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Jacques W. (Jacques Wardlaw) Redway.

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stances the stream valleys and flood plains cover an area equal
to the inter-streaui uplands. In strong contrast are the low,



78 PHYSICAL GEOGRAPHY

rt^cently formetl murine plains along the southern coast of New
Jersey, and the still younger tule plains of the Sacramento
Valley. In these the rivers have hardly been able to select their
channels, much less to extend them.

* A high plateau sparsely covered with vegetation is much more
readily liissected by streams than a grass-covered surface. The
region through which the middle course of the Colorado River
flows is an example. Here the plateau has been cut to a depth
ranging ifrom three thousand to six thousand feet. The region is
one of deficient rainfall, however. Extensive corrasion is shown
along the beds of the streams that rise at a distance in snow-
clad mountains. Only a small part of the plateau as yet has
been removed, and large areas show but little signs of dissec-
tion. In other parts, however, such as the "Land of Standing
Rocks," denudation has been enormous, and only the towers of
hardei- rocks remain. A complex dissection may be seen in vari-
ous parts of the Appalachian highlands. Here, because of a
greater rainfall, the streams have formed a network of canons
throughout the regions.

' Such formations are very common in the lava-covered regions
of the Sierra Nevada and Cascade Mountains ; they are also found
in the Piedmont lands of western Texas.

'"There are many examples of isolated peaks, or '"'monad-
noeks, " in those mountain-ranges that have been very greatly
worn. Mount Holyoke is one of several examples in Massachu-
setts. It was not thrown up in its present form ; on the con-
trary, it was left when the rest of the range, being softer, was
worn away. Mount Monadnock, New Hampshire, is a similar
example. I.solated ridges or ranges are more common, and ex-
cellent examples may be found in the Great Basin.

"Not only were the deposits that became sedimentary rock
thicker before the folding took place, but they were made still
thicker by side pressure and crumpling.

'^ At the mouth of every canon there will be found a fan-shaped
pile of coarser material called talus. A pile of talus is usually
found at the bottom of every steep, rocky cliff.

" A large part of Rhode Island and Connecticut constitutes the
base of an old mountain highland that has been worn down al-
most to sea-level.

" Both valleys have been modified by water, the depression



PLAINS, PLATEAUS, AND MOUNTAINS 79

having been submerged, partly filled with sediment, and re-ele-
vated.

'^ It is not unlikely that the process has Ijeen more complex,
and that periods of elevation have alternated with those of rest.
Old sliorelines and deposits of river gravel occur all along the
lower river. The numerous clay banks seem also to have been
deposited by slack water. The lower part of this valley is now
practically an estuary.

'^ Among the famous passes are Argentine, 13,100 feet, the
highest wagon road pass in the world ; Marshall Pass, 10,900
feet, one of the highest railway passes in the world ; Alpine Pass,
13,550 feet, and Mosquito Pass, 13,700 feet— all in Colorado. Sim-
plon, St. Bernard, and Brenner are famous passes across the Alps,
and for centuries they have been highways of commerce. A rail-
Avay pass across the Andes is nearly 14,000 feet above sea-level.

In many instances the pass is not fully surmounted ; instead
of building the railway over the divide, it is more economical to
construct a tunnel under it. Some of these tunnels are marvels
of engineering skill. St. Gotthard and Mont Cenis tunnels
through the Alps ; Hoosac tunnel through the range of the same
name in Massachusetts ; San Fernando tunnel, in California ;
and the tunnel of the Transandine Railway are examples : each
is one niile or more in length. In other cases the railway sur-
mounts the range by a series of long and intricate loops, crossing
and recrossing itself through tunnels that often are sharply
curved. >'^ear Caliente, California, the Southern Pacific Rail-
way is built in sinuous loops aggregating about twenty miles in
order to cross a divide scarcely two miles from the head of the
valley. The famous loops of the Colorado Midland over Hager-
mans Pass is also a well-known example of the railway builders'
skill.



CHAPTEE V



DESTRUCTIVE MOVEMENTS OF THE ROCK ENVELOPE :
VOLCANOES AND THEIR PHENOMENA

Of the various phenomena that attend changes in the
level of the rock envelope, two of them, volcanoes and
earthquakes, are noteworthy because the results are more




VESUVIUS, A TYPICAL CINDER CONE
From a model.— tAfter O^asmjitb.



or less destructive. In the one case, great quantities of
molten matter are ejected from fissures or vents, covering
very large areas; in the other, there is a movement at

80



VOLCANOES AND THEIR PHENOMENA 81

some part or other of the rock envelope, so sudden that a
tremor, or even a severe shock, occurs.

Volcanoes. — A channel or vent in the rock envelope
from which great quantities of steam and molten rock are
ejected constitutes a volcano} In most instances a great
deal of material, in the form of clots of half-molten rock,
fall about the vent and build up a conical pile, sometimes
called a "volcano," but more properly, a cinder cone. At
the top of the latter is a cup-shaped depression called the
crater or, if very large, the calderar

Volcanoes shoAviug any display of energy are said to be
active, quiescent, or inactive, according to the character of
their energ}^ ; those in which all signs of activity seem to
have disappeared are said to be extinct.^ In a few in-
stances the activity seems to be continuous. Thus the
caldera of Mauua Loa nearly always contains lava in its
molten condition, and Stromboli, " the Lighthouse of the
Mediterranean," has been a mariner's beacon for more
tlum two thousand years. Most active volcanoes, how-
ever, are intermittent in action, alternating their eruptions
with long periods of rest.

Phenomena of Eruption. — In certain respects all vol-
canic outbursts are similar ; that is, lava and steam are
ejected from a subterranean source, and the matter ejected
is forced out of a vent or channel in the rock envelope.
Beyond this, however, the various types of eruption have
but little in common. In most cases the eruptions are
very destructive. Frequently they are preceded by earth-
quakes, though these warnings are by no means always
present. Generally they begin with explosions that rend
the top of the cinder cone in fragments. In some instances
the plug of hardened lava that fdlcd the channel is blown
out, but cpiite likely a new channel is formed at the one
side or the other.^



B2



imiysi(;al ceocraphy



A vent once made, the water tliat had accumnkited about
the cinder cono, together with mud and fragments of rock,
are hurk?d upward ; an outrush of steam mingled with
mud and rock waste follows, and a cloud of inky blackness
quickly envelopes the cone. The condensing steam, with
which sulphureous vapors are sometimes mingled, produces
luiavy rains ; and if sulphur gases are present, the rain
may become so corrosive that vegetation is blighted and
in many instances the crops are destroyed.^

A flow of lava follows. At first the lava is ejected with
almost explosive violence, but after aAvhile the How be-
comes steady and



regular." The
ejection of mate-
rial takes place,
not only at the
main vent, but at
the score of new
ones formed on






mm



JTOi



IDEAL StCTlON OF A VOLCANO

(Minor crtiptwits are lakiitg place through /i^siira tii I hr flanks the HankS Ot the
q/" the cinder cone, building parasitic cones.

old. At each vent
small monticules, or parasitic cones, are quickly formed,
and the eruption from them does not differ materially from
that at the main vent.

Volcanoes such as Stromboli display but comparatively
little explosive energy. From an overhanging crag of
this volcano the eruption may be safely studied. At inter-
vals of fifteen or twenty minutes a gigantic bubble begins
to form in the caldron of seething lava. In a few mo-
ments it rises to the toD and bursting, hurls a shower of
lava clots into the air.'' The eruptions of the Hawaiian
volcanoes are materially different from those of the Strom-
bolian or the Vesuvian type. Instead of the intermittent
bubbles of Stromboli, or the violent outburst of Vesuvius,



VOLCANOES AND THEIR PHENOMENA



83



the lava rises in the caklera until it overflows the lowest
part of the rim.^ The flow of lava — often an enormous
quantity — continues for several clays, or perhaps for sev-
eral Aveeks, and then subsides as quietl}^ as it began.

The fissure eruptions that occurred in previous geologi-
cal periods seem to have somewhat resembled those of the
Hawaiian volcanoes. In these eruptions there were ap-
parently none of the phenomena that mark outbursts of
the Yesuvian type. Great fissures were formed, and
through these

the lava Avas ,-'"'" """■ - ,.

forced.' In some
instances there
was an enormous
flow of lava ; in
others the lava
merely filled the
fissure and hard-
ened, leaving ^,4 scclwn through one of the Henry Mountains.

lines indicate the strata removed by erosion

vol-







A LACCOLITE

'ountains. The dotted

dykes of

canic rock. The plains of the Columbia are the rem-
nants of a flood of lava from fissures in the Sierra Nevada
mountains. The Palisades of the Hudson form a dyke
of similar character.

In a few instances a flow of lava, thrust upward, has
raised the outer strata of the rock envelope in much the
same manner that a blister of the skin is formed. No
extrusion of lava took place, and, as a rule, none reached
the surface. Irruptions of this kind form what are com-
monly known as laccoUfes. The Henry Mountains, a de-
tached group of knolls in Utah, are examples.

Products of Eruption. — Excepting the very small
amount of sulphur gases emitted, practically but two sub-
stances are ejected from volcanoes — steam and lava. In



84 rilYSKiAL GEOGRAPHY

the eruption of Vesuvius that occurred in 1872, it is esti-
mated that ninety-eight per cent, of the material ejected
consisted of steam. From the Hawaiian volcanoes, how-
ever, the matter thrown out consists almost wholly of
great quantities of lava.

The term lava includes every form of molten rock of
volcanic origin. Lavas, therefore, differ not only in ap-
pearance, but in chemical composition as well. In many
instances the lava resembles furnace slag, and has about
the same comi)osition. Sometimes it is vesicular, or
spongy ; pumice-stone, or " volcanic froth," is so porous
that it floats on water. Obsidian, or " volcanic glass," an-
other form, does not differ materially from black bottle-
glass. The sponge-like clots of lava that accumulate about
volcanoes form scoria; they are suggestive of furnace
" clinkers."

A misunderstanding of volcanic phenomena has led to
the adoption of certain names that often give erroneous
ideas of volcanic action. There are no " flames " about
volcanic outbursts; the so-called flames are merely the
reflection of the white-hot lava from the under surface of
the dense clouds of steam.^^ " Smoke " is also absent,
except as the clouds of dust and steam can be thus
called. Volcanic " ashes " are not ashes at all ; they con-
sist merely of finely divided lava. It is thought that this
form of lavc„ results from the action of steam which, forced
through the lava by intense pressure, carries much of it
along in a fine, powdery state.

Most lavas are readily decomposed by the action of air
and moisture, and the Hawaiian lavas make excellent soil
in the course of a very few years. The economic value of
lavas, therefore, may be considerable. Sulphur, or " brim-
stone," is a common mineral in and about the craters of
volcanoes. It is formed by the action of certain sulphur



VOLCANOES AND THEIR PHENOMENA



85



gases that, on mixing, decompose each other and deposit
the sulphur in the shape of crystals.

Nature of Volcanoes. — That the cause of volcanic ac-
tion is due indirectly to the gradual shriidvage of the crust
of the earth is admitted by most geographers. To what
extent the process of contraction becomes a direct cause,
however, is a matter of uncertainty, and one upon which
there is a great diversity of opinion. It is generally con-
ceded, also, that the material ejected comes, not from an
assumed " liquid interior " of the earth, but is formed
at a very moderate depth below the seat of eruption.




jv^



FORMS OF ERUPTION



/}, a dyke; B, E, subterranean inlrusiom; C, a cinder cone; D, a laccolite; F, a lava sheet;

G, granite core of a range.

Various theories have been advanced to account for the
possible causes of eruption, but of these only one or two
are supported by positive evidence. The pressure that
results when the rock layers fit themselves about a shrink-
ing interior is sufficient to heat the parts upon which the
pressure is exerted, far beyond the temperature of fusion ;
and if a break or fracture takes place, the pressure being
relieved at that i)oint, the superheated rock at once
liquefies and is forced out of the fissure. The intrusion
of water upon molten matter undoubtedly causes the ex-
plosive features of the eruption, but it is improbable that
this is the prime cause.



8G PTTYSICAL GEOGRAPHY

In ;i low iiist;iiicos there seems to l)e more or less re-
lation between volcanic vents situated at no great dis-
tance from one another. Thus, while Vesuvius was so
long- inactive, Epomeo on the island of Ischia was active ;
but after the eruptions of Vesuvius began again, Epomeo
became dormant. A similar condition possibly obtained
in past times, for the Plilegrean Fields, an area south of
Vesuvius, is honeycombed with old craters through which
eruptions took place at successive intervals.

The same phenomenon is observed in the case of the
Hawaiian and the Ecuadorean groups. Activity is us-
ually confined to a single caldera, and if this becomes
dormant for any length of time the seat of activity is
transferred to another vent. In the cases of the Italian
and the Ecuadorean groups, the cessation of all activity is
usually followed by a period of frequent and destructive
earthquakes.

Results of Vulcanism. — ^Notwithstanding their stu-
pendous display of energy, the physiogi-aphic effects of
volcanic outbursts are comparatively unimportant, and as
a rule they are confined to the vicinity of the volcano.
The most noticeable feature is the cone or dome that pop-
ularly is called a volcano or volcanic peak. Each volcano
builds its own cone, and in many instances the cones
have been built along the folds of mountain-ranges. In
several cases they have been formed successively along the
line of the fold at no great distance apart.

The lava usually collects at the vents, extending later-
ally outward, and at the same time building the cone high-
er and higher. The successive eruptions of the calderas
of Hawaii have formed a mass 14,000 feet high that covers
an area as large as the State of Connecticut. Most of the
volcanic mountains of the Hawaiian Islands are dome-
shaped rather than conical, the shape resulting from the



VOLCANOES AND THEIR PHENOMENA



87



-^eiy liquid conditiou of the lava aud the absence of ashes
aud scoria.

Some of the lava flows of the Iceland volcanoes have
been extensive. Of the thirteen or more cinder cones in
the island Hekla and Skaptar Joknl are the best known
because of the frequency of their eruptions. In 1783,
there occurred a flow of lava from the latter that contin-
ued for two years. Two streams flowed in nearly oppo-
site directions from the crater, one forty, the other fifty
miles in length. More than 1,000 square miles in area
were covered by the lava. A score of villages was swept
out of existence. Streams were dammed by the lava and








A LAVA FLOOD, HAWAIIAN ISLANDS



their floods added to the destruction. Thousands of cat-
tle were killed, and a large ]jart of the po]>ulatiou perished
m tlie famine that resulted from the eruption.

The ashes sometimes accomplish moi-e ruin tlian th;it
which results from the lava flow and Ihr coiiosive rain.



88 PHYSICAL GEOGRAPHY

Herculanenm and Pompeii were destroyed by the erup-
tion of Vesuvius A.D. 79. Pompeii was covered with loose
nniterial, and much of the city has been excavated in re-
cent years. Herculaneum received a heavy fall of rain
in addition to the ashes, and the latter were cemented
into a tolerably hard rock.

In many instances the ashes have been hurled to a great
distance, being in part carried by the wind. During the
eruption of Tomboro, in Sunda Strait, dwellings forty
miles distant were crushed and large areas of forestry were
destroyed. Similar, but more appalling effects resulted
from the eruption of Krakatoa, also in Sunda Strait. The
explosions lasted for two days and culminated with the
disa[)pearance of half of the island. Forestry seventy-five
miles away was crushed by the falling mud and rain, and
the fine material covered the city of Batavia to a depth of
several inches. Some of the lighter dust was carried by
the wind to a distance of more than 1,000 miles.

Islands are both formed and destroyed by the outbursts
of marine volcanoes. Off the coast of Tunis, near the
site of Carthage, a reef called Graham's Island was formed
during an eruption, and remained in existence for several
years. It then gradually settled below sea-level and dis-
appeared. Several new islands appeared in the group of
the Azores, during eruptions, but they gradually disap-
peared. A more remarkable case is that of Santorini,''
an island in the Greek Archipelago, which was formed as a
result of eruptions. It is now inhabited.

Fissure eruptions are noted mainly for the enormous
flows of lava. From one or more of these fissures in the
Sierra Nevada ranges there occurred a flood of lava that
covered more than one hundred thousand square miles.
Large areas of California, Oregon, Washington and Idaho
were engulfed, and in several places the Columbia River



VOLCANOES AND THEIR PHENOMENA 89

was pushed out of its chanuel. In inaiij places small
cinder cones have been formed on the surface of the
lava, each being an eruption upon an eruption. In places,
the sea of lava is nearly four thousand feet deep, and the
average depth is not far from one thousand feet.

Vulcanism seems to be a trustworthy index of processes
going on within the earth's crust which affect the level of a
region. Careful measurements have shown that, in regions
of volcanic activity, an elevation of the surface is taking
place. Thus, along much of the Mexican and South Amer-
ican coast, where volcanic forces are active, upheaval is
taking i)lace. In the South Pacific Ocean, on the con-
trary, where vulcanism seems to have recently ceased,
there has been a considerable subsidence. It cannot be
said with certainty, however, that these are matters of
cause and effect.

Distribution of Volcanoes. — Volcanoes are commonly
found along the lines of the younger mountain folds, and
they are almost alwaj'S near the sea. The Pacific Ocean
is nearly girdled by chains of mountains that are com-
paratively 3'oang, and in these fcjlds are situated a majority
of the active and dormant volcanoes of the earth.

Another short chain extends along Java and the re-
maining Sunda Islands tf) New Zealand. It contains
about one hundred active and dormant volcanoes, and is
the chief seat of volcanic activity on the earth. The Ha-
waiian group is about the only one situated in mid-ocean.
In what direction does it extend ? This chain is about a
thousand miles long. The seat of activity, however, is con-
fined mainly to the island of Hawaii, on which there are
three calderas — Kea, Loa, and Kilauoa.

A chain of volcanic islands (ixtends from Jan Ma^yen
island through Iceland, the Azores, Canary, and Cape
Verd Islands, southward as far as Tristan da Cuuha. An-



00 PHYSICAL GEOGEAPHY

otlior extends through the West Indies, but it contains no
volcanoes at present active. Graham Land, in the Ant-
arctic Continent, contains at least two volcanoes that
have been active in recent times.

Among American volcanoes the Peruvian and Ecuado-
reau groups are famous for their great height. Name three
of them. The Mexican group contains four of interest,
because they are so far inland. Find them ; in what
direction does the line extend? Thej are active or quies-
cent at short intervals.

The North American group contains a great many
dormant and extinct cones ; but at least four — Shasta, Ta-
coma (or Rainier), and Lassen must have been active at
no greatly remote time. A small cone near Lassen Peak
has been in eruption within fifty or sixty years, and the
stumps of trees, many of them in a good state of preserva-
tion, are still protruding through the sheet of lava.

Cinder cones and volcanic " necks " are abundant all
through the plateaus of the Western Highlands. In Ari-
zona there are several hundred. One of the most imposing,
San Francisco Peak, has been in eruption within recent
times. In New Mexico there are also many small cones.
Almost all the high peaks of the Cascade and Sierra Ne-
vada ranges are cinder cones.

The Aleutian group contains about thirty cones, quies-
cent and active. One of these, Bogoslov, north of Una-
laska, has been in eruption almost constantly since 1880.
Many of the peaks of the West Indies are cinder cones,
but none has been active in recent times. The remains of
old cones are abundant in the Appalachian and Laurentian
Mountains, but they seem to have been extinct since early
geological times. One of them. Mount Eoyal, has given
to the city of Montreal its name.



VOLCANOES AND THEIR PHENOMENA 91

QUESTIONS AND EXERCISES.— Explain the nature of the so-
called smoke, flames, and ashes of volcanic eruptions. Why are these
terms inapplicable ?

Prepare a written description of the geographic distribution of vol-
canoes, taking into consideration their position with reference to moun-
tain-ranges, proximity to the sea, latitude, and situation with reference
to continents and islands. Consult the map, p. 92.

Note the features in the diagram, p. 88, and prepare a brief descrip-
tion of the various ways in which lava is extruded.



COLLATERAL READING AND REFERENCE

PLHry. — Letters — Book vi., 16-vi. 20.
Shaler. — Aspects of the Earth, pp. 46-97.

" First Book of Geology, pp. 88-97.

Le Coxte.— Elements of Geology, pp. 89-103.
Redway and HixMAJf.— Natural Advanced Geography, p. 13.
United States GEOLOGiCAii Survey.— Shasta and Lassen
sheets.

NOTES

' The channel or tube is the essential part of the volcano, and
the " mountain " or cinder cone is merely an incidental feature.
The latter is rarely absent.

^ The craters of the earth are exceedingly small, compared with
those of the moon. Terrestrial craters are rarely more than half
a mile in diameter ; lunar craters, on the contrary, frequently ex-
ceed twenty or thirty miles in diameter ; Tyeho and Copernicus,
are each more than forty miles.

' As a rule, such volcanoes are rarely distinguishable, except by
most careful investigation. Usually the cone has been almost
obliterated, nothing remaining except such masses of lava as are
not easily altered by the action of moisture and atmospheric ele-
ments. Mount Tom, Massachusetts, is an excellent example
of an old volcano.

* The eru[)tion of Vesuvius in 1750 took place, not at the former
crater, but a little to one side. One of the old crater walls re-
mained standing, and for many years was called Monte Summa.
During the eruption of 1872 a large number of vents was formed,



VOLCANOES AND THEIR PHENOMENA



93






and the flanks of the mountain were dotted with monticules.

Professor Palmieri, who remained in his observatory on the

mountain during the

entire period, said that

the whole side of the

cone ' * seemed to sweat

fire at every pore."

' The sulphur com-
pounds combine with
the steam, making sul-
phurous acids, and not
infrequently the acid
dissolved in the rain is
strong enough to de-
stroy vegetation.

• It behaves exactly
as though it were forced
out by gases under ex-
tremely high pressure,
the elasticity of the
medium that consti-
tutes the power being
the most noticeable
feature.

^ The phenomena are
simply those exhibited

by a viscous body in a state of slow boiling, and are perfectly
illustrated in the slow cooking of oatmeal. It is a significant



Online LibraryJacques W. (Jacques Wardlaw) RedwayElementary physical geography : an outline of physiography → online text (page 6 of 25)