VOLCANIC MOUNTAINS.
'Tis said Enceladus' huge frame,
Heart-stricken by the avenging flame,
Is prisoned here, and underneath
Gasps through each vent his sulphurous breath;
And still as his tired side shifts round,
Trinacia echoes to the sound
Through all its length, while clouds of smoke
The living soul of ether choke.
VIRGIL: _Æneid iii._
In some parts of the world we meet with mountains of a very different
kind from any we have yet considered,--mountains that are known
at times to send forth fiery streams of glowing lava, and to emit
with terrific force great clouds of steam. Such mountains have
long been known, in popular but unscientific language, as "burning
mountains,"[25]--a term which is unfortunate, because they do not
burn in the proper sense of the word, like candles or gas-jets. They
are better known as volcanoes. There are about three hundred and
fifty known active volcanoes; and if we include all mountains that
once were in that state, the number is about one thousand.
[25] See papers by the writer on Volcanoes and Volcanic Action
in "Knowledge" for May and June, 1891, on which this chapter is
partly based.
Such mountains are connected in a curious way with those upheaved
ridges of the world known as mountain-chains (see chap. vi., p.
191). And not only are many mountains more or less penetrated and
intersected by rocks of an igneous origin (see chap. v., p. 155),
but some have been largely formed by the action of old volcanoes.
In fact, there are hills in Great Britain and parts of Europe, in
America, and other countries, that once were actual volcanoes (see
page 277).
We must briefly consider these strange mountains so different from
others, and see what we can find out about them. Let us first inquire
how a volcano is made, then consider what a volcano does; that is, we
must view it as a geological agent that has a certain definite part
to play in the economy of the world. And lastly, we may glance at
some of the old volcanoes, and see what they were doing in those long
ages of the world during which the great series of the stratified
rocks were formed,--which rocks are, as it were, the book in which
the earth has written her autobiography.
In old days volcanoes were regarded with superstitious awe; and any
investigation of their actions would have been considered rash and
impious in the highest degree. Mount Etna, as Virgil tells us, was
supposed to mark the spot where the angry gods had buried Enceladus,
one of the rebellious giants. Volcano, a certain "burning mountain"
in the Lipa Islands, was likewise called the forge, or workshop,
of Vulcan (or Volcan), the god of fire. And so it comes about that
all "burning mountains" take their name from this one Mediterranean
island, and at the same time tell us of the mythological origin of
the word. It has been said that words are "fossil thoughts;" and we
have here an old and very much fossilised thought,--a kind of thought
long since extinct among civilised peoples, and one which is never
likely to come to life again.
A volcanic mountain consists of alternating sheets of lava and
volcanic ashes, mantling over each other in an irregular way,
and all sloping away from the centre. In the centre is a pit or
chimney, widening out towards the top so as to resemble a funnel or
cup; hence the name "crater," which means a cup. In the centre of
this crater a very small cone ("minor cone") is frequently found;
and it is interesting to find that many of the moon's volcanic
craters possess these "minor cones." A number of cracks or fissures
intersect the volcano. These frequently spread out from the centre
of the mountain in all directions, like the spokes of a wheel.
They generally get filled with lava that wells up from below, thus
forming "dykes," which may be regarded as so many sheets of igneous
rock, such as basalt, that have forced their way while still liquid
in among the layers of lava and ashes. The word "ash" is used by
geologists in a special sense; and volcanic ash is not, as might
be supposed, a deposit of cinders, but mostly of dust of various
degrees of fineness, and sometimes it is very fine indeed. Pieces
of pumice-stone may be embedded in a layer of volcanic ash, and
sometimes great blocks of stone that have been shot out of the
volcano as from a big gun, but these only form a small part of the
layer. Dykes strengthen the mountain, and tend to hold it together
when violently shaken during an eruption.
The shape and steepness of a volcano depend on the nature of the
materials ejected. The finer the volcanic ash, the steeper and more
conical is the mountain. The building up of a volcano may be fairly
illustrated by the little cone of sand formed in an hourglass as the
sand-grains fall. These settle down at a certain slope, or angle,
at which they can remain, instead of falling down to the bottom, as
they do directly this slope is exceeded. Some volcanoes are built up
almost entirely of volcanic ash and its embedded blocks. Vesuvius,
Teneriffe, Jorullo, in Mexico, and Cotopaxi, in the Andes, are
examples of steep volcanic cones built up in this way. Others, less
steep and more irregular in shape, are chiefly formed of successive
lava-flows. Little minor cones are frequently formed on the side of
a volcano; and these during an eruption give rise to small outbursts
of their own. They are easily accounted for by the dykes which are
mentioned just now; for when molten rock forces its way through
fissures, it sometimes finds an outlet at the surface, and being full
of steam, as soda-water is full of gas, it gives rise to an eruption.
The great opening in the centre of a volcano, with its molten lava,
is like a very big dyke that has reached the surface and so succeeded
in producing an eruption.
The opening of a soda-water bottle not infrequently illustrates a
volcanic eruption; for when the pent-up carbonic acid cannot escape
fast enough, it forces out some of the water, even when the bottle is
held upright.
Every volcano has been built up on a platform of ordinary stratified
rocks; and at some period _after_ these had been laid down in water
and raised up into dry land, molten rock found its way through them,
and so the volcano was built up by successive eruptions during many
years. It is probable that earthquake shocks, preceding the first
eruption, cracked up these strata, and so made a way for the lava to
come up.
The main point we wish to emphasize is that _volcanoes are never
formed by upheaval_. In this way they differ from all other
mountains. They have not been made by the heaving up of strata, but
have been gradually piled up, something like rubbish heaps that
accumulate in the Thames barges as the dustmen empty their carts into
them, only in the case of volcanoes the "rubbish" comes from below.
It is not necessary to suppose that the reservoir down below, from
which the molten rock is supplied, exists at any very great depth
below the original land surface on which the volcano grows up.
The old "upheaval theory" of volcanoes, once advocated by certain
authorities, instead of being based on actual evidence or on
reasoning from facts, was a mere guess. Moreover, if the explanation
we have given should not be sufficiently convincing, there is good
proof furnished by the case of a small volcano near Vesuvius, the
building of which was actually witnessed. It is called Monte Nuovo,
or the New Mountain. It is a little cone 430 feet high, on the bank
of Lake Averno, with a crater more than a mile and a half wide at the
base. It was almost entirely formed during a single night in the year
1538, A. D. We have two accounts of the eruption to which it owes
its existence; and each writer says distinctly that the mountain was
formed by the falling of stones and ashes.
One witness says,--
"Stones and ashes were thrown up with a noise like the
discharge of great artillery, in quantities which seemed as if
they would cover the whole earth; and in four days their fall
had formed a mountain in the valley between Monte Barbaro and
Lake Averno, of not less than three miles in circumference, and
almost as high as Monte Barbaro itself,--a thing incredible
to those who have not seen it, that in so short a time so
considerable a mountain should have been formed."
Another says,--
"Some of the stones were larger than an ox. The mud (ashes
mixed with water) was at first very liquid, then less so, and
in such quantities that with the help of the afore-mentioned
stones a mountain was raised one thousand paces in height."
(The writer's astonishment led him greatly to exaggerate the height.)
These accounts are important as showing how in a much longer time
a big volcano may be built up. From such small operations we learn
how Nature works on a large scale. The great volcano in Mexico known
as Jorullo was probably built up in a very similar way. There is a
tradition among the natives that it was made in two or three days;
but we can hardly believe that. Volcanoes, as they get older, tend
to grow taller and bigger; but every now and then a large portion may
be blown away by some great eruption, and they have, as it were, to
begin again.
[Illustration: THE ERUPTION OF VESUVIUS IN 1872. FROM AN
INSTANTANEOUS PHOTOGRAPH.]
Let us now consider volcanoes as geological agents, and see what
they do. A volcanic eruption may be described in a general way
as follows: Its advent is heralded by earthquakes affecting the
mountain and the whole country round; loud underground explosions
are heard, resembling the fire of distant artillery. The vibrations
are chiefly transmitted through the ground; the mountain seems
convulsed by internal throes, due, no doubt, to the efforts of the
imprisoned steam and liquid rock to find an opening. These signs are
accompanied by the drying up of wells and disappearance of springs,
since the water finds its way down new cracks in the rocks, caused
by the frequent shocks and quiverings. When at last an opening has
been made, the eruption begins,--generally with one tremendous
burst that shakes the whole mountain down to its foundations. After
this, frequent explosions follow with great rapidity and increasing
violence, generally from the crater. These are indicated by the
globular masses of steam which are to be seen rising up in a tall
column like that which issues from the funnel of a locomotive. But
sometimes the whole mountain seems to be more or less engaged in
giving out steam, and thus to be partly enveloped in it. This is
illustrated by our engraving from an instantaneous photograph of
Vesuvius in eruption in the year 1872. The steam and other gases, in
their violent ascent, hurl up into the air a great deal of solid rock
from the sides of the central opening, after first blowing out the
stones which previously stopped up the orifice.
Blocks of stone falling down meet with others coming up; and so a
tremendous pounding action takes place, the result of which is that
great quantities of volcanic dust and ashes are produced, generally
of extreme fineness. Winds and ocean currents transport these light
materials for long distances. The observations made during the
famous and fruitful voyage of H. M. S. "Challenger" showed that
fine volcanic dust is carried by wind and marine currents to almost
all parts of the oceans. The darkness so frequently mentioned in
accounts of eruptions--sometimes at a very great distance from the
volcano--is entirely caused by clouds of volcanic dust hiding the
light of the sun. Perhaps the best example of this is the case of the
eruption of Krakatoa (in the Strait of Sunda, between Sumatra and
Java) in 1883. Its explosions were heard in all directions for two
thousand miles, and a perceptible layer of volcanic dust fell at all
places within one thousand miles; while the finest dust and vapour,
shot up fifteen or twenty miles high, were spread all over the globe,
causing, while still suspended in the atmosphere, the peculiar red
sunsets noticed in all parts of the world for some months after the
eruption.
Again, those very curious deposits of "red clay" found in the very
deepest parts of the Pacific and Atlantic oceans (at depths of about
four thousand fathoms, or twenty-four thousand feet) have been shown
to be chiefly composed of volcanic dust, their red colour being due
to oxidised iron.
But there is another way in which a good deal of fine volcanic dust
is made; and it is this: the lava is so full of steam intimately
mixed up with it that the steam, in its violent effort to escape,
often blows the lava into mere dust.
Another interesting phenomenon may be thus described: Portions of
liquid, or half liquid, lava are caught up by the steam and hurled
into the air. These assume a more or less round form, and are known
as "bombs." At a distance they give rise to the appearance of
flames. And here we may remark that the flaring, coloured pictures
of Etna or Vesuvius in eruption, which frequently may be seen, are
by no means correct. The huge flames shooting up into the air are
quite imaginary, but are probably suggested by the glare and bright
reflection from glowing molten lava down in the crater.
So great is the force of the pent-up steam trying to escape that it
frequently blows a large part of the volcano bodily away; and in some
cases a whole mountain has been blown to pieces.
Finally, torrents of rain follow and accompany an eruption,--a result
which clearly follows from the condensation of large volumes of
steam expanding and rising up into the higher and cooler layers of
the atmosphere. Vast quantities of volcanic ash are caught up by the
rain, and in this way very large quantities of mud are washed down
the sides of the mountain.
Sometimes the mud-flows are on a large scale, and descending with
great force, bury a whole town. It was mostly in this way that the
ancient cities of Herculaneum and Pompeii were buried by the great
eruption of Vesuvius in the year 79 A. D., in which the elder Pliny
lost his life. The discoveries made during excavations at Pompeii are
of very great interest as illustrating old Roman life. The Italians
give the name _lava d'acqua_, or water-lava, to flows of this kind,
and they are greatly dreaded on account of their great rapidity. An
ordinary lava-stream creeps slowly along, so that people have time
to get out of the way; but in the case of mud-flows there is often
no time to escape. No lava-stream has ever reached Pompeii since it
was first built, although the foundations of the town stand upon an
old lava-flood. Herculaneum is nearer to Vesuvius, and has at times
been visited by lava-streams. Mud-lavas, ashes, and lava-streams have
accumulated over this city to a depth of over seventy feet.
Lava-streams vary greatly in size; in some cases the lava, escaping
from craters, comes to rest before reaching the base of the slopes of
the volcano; in other cases a lava-flow not only reaches the plains
below, but extends for many miles over the surrounding country.
Hence lava-streams are important geological agents. Let us look at
some famous instances. The most stupendous flow on record was that
which took place from Skaptar Jökull in Iceland, in the year 1783.
In this case a number of streams issued from the volcano, flooding
the country far and wide, filling up river gorges which were in some
cases six hundred feet deep and two hundred and fifty feet broad, and
advancing into the alluvial plains in lakes of molten rock twelve to
fifteen miles wide and one hundred feet deep. Two currents of lava
which flowed in nearly opposite directions spread out with varying
thickness according to the nature of the ground for forty and fifty
miles respectively. Had this great eruption taken place in the south
of England, all the country from the neighbourhood of London to
that of Gloucester might have been covered by a flood of basalt of
considerable thickness.
Sometimes, when the lava can only escape at a point low down on the
mountain, a fountain of molten rock will spout high into the air.
This has happened on Vesuvius and Etna. But in an eruption of Mauna
Loa, in the Sandwich Islands, an unbroken fountain of lava, from two
hundred to seven hundred feet high and one thousand feet broad, burst
out at the base of the mountain; and again in April, 1888, the same
thing happened on a still grander scale. In this case four fiery
fountains continued to play for several weeks, sometimes throwing
the glowing lava to a height of one thousand feet in the air. Surely
there can be no more wonderful or awful sight than this in the world.
The volcanoes of Hawaii, the principal island in the Sandwich
Islands, often send forth lava-streams covering an area of over
one hundred square miles to a depth of one hundred feet or more;
but they are discharged quite quietly, like water welling out of a
spring. Repeated flows of this kind, however, have in the course of
ages built up a great flat cone six miles high from the floor of the
ocean, to form this lofty island, which is larger than Surrey; and
it is calculated that the great volcanic mountain must contain enough
material to cover the whole of the United States with a layer of rock
fifty feet deep.
But it is not only on the surface of the land that volcanic eruptions
take place; for in some cases the outbreak of a submarine eruption
has been witnessed, and it is highly probable that in past geological
ages many large eruptions of this nature have taken place. In the
year 1783, an eruption took place about thirty miles off the west
coast of Iceland. An island was built up from which glowing vapour
and smoke came forth; but in a year or less the waves had washed
everything away, leaving only a submerged reef. The island of
Santorin, in the Greek Archipelago, is a partly submerged volcano.
But in some cases enormous outpourings of lava have taken place,
not from volcanoes, but from openings of the ground here and there,
and more usually from long fissures or cracks in the rocks lying at
the surface. In many cases so much lava has quietly welled out in
this way that the old features of the landscape have been completely
buried up, and wide plains and plateaux formed over them. Sir A.
Geikie says,--
"Some of the most remarkable examples of this type of volcanic
structure occur in western North America. Among these that of
the Snake River plain in Idaho may be briefly described.
"Surrounded on the north and east by lofty mountains, it
stretches westward as an apparently boundless desert of sand
and bare sheets of black basalt. A few streams descending into
the plain from the hills are soon swallowed up and lost. The
Snake River, however, flows across it, and has cut out of its
lava bed a series of picturesque gorges and rapids.
"The extent of country which has been flooded with basalt in
this and adjoining regions of Oregon and Washington has not
yet been accurately surveyed, but has been estimated to cover
a larger area than France and Great Britain combined. Looked
at from any point on its surface, one of these lava plains
appears as a vast level surface, like that of a lake bottom.
This uniformity has been produced either by the lava rolling
over a plain or lake bottom, or by the complete effacement of
an original, undulating contour of the ground under hundreds of
feet of lava in successive sheets. The lava, rolling up to the
base of the mountains, has followed the sinuosities of their
margin, as the waters of a lake follow its promontories and
bays."
A few further examples of mud-lavas may be mentioned here. Cotopaxi,
a great volcano in Ecuador, South America, with a height of 17,900
feet, reaches so high into the atmosphere that the higher parts are
capped with snow. In June, 1877, a great eruption took place, during
which the melting of snow and ice gave rise to torrents of mud and
water, which rushed down the steep sides of the mountain, so that
large blocks of ice were hurried along. The villages around to a
distance of about seventy miles were buried under a deposit of mud,
mixed with blocks of lava, ashes, pieces of wood, etc.
Sometimes a volcano discharges large quantities of mud directly
from the crater. In this case the mud is not manufactured by the
volcano itself, but finds its way through fissures and cracks from
the bed of the neighbouring sea or rivers to the crater. Thus, in
the year 1691, Imbaburu, one of the Andes of Quito, sent out floods
of mud containing dead fish, the decay of which caused fever in the
neighbourhood. In the same way the volcanoes of Java have often
buried large tracts of fertile country under a covering of volcanic
mud, thus causing great devastation.
Vast quantities of dust are produced, as already explained, by the
pounding action that takes place during an eruption, as portions
of rock in falling down meet others that are being hurled into the
air. Striking instances of this have occurred not far from Great
Britain. Thus in the year 1783, during an eruption of Skaptar Jökull,
so great was the amount of dust thus created that the atmosphere in
Iceland was loaded with it for several months. Carried by winds, it
even reached the northern parts of Scotland, and in Caithness so
much of it fell that the crops were destroyed. This is remarkable,
considering that the distance was six hundred miles. Even in Holland
and Norway there are traces of this great shower of dust from the
Icelandic volcano.
During the fearful eruption of Tomboro, a volcano in the island of
Sumbawa, in the Eastern Archipelago, in 1815, the abundance of ashes
and dust ejected caused darkness at midday at Java, three hundred
miles away, and even there the ground was covered to a depth of
several inches. In Sumbawa itself the part of the island joining
the mountain was entirely desolated, and all the houses destroyed,
together with twelve thousand inhabitants. Trees and herbage were
overwhelmed with pumice and volcanic dust. The floating pumice on the
sea around formed a layer two feet, six inches thick, through which
vessels forced their way with difficulty. From such facts as these
it is clear that if in past ages volcanoes have been so powerfully
active as they are now, we should expect to find lava-flows, dykes,
and great deposits of volcanic ash deposited in water among the
stratified rocks; and such is the case. Many large masses of rock
familiar to the geologist, and often forming parts of existing
mountains, are to be accounted for either as great lava-flows, or
dykes that have forced their way in among the strata, or as extensive
deposits of volcanic ash.
But perhaps the reader would like to know what the inside of a
volcanic crater is like during an eruption. Let us, then, take a peep
into that fearful crater of Kilauea, in the Sandwich Islands. For
this purpose we cannot do better than follow Miss Bird's admirable
description of her adventurous expedition to this crater:--
"The abyss, which really is at a height of four thousand feet,
on the flank of Mauna Loa, has the appearance of a pit on
a rolling plain. But such a pit! It is quite nine miles in
circumference, and at its lowest area--which not long ago fell
about three hundred feet, just as ice on a pond falls when
the water below is withdrawn--covers six square miles. The
depth of the crater varies from eight hundred to one thousand
feet, according as the molten sea below is at flood or ebb.
Signs of volcanic activity are present more or less throughout
its whole depth, and for some distance round its margin, in
the form of steam-cracks, jets of sulphurous vapour, blowing
cones, accumulating deposits of acicular crystals of sulphur,
etc., and the pit itself is constantly rent and shaken by
earthquakes. Grand eruptions occurred with circumstances
of indescribable terror and dignity; but Kilauea does not
limit its activity to these outbursts, but has exhibited its
marvellous phenomena through all known time in a lake or lakes
on the southern part of the crater three miles from this side.
"This lake--the _Hale-mau-mau_, or 'House of Everlasting Fire,'
of the Hawaiian mythology, the abode of the dreaded goddess
Pele--is approachable with safety, except during an eruption.
The spectacle, however, varies almost daily; and at times the
level of the lava in the pit within a pit is so low, and the
suffocating gases are evolved in such enormous quantities, that
travellers are unable to see anything. There had been no news
from it for a week; and as nothing was to be seen but a very
faint bluish vapour hanging round its margin, the prospect was
not encouraging.... After more than an hour of very difficult
climbing, we reached the lowest level of the crater, pretty
nearly a mile across, presenting from above the appearance
of a sea at rest; but on crossing it, we found it to be an
expanse of waves and convolutions of ashy-coloured lava, with
huge cracks filled up with black iridescent rolls of lava only
a few weeks old. Parts of it are very rough and ridgy, jammed
together like field-ice, or compacted by rolls of lava, which
may have swelled up from beneath; but the largest part of the
area presents the appearance of huge coiled hawsers, the ropy
formation of the lava rendering the illusion almost perfect.
These are riven by deep cracks, which emit hot sulphurous
vapours....
"As we ascended, the flow became hotter under our feet, as well
as more porous and glistening. It was so hot that a shower of
rain hissed as it fell upon it. The crust became increasingly
insecure, and necessitated our walking in single file with the
guide in front, to test the security of the footing. I fell
through several times, and always into holes full of sulphurous
steam so malignantly acid that my strong dogskin gloves were
burned through as I raised myself on my hands.
"We had followed the lava-flow for thirty miles up to the
crater's brink, and now we had toiled over recent lava for
three hours, and by all calculation were close to the pit; yet
there was no smoke or sign of fire, and I felt sure that the
volcano had died out for once for our special disappointment....
"Suddenly, just above, and in front of us, gory drops were
tossed in the air, and springing forwards we stood on the brink
of _Hale-mau-mau_, which was about thirty-five feet below
us. I think we all screamed. I know we all wept; but we were
speechless, for a new glory and terror had been added to the
earth. It is the most unutterable of wonderful things. The
words of common speech are quite useless. It is unimaginable,
indescribable; a sight to remember for ever; a sight which
at once took possession of every faculty of sense and soul,
removing one altogether out of the range of ordinary life.
Here was the real 'bottomless pit,' 'the fire which is not
quenched,' 'the place of Hell,' 'the lake which burneth with
fire and brimstone,' 'the everlasting burnings,' 'the fiery
sea whose waves are never weary.'[26] There were groanings,
rumblings, and detonations, rushings, hissings, splashings,
and the crashing sound of breakers on the coast; but it was
the surging of fiery waves upon a fiery shore. But what can
I write? Such words as jets, fountains, waves, spray, convey
some idea of order and regularity, but here there was none.
The inner lake, while we stood there, formed a sort of crater
within itself; the whole lava sea rose about three feet; a
blowing cone about eight feet high was formed; it was never the
same two minutes together. And what we saw had no existence
a month ago, and probably will be changed in every essential
feature a month hence.... The prominent object was fire in
motion; but the surface of the double lake was continually
skimming over for a second or two with a cooled crust of a
lustrous grey-white, like frosted silver, broken by jagged
cracks of a bright rose-colour. The movement was nearly always
from the sides to the centre; but the movement of the centre
itself appeared independent, and always took a southerly
direction. Before each outburst of agitation there was much
hissing and throbbing, internal roaring, as of imprisoned
gases. Now it seemed furious, demoniacal, as if no power on
earth could bind it, then playful and sportive, then for a
second languid, but only because it was accumulating fresh
force.... Sometimes the whole lake ... took the form of mighty
waves, and surging heavily against the partial barrier with
a sound like the Pacific surf, lashed, tore, covered it, and
threw itself over it in clots of living fire. It was all
confusion, commotion, forces, terror, glory, majesty, mystery,
and even beauty. And the colour, 'eye hath not seen' it! Molten
metal hath not that crimson gleam, nor blood that living
light."[27]
[26] Perhaps these Scripture phrases were suggested long before
the Bible was written, by the sight of some crater in active
eruption.
[27] The Hawaiian Archipelago.
Continued observation of volcanoes, together with evidence derived
from history, teaches that there are different stages of volcanic
action. There are three pretty well-marked phases. First, the state
of permanent eruption; this is not a dangerous state, because the
steam keeps escaping all the time: the safety-valve is at work, and
all goes smoothly. The second state is one of moderate activity, with
more or less violent eruptions at brief intervals; this is rather
dangerous, because at times the safety-valve does not work.
And thirdly, we have paroxysms of intense energy, alternating with
long periods of repose sometimes lasting for centuries. These
eruptions are extremely violent, and cause widespread destruction;
the safety-valve has got jammed, and so the boiler bursts.
No volcano has been so carefully watched for a long time as Vesuvius.
Its history illustrates the phases we have just mentioned. The first
recorded eruption is that of A. D. 79, a very severe one of the
violent type, by which Herculaneum, Pompeii, and Stabiæ were buried.
We have an interesting account by the younger Pliny. Before this
great eruption took place, Vesuvius had been in a state of repose
for eight hundred years, and if we may judge from the Greek and Roman
writings, was not even suspected of being a volcano. Then followed
an interval of rest until the reign of Severus, the second eruption
taking place in the year 203. In the year 472, says Procopius, all
Europe was covered more or less with volcanic ashes. Other eruptions
followed at intervals, but there was complete repose for two
centuries; that is, until the year 1306. In 1500 it was again active,
then quiet again for one hundred and thirty years. In 1631 there took
place another terrific outburst. After this many eruptions followed,
and they have been frequent ever since. Vesuvius is therefore now in
the second stage of moderate activity.
But geologists can take a wider view than this. They can sum up the
history of a volcanic region of the earth; and the result is somewhat
as follows: Volcanoes, like living creatures, go through different
periods or phases, corresponding roughly to youth, middle age, old
age, and finally decay. The invasion of any particular area of the
earth's surface by the volcanic forces is heralded by underground
shocks, or earthquakes. A little later on cracks are formed, as
indicated by the rise of saline and hot springs, and the issuing of
carbonic acid and other gases at the surface of the earth. As the
underground activity becomes greater, the temperature of the springs
and emitted gases increases; and at last a visible rent is formed,
exposing highly heated and glowing rock below. From the fissure thus
formed, the gas and vapours imprisoned in the molten rocks escape
with such violence as to disperse the latter in the form of pumice
and volcanic ash, or to cause them to pour out as lava-streams.
The action generally becomes confined to one or more points along the
line of action (which is a line of fissures and cracks). In this way
a chain of volcanoes is formed, which may become the seat of volcanic
action for a long time.
When the volcanic energies have become somewhat exhausted, so that
they cannot raise up the lava and expel it from the volcanic crater,
nor rend the sides of the volcano and cause minor cones to grow up
on their flanks, small cones may be formed at a lower level in the
plains around the great central chain. These likewise are fed from
fissures.
Later on, as the heated rock below cools down, the fissures are
sealed up by lava that has become solid; and then the volcanoes
fall, as it were, into the "sere and yellow leaf," and remain in a
peaceful, quiet state befitting their old age.
After this they begin to suffer from long exposure to the atmospheric
influences of decay, and rain and rivers wash them away more or less
completely.
But still the presence of heated rocky matter at no great depth
below is proved by the outbursts of gases and vapours, the forming
of geysers and ordinary hot springs. Gradually, however, even these
signs of heat below disappear; and the cycle of volcanic phases is at
an end. Such a series of changes may require millions of years; but
by the study of volcanoes in every stage of their growth and decline
it is possible thus to sketch out an outline of their history.
It must be confessed that in the present state of scientific
knowledge no full and complete explanation of volcanic action is
possible. Geologists and others are as yet but feeling their way
cautiously towards the light which, perhaps before long, will
illumine the dark recesses of this mysterious subject. Many theories
and ideas have been put forward, but in the opinion of the writer the
most promising explanation is one that may be briefly expressed as
follows:
There are below the crust of the earth large masses of highly
heated rock that are _kept solid_ by the enormous pressure of the
overlying rocks, or otherwise they would melt,--for it is a known
fact that pressure tends to prevent the melting of a solid body. But
when earth-movements taking place within the earth's crust--such
as the upheaving of mountain-chains--take off some of the weight,
the balance between internal heat and the pressure from above is no
longer maintained; and so these highly heated rocks run off into
the liquid state, and finding their way to the surface through the
fissures mentioned above, give rise to volcanic action. There is
much to be said in favour of this view. It rightly connects volcanic
action with movements of upheaval, with mountain-chains and lines of
weakness in the earth's crust.
There is very good reason to believe that the earth was once in a
highly heated state, and has been slowly cooling down for ages. The
increase of temperature observed in penetrating mines tells us that
it still retains below the surface some of its old heat. We need not
therefore be surprised at the existence of heated masses of rock down
below, or seek, as some have done, an entirely different source for
the origin of volcanic heat than that which remains from the earth's
once molten condition. It would take too long to state the reasons
on which this idea of the former state of our planet is based, and
moreover, it would bring us into the region of astronomy, with which
we are not concerned at present.
In various parts of Great Britain and Ireland we meet with old
volcanic rocks,--lavas, intrusive dykes, and sheets of basalt, etc.,
together with vast deposits of volcanic ash, which, sinking into
the old neighbouring seas, became stratified, or arranged in layers
like the ordinary sedimentary rocks. In some cases we see embedded
in these layers the very "bombs" that were thrown out by the old
volcanoes (see page 253). And besides these purely volcanic rocks, we
often meet in these areas with great bosses of granite, which must
have been in some way connected with the old volcanoes, and probably
were in many cases the source from which much of the volcanic rock
was derived. But more than this, in a few instances we have the site
of the old volcano itself marked out by a kind of pipe, or "neck,"
now filled with some of its volcanic débris in the shape of coarse,
rounded fragments (see page 277).
During a very ancient period, known to geologists as the Silurian
Period, great lava-flows took place from volcanoes situated where
North and South Wales and the Lake District now are; and by their
eruptions a vast amount of volcanic ash was made, which fell into
the sea and slowly sank to the bottom, so that the shell-fish living
there were buried in the strata thus formed, and may now be seen in a
fossilised condition.
[Illustration: Fig. 1. THE RANGES OF THE GREAT BASIN, WESTERN
STATES OF NORTH AMERICA, SHOWING A SERIES OF GREAT FRACTURES AND
TILTED MASSES OF ROCK.]
[Illustration: Fig. 2. SECTION THROUGH SNOWDON.]
Thus Snowdon, Cader Idris, the Arans, Arenig Mountain, and others,
are very largely made up of these ancient volcanic materials. The
writer has picked up specimens of fossil shell-fish near the summit
of Snowdon from a bed of fine volcanic ash that forms the summit.
Fig. 2 represents a section through Snowdon, from which it will be
seen that we have first a few sedimentary strata, _S_, then a great
lava-flow, _L_; and that volcanic ashes accumulated on the top of
this, of which _A A_ are patches still left. _B_ is an intrusive dyke
of a basaltic rock that forced its way through afterwards. Again, in
the Lake District there is a well-known volcanic series of stratified
rocks of the same age, consisting mostly of lavas and ashes, the
total thickness of which is about twelve thousand feet (known as the
"Green Slates and Porphyries"), so that a large part of some of the
mountains there have also been built up by volcanic action; but no
traces of the old volcanoes remain.
Going farther north we find abundant proof that volcanic action on
a prodigious scale took place in Scotland during the very ancient
period of the Old Red Sandstone, with which the name of Hugh Miller
will always be associated. In Central Scotland we see lava-flows and
strata formed of volcanic ash, with a thickness of more than six
thousand feet, fragments of which, having escaped the destructive
agents of denudation, now form important chains of hills, such
as the Pentland, Ochil, and Sidlaw ranges. Nor was the volcanic
action confined to this region. In the district of the Cheviot
Hills similar volcanic rocks are to be seen. But here again the old
volcanoes have long since been swept away, leaving us only portions
of their outpourings buried in the hills.
There can be no doubt that the present area of the Grampian Hills was
once the site of a considerable number of volcanoes, only at a much
higher level than their present surface, elevated though that is to
the region of the clouds; but in this case subsequent denudation has
been so enormous that the old mountain surface has been planed away
until all we can now see is a series of separate patches of granite,
that were once in a fused and highly heated state far below the
surface, and formed part of the subterranean reservoirs from which
the volcanoes derived their great supplies of lava and steam. It is
indeed difficult to imagine the enormous amount of denudation which
has taken place in the Highlands of Scotland, and to realise that the
magnificent range of the Cairngorms, for instance, has been for ages
worn down until now they are but a remnant of what they once were.
In this region we see the once boiling and seething masses of
rock which fed the old volcanoes, now no longer endowed with
life-like power by the force of steam, but lying in deathlike cold
and stiffness, with their beautiful crystals of mica and felspar
sparkling in the sun. The volcanic fires have died out; but the
traces of their work are unmistakable, among which we must not forget
to reckon the beautiful minerals made by the action of heated water
upon the surrounding rocks.
The beautiful cairngorm stones are still sometimes found on the
mountain from which they take their name, and in all volcanic regions
minerals are plentiful.
The well-known hill called Arthur's Seat, close to Edinburgh, marks
the site of an old volcano. The "neck," or central opening, may be
seen at the top of the hill, but choked up with volcanic rocks and
débris. The crater has long since disappeared, but Salisbury Craigs
and St. Leonard's Craigs are formed of a great sheet of basalt that
intruded itself among the stratified rocks that had been formed
there, and so belong really to a great intrusive dyke. In the Castle
Rock we see the same basalt again.
During a much later age, known as the Miocene Period (see chap. x.,
p. 324), enormous outpourings of lava took place in Western Europe,
covering hundreds of square miles. Of these the most important is
that which occupies a large part of the northeast of Ireland, and
extends in patches through the Inner Hebrides and the Faröe Islands
into Iceland. These eruptive rocks, unlike those above referred to,
must have poured out at the surface, and have taken the form of
successive sheets, such as we now see in the terraced plateaux of
Skye, Eigg, Canna, Muck, Mull, and Morven. These, then, are patches
of what once formed a great plain of basalt. During later times
this volcanic platform has been so greatly cut up by the agents of
denudation that it has been reduced to mere scattered fragments;
thousands of feet of basalt have been worn away from it; deep and
wide valleys have been carved out of it; and in many cases it has
been almost entirely stripped off from the wide areas it once
covered. Where, as in the Isle of Eigg, the lava has been piled up
in successive sheets, with some layers of volcanic ash between, the
latter has been worn away rather faster than the hard layers of
basalt, and each lava-flow is clearly marked by a terrace. These
volcanic eruptions have thus had a great influence in moulding the
scenery of this region. In Ireland the old basalts are well seen at
the Giant's Causeway, and on the Scottish coast we see them again at
the well-known Fingal's cave at Staffa. This island, like the others,
is just a patch of the old lava-streams.
Its curious six-sided columns illustrate a fact with regard to the
subsequent cooling of lava-flows. Some internal forces, analogous to
that which regulates the shapes of crystals, have caused it to crack
along three sets of lines, so placed with regard to each other as to
produce six-sided columns.
In Ireland the basalts attain a thickness of nine hundred feet; in
Mull they are about three thousand feet thick. It has been clearly
proved that Mull is the site of one of the old volcanoes of this
period, but very few others have as yet been detected. Perhaps the
eruptions took place mainly from large fissures, instead of from
volcanic cones, for it is known that the ground below the lava-sheets
has been rent by earthquakes into innumerable fissures, into which
the basalt was injected from below.
In this way a vast number of "dykes" were formed. These have been
traced by hundreds eastwards from this region across Scotland,
and even the north of England. In this case the molten rock was
struggling to get through the overlying rocks and escape at the
surface; but apparently it did not succeed in so doing, for we do not
find lava-flows to the east and south. These basalt dykes are found
as far south as Yorkshire, and can be traced over an area of one
hundred thousand square miles.
It is thus evident that in the Miocene Period a great and extensive
mass of molten basalt was underlying a large part of the British
Isles, and probably the weight of the thick rocks overlying it was
sufficient to prevent its escape to the surface. If it had succeeded
in so escaping and overflowing, how different the scenery of much of
Scotland and Northern England might have been!
[Illustration: COLUMNAR BASALT AT CLAMSHELL CAVE, STAFFA. FROM A
PHOTOGRAPH BY J. VALENTINE.]