over all the continents ; in Nova Scotia, Canada, New England,
New Jersey and the States south, the region of Lake Superior, the
Rocky Mountains, and western America; in Ireland, Scotland,
and various parts of Europe ; and so over much of the globe.
These evidences combine to prove that the interior of the earth
, is a source of heat.
(It is, however, still an open question whether the internal heat
is that of fusion ; or, if there is fusion, whether the w^iole interior
is fused, or whether there are only interior seas of liquid rock ; or,
j if the interior is fused, what is the thickness of the crust. From
a survey of the facts, the most probable conclusion is that the
; crust is not over 100 miles thick. Within the crust there may be
isolated seas of melted rock, feeding volcanoes.
Professor Perrey, of Dijon, has inferred from his extended re-
searches that there is a periodicity in earthquakes dependent on
tides in the internal igneous material of the globe. (See beyond,
on Earthquakes.)
Recent mathematical calculations have made the thickness of the crust to be
at least 800 miles. But the results of figures should not be allowed to suspend
or throw discredit on observations until it is absolutely certain that all the
data required for them are known and thoroughly understood in their various
bearings.
2. EFFECTS OF HEAT.
The effects of heat considered in this place are the following : —
1. Volcanoes and related phenomena ; 2. Non-volcanic igneous
ejections; 3. MetamorjDhism and production of mineral veins.
Heat is also one at least of the causes of the elevation of mountains, and
of earthquakes. These topics are considered in the following chapter. It is
an important agent also in most chemical changes ; and hence its effects belong
in part to Chemical Geology.
1. VOLCANOES.
The facts relating to volcanoes are here presented under the fol-
lowing heads : — (1) General nature of volcanoes, and their geogra-
phical distribution ; (2) Kinds of volcanic cones ; (3) Volcanic action ;
HEAT— VOLCANOES. 685
(4) Origin of the forms of volcanic cones ; (5) Subordinate volcanic
phenomena ; (6) Source of volcanoes.
1. General nature of volcanoes, and their geographical
distribution.
1. Volcanoes. — Volcanoes are mountains or hills, of a more or
less conical shape, in a state of igneous action, and consequently
emitting vapors, and occasionally melted rock or lava, with showers
of fragments or cinders, from a central opening called the crater.
They are conduits of fire, opening outward from within or beneath
the earth's crust. An extinct volcano is a volcanic mountain that
has ceased to be active, — the body with the fire out.
The lavas flow out either over the edge or lip of the crater, or,
more commonly, through fissures in the sides or about the base of
the mountain. The cinders are thrown upward from the vent or
crater to a great height, as a jet of sparks or fiery masses, and fall
around in cooled particles or fragments, which are only granulated
lava : they may build up a conical elevation around the vent, or be
carried to a distance by the winds.
When rain or moisture from any source descends with the cin-
ders, the mass forms tufa, — a stratified, somewhat earthy, granular,
and rather soft rock, of gray, yellowish-brown, and brownish colors.
2. Geographical distribution. — Volcanoes occur (1) over the
border regions of the continents, — that is, the regions between the
oceans and the summit of the border range of mountains, as be-
tween the Pacific and the summit of the Rocky Mountains ; (2) in
the continental islands, or those near sea-coasts ; (3) in oceanic
islands, nearly all of which, excepting a few of very large size and
the coral islands, are throughout volcanic, — and the coral islands
have probably a volcanic basis. (4.) Volcanoes are mostly confined
to the borders of the larger ocean, the Pacific and the vicinity of
the seas separating the northern from the southern continents,
namely, the West Indies between North and South America, — the
Mediterranean between Europe and Africa, — the Red Sea between
Asia and Africa, — the East Indies between Asia and Australia.
There are but few about the Atlantic, excepting those of the islands ;
and over the interior of continents, remote from the regions men-
tioned, they are almost unknown.
(5.) Volcanoes are very commonly in linear series or groups.
1. Borders of the Pacific. — The Pacific is almost completely belted with vol-
canic mountains. They occur in Fuegia, the southern extremity of the Andes ;
in Patagonia; 32 in Chili, — that of Aconcagua 23,000 feet high ; 7 or 8 in Bolivia
686
DYNAMICAL GEOLOGY.
and southern Peru, — Arequipa 18,000 feet ; 19 or 20 about Quito, nearly all over
14,000 feet, and among them Cotopaxi, 18,876 feet in altitude; in Central
America there are 39 ; in Mexico, 7 of large size, with others smaller ; in Cali-
fornia, Oregon, and northwest America, 12, making a lofty series of snowy sum-
Fig. 966.
Volcano of Cotopaxi.
mits, 12,000 to 18,000 feet high,— St. Helen's, in Oregon, 16,000 feet; Mt. Hood,
14,000 ; Mt. Shasta, 14,000. In the Aleutian Islands, which form a curve like
a festoon across the Northern Pacific, there are 21 islands with volcanoes ; in
Kamtchatka, 15 to 20 ; in the Kuriles, 13 ; in the Japan group, 24, some 10,000
feet high ; in the Philippines, 15 to 20 ; several along the north coast of New
Guinea; a number in New Zealand ; in the Antarctic, on the parallel of 76° 5', and
near the meridian of 168° E., Mts. Erebus and Terror, 12,400 and 10,900 feet high,
both in full action when seen by Ross in 1841 ; and more to the east, south of
Cape Horn, Deception Island and Bridgeman's.
2. Over the Pacific. — At the Hawaian Islands, there are remains of ten or more
volcanic mountains, and two on Hawaii are now active, — Mt. Loa, 13,760 feet
high, and Mt. Hualalai, about 10,000 feet; while Mt. Kea, on the same island,
13,950 feet high, has not been very long extinct (fig. 24. p. 31, and fig. 973, p. 695).
There are other volcanic mountains at the Society group, Marquesas, Navigator,
Friendly Islands, Feejees, Santa Cruz group. New Hebrides, Ladrones; among
which Tauna and Ambrym in the New Hebrides, Tafoa and Amargura in the
Friendly group, Tinakoro in the Santa Cruz group, and two or three in the
Ladrones, are in action.
3. Over the seas that divide the northern and southern continents from one an-
other, and the regions in their vicinity. — (a.) The West Indies, where ten islands
are eminently volcanic. (6.) The Mediterranean and its borders, as in Sicily
HEAT — VOLCANOES. 687
and the islands north; Vesuvius, and other parts of Italy; Spain, central
France, Germany, etc., in Europe; the Grecian Archipelago, which contains
five volcanic islands, — Santorin, Milo, Cimolos, Polenos, and Misyros; in Asia
Minor, where are the Catacecaumene and other volcanic regions ; and more to
the eastward, towards the Caspian, Mt. Ararat, 16,950 feet high; Little Ararat,
12,800 feet ; Demavend, on the south shore of the Caspian, 20,000 feet, (c.) The
Red Sea, along its southern borders, where there are a number of lofty volcanic
summits, {d.) The East Indies, where there are 200 or more volcanoes, of
which there are nearly 50 in Java alone, according to Dr. Junghuhn, and 28 out
of the 50 now active, nearly as many in Sumatra, 109 in the small islands near
Borneo, a number in the Philippines, etc.
4. Tn the Indian Ocean. — A few in Madagascar ; also the Isle of Bourbon,
Mauritius, and the Comoro Islands, and, to the south, Kerguelen Laud, etc.
5. On the Atlantic borders. — Only in the Bight of Benin on the African coast,
where one in the Cameroons Mountains is said to be 14,000 feet high, and the
neighboring islands from Fernando Po to Annabon.
6. In the Atlantic Ocean. — St. Helena, the Cape Verdes, Canaries, Madeira,
Azores, and Iceland. All the islands of the deep part of the ocean (that is, not
on the European or American borders) are volcanic.
7. Over the interior of the continents. — In America,- north and south, there are
none east of the Rockj'^ Mountains and Andes. In Africa, none are known. In
Asia, there is a small volcanic region in the Thian-chan Mountains, at Pe-schan
and Turfan, besides hot springs near Alak-tu-kul, and some other spots in that
vicinity. In Australia, none are known over the interior, the few observed being
situated near its southern border.
2. Kinds of volcanic cones.
As the volcanic mountain is made from its own ejections, it may
consist either (1) of lava alone; (2) of tufa alone; (3) of cinders alone;
(4) of combinations of lavas with either cinders or tufas, or loth. The last
is the more common kind.
1. Lava-cones. — Lavas, when quite liquid, flow off naturally at a
small angle. The average slope of lava-cones is, therefore, very
gentle, — usually between 3 and 10 degrees.
Fig. 967. L
A J — B
K
A, B, B, C, profile of Hawaii, as seen from the eastward; L, Mt. Loa; K, Mt. Kca.
The great volcanoes of Hawaii (Sandwich or Hawaian Islands),
Mt. Loa and Mt. Kea, show^n in the map (fig. 968), and sections of
688
dynajvlical geology.
which are given in figure 967, are mainly lava-cones, and. the gene-
ral slope is 6 to 8 degrees. (These two figures are parts of one pro-
Fig. 968.
Map of part of Ilawaii.
file view of the island, the two joining at B.) Etna has a similar
Fig. 969.
g B fta) i
T- ^ W % "^-^ - J — - on
^nouuM^'.^
Crater of Kilauea, in 1840: a, large boiling lake of lava; at o and near e, snlphur-banks ; r,
an adjoining small crater; ^j, neck between Kilauea and the crater r.
low inclination. A horizontal section of Mt. Loa, 1800 feet Lelow
its top, would be nearly twenty miles broad.
In true lava-cones, like IslX. Loa, the crater is generally a pit-crater,
HEAT VOLCANOES.
689
— a great depressed area in the surface of the mountain, like a pit
or quarry-hole in a plaiih, as in the summit-crater of Mt. Loa and
in Kilauea, the latter 4000 feet above the sea. A larger bird's-eye
view of Kilauea (with an adjoining small crater, r) is shown in fig. 969,
and a vertical transverse section of the same, more enlarged, in fig.
970. The pits have precipitous walls of stratified rocks ; for the
lavas are in layers, and the layers are nearly horizontal.
Fig. 970.
Vertical section of crater of Kilauea, 1840.
At Mt. Loa, the summit-crater is 13,000 feet' in its longer diameter, and 780
feet deep. Kilauea is 16,000 feet in its greatest length, 7h miles in circuit, nearly
four square miles in area, and 600 to 1000 feet deep, — the latter after one of its great
eruptions. It is as much open to the day as a city of two miles square would be
within an encircling wall of 600 feet (the present depth) ; and the pools of boil-
ing lavas and vapors (one of which is at a, fig. 969) may be as leisurely sur-
veyed from the brink as if the objects were gardens and cathedrals.
2. Tufa-cones. — Flowing mud from a boiling basin, or cinders wet
with water and steam, take a larger angle of flow than lavas ; and
tufa-cones, therefore, have commonly an angle of between 15 and 30
degrees. The layers usually slope inwards towards the bottom of
the crater (fig. 971), as well as outwards down the sides. The tufa
Fig. 971.
Fig. 972.
Section of a tufa-cone.
Assumption Island, one of the Ladrones.
has a brownish-yellow color, owing to the action of the steam or hot
water on the cinders, peroxydizing part of the iron in the minerals
(pyroxene mainly) of the lavas, and making a hydrous peroxyd
(p. 65). The crater has generally a saucer shape. A tufa-cone on
Oahu (called Diamond Hill) has a height of 1000 feet. Such cones
are among the results of lateral eruptions about a great volcano
near the sea.
3. Cinder-cones. — Falling cinders, like sand, may make a declivity
of 40 to 45 degrees. The eruption of cinders, therefore, produces a
45
690 DYNAMICAL GEOLOGY.
crater with a narrow throat, a narrow rim above, steep sides, the
slope 35 to 45 degrees (tig. 972), If the voltano is in brisk action,
the space within the crater is dark with the rising vapors, and
the explosions attending the ejection of cinders occur usually at
short intervals.
The cone is at first nearly black or brownish black, but, if not soon covered
with vegetation, it often becomes, through atmospheric agencies, of a red color,
from the peroxydation of the protoxyd of iron in the lava : the peroxyd of iron
formed differs from that of the tufa-cone in not containing water, and hence
the difference of color. The growth of vegetation tends to change back the
red color to brownish black, since the carbon deoxydizes the peroxyd, making
protoxyd and carbonic acid.
4. Mixed cones. — The cones which, like Vesuvius, are formed partly
of lava and partly of cinders or tufa, may have any angle of slope
up to 35 degrees. They may be lava below, and terminate in a
lofty cone of cinders of 40 to 45 degrees. The crater may be nearly
like that of the cinder-cone, — a deep cavity, with the walls thin,
compared with those of the simple lava-cone. There is no fixed
order in the alternations of lavas and cinder or tufa layers : the
lavas are apt to prevail most in the early stages of a volcano.
3. Volcanic action.
The agents concerned in volcanoes are (1) lava; and (2) over-
heated steam and atmospheric air, with vapors of sulphur, and some
other gases.
The phenomena are (1) Rising and projectile effects of escaping
vapors ; (2) Movements of the lavas in the crater: (3) Eruptions.
The facts presented in illustration of this subject are taken mainly from
the volcanoes of Kilauea and Vesuvius, both of which have been visited by the
author.
1. Agents.
1. Kinds of volcanic rocks or lavas. — The fused rock-mate-
rial is, in all cases, called lava. When solidified, it is lava still, and
is often so termed, whatever its texture ; but in general the name is
restricted to those volcanic rocks which are more or less cellular.
The cellules are usually ragged, and not smooth and almond-shaped
like those of an amygdaloid. The solid kinds, with rarely a cellule
or with none at all, come under the general designation of volcanic
rocks. A very light cellular lava is a scoria, or volcanic slag, or is said
to be scoriaceous.
The principal kinds of volcanic rocks and lavas have been described on pp.
87-89, to which reference may here be made. The most common are dolente,
doleritic lava, basalt, basaltic lava, clinkstone, trachyte.
HEAT — VOLCANOES. 691
The rock of Vesuvius is leticitopTiyr, it containing the white mineral leueite
disseminated through it; that of Mt. Loa is mostly of the first four of the kinds
just mentioned. But about some parts, and even at the summit, of Mt. Loa, there
are clinkstone and jyorphiji-y, — compact light-colored feklsjmthic rocks without
cellules. It is not an uncommon fact that, while the ordinary rocks of the ex-
terior of a volcanic mountain are the heavy cellular dolerites and basalt, those
of the interior (as best seen when the mountain-mass is intersected by profound
gorges) are of these compact feldspathic kinds having no resemblance to ordi-
nary lavas.
2. Liquidity of lava. — The liquid lava flows usually with nearly
the mobility of melted iron or glass. The whole of the flowing
mass does not, however, appear to be properly in a liquid or melted
condition ; a portion, in unfused grains, is suspended in a fused por-
tion. As the heat just below the surface has the intensity of what
is called white heat, any part of the rock-material which is fusible
at this temperature, or, rather, which is not consolidated at this
temperature (for the material has come from the depths below,
where the heat is much greater, it increasing with the depth or
pressure), will be in a melted state. In the crater of Kilauea, the
liquid lava cools at surface into a scoriaceous glass, and this glass was,
beyond doubt, in fusion, like the glass of a glass-furnace, — though
perhaps less perfectly, as stony unfused grains may be disseminated
through it. Below the surface, six inches more or less, the lava
has the aspect of a cellular rock ; but even glass takes this form if
very slowly cooled, and would do so all the more readily if it con-
tained a large amount of unmelted grains of any stony material.
At Kilauea the liquidity is so complete that jets, but a quarter of an inch
through, are sometimes tossed up from a tiny vent, and as they fall back on one
another make a column of hardened tears of lava. Again, the winds draw
out the glass of the lava-jets in the boiling pools into fine threads, by carrying
off small fragments, and thus make what is called Pele's hair, the crater being
the residence, in native mythology, of the goddess Pele.
The mobility is also very largely promoted by the vapors rising
in the lava, especially the overheated steam. This is considered its
sole cause by Scrope.
3. Vapors or gases. — Besides air, steam (vapor of water), and
sulphurous vapors (either sulphurous acid or sulphur), there are
sometimes (1) Carbonic acid gas, derived from limestone, and perhaps
from other sources below ; (2) Muriatic acid gas, derived from sea-
water, but probably not exclusively.
But these two gases, along with nitrogen and sulphuretted hydrogen, are
mostly emanations irova. fumaroles, — vents of hot air, steam, or sulphurous fumes,
in the neighborhood of a volcano, — rather than from the liquid lava. Further
examinations of the gases which escape from the liquid lavas in the crater are
692 DYNAMICAL GEOLOGY.
required. About Vesuvius and many other volcanoes incrustations of com-
mon salt and other chlorids form during an eruption in places a little distant
from the scenes of intensest action ; and these, as well as the muriatic acid,
appear to show that sea-water gains access to the lavas; and, if so, fresh waters
also may. The steam may come jjartly from the depths of the lava, and partly
from superficial waters.
2. Volcanic Phexomexa.
1. Rising and projectile effects of escaping vapors. — The water
and other vaporizable substances within the lava are under a press-
ure of about 125 pounds to a square inch for every 100 feet of
depth. Owing to the heat and their consequent expansion, they
slowly rise in the heavy, viscid liquid ; as they rise, they keep ex-
panding, until, nearing the surface, they begin to take the form of
vapors, and finally break through.
The bubble or vapor in boiling water has projectile force enough,
as it breaks at the surface, to throw up water in jets to a height of
two or three inches. Were the resistance greater, as in a more
dense and viscid liquid, the bubbles would become larger by addi-
tions before they could escape ; the force would therefore be greater
and the jets higher. In lavas which have the freest liquidity, as
those of Kilauea, the jets are thirty to forty feet high. Conse-
quently, a surface of liquid lava, as in the lakes of lava in Kilauea,
is covered throughout with jets, like a vat of boiling water, and
there is only a muttering noise from the action. It looks like ordi-
nary ebullition, only the jets are jets of fiery liquid rock. They
rise vertically, and fall back into the pool, or on its sides, before they
have cooled. A lake 1000 feet in diameter (at a, fig. 969) was there
in brilliant play over its whole surface when visited by the author
in 1840 ; and, in more active times, a large part of the area of four
square miles has been in this boiling state.
If the lavas be less liquid, the vapors are kept from escaping, by
the resistance, until they have collected in far larger bubbles, and,
when such bubbles burst, the projectile force may be enormous ; it
carries the fragments far aloft, to descend in a shower of cinders of
great extent.
Such bubbles, rising and bursting, were seen by Spallanzani in the crater of
Stromboli, a high cinder-cone in the Mediterranean, north of Sicily. In times
of moderate action at Vesuvius, the outbursts of cinders occur every three to
ten minutes ; but in a period of eruption they are almost incessant. Accord-
ing to Sir Wm. Hamilton, the cinders rose to a height of 10,000 feet at the erup-
tion of 1779, — a height indicating a vast projectile force. Occasionally masses
of lava are thi'own up which descend like huge cannon-balls, having been
rounded by the rotation before they had cooled, and rendered compact exter-
HEAT VOLCANOES. 693
nally, while usually cellular within. Such masses are called volcanic bombs.
They may have lenticular as well as spheroidal shapes.
2. Movements of the lavas in the crater. — {a.) Upward move-
ment. — As the vaporizable substances (water, sulphur, etc.) and at-
mospheric air expand while rising in the volcanic vent, they displace
correspondingly the lava, and so cause a general expansion of the
mass. This alone produces a rise of the lavas in the conduit.
When the boiling of a viscid fluid in a tube causes its upper surface to
ascend, because the liquid at top becomes inflated or frothy with vapor, it
exemplifies the same principle, although the degree of inflation very far exceeds
that in a dense lava. The fact of a rising in the volcano from this cause is
beyond question.
This rising becomes apparent in overflowings from the pools of
the crater, over its bottom, in streams which cool and become solid
lava. Whether the whole rising is due to this cause is not ascer-
tained. The risings and overflowings are repeated from time to
time, until the material within the crater has reached a height and
an intensity of action that lead to an eruption.
At Kilauea (the bottom of which, when at its lowest mark, is 3000 feet above
the sea) the conduit of liquid lava descending downward below the crater is
3000 feet long to the sea-level; and it may extend many miles, or perhaps scores
of miles, bcluAv this. Nineteen miles would correspond to about 100,000 feet.
A rise of the lavas within the crater of 400 to 500 feet in the manner above ex-
plained is all that in three eases of eruption at Kilauea preceded the outbreak.
Five hundred feet in 100,000 is an average expansion of only a half of one per
cent. But it is probable that the vapors which produce this result are com-
paratively superficial; they may be from the fresh or salt waters of the sur-
rounding region.
The increase of activity as the lavas rise in a crater has two
obvious causes : (1) the temperature of the lavas increases with the
pressure ; and, consequently, a rise of 100 feet would have increased
very much the temperature at the bottom of that 100 feet, and so
on for greater depths ; (2) the rise exposes a higher column of
liquid lava above to the action of external waters.
{b.) Circulating movement. — In the lava-conduit the greatest heat is
along the centre, most remote from the cold sides. Hence, as in
any cauldron, the ascent from inflation by rising vapors would be
greatest at the centre ; there would therefore be at the surface a
flow from the centre to the sides, and a system of circulation. This
was exhibited on a grand scale at Kilauea in 1840, where the liquid
lava in the great lake (1000 feet across, a, fig. 9G9) seemed like a
river that came to the surface for a moment and then disappeared.
694 DYNAMICAL GEOLOGY.
The area of greatest heat was near the northeast side of the lake,
and the stream seemed to flow to the southwest.
3. Eruptions. — (a.) General facts. — The rising of the lavas within
the crater, and the activity of the vapors from one cause or another,
reach such a height and have so great power that an eruption takes
place, — either over the brim of the crater, or through the fractured
mountain. The lavas flow off to a distance sometimes of sixty
miles or more. Examples are given beyond.
The outflow of lavas is attended in most volcanoes, as in Vesuvius,
with the ejection of cinders, and they continue to be thrown out
long after the flow has ceased. They thus build up a cinder-cone
immediately around the open vent.
Most of the small cones about volcanic mountains — called often jiarasitic
cones — are formed in this manner about a point in some opened fissure from
which lavas were ejected. Cinder and vapor eruptions are the last effects of the
subsiding fires of a volcano, Mt. Kea is an example of a mountain-cone finish-
ing its career as an eruptive volcano by the formation of a number of cinder-
cones at summit : their height is 300 to 500 feet. In other cases, the central