glaciers consist, is produced by slow and gradual transformation of
snow into ice; and though the ice thus formed may eventually be as
clear and transparent as the present pond or river ice, its structure
is, nevertheless, entirely distinct.
We may compare these different processes during any moderately cold
winter in the ponds and snow meadows immediately about us. We need
not join an Arctic exploring expedition, or even undertake a more
tempting trip to the Alps, in order to investigate these phenomena
ourselves, if we have any curiosity to do so. The first warm day
after a thick fall of light, dry snow, such as occurs in the coldest
of our winter weather, is sufficient to melt its surface.
As this snow is porous, the water readily penetrates it, having also
a tendency to sink by its own weight, so that the whole mass becomes
more or less filled with moisture in the course of the day. During
the lower temperature of the night, however, the water is frozen
again, and the snow is now filled with new ice particles. Let this
process be continued long enough and the mass of snow is changed into
a kind of ice gravel, or, if the grains adhere together, to something
like what we call pudding-stone, allowing, of course, for the
difference of material; the snow, which has been rendered cohesive
by the process of partial melting and regelation, holding the ice
globules together, just as the loose materials of the pudding-stone
are held together by the cement which unites them.
Within this mass air is intercepted and held inclosed between the
particles of ice. The process by which snowflakes or snow crystals
are transformed into grains of ice, more or less compact, is easily
understood. It is the result of a partial thawing under a temperature
maintained very nearly at thirty-two degrees, falling sometimes a
little below and then rising a little above the freezing-point, and
thus producing constant alternations of freezing and thawing in the
same mass of snow. This process amounts to a kind of kneading of
the snow, and when combined with the cohesion among the particles
more closely held together in one snowflake, it produces granular
ice. Of course, the change takes place gradually, and is unequal in
its progress at different depths in the same bed of fallen snow. It
depends greatly on the amount of moisture infiltrating the mass,
whether derived from the melting of its own surface, or from the
accumulation of dew, or the falling of rain or mist upon it.
The amount of water retained within the mass will also be greatly
affected by the bottom on which it rests and by the state of the
atmosphere. Under a certain temperature the snow may only be glazed
at the surface by the formation of a thin, icy crust, an outer
membrane, as it were, protecting the mass below from a deeper
transformation into ice; or it may be rapidly soaked throughout its
whole bulk, the snow being thus changed into a kind of soft pulp,
what we commonly call slush, which, upon freezing, becomes at once
compact ice; or, the water sinking rapidly, the lower layers only
may be soaked, while the upper portion remains comparatively dry.
But, under all these various circumstances, frost will transform the
crystalline snow into more or less compact ice, the mass of which
will be composed of an infinite number of aggregated snow particles,
very unequal in regularity of outline, and cemented by ice of another
kind, derived from the freezing of the infiltrated moisture, the
whole being interspersed with air.
Let the temperature rise, and such a mass, rigid before, will resolve
itself again into disconnected ice particles, like grains more or
less rounded. The process may be repeated till the whole mass is
transformed into very compact, almost uniformly transparent and blue
ice, broken only by the intervening air-bubbles. Such a mass of ice,
when exposed to a temperature sufficiently high to dissolve it, does
not melt from the surface and disappear by a gradual diminution of
its bulk, like pond ice, but crumbles into its original granular
fragments, each one of which melts separately. This accounts for the
sudden disappearance of icebergs, which, instead of slowly dissolving
into the ocean, are often seen to fall to pieces and vanish at once.
Ice of this kind may be seen forming every winter on our sidewalks,
on the edge of the little ditches which drain them, or on the summits
of broad gate posts when capped with snow. Of such ice glaciers are
composed; but, in the glacier, another element comes in which we have
not considered as yet--that of immense pressure in consequence of
the vast accumulations of snow within circumscribed spaces. We see
the same effects produced on a small scale when snow is transformed
into a snowball between the hands. Every boy who balls a mass of snow
in his hands illustrates one side of glacial phenomena. Loose snow,
light and porous, and pure white from the amount of air contained in
it, is in this way presently converted into hard, compact, almost
transparent, ice. This change will take place sooner if the snow be
damp at first, but if dry, the action of the hand will presently
produce moisture enough to complete the process. In this case,
mere pressure produces the same effect which, in the cases we have
been considering above, was brought about by alternate thawing and
freezing, only that, in the latter, the ice is distinctly granular,
instead of being uniform throughout, as when formed under pressure.
In the glaciers, we have the two processes combined. But the
investigators of glacial phenomena have considered too exclusively
one or the other: some of them attributing glacial motion wholly to
the dilatation produced by the freezing of infiltrated moisture in
the mass of snow; others accounting for it entirely by weight and
pressure. There is yet a third class, who, disregarding the real
properties of ice, would have us believe that, because tar, for
instance, is viscid when it moves, therefore ice is viscid because it
moves.
There is no chain of mountains in which the shape of the valleys is
more favorable to the formation of glaciers than the Alps. Contracted
at their lower extremity, these valleys widen upward, spreading
into deep, broad, trough-like depressions. Take, for instance, the
valley of Hassli, which is not more than half a mile wide where you
enter it above Meyringen; it opens gradually upward till, above the
Grimsel, at the foot of the Finster-Aarhorn, it measures several
miles across. These huge mountain-troughs form admirable cradles for
the snow, which collects in immense quantities within them, and as it
moves slowly down from the upper ranges is transformed into ice on
its way, and compactly crowded into the narrower space below. At the
lower extremity of the glacier the ice is pure blue and transparent,
but as we ascend it appears less compact, more porous and granular,
assuming gradually the character of snow, till in the higher regions
the snow is as light, as shifting, as incoherent as the sand of the
desert. A snowstorm on a mountain summit is very different from
a snowstorm on the plain on account of the different degrees of
moisture in the atmosphere. At great heights there is never dampness
enough to allow the fine snow crystals to coalesce and form what are
called snowflakes. I have even stood on the summit of the Jungfrau
when a frozen cloud filled the air with ice-needles, while I could
see the same cloud poring down sheets of rain upon Lauterbrunnen
below. I remember this spectacle as one of the most impressive I
have ever witnessed in my long experience of Alpine scenery. The
air immediately about me seemed filled with rainbow dust, for the
ice-needles glittered with a thousand hues under the decomposition of
light upon them, while the dark storm in the valley below offered
a strange contrast to the brilliancy of the upper region in which I
stood. One wonders where even so much vapor as may be transformed
into the finest snow should come from at such heights. But the warm
winds creeping up the sides of the valley, the walls of which become
heated during the middle of the day, come laden with moisture which
is changed to a dry snow like dust as soon as it comes into contact
with the intense cold above.
Currents of warm air affect the extent of the glaciers and influence
also the line of perpetual snow, which is by no means at the same
level, even in neighboring localities. The size of glaciers, of
course, determines to a great degree the height at which they
terminate, simply because a small mass of ice will melt more
rapidly, and at a lower temperature, than a larger one. Thus the
small glaciers, such as those of the Rothhorn or of Trift, above the
Grimsel, terminate at a considerable height above the plain, while
the Mer de Glace, fed from the great snow-caldrons of Mont Blanc,
forces its way down to the bottom of the Valley of Chamouni, and the
glacier of Grindelwald, constantly renewed from the deep reservoirs
where the Jungfrau hoards her vast supplies of snow, descends to
about four thousand feet above the sea level. But the glacier of the
Aar, though also very large, comes to a pause at about six thousand
feet above the level of the sea; for the south wind from the other
side of the Alps, the warm sirocco of Italy, blows across it, and it
consequently melts at a higher level than either the Mer de Glace or
the Grindelwald. It is a curious fact that, in the Valley of Hassli,
the temperature frequently rises instead of falling, as you ascend;
at the Grimsel the temperature is at times higher than at Meyringen,
below, where the warmer winds are not felt so directly. The glacier
of Aletsch, on the southern slope of the Jungfrau, and into which
many other glaciers enter, terminates also at a considerable height,
because it turns into the Valley of the Rhone, through which the
southern winds blow constantly. Under ordinary conditions, vegetation
fades in these mountains at the height of six thousand feet, but,
in consequence of prevailing winds and the sheltering influence of
the mountain walls, there is no uniformity in the limit of perpetual
snow and ice. Where currents of warm air are very constant, glaciers
do not occur at all, even where other circumstances are favorable to
their formation.
There are valleys in the Alps far above six thousand feet which have
no glaciers, and where perpetual snow is seen only on their northern
sides. These contrasts in the temperature lead to the most wonderful
contrasts in the aspect of the soil; summer and winter lie side by
side, and bright flowers look out from the edge of snows that never
melt. Where the warm winds prevail there may be sheltered spots at
a height of ten or eleven thousand feet, isolated nooks opening
southward where the most exquisite flowers bloom in the midst of
perpetual snow and ice; and occasionally I have seen a bright little
flower with a cap of snow over it that seems to be its shelter.
The flowers give, indeed, a most peculiar charm to these high
Alpine regions. Occurring often in beds of the same kind, forming
green, blue, or yellow patches, they seem nestled close together in
sheltered spots, or even in fissures and chasms of the rock, where
they gather in dense quantities.
Even in the sternest scenery of the Alps some sign of vegetation
lingers; and I remember to have found a tuft of lichen growing on
the only rock which pierced through the ice on the summit of the
Jungfrau. It was a species then unknown to botanists, since described
under the name of Umbelicarus Higinis. The absolute solitude, the
intense stillness of the upper Alps is most impressive; no cattle,
no pasturage, no bird, nor any sound of life--and, indeed, even if
there were, the rarity of the air in these high regions is such that
sound is hardly transmissible. The deep repose, the purity of aspect
of every object, the snow, broken only by ridges of angular rocks,
produce an effect no less beautiful than solemn. Sometimes, in the
midst of the wide expanse, one comes upon a patch of the so-called
red snow of the Alps. At a distance one would say that such a spot
marked some terrible scene of blood, but as you come nearer the hues
are so tender and delicate, as they fade from deep red to rose, and
so die into the pure colorless snow around, that the first impression
is completely dispelled. This red snow is an organic growth, a plant
springing up in such abundance that it colors extensive surfaces,
just as the microscopic plants dye our pools with green in the
spring. It is an Alga (Protocoites nivalis), well known in the
Arctics, where it forms wide fields in the summer.
In ordinary times, layers from six to eight feet deep are regularly
added annually to the accumulation of snow in the higher regions--not
taking into account, of course, the heavy drifts heaped up in
particular localities, but estimating the uniform average over wide
fields. This snow is gradually transformed into more or less compact
ice, passing through an intermediate condition analogous to the slush
of our roads, and in that condition chiefly occupies the upper part
of the extensive troughs into which these masses descend from the
loftier heights. This region is called the region of the _névé_. It
is properly the birthplace of the glaciers, for it is here that the
transformation of the snow into ice begins. The _névé_ ice, though
varying in the degree of its compactness and solidity, is always very
porous and whitish in color, resembling somewhat frozen slush, while
lower down in the region of the glacier proper the ice is close,
solid, transparent, and of a bluish tint.
In consequence of the greater or less rapidity in the movement of
certain portions of the mass, its centre progressing faster than its
sides, and the upper, middle and lower regions of the same glacier
advancing at different rates, the strata, which in the higher ranges
of the snow fields were evenly spread over wide expanses, become
bent and folded to such a degree that the primitive stratification
is nearly obliterated, while the internal mass of the ice has also
assumed new features under these new circumstances. There is, indeed,
as much difference between the newly formed beds of snow in the
upper region and the condition of the ice at the lower end of a
glacier as between a recent deposit of coral sand or a mud bed in
an estuary and the metamorphic limestone or clay slate twisted and
broken as they are seen in the very chains of mountains from which
the glacier descended.
VOLCANIC ACTION
--SIR ARCHIBALD GEIKIE
Large quantities of water accompany many volcanic eruptions. In some
cases, where ancient crater-lakes or internal reservoirs, shaken by
repeated detonations, have been finally disrupted, the mud which has
thereby been liberated has issued from the mountain. Such “mud-lava”
(_lava d’aqua_), on account of its liquidity and swiftness of motion,
is more dreaded for destructiveness than even the true melted lavas.
On the other hand, rain or melted snow or ice, rushing down the cone
and taking up loose volcanic dust, is converted into a kind of mud
that grows more and more pasty as it descends. The mere sudden rush
of such large bodies of water down the steep declivity of a volcanic
cone can not fail to effect much geological change. Deep trenches
are cut out of the loose volcanic slopes, and sometimes large areas
of woodland are swept away, the débris being strewn over the plains
below.
One of these mud-lavas invaded Herculaneum during the great eruption
of 79, and by quickly enveloping the houses and their contents,
has preserved for us so many precious and perishable monuments of
antiquity. In the same district, during the eruption of 1622, a
torrent of this kind poured down upon the villages of Ottajano and
Massa, overthrowing walls, filling up streets, and even burying
houses with their inhabitants. During the great eruption of Cotopaxi,
in June, 1877, enormous torrents of water and mud, produced by
the melting of the snow and ice of the cone, rushed down from the
mountain. Huge portions of the glaciers of the mountain were detached
by the heat of the rocks below them and rushed down bodily, breaking
up into blocks. The villages all round the mountain to a distance
of sometimes more than ten geographical miles were left deeply
buried under a deposit of mud mixed with blocks of lava, ashes,
pieces of wood, lumps of ice, etc. Many of the volcanoes of Central
and South America discharge large quantities of mud directly from
their craters. Thus, in the year 1691, Imbaburu, one of the Andes of
Quito, emitted floods of mud so largely charged with dead fish that
pestilential fevers arose from the subsequent effluvia. Seven years
later (1698), during an explosion of another of the same range of
lofty mountains, Carguairazo (14,706 feet), the summit of the cone
is said to have fallen in, while torrents of mud containing immense
numbers of the fish _Pymelodus Cyclopum_ poured forth and covered
the ground over a space of four square leagues. The carbonaceous mud
(locally called _moya_) emitted by the Quito volcanoes sometimes
escapes from lateral fissures, sometimes from the craters. Its
organic contents, and notably its siluroid fish, which are the same
as those found living in the streams above ground, prove that the
water is derived from the surface, and accumulates in craters or
underground cavities until discharged by volcanic action. Similar but
even more stupendous and destructive outpourings have taken place
from the volcanoes of Java, where wide tracts of luxuriant vegetation
have at different times been buried under masses of dark gray mud,
sometimes 100 feet thick, with a rough hillocky surface from which
the top of a submerged palm-tree would here and there protrude.
A volcano, as its activity wanes, may pass into the Solfatara
stage, when only volatile emanations are discharged. The well-known
Solfatara near Naples, since its last eruption in 1198, has
constantly discharged steam and sulphurous vapors. The island of
Volcano has now passed also into this phase, though giving vent to
occasional explosions. Numerous other examples occur among the old
volcanic tracts of Italy, where they have been termed _soffioni_.
Another class of gaseous emanations betokens a condition of volcanic
activity further advanced toward final extinction. In these, the
gas is carbon-dioxide, either issuing directly from the rock or
bubbling up with water which is often quite cold. The old volcanic
districts of Europe furnish many examples. Thus on the shores of the
Laacher See--an ancient crater-lake of the Eifel--the gas issues
from numerous openings called _moffette_, round which dead insects,
and occasionally mice and birds, may be found. In the same region
occur hundreds of springs more or less charged with this gas. The
famous Valley of Death in Java contains one of the most remarkable
gas-springs in the world. It is a deep, bosky hollow, from one small
space on the bottom of which carbon-dioxide issues so copiously as
to form the lower stratum of the atmosphere. Tigers, deer, and wild
boars, enticed by the shelter of the spot, descend and are speedily
suffocated. Many skeletons, including those of man himself, have been
observed.
As a distinct class of gas-springs, we may group and describe here
the emanations of volatile hydrocarbons which, when they take fire,
are known as Fire-wells. These are not of volcanic origin, but arise
from changes within the solid rocks underneath. They occur in many of
the districts where mud-volcanoes appear, as in northern Italy, on
the Caspian, in Mesopotamia, in southern Kurdistan, and in many parts
of the United States.
In the oil regions of Pennsylvania, certain sandy strata occur at
various geological horizons whence large quantities of petroleum and
gas are obtained. In making the borings for oil-wells, reservoirs
of gas as well as subterranean courses or springs of water are met
with. When the supply of oil is limited, but that of gas is large, a
contest for possession of the bore-hole sometimes takes place between
the gas and water. When the machinery is removed and the boring is
abandoned, the contest is allowed to proceed unimpeded, and results
in the intermittent discharge of columns of water and gas to heights
of 130 feet or more. At night, when the gas has been lighted, the
spectacle of one of these “fire-geysers” is inconceivably grand.
Eruptive fountains of hot water and steam, to which the general name
of Geysers (_i. e._, gushers) is given, from the examples in Iceland,
which were the first to be seen and described, mark a declining phase
of volcanic activity. The Great and Little Geysers, the Strokkr,
and other minor springs of hot water in Iceland, have long been
celebrated examples. More recently another series has been discovered
in New Zealand. But probably the most remarkable and numerous
assemblage is that which has been brought to light in the northwest
part of the Territory of Wyoming, and which has been included within
the “Yellowstone National Park.” In this singular region the ground
in certain tracts is honeycombed with passages which communicate with
the surface by hundreds of openings, whence boiling water and steam
are emitted. In most cases, the water remains clear, tranquil, and
of a deep green-blue tint, though many of the otherwise quiet pools
are marked by patches of rapid ebullition. These pools lie on mounds
or sheets of sinter, and are usually edged round with a raised rim
of the same substance, often beautifully fretted and streaked with
brilliant colors. The eruptive openings usually appear on small, low,
conical elevations of sinter, from each of which one or more tubular
projections rise. It is from these irregular tube-like excrescences
that the eruptions take place.
The term geyser is restricted to active openings whence columns of
hot water and steam are from time to time ejected; the non-eruptive
pools are only hot springs. A true geyser should thus possess an
underground pipe or passage, terminating at the surface in an opening
built round with deposits of sinter.
At more or less regular intervals, rumblings and sharp detonations
in the pipe are followed by an agitation of the water in the basin,
and then by the violent expulsion of a column of water and steam to a
considerable height in the air. In the Upper Fire Hole basin of the
Yellowstone Park, one of the geysers, named “Old Faithful,” has ever
since the discovery of the region sent out a column of mingled water
and steam every sixty-three minutes or thereabout. The column rushes
up with a loud roar to a height of more than 100 feet, the whole
eruption not occupying more than about five or six minutes. The other
geysers of the same district are more capricious in their movements,
and some of them more stupendous in the volume of their discharge.
The eruptions of the Castle, Giant, and Beehive vents are marvelously
impressive.
In course of time, the network of underground passages undergoes
alteration. Orifices that were once active cease to erupt, and even
the water fails to overflow them. Sinter is no longer formed round
them, and their surfaces, exposed to the weather, crack into fine
shaly rubbish like comminuted oyster-shells. Or the cylinder of
sinter grows upward until, by the continued deposit of sinter and the
failing force of the geyser, the tube is finally filled up, and then
a dry and crumbling white pillar is left to mark the site of the
extinct geyser.
Mud-Volcanoes are of two kinds: 1st, where the chief source of
movement is the escape of gaseous discharges; 2d, where the active
agent is steam.