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Jacques W. (Jacques Wardlaw) Redway.

Elementary physical geography : an outline of physiography online

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fact that when the barometer is low, the level of the lava is
higher than at other times.

' There is evidence of the presence of gases in the Hawaiian
lavas, not under pressure and endeavoring to escape, but in a con-
dition of absorption or occlusion. Occasionally, clots of lava are
shot into the air, and as soon as the ejected mass perceptibly cools,
its absorptive power is lessened, the escaping steam or other vapor
blowing the viscous lava into the fine, tenuous threads known as
'* Pele's hair." The threads thus formed are so gossamer-like
that they are carried a long distance by the wind.

° There is a tendency to consider the vulcanism of past epochs
as crater eruptions only. That such eruptions have occurred in




ALTERATIONS IN THE SHAPE OF VESUVIUS
^.T). 6j, 79 to i6ji, lyOy, 1822, 1868.



U PHYSICAL GEOGRAPHY

prior epochs cannot be denied ; old craters and the lava plugs
that filled them are found in great numbers in many parts of the
earth. Most, if not all, of the great lava floods, however, came,
not from craters, but from fissures. No crater in the world is large
enough to have ejected a lava flood in the manner in which that
of the Oregon and Washington flood was spread. Calderas like
those of Hawaii would have built up a dome-shaped mass of
eijecta. The lava flood in question was a sheet. It could have
come from nothing but a fissure, and the fissure must have been
many miles in length. Cinder cones and craters ai"e found here
and there on the surface of this vast sheet. In each case the cone
and its crater represent a volcano that formed on the lava flood
after the surface had hardened. This fact indicates that vulcan-
ism occurs just as readily with a swpramontane as a siih-moun-
taiii reservoir. In many instances there has been nothing more
than a mere filling of the fissure — axi intrusion oi lava, but no ex-
trusion. Not infrequently the vipper edges of the fissure walls
have been worn away, leaving the harder volcanic rock in the
form of a ridge or dyke. The Palisades of the Hudson are an ex-
ample. The Devil's Slide, in Weber Cailon, Utah, is also an illus-
tration. In this instance there are two dykes about twenty feet
apart, the groove between them being of softer rock.

'° This may be illustrated by a very familiar example. When
a train of railway coaches passes through a long tunnel, a flood
of mellow light now and then illuminates the tunnel and the in-
terior of the coaches. The light comes from the fire-box of the
locomotive. When the furnace door is opened the light of the
glowing coal is reflected from the steam that fills the tunnel.
Each globule of water dust is a tiny mirror, and as a result the
tunnel is flooded with light. In the case of the volcanic " fires "
the light is reflected from the under side of the cloud of steam.

"This island, better known as Thera, is a few miles north of
Crete. According to one myth it grew from a clod of earth hurled
from the ship Argo ; according to another it was the product of
submarine fires. Both legends are a testimony to its volcanic
origin. The topography of the island was considerably altered
by an eruption that occurred in 1866. The area covered by ashes
and .scoria quickly became cultivable, and has since added no lit-
tle wealth to the island.



CHAPTER VI

DESTEUCTIVE MOVEMENTS OF THE EOCK ENVELOPE :

EAKTHQUAKES

EiGiD and solid as thej seem, the substances that form
the rock envelope are more or less elastic. This is notice-
able when an underground explosion ' occurs, or even when
a very heavy weight falls to the ground ; the latter trem-
bles for an instant, causing a slight shock.

Any instantaneous disturbance, therefore, such as a sub-
terranean explosion, the collapse of a cavernous space, or




THE PROGRESSION OF EARTHaUAKE WAVES

the sudden breaking of strata, causes a vibration or trera-
1»liiig of the surrounding rock. These tremors or earth-
quakes may be perceptible for several seconds, or even
for so long as a minute. The shock, moreover, may in-
volve an area of sfncral tlionsand S(piare miles.

Nature of Earthquakes. — No matter how far below

95



96 PHYSICAL GEOGEAPIIY

the surface of the rock envelope the centre of the disturb-
ance ma}' be, as soon as the vibrations reach the surface
they behave just as do the circuhir waves that form when
a stone is thrown into still water.- In the diagram on
page 95 the shock originates at O ; at what place will the
resulting wave have an up-and-down motion ? These are
called vertical waves. As the successive waves move out-
ward, little by little the vertical movement gives place to
one that is both horizontal and progressive, and the latter
may be called a horizontally progressive wave. At what
part of the diagram are the waves most nearly horizontal ?
Where do ihej partake both of the vertical and the pro-
gressive character '? ^

The effects that have been observed, however, indicate
that the tremors or vibrations do not always spread out so
evenly from the centre of disturbance as is the case with
the waves resulting when a stone is thrown into water.
Some kinds of rock seem more elastic than others, and so
the concentric waves, instead of remaining circular in form,
become irregular in shape. If the waves of water strike
an unyielding surface, they are rejected, the reflected
wave often crossiug the original at oblique angles. Rock
waves, it is thought, are similarly reflected, and some-
times they produce efiects that would seem as though
there had been a vorticose, or whirling movement.^

Although the surface waves of earthquakes bear a close
resemblance to the circular Avaves formed by dropping a
stone in water, it must be remembered that they ditier
greatly in velocity and energy. The latter progress only
a few jards a minute ; the former have the velocit}' more
thaij double that of the swiftest projectile fired from a
modern gnn, travelling at a rate that varies from thirty
to forty or more miles a minute.^ The velocity of the
wave depends partly on the elasticity of the material



EARTHQUAKES



97



through which it travels, and partly ou the energy with
which it is propagated. In hard, crystalline rock it travels
rapidly and extends a great distance ; in sand and loosely
coherent rock the velocity is much slower, and the waves
quickly lose their
energy.^

In the case of se-
vere earthquakes a
series of shocks fol-
low one upon another
with increasing in-
tervals of time.*' The
first shocks are com-
monly the most vio-
lent. The duration
of the shock is not
perceptible to the
senses for more than
four or five seconds,
hut careful measure-
ments by the seismo-
graph, an instrument
for the detection of
shocks, show that it
may last for more ^ Rock column likely to be overturned

' "^ . BY AN EARTHQUAKE

than a minute. In

ID any instances a The rock has broken away from the cliff, splitting along a

naturally formed plane. I^ock waste, falling into the

shock seems to con- crevice, has become saturated zcith water, which by /reel-
ing, has expanded and pushed the mass farther and

Sist of a single Vio- farther from the cUff.

lent thump.'

The focus of the shock may vary from a short distance
to several miles below the surface of the earth. The
average distance is not far from six miles. The area
involved in the earth- waves may be either circular or




V Wif'^^''i^^l''^'j



98 PHYSICAL GEOGRAPHY

elliptical.^ Tlie diameter of the area seldom exceeds one
thousand miles.^

Attending Phenomena.— Earthquakes are frequently
attended bv sounds. Sometimes the latter resemble low,
rumbling thunder ; more commonly, however, the noise
is like that produced when a heavily loaded wagon goes
rajndly down a gravelled incline.

lu the great majority of earthquakes the effects are
not severe ; they rarely extend beyond the stopping of
clock pendulums, and the swinging of chandeliers, or the
breaking of delicate substances. In severe shocks the
walls of houses are wrenched and cracked, and the ground
is fissured. In disastrous shocks buildings are shattered
and the surface of the earth is seamed with deep fis-
sures and chasms. In several instances lakes ^° have been
formed or, perhaps drained, and stream channels changed.

If the centre of the shock is in or near the ocean it is
commonly followed by a series of gigantic waves, incor-
rectly called " tidal" waves. Following the Lisbon earth-
quake in 1755, enormous Avaves rolled in from the sea, and
wrecked whatever the earthquake had left.'' The ocean-
waves that followed the earthquake at Ai'ica, Peru,''^ car-
ried the United States Steamship Waieree nearly seven
miles inland, leaving her stranded in a dry stream bed.

Cause of Earthquakes.— It is generally believed that
earthquakes are the result of similar, but very rapid
movements of the rock envelope that fold the strata into
mountain -ranges and force molten lava from volcanic
fissures. If the strata are slowly bent, no vibratory effect
is noticeable, but if the strain increases until a fracture
or a collapse takes place, the shock produces the vibra-
tions that constitute the earthquake.

When fissures are formed, usually one wall slips upon
the other, so that the two edges are no longer in the same



EARTHQUAKES



99



level.^^ The resulting inequality is called a fault, and
wherever such faultiugs are found, they indicate, if not
an earthquake, at least a surface disturbance. The ex-
istence of such faults, therefore, is evidence that the outer
shell of the earth is constantly under stress " at some
point or another, and that the release of the strain pro-
duces the earthquuk



Ke.




AN hhHh(.i OF THE HAK I HOb'AKi: AT CllAKLLbiaN
The crack when first formed irai about two feet wide. From a photograph.

Distribution and Occurrence of Earthquakes.— No
part of tht! earth is free from earthquakes, and recent ob-
servations have shown that, in some part or other, they
are of almost daily occurrence. As a rule, however, they
are so feeble that scarcely one in fifty is noticeable, or
even perceptible, without the aid of instrumental measure-
ments.'^

As in the distribution of volcanoes, earthquakes are of
more frequent occurrence in younger mountain -ranges



100 PHYSICAL GEOGRAPHY

than in the older ones. They are still less frequent in
plains, unless the latter are undergoing a process of uplift
or depression. They also accompany most volcanic dis-
turbances.'*

The study of several thousand earthquakes shows that
shocks are a little more frequent when the earth is nearest
the sun, and that they are also more prevalent when the
moon is nearest the earth." An explanation for this is
not hard to find. Owing to the tendency to adjust itself,
some part or other of the rock envelope is constantly under
an increasing stress. But when the earth approaches
either the sun or the moon, the increased mutual attrac-
tion adds its force to the strain ; the latter is overcome,
and a shock results.

QUESTIONS AND EXERCISES.— If you live in the vicinity of a
body of water, study the waves that form when a good-sized stone is
tossed so that it falls vertically into still water.

What is the relative position of the vertical and the horizontally
progressive waves ? Repeat the experiment until the results obtained
are familiar.

If possible, clamp a brass or metal plate, about a foot square, to a firm
table, so that the clamp holds the plate at its centre. Sprinkle dry
sand on the plate and draw a violin bow across the edge. From the
figures produced by the sand note the direction and character of the
vibrations.

COLLATERAL READING AND REFERENCE

Rock WOOD.— Notes on American Earthquakes.

Shaler.— Aspects of the Earth, pp. 1-45.

Le Conte.— Elements of Geology, pp. 154-171.

NOTES

'Thus, the explosion under Flood Rock, for the purpose of
clearing and widening Hell Gate Channel, produced an earth
shock that differed in no material principle from those produced



EARTHQUAKES 101

by natural causes. The earth shock resulting from this explosion
was recorded at a distance of nearly forty miles from Hell Gate.
The velocity of the wave varied from 5,000 to 8,000 feet per
second in the vicinity of the explosion.

^ The vibrations as tliey form underground are spherical waves
and much like those formed in the air by the discharge of a fire-
arm or the ringing of a bell. When the waves reach the surface
of the rock envelope they spread out in the form of circular
waves.

' Such waves have a terrific shattering force ; but those in
which the horizontal and vertical components are combined are
even more destructive : they not only shatter, but they produce a
rocking motion as well. Vertical vibrations may only shatter a
building ; a " roller " will not only shatter, but overthrow it.

* It has been calculated that the amplitude, or up-and-down
motion, rarely exceeds one-quarter of an inch in height ; and or-
dinarily, in severe shocks, it is seldom more than one-twentieth
of an inch. The horizontal oscillation is scarcely more than half
an inch, and even when it is not more than half as much, the
shock has considerable shattering power.

* During the earthquake at Riobamba, Ecuador, a vertical
movement of more than two feet is said to have been observed.
The statement, however, is not considered authentic. At all
events, the energy was sufficient to hurl heavy objects a hundred
feet into the air. The bodies of men were thrown several hun-
dred feet across tlie river.

* At St. Thomas, one of the Lesser Antilles, the shocks of 18(58
aggregated nearly three hundred in number. The earthquakes
tliat shattered San Salvador, the capital of the State of Salvador,
lasted for about ten days. 'J'he Charleston earthquakes did not
cease for nearly a month, and a hundred similar instances might
also be added. All this accords with the well-known law that a
mass of rock envelope, in changing its foundations, cannot adapt
itself to its new position at once, but does so little by little.

■' Many of the California earthquakes are of this character.

" The elliptical form is especially noticeable in mountainous
areas, anfl in nearly every instance the major, or long diameter
of the ellip.se, coincides with the trend of the range or system.
The reason therefor is the fact that the strata of rock are more
elastic along than across their nuisses.



102 PHYSICAL GEOGKAPIIY

"In several iiistaiu'o.s, however, the area involved has far ex-
ceedeil this. Thus the shock that in 1755 destroyed Lisbon was
felt at a distance of about twenty-five hundred miles. The sea-
wave is proi)aj?atP(l to a much greater distance.

'"' Till' earthquake that destroyed the city of San Salvador broke
down the rim of a small lake and drained it. The famous earth-
quake of New Madrid, Missouri, changed the level of the land to
such an extent that a permanent swamp Avas formed in land that,
before the shock, was high and dry. This area has since been
known as the "Sunk Region." During the severest shock the
current of the Mississippi is said to have been temporarily re-
versed ; that it was greatly disturbed is shown by changes in its
channel occurring at that time. Reelfoot Lake, in Tennessee, was
consideraljly enlarged at the same time.

" Probably the most disastrous waves ever known to written
history, however, followed this earthquake. After the town had
been felled by shocks so terrific that thirty thousand people
perished, most of the survivors took refuge on the massive sea-
wall. Hardly had they reached it when the water began to re-
cede, leaving the harbor dry. Then an enormous wave, sixty
feet high, rolled in and completed the destruction, and thirty
thousand more lives were swept out of existence before the
waves ceased. At Cadiz the waves were thirty feet high, at
Madeira eighteen, and along the Iri.sh coast they were four or
five feet in height.

" The sea-wave resulting from this earthquake crossed the
Pacific Ocean and was recorded at Yokohama, Japan, twenty
hours afterward. On the American coast the wave was observed
as far north as Alaska, and to the westward as far as Australia.
Tlie earthquake that in 1854 devastated a part of Japan was fol-
lowed by a destructive wave. At Simoda the wave was thirty
feet high ; at Peel's Island, one thousand miles away, it was
fifteen feet ; on the California coast it was from twelve to
eighteen inches in height.

'" The destruction of Babispe, a small village in northern Mex-
ico, is an excellent illustration. This disturbance, alleged to be
a volcanic eruption, was in reality nothing more than a severe
earth(|uake that levelled the buildings of the town. During
the series of shocks a fissure was made, extending several miles
in length, and when equilibrium was restored, the fissure had be-



EARTHQUAKES 103

come a fault — one side or wall being, in places, from ten to four-
teen feet below the other,

'* At Monson, Massachusetts, the rock in the granite quarries
usually exhibits signs of heavy strain. Professor Jsiles observed
that ijieces, before their ends had been detached, were split along
a horizontal plane and bent upward at the middle. One mass,
measuring 354 x 11 x 3 feet, increased an inch and one-half in
length after it had been detached. These facts indicate the enor-
mous pressure to which rocks may be subjected ; incidentally
they show that even the hardest rocks are decidedly elastic.

'* An instrument for measuring any of the elements of an
earthquake shock is called a seismojneter.; if it merely records a
slKJck it is a seismof/rajjh. The horizontal element of the shock
is recorded by means of a delicate pendulum carrying a pencil or
stylus. The jar sets the pendulum in vibration, and the pencil
records the direction of the oscillations.

'* The sudden formation of gases on their rapid motion from
one part of the volcanic district to another, will accomnt for
earth shocks at such times.

" Of a total of 364 shocks, 147 occurred in the Atlantic High-
lands and Coast Plain, 06 in the Great Central Plain, and 151 in
the Pacific Highlands. These figures have only an approximate
value, however, inasmuch as many of the earth shocks occurring
in the sparsely settled regions of the Pacific Highlands escape
notice altogether. Of 66 shocks recorded in Canada, the United
States and the West Indies during one year, 24 were in the At-
lantic slope and the West Indies ; 3 were in the Great Central
Plain ; and :'.!) in tlie Pacific Highlands, including Mexico and
Central America.







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CHAPTEE VII

THE WASTING OF THE LAND : THE WORK OP

EIYERS

While various forces are at work wrinkling and folding
tlie strata of tlie rock envelope, other agents are constant-
ly^ at work wearing away those same folds and irregular-
ities and wasting or degrading the surface of the land to
its lowest, or base level.

The principal agent in producing these effects is water,
in one or another of its different forms. Falling on the
land as rain it removes line and loose particles of earth.
It also sinks into the pores of the rock, perhaps dissolving
some of it or, perhajDS, freezing and breaking oft' small
pieces. This process of degradation is called erosion.
Gathering into swift torrents, the latter cut their channels
deej) into the surface, producing the effects called cor-
rosion. Flowing against clifts and banks or, perhaps,
through underground channels, it saps the foundations of
masses of earth and breaks them down by undermining.

Gravitation is an aid in the 2)rocess of degradation,
for not only does the water invariably flow downward,
Ijut the detritus, or rock waste resulting, is likewise mov-
ing to lower levels. Perhaps, for a time, it lodges in a
hollow, or Ijasin-shaped depression, until the latter is
tilled ; then the downward progress again begins. Of the

lofj



lOG



PHYSICAL GEOGRAPHY



water that falls from the clouds upon the land,' some
evaporates and mingles with the air ; a part sinks into
the ground, filling up the underground channels and res-
ervoirs ; the remainder gathers into channels and flows
back to the sea.

Streams of water flowing upon the land are variously




THE BEGINNING OF A LOOP, CUMBERLAND RIVER, KENTUCKY
The river has built a flood plain on the vest side ami is cutting into the east bank.

called rills, rivulets, brooks, creeks, and rivers— the name
usually depending on the size of the stream. The largest
streams are rivers. Almost every river is made up of
branches and tributaries, and these, in turn, are fed by
smaller branches — all together comprising the river sys-
tem. The area drained by the river system is its ivater-
shed ° or basin, and usually the latter is surrounded by a



THE WASTING OF THE LAND: RIVERS 107

well-defined height of land, the ridge or divide that sep-
arates it from adjacent basins.

In some instances the crest of a mountain-range forms
a divide, but in very many cases the latter is an almost
imperceptible rise only a few feet high.^ Thus, at Chi-
cago, the divide between Lake Michigan and a tribu-
tary of the Illinois River is only ten or fifteen feet high-
er than the level of the lake. It must be borne in mind,
however, that a high mountain-range is not necessarily a
divide, for there are many instances where ranges are
crossed hy rivers. From any good map find the divide
between the Susquehanna and Allegheny Rivers ; between
the Great Kanawha and Ohio Rivers. Compare the divides
with the ranges.

Physiography of Rivers. — The beginnings of most
large rivers are high in the mountains, where the rainfall
is heaviest and the greatest accumulation of snow is found.
The water that is let loose from a spring or from a
winter's snowdrift trickles down the slope in tiny rills.
On their way the rills unite into rivulets and brooks that
tumble down the mountain slopes in self-made, pebbled
gullies.

Other streams join the brook and swell its volume into
a mountain torrent that rushes down the steep incline,
cutting its channel into hard rock and tossing to the one
side or the other the obstacles in its way. Almost always
it fiows in a deep cafion or gorge, the cTitting of which is
the principal part of its work.^ AVhen the stream emerges
from the mountain canon it is burdened with rock waste
brought from the mountain side and, no longer able to
carry all of this, because of the lessened slope, it drops
the coarser material, forming a fan-shaped pile. Thence-
forth, because it is no longer a swift torrent, it cannot re-
move the heavier obstacles, but must flow around them.



108



PHYSICAL GEOGEAPHY



The lighter rock waste, called sediment or silt, ^ is still
carried by the flood of the river. Perhaps a little of it is
drojiped here and there, but the greater part is borne to

the coast plain,
which in many iu-
stances is the
" m ade - la nd "
formed of river
sediments. After
reaching the latter
the silt is gradual-
ly dropped until
the river reaches
tide-water. There,
about all the rest
of the silt is de-
posited — either to
be spread out in
the form of a delta,
or to be piled up
near the shore in
spits and bars.

It is evident,
therefore, that in
streams which are
degrading the land three processes are usually going on,
namely — corrasion and undermining, transportation, and
deposition. That is, from the moment the water touches
the rock envelope it is picking up particles of earth ; it is
carrying them downward; or else it is dropping them.
AVhichever it does, depends on the current. Increase its
velocity and the water will pick up more particles ; de-
crease the velocity and it will begin to drop them and
flow around them. In the upper, or torrential part, most




LOOPS AND CUT-OFFS OF THE LOWER MIS-
SISSIPPI

The abandoned channels are sometimes called " Bavous ;'
they form an intricate net-work of passages.



THE WASTING OF THE LAND : RIVERS 109



streams emphasize their right of possession by cutting
their channels deeper. In the lower course the reverse
is apt to be true; the stream clogs its channel with silt*
and is therefore compelled to make a new one on the one
side or the other.

In the study of such rivers as the Mississippi the rea-
sons therefor are not hard to find. Because the slope of



Online LibraryJacques W. (Jacques Wardlaw) RedwayElementary physical geography : an outline of physiography → online text (page 7 of 25)