THE HISTORY OF A LAVA STREAM
[Illustration]
It is now just twenty-two years ago, boys, since I saw a wonderful
sight, which is still so fresh in my mind that I have to look round and
remember that it was before any of you were born, in order to persuade
myself that it can be nearly a quarter of a century since I stood with
my feet close to a flowing stream of red-hot lava.
It happened in this way. I was spending the winter with friends in
Naples, and we were walking quietly one lovely afternoon in November
along the Villa Reale, the public garden on the sea-shore, when one of
our party exclaimed, "Look at Vesuvius!" We did so, and saw in the
bright sunlight a dense dark cloud rising up out of the cone. The
mountain had been sending out puffs of smoke, with a booming noise, for
several days, but we thought nothing of that, for it had been common
enough for slight eruptions to take place at intervals ever since the
great eruption of 1867. This cloud, however, was far larger and
wider-spread than usual, and as we were looking at it we saw a thin red
line begin some way down the side of the mountain and creep onwards
toward the valley which lies behind the Hermitage near where the
Observatory is built (see Fig. 37). "A crater has broken out on the
slope," said our host; "it will be a grand sight to-night. Shall we go
up and see it?" No sooner proposed than settled, and one of the party
started off at once to secure horses and men before others engaged them.
[Illustration: Fig. 37.
Somma. Vesuvius.
Vesuvius, as seen in eruption by the author, November 1868.]
It was about eight o'clock in the evening when we started in a carriage
for Resina, and alighting there, with buried Herculaneum under our feet,
mounted our horses and set forward with the guides. Then followed a long
ascent of about two hours and a half through the dark night. Silently
and carefully we travelled on over the broad masses of slaggy lava of
former years, along which a narrow horse-path had been worn; and ever
and anon we heard the distant booming in the crater at the summit, and
caught sight of fresh gleams of light as we took some turning which
brought the glowing peak into view.
Our object was to get as close as possible to the newly-opened crater in
the mountain-side, and when we arrived on a small rugged plain not far
from the spot, we alighted from our horses, which were growing
frightened with the glare, and walked some distance on foot till we came
to a ridge running down the slope, and upon this ridge the lava stream
was flowing.
Above our heads hung a vast cloud of vapour which reflected the bright
light from the red-hot stream, and threw a pink glow all around, so
that, where the cloud was broken and we could see the dark sky, the
stars looked white as silver in contrast. We could now trace clearly the
outline of the summit towering above us, and even watch the showers of
ashes and dust which burst forth from time to time, falling back into
the crater, or on to the steep slopes of the cone.
If the night had not been calm, and such a breeze as there was blowing
away from us, our position would scarcely have been safe; and indeed we
were afterwards told we had been rash. But I would have faced even a
greater risk to see so grand a spectacle, and when the guide helped me
to scramble up on to the ledge, so that I stood with my feet within a
few yards of the lava flow, my heart bounded with excitement. I could
not stay more than a few seconds, for the gases and vapour choked me;
but for that short time it felt like a dream to be standing close to a
river of molten rock, which a few hours before had been lying deep in
the bowels of the earth. Glancing upwards to where this river issued
from the cone in the mountain-side, I saw it first white-hot, then
gradually fading to a glowing red as it crept past my feet; and then
looking down the slope I saw it turn black and gloomy as it cooled
rapidly at the top, while through the cracks which opened here and there
as it moved on, puffs of gas and vapour rose into the air, and the red
lava beneath gleamed through the chinks.
We did not stay long, for the air was suffocating, but took our way back
to the Hermitage, where another glorious sight awaited us. Some way
above and behind the hill on which the Observatory stands there is, or
was, a steep cliff, and over this the lava stream, now densely black,
fell in its way to the valley below, and as it fell it broke into huge
masses, which heeling over exposed the red-hot lava under the crust,
thus forming a magnificent fiery cascade in which black and red were
mingled in wild confusion.
This is how I saw a fresh red-hot lava stream. I had ascended the
mountain some years before, when it was comparatively quiet, with only
two small cones in its central crater sending out miniature flows of
lava (see Fig. 38). But the crater was too hot for me to cross over to
these cones, and I could only marvel at the mass of ashes of which the
top of the mountain was composed, and plunge a stick into an old lava
stream to see how hot it still remained below. Peaceful and quiet as the
mountain seemed then, I could never have imagined such a glorious
outburst as that of November 1868 unless I had seen it, and yet this was
quite a small eruption compared to those of 1867 and 1872, which in
their turn were nothing to some of the older eruptions in earlier
centuries.
[Illustration: Fig. 38.
The top of Vesuvius in 1864. (After Nasmyth.)]
Now it is the history of this lava stream which I saw, that we are going
to consider to-day, and you will first want to know where it came from,
and what caused it to break out on the mountain-side. The truth is, that
though we know now a good deal about volcanoes themselves, we know very
little about the mighty cauldrons deep down in the earth from which
they come. Our deepest mines only reach to a depth of a little more than
half a mile, and no borings even have been made beyond three-quarters of
a mile, so that after this depth we are left very much to guesswork.
We do know that the temperature increases as we go farther down from the
surface, but the increase is very different in different districts--in
some places being five times greater than it is in others at an equal
depth, and it is always greatest in localities where volcanoes have been
active not long before. Now if there were an ocean of melted rock at a
certain distance down below the crust all over the globe, there could
scarcely be such a great difference between one place and another, and
for this and many other reasons geologists are inclined to think that,
from some unknown cause, great heat is developed at special points below
the surface at different times. This would account for our finding
volcanic rocks in almost all parts of the world, even very far away from
where there are any active volcanoes now.
But, as I have said, we do not clearly know why great reservoirs of
melted rock occur from time to time deep under our feet. We may perhaps
one day find the clue from the fact that nearly all, if not all,
volcanoes occur near to the water's edge, either on the coast of the
great oceans or of some enormous inland sea or lake. But at present all
we can say is, that in certain parts of the globe there must be from
time to time great masses of rock heated till they are white-hot, and
having white-hot water mingled with them. These great masses need not,
however, be liquid, for we know that under enormous pressure white-hot
metals remain solid, and water instead of flashing into steam is kept
liquid, pressing with tremendous force upon whatever keeps it confined.
But now suppose that for some reason the mass of solid rock and ground
above one of these heated spots should crack and become weak, or that
the pressure from below should become so great as to be more powerful
than the weight above, then the white-hot rock and water quivering and
panting to expand, would upheave and burst the walls of their prison.
Cannot you picture to yourselves how when this happened the rock would
swell into a liquid state, and how the water would force its way upwards
into cracks and fissures expanding into steam as it went. Then would be
heard strange rumbling noises underground, as all these heavily
oppressed white-hot substances upheaved and rent the crust above them.
And after a time the country round, or the ground at the bottom of the
sea, would quake and tremble, till by and by a way out would be found,
and the water flashing into vapour would break and fling up the masses
of rock immediately above the passage it had made for itself, and
following after these would come the molten rock pouring out at the new
opening.
Such outbursts as these have been seen at sea many times near volcanic
islands. In 1811 a new island called Sabrina was thrown up off St.
Michael's in the Azores, and after remaining a short time was washed
away by the waves. In the same way Graham's Island appeared off the
coast of Sicily in 1831, and as late as 1885 Mr. Shipley saw a
magnificent eruption in the Pacific near the Tonga Islands when an
island about three miles long was formed.
Another very extraordinary outburst, this time on land, took place in
1538 on the opposite side of the Bay of Naples to where Vesuvius stands.
There, on the shores of the Bay of Baiae, a mountain 440 feet high was
built up in one week, where all had before been quiet in the memory of
man. For two years before the outburst came, rumblings and earthquakes
had alarmed the people, and at last one day the sea drew back from the
shore and the ground sank about fourteen feet, and then on the night of
Sunday, September 29, 1538, it was hurled up again, and steam, fiery
gases, stones, and mud burst forth, driving away the frightened people
from the village of Puzzuoli about two miles distant. For a whole week
jets of lava, fragments of rock, and showers of ashes were poured out,
till they formed the hill now called Monte Nuovo, 440 feet high and
measuring a mile and a half round the base. And there it has remained
till the present day, perfectly quiet after the one great outburst had
calmed down, and is now covered with thickets of stone-pine trees.
These sudden outbursts show that some great change must occur in the
state of the earth's crust under the spots where they take place, and we
know that eruptions may cease for centuries in any particular place and
then begin afresh quite unexpectedly. Vesuvius was a peaceable mountain
overgrown with trees and vines in the time of the Greeks till in the
year A.D. 79 occurred the terrific outburst which destroyed Herculaneum
and Pompeii, shattering old Vesuvius to pieces, so that only the cliffs
on the northwest remain and are called Somma (see Fig. 37), while the
new Vesuvius has grown up in the lap, as it were, of its old self. Yet
when we visit the cliffs of Somma, and examine the old lava streams in
them, we see that the ancient peaceful mountain was itself built up by
volcanic outbursts of molten rock, and showers of clinkers or scoriae,
long before man lived to record it.
Meanwhile, when once an opening is made, we can understand how after an
eruption is over, and the steam and lava are exhausted, all quiets down
for awhile, and the melted rock in the crater of the mountain cools and
hardens, shutting in once more the seething mass below. This was the
state of the crater when I saw it in 1864, though small streams still
flowed out of two minute cones; but since then at least one great
outburst had taken place in 1867, and now on this November night, 1868,
the imprisoned gases rebelled once more and forced their way through the
mountain-side.
At this point we can leave off forming conjectures and really study what
happens; for we do know a great deal about the structure of volcanoes
themselves, and the history of a lava-flow has been made very clear
during the last few years, chiefly by the help of the microscope and
chemical experiments. If we imagine then that on the day of the eruption
we could have seen the inside of the mountain, the diagram (Fig. 39)
will fairly represent what was taking place there.
[Illustration: Fig. 39.
Diagrammatic section of an active volcano.
_a_, Central pipe or funnel. _b_, _b_, Walls of the crater or cup.
_c_, _c_, Dark turbid cloud formed by the ascending globular clouds
_d_, _d_. _e_, Rain-shower from escaped vapour. _f_, _f_, Shower of
blocks, cooled bombs, stones, and ashes falling back on to the cone.
_g_, Lava escaping through a fissure, and pouring out of a cone opened
in the mountain side.]
In the funnel _a_ which passes down from the crater or cup _b_, _b_,
white-hot lava was surging up, having a large quantity of water and
steam entangled in it. The lava, or melted rock, would be in much the
same state as melted iron-slag is, in the huge blast-furnaces in which
iron-rock is fused, only it would have floating in it great blocks of
solid rock, and rounded stones called bombs which have been formed from
pieces of half-melted rock whirled in air and falling back into the
crater, together with clinkers or scoriae, dust and sand, all torn off
and ground down from the walls of the funnel up which the rush was
coming. And in the pipe of melted rock, forcing the lava upwards,
enormous bubbles of steam and gas _d_, _d_ would be rising up one after
another as bubbles rise in any thick boiling substances, such as boiling
sugar or tar.
In the morning before the eruption, when only a little smoke was issuing
from the crater, these bubbles rose very slowly through the loaded
funnel and the half-cooled lava in the basin, and the booming noise,
like that of heavy cannon, heard from time to time, was caused by the
bursting of one of these globes of steam at the top of the funnel, as it
brought up with it a feeble shower of stones, dust, and scoriae.
Meanwhile the lava surging below was forcing a passage _g_ for itself in
a weak part of the mountain-side and, just at the time when our
attention was called to Vesuvius, the violent pressure from below rent
open a mouth or crater at _h_, so that the lava began to flow down the
mountain in a steady stream. This, relieving the funnel, enabled the
huge steam bubbles _d_, _d_ to rise more quickly, and to form the large
whitish-grey cloud _c_, into which from time to time the red-hot blocks,
scoriae, and pumice were thrown up by the escaping steam and gases. These
blocks and fragments then fell back again in a fiery shower _f_, _f_
either into the cup, to be thrown up again by the bursting of the next
bubble, or on to the sides of the cone, making it both broader and
higher.
Only one feature in the diagram was fortunately absent the evening we
went up, namely, the rain-shower _e_. The night, as I said, was calm,
and the air dry, and the steam floated peacefully away. The next night,
however, when many people hurried down from Rome to see the sight they
were woefully disappointed, for rain-showers fell heavily from the
cloud, bringing down with them the dust and ashes, which covered the
unfortunate sight-seers.
This was what happened during the eruption, and the result after a few
days was that the cone was a little higher, with a fresh layer of rough
slaggy scoriae on its slopes, and that on the side of the mountain behind
the Hermitage a new lava stream was added to the many which have flowed
there of late years. What then can we learn from this stream about the
materials which come up out of the depths of the earth, and of the
manner in which volcanic rocks are formed?
The lava as I saw it when coming first out of the newly-opened crater
is, as I have said, like white-hot iron slag, but very soon the top
becomes black and solid, a hard cindery mass full of holes and cavities
with rough edges, caused by the steam and sulphur and other gases
breaking through it.[1] In fact, there are so many holes and bubbles in
it that it is very light and floats on the top of the heavier lava
below, falling over it on to the mountain-side when it comes to the end
of the stream. Still, however, the great mass moves on, so that the
stream slides over these fallen clinkers or scoriae. Thus after an
eruption a new flow consists of three layers; at the top the cooled and
broken crust of clinkers, then the more solid lava, which often remains
hot for years, and lastly another cindery layer beneath, formed of the
scoriae which have fallen from above (see Fig. 40).
[1] For the cindery nature of the surface of such a stream see the
initial letter of this chapter.
[Illustration: Fig. 40.
Section of a lava-flow. (J. Geikie.)
1, Slaggy crust, formed chiefly of scoriae of a glassy nature. 2, Middle
portion where crystals form. 3, Slaggy crust which has slipped down and
been covered by the flow.]
You would be surprised to see how quickly the top hardens, so that you
can actually walk across a stream of lava a day or two after it comes
out from the mountain. But you must not stand still or your shoes would
soon be burnt, and if you break the crust with a stick you will at once
see the red-hot lava below; while after a few days the cavities become
filled with crystals of common salt, sulphur or soda, as the vapour and
gases escape.
Then as time goes on the harder minerals gradually crystallise out of
the melted mass, and iron-pyrites, copper-sulphate, and numerous other
forms of crystal appear in the lower part of the stream. In the clinkers
above, where the cooling goes on very rapidly, the lavas formed are
semi-transparent and look much like common bottle-glass. In fact, if you
take this piece of obsidian or volcanic glass in your hand, you might
think that it had come out of an ordinary glass manufactory and had
nothing remarkable in it.
[Illustration: Fig. 41.
A slice of volcanic glass showing the lines of crystallites and
microliths which are the beginnings of crystals.[1] (J. Geikie.)]
[1] This arrangement in lines is called _fluidal structure_ in lava.
But the microscope tells another tale. I have put a thin slice under the
first microscope, and this diagram (Fig. 41) shows what you will see.
Nothing, you say, but a few black specks and some tiny dark rods. True,
but these specks and rods are the first beginnings of crystals forming
out of the ground-mass of glassy lava as it cools down. They are not
real crystals, but the first step toward them, and by a careful
examination of glassy lavas which have cooled at different rates, they
have been seen under the microscope in all stages of growth, gradually
building up different crystalline forms. When we remember how rapidly
the top of many glassy lavas cool down we can understand that they have
often only time to grow very small.
[Illustration: Fig. 42.
A slice of volcanic glass under the microscope, showing well-developed
microliths. (After Cohen.)]
The smaller specks are called _crystallites_, the rods are called
_microliths_.[1] Under the next microscope you can see the microliths
much more distinctly (Fig. 42) and observe that they grow in very
regular shapes.
[1] _Micros_, little; _lithos_, stone.
Our first slice, however (Fig. 41), tells us something more of their
history, for the fact that they are arranged in lines shows that they
have grown while the lava was flowing and carrying them along in
streams. You will notice that each one has its greatest length in the
direction of the lines, just as pieces of stick are carried along
lengthways in a river. In the second specimen (Fig. 42) the microliths
are much larger and the stream has evidently not been flowing fast, for
they lie in all directions.
This is what we find in the upper part of the stream, but if we look at
a piece of underlying lava we find that it is much more coarse-grained,
and the magnifying-glass shows many crystals in it, as well as a number
of microliths. For this lava, covered by the crust above, has remained
very hot for a long time, and the crystals have had time to build
themselves up out of the microliths and crystallites.
Still there is much glassy groundwork even in these lavas. If we want to
find really stony masses such as porphyry and granite made up entirely
of crystals we must look inside the mountain where the molten rock is
kept intensely hot for long periods, as for example in the fissure _g_,
Fig. 39.
Such fissures sometimes open out on the surface like the one I saw, and
sometimes only penetrate part of the way through the hill; but in either
case when the lava in them cools down, it forms solid walls called dykes
which help to bind the loose materials of the mountain together. We
cannot, of course, examine these in an active volcano, but there are
many extinct volcanoes which have been worn and washed by the weather
for centuries, so that we can see the inside. The dykes laid bare in the
cliffs of Somma are old fissures filled with molten rock which has
cooled down, and they show us many stony lavas; and Mr. Judd tells us of
one beautiful example of a ruined volcano which composes the whole
island of Mull in the Hebrides, where such dykes can be traced right
back to a centre. This centre must once have been a mass of melted
matter far down in the earth, and as you trace the dykes back deeper and
deeper into it, the rocks grow more and more stony, till at last they
are composed entirely of large crystals closely crowded together
without any glassy matter between them. You know this crystalline
structure well, for we have plenty of blocks of granite scattered about
on Dartmoor, showing that at some time long ago molten matter must have
been at work in the depths under Devonshire.
We see then that we can trace the melted rock of volcanoes right
back--from the surface of the lava stream which cools quickly at the
top, hurrying the crystallites and microliths along with it--down
through the volcano to the depths of the earth, where the perfect
crystals form slowly and deliberately in the underground lakes of
white-hot rock which are kept in a melted state at an intense heat.
[Illustration: Fig. 43.
A piece of Dartmoor Granite, drawn from a specimen.]
But I promised you that we would have no guesswork here, and you will
perhaps ask how I can be certain what was going on in the depths when
these crystals were formed. A few years ago I could not have answered
you, but now chemists, and especially two eminent French chemists, MM.
Fouque and Levy, have actually _made_ lavas and shown us how it is done
in Nature.
By using powerful furnaces and bellows they have succeeded in getting
temperatures of all degrees, from a dazzling white heat down to a dull
red, and to keep any temperature they like for a long time, so as to
imitate the state of a mass of melted rock at different depths in the
earth, and in this way they have actually _made_ lavas in their
crucibles. For example, there is a certain whitish rock common in
Vesuvius called _leucotephrite_,[1] which is made up chiefly of crystals
of the minerals called leucite, Labrador felspar, and augite. This they
proposed to make artificially, so they took proper quantities of silica,
alumina, oxide of iron, lime, potash, and soda, and putting them in a
crucible, melted them by keeping them at a white heat. Then they lowered
the temperature to an orange-heat, that is a heat sufficient to melt
steel. They kept this heat for forty-eight hours, after which they took
out some of the mixture and, letting it cool, examined a slice under the
microscope. Within it they found crystals of _leucite_ already formed,
showing that these are the first to grow while the melted rock is still
intensely hot. The rest of the mixture they kept red-hot, or at the
melting-point of copper, for another forty-eight hours, and when they
took it out and examined it they found that the whole of it had been
transformed into microliths of the two other forms of crystals, Labrador
felspar and augite, except some small eight-sided crystals of magnetite
and picotite which are also found in the natural rock.
[1] _Leucos_, white; _tephra_, ashes.
There is no need for you to remember all these names. What I do want you
to remember is, that, at the different temperatures, the right crystals
and beginnings of crystals grew up to form the rock which is found in
Vesuvius. And what is still more interesting, they grew exactly to the
same stages as in the natural rock, which is composed of _crystals_ of
leucite and _microliths_ of the two other minerals.
This is only one among numerous experiments by which we have learnt how
volcanic rocks are formed and at what heat the crystals of different
substances grow. We are only as yet at the beginning of this new study,
and there is plenty for you boys to do by and by when you grow up. Many
experiments have failed as yet to imitate certain rocks, and it is
remarkable that these are usually rocks of very ancient eruptions, when
_perhaps_ our globe may have been in a different state to what it is
now; but this remains for us to find out.
Meanwhile I have still another very interesting slide to show you which
tells us something of what is going on below the volcano. Under the
third microscope I have put a slice of volcanic glass (Fig. 44) in which
you will see really large crystals with dark bands curving round them.
These crystals have clearly not been formed in the glass while the lava
was flowing, first because they are too large to have grown up so
rapidly, and secondly because they are broken at the edges in places and
sometimes partly melted. They have evidently come up with the lava as it
flowed out of the mountain, and the dark bands curving round them are
composed of microliths which have been formed in the flow and have swept
round them, as floating straws gather round a block of wood in a stream.
Such crystals as these are often found in lava streams, and in fact they
make a great difference in the rate at which a stream flows, for a
thoroughly melted lava shoots along at a great pace and often travels
several miles in a very short time; but an imperfectly melted lava full
of crystals creeps slowly along, and often does not travel far from the
crater out of which it flows.
[Illustration: Fig. 44.
Slice of volcanic glass under the microscope, showing large included
crystals brought up from inside the volcano in the fluid lava. The dark
bands are lines of microliths formed as the lava cooled. (J. Geikie.)]
So you see we have proof in this slice of volcanic glass of two separate
periods of crystallisation--the period when the large crystals grew in
the liquid mass under the mountain, and the period when the microliths
were formed after it was poured out above ground. And as we know that
different substances form their crystals at very different temperatures,
it is not surprising that some should be able to take up the material
they require and grow in the underground lakes of melted matter, even
though the rest of the lava was sufficiently fluid to be forced up out
of the mountain.
And here we must leave our lava stream. The microscope can tell us yet
more, of marvellous tiny cavities inside the crystals, millions in a
single inch, and of other crystals inside these, all of which have their
history; but this would lead us too far. We must be content for the
present with having roughly traced a flow of lava from the depths below,
where large crystals form in subterranean darkness, to the open air
above, where we catch the tiny beginnings of crystals hardened into
glassy lava before they have time to grow further.
If you will think a little for yourselves about these wonderful
discoveries made with the magic-glass, you will see how many questions
they suggest to us about the minerals which we find buried in the earth
and running through it in veins, and you will want to know something
about the more precious crystals, such as rubies, diamonds, sapphires,
and garnets, and many others which Nature forms far away out of our
sight. All these depend, though indirectly, upon the strange effects of
underground heat, and if you have once formed a picture in your minds of
what must have been going on before that magnificent lava stream crept
down the mountain-side and added its small contribution to the surface
of the earth, you will study eagerly all that comes in your way about
crystals and minerals, and while you ask questions with the spectroscope
about what is going on in the sun and stars millions of miles away, you
will also ask the microscope what it has to tell of the work going on at
depths many miles under your feet.