an earthquake, is probably due to the
grinding of the rocks together below the surface.
Earthquakes generally accompany volcanic outbursts of the violent
or explosive type, and in such cases subterranean explosions cause
both the eruptions and the quakings of the earth. It is well known
that the principal volcanic districts or belts of the earth are also
the belts of most frequent earthquakes, but this does not mean that
volcanic action causes most of the earthquakes. Active volcanoes and
earthquakes are so commonly associated in the same belts because those
belts no doubt represent portions of the crust which are now most
actively yielding to the forces directly resulting from the shrinkage
of the earth. Within the volcanic belts many earthquakes take place
unaccompanied by any volcanic action, and many others take place
far from volcanoes. Some earthquakes have been caused by the impact
of great landslides or avalanches, or by the sudden caving in of
underground openings.
Brief descriptions of a few typical carefully studied earthquakes
during recent years will serve to make the main features of earthquakes
still clearer to the reader.
The violent Japanese earthquake of 1891 was caused by the sinking of
a block of earth forty miles long from two to thirty feet below that
on the other side of a fault fracture. There was also considerable
horizontal shifting, and cracks developed in the adjacent region. A
distinct fault scarp, fifteen to twenty feet high, developed, and in
some cases extended right across cultivated fields.
[Illustration: Fig. 13.--Map of the United States, showing the large
areas over which three of the greatest of our earthquakes were actually
felt by people. These earthquakes were recorded in many parts of the
world by delicate instruments: New Madrid, 1811; Charleston, 1886; San
Francisco, 1906.]
[Illustration: Fig. 14.--Sketch map showing the trace of the great
fault fracture along which a renewed sudden movement of as much as
twenty feet took place to cause the San Francisco earthquake of 1906.
(After U. S. Geological Survey.)]
The San Francisco earthquake of 1906 was produced by renewed movement
along the great fault which extends lengthwise through the Coast Range
Mountains for several hundred miles. It is literally correct to say
that, for 250 miles along this great earth fracture, one part of the
Coast Range instantaneously slipped from two to twenty-two feet past
the other. More or less of the movement extended at least several
thousand feet down into the earth. In this case both sides slipped and
the movement was very largely horizontal rather than vertical. The
land on the east side of the fault moved south and that on the west
side moved north, the amount diminishing away from the fault on each
side so that some miles out the actual crustal movement was only a few
inches. When one thinks of the tremendous volumes of earth material
involved in this shifting of the earth's crust, is it any wonder
that such destructive earthquake waves were produced? Many buildings
were wrecked, several hundred people were killed, the disastrous San
Francisco fire resulted, water mains were broken, and fences and roads
crossed by the fault were dislocated as much as fifteen to twenty feet.
[Illustration: Plate 5.--Swift Current Valley in Glacier National Park,
Montana. This was once a deep V-shaped canyon carved out (eroded)
by stream action. Then a great valley glacier slowly plowed its way
through it during the Ice Age and, by ice erosion, the present nearly
straight U-shaped canyon has resulted. (_Photo by Campbell, U. S.
Geological Survey._)]
[Illustration: Plate 6.--View in the Yosemite Valley from Near the
Western Entrance. The great rock called "El Capitan," on the left rises
3,500 feet above the river, and Bridal Veil Falls on the right is 620
feet high. All the rock is granite, the nearly vertical walls of which
have resulted from the action of a great glacier which plowed its way
through the valley during the Ice Age; the valley walls have been cut
back by the removal of large vertical joint blocks. The flat bottom of
the valley has resulted from the filling with sediment of a postglacial
lake in the valley. (_Photo by F. N. Kneeland, Northampton, Mass._)]
During the great earthquake on the coast of Alaska in 1899 notable
changes took place along the shore for some miles, one portion having
suddenly risen as much as forty-seven feet, while another portion sank
below sea level.
[Illustration: Fig. 15.--Map showing the principal earthquake regions
of the world.]
In 1886 the earthquake centering near Charleston, S. C., was preceded
by rumbling and roaring noises and the slight quaking increased to
violent shaking which lasted more than a minute. Eight minutes later
a rather violent earthquake shock took place, followed during the
next ten or twelve hours by less severe shocks. Most buildings in the
city were wrecked or more or less badly damaged, and some people
were killed. The shocks were so violent that the quaking was actually
felt by people over an area of more than 2,000,000 square miles, the
disturbance having spread at the rate of about 150 miles per minute.
Near Charleston openings and fissures were formed through which sand
and muddy water were ejected, but the cause of the disturbance was most
likely slipping of the old very hard rocks below the loose deposits of
the Coastal Plain.
From 1811 to 1813 a series of violent earthquakes developed in the
general vicinity of New Madrid, Missouri. In an area of over 2,000
square miles, now called the "sunk country," many portions suddenly
sank giving rise to small fault scarps or cliffs, and various lake
basins were formed. Development of a fissure caused a local change in
the course of the Mississippi River.
In 1897, Assam, India, was shaken by an earthquake of unusual
magnitude, which lasted 2-1/2 minutes. An area of 150,000 square miles
was disastrously shaken, and the shocks were distinctly felt over
an area of 750,000 square miles. A number of notable fault scarps
developed, the movement on one having been thirty-five feet.