Chapter 6 (third part)

Image via Wikipedia

ON THE ABSENCE OF ACTIVE VOLCANOES
IN THE AREAS OF SUBSIDENCE, AND ON
THEIR FREQUENT PRESENCE IN THE AREAS
OF ELEVATION.

Before making some concluding remarks
on the relations of the spaces
coloured blue and red, it will be
convenient to consider the position on
our map of the volcanoes historically
known to have been in action.  It is
impossible not to be struck, first with
the absence of volcanoes in the great areas of subsidence tinted pale
and dark blue,--namely, in the central parts of the Indian Ocean,
in the China Sea, in the sea between the barriers of Australia and
New Caledonia, in the Caroline, Marshall, Gilbert, and Low Archipelagoes;
and, secondly, with the coincidence of the principal volcanic chains
with the parts coloured red, which indicates the presence of
fringing-reefs; and, as we have just seen, the presence in most
cases of upraised organic remains of a modern date.  I may here remark
that the reefs were all coloured before the volcanoes were added to the map, or
indeed before I knew of the existence of several of them.

The volcano in Torres Strait, at the northern point of Australia, is that
which lies nearest to a large subsiding area, although situated 125 miles
within the outer margin of the actual barrier-reef.  The Great Comoro
Island, which probably contains a volcano, is only twenty miles distant
from the barrier-reef of Mohila; Ambil volcano, in the Philippines, is
distant only a little more than sixty miles from the atoll-formed Appoo
reef: and there are two other volcanoes in the map within ninety miles of
circles coloured blue.  These few cases, which thus offer partial
exceptions to the rule, of volcanoes being placed remote from the areas of
subsidence, lie either near single and isolated atolls, or near small
groups of encircled islands; and these by our theory can have, in few
instances, subsided to the same amount in depth or area, as groups of
atolls.  There is not one active volcano within several hundred miles of an
archipelago, or even a small group of atolls.  It is, therefore, a striking
fact that in the Friendly Archipelago, which owes its origin to the
elevation of a group of atolls, two volcanoes, and, perhaps, others are
known to be in action: on the other hand, on several of the encircled
islands in the Pacific, supposed by our theory to have subsided, there are
old craters and streams of lava, which show the effects of past and ancient
eruptions.  In these cases, it would appear as if the volcanoes had come
into action, and had become extinguished on the same spots, according as
the elevating or subsiding movements prevailed.

There are some other coasts on the map, where volcanoes in a state of
action concur with proofs of recent elevation, besides those coloured red
from being fringed by coral-reefs.  Thus I hope to show in a future volume,
that nearly the whole line of the west coast of South America, which forms
the greatest volcanic chain in the world, from near the equator for a space
of between 2,000 and 3,000 miles southward, has undergone an upward
movement during a late geological period.  The islands on the north-western
shores of the Pacific, which form the second greatest volcanic chain, are
very imperfectly known; but Luzon, in the Philippines, and the Loo Choo
Islands, have been recently elevated; and at Kamtschatka (At Sedanka, in
latitude 58 deg N. (Von Buch's "Descrip. des Isles Canaries," page 455).
In a forthcoming part, I shall give the evidence referred to with respect
to the elevation of New Zealand.) there are extensive tertiary beds of
modern date.  Evidence of the same nature, but not very satisfactory, may
be detected in Northern New Zealand where there are two volcanoes.  The
co-existence in other parts of the world of active volcanoes, with upraised
beds of a modern tertiary origin, will occur to every geologist.  (During
the subterranean disturbances which took place in Chile, in 1835, I have
shown ("Geolog. Trans." 2nd Ser., vol. v., page 606) that at the same
moment that a large district was upraised, volcanic matter burst forth at
widely separated points, through both new and old vents.)  Nevertheless,
until it could be shown that volcanoes were inactive, or did not exist in
subsiding areas, the conclusion that their distribution depended on the
nature of the subterranean movements in progress, would have been
hazardous.  But now, viewing the appended map, it may, I think, be
considered as almost established, that volcanoes are often (not necessarily
always) present in those areas where the subterranean motive power has
lately forced, or is now forcing outwards, the crust of the earth, but that
they are invariably absent in those, where the surface has lately subsided
or is still subsiding.  (We may infer from this rule, that in any old
deposit, which contains interstratified beds of erupted matter, there was
at the period, and in the area of its formation, a TENDENCY to an upward
movement in the earth's surface, and certainly no movement of subsidence.)

ON THE RELATIONS OF THE AREAS OF SUBSIDENCE AND ELEVATION.

The immense surfaces on the map, which, both by our theory and by the plain
evidence of upraised marine remains, have undergone a change of level
either downwards or upwards during a late period, is a most remarkable
fact.  The existence of continents shows that the areas have been immense
which at some period have been upraised; in South America we may feel sure,
and on the north-western shores of the Indian Ocean we may suspect, that
this rising is either now actually in progress, or has taken place quite
recently.  By our theory, we may conclude that the areas are likewise
immense which have lately subsided, or, judging from the earthquakes
occasionally felt and from other appearances, are now subsiding.  The
smallness of the scale of our map should not be overlooked: each of the
squares on it contains (not allowing for the curvature of the earth)
810,000 square miles.  Look at the space of ocean from near the southern
end of the Low Archipelago to the northern end of the Marshall Archipelago,
a length of 4,500 miles, in which, as far as is known, every island, except
Aurora which lies just without the Low Archipelago, is atoll-formed.  The
eastern and western boundaries of our map are continents, and they are
rising areas: the central spaces of the great Indian and Pacific Oceans,
are mostly subsiding; between them, north of Australia, lies the most
broken land on the globe, and there the rising parts are surrounded and
penetrated by areas of subsidence (I suspect that the Arru and Timor-laut
Islands present an included small area of subsidence, like that of the
China Sea, but I have not ventured to colour them from my imperfect
information, as given in the Appendix.), so that the prevailing movements
now in progress, seem to accord with the actual states of surface of the
great divisions of the world.

The blue spaces on the map are nearly all elongated; but it does not
necessarily follow from this (a caution, for which I am indebted to Mr.
Lyell), that the areas of subsidence were likewise elongated; for the
subsidence of a long, narrow space of the bed of the ocean, including in it
a transverse chain of mountains, surmounted by atolls, would only be marked
on the map by a transverse blue band.  But where a chain of atolls and
barrier-reefs lies in an elongated area, between spaces coloured red, which
therefore have remained stationary or have been upraised, this must have
resulted either from the area of subsidence having originally been
elongated (owing to some tendency in the earth's crust thus to subside), or
from the subsiding area having originally been of an irregular figure, or
as broad as long, and having since been narrowed by the elevation of
neighbouring districts.  Thus the areas, which subsided during the
formation of the great north and south lines of atolls in the Indian
Ocean,--of the east and west line of the Caroline atolls,--and of the
north-west and south-east line of the barrier-reefs of New Caledonia and
Louisiade, must have originally been elongated, or if not so, they must
have since been made elongated by elevations, which we know to belong to a
recent period.

I infer from Mr. Hopkins' researches ("Researches in Physical Geology,"
Transact. Cambridge Phil. Soc., volume vi, part i.), that for the formation
of a long chain of mountains, with few lateral spurs, an area elongated in
the same direction with the chain, must have been subjected to an elevatory
movement.  Mountain-chains, however, when already formed, although running
in very different directions, it seems (For instance in S. America from
latitude 34 deg, for very many degrees southward there are upraised beds
containing recent species of shells, on both the Atlantic and Pacific side
of the continent, and from the gradual ascent of the land, although with
very unequal slopes, on both sides towards the Cordillera, I think it can
hardly be doubted that the entire width has been upraised in mass within
the recent period.  In this case the two W.N.W. and E.S.E. mountain-lines,
namely the Sierra Ventana and the S. Tapalguen, and the great north and
south line of the Cordillera have been together raised.  In the West Indies
the N. and S. line of the Eastern Antilles, and the E. and W. line of
Jamaica, appear both to have been upraised within the latest geological
period.) may be raised together by a widely-acting force: so, perhaps,
mountain-chains may subside together.  Hence, we cannot tell, whether the
Caroline and Marshall Archipelagoes, two groups of atolls running in
different directions and meeting each other, have been formed by the
subsidence of two areas, or of one large area, including two distinct lines
of mountains.  We have, however, in the southern prolongation of the
Mariana Islands, probable evidence of a line of recent elevation having
intersected one of recent subsidence.  A view of the map will show that,
generally, there is a tendency to alternation in the parallel areas
undergoing opposite kinds of movement; as if the sinking of one area
balanced the rising of another.

The existence in many parts of the world of high table-land, proves that
large surfaces have been upraised in mass to considerable heights above the
level of the ocean; although the highest points in almost every country
consist of upturned strata, or erupted matter: and from the immense spaces
scattered with atolls, which indicate that land originally existed there,
although not one pinnacle now remains above the level of the sea, we may
conclude that wide areas have subsided to an amount, sufficient to bury not
only any formerly existing table-land, but even the heights formed by
fractured strata, and erupted matter.  The effects produced on the land by
the later elevatory movements, namely, successively rising cliffs, lines of
erosion, and beds of literal shells and pebbles, all requiring time for
their production, prove that these movements have been very slow; we can,
however, infer this with safety, only with respect to the few last hundred
feet of rise.  But with reference to the whole vast amount of subsidence,
necessary to have produced the many atolls widely scattered over immense
spaces, it has already been shown (and it is, perhaps, the most interesting
conclusion in this volume), that the movements must either have been
uniform and exceedingly slow, or have been effected by small steps,
separated from each other by long intervals of time, during which the
reef-constructing polypifers were able to bring up their solid frameworks
to the surface.  We have little means of judging whether many considerable
oscillations of level have generally occurred during the elevation of large
tracts; but we know, from clear geological evidence, that this has
frequently taken place; and we have seen on our map, that some of the same
islands have both subsided and been upraised.  I conclude, however, that
most of the large blue spaces, have subsided without many and great
elevatory oscillations, because only a few upraised atolls have been
observed: the supposition that such elevations have taken place, but that
the upraised parts have been worn down by the surf, and thus have escaped
observation, is overruled by the very considerable depth of the lagoons of
all the larger atolls; for this could not have been the case, if they had
suffered repeated elevations and abrasion.  From the comparative
observations made in these latter pages, we may finally conclude, that the
subterranean changes which have caused some large areas to rise, and others
to subside, have acted in a very similar manner.

RECAPITULATION.

In the three first chapters, the principal kinds of coral-reefs were
described in detail, and they were found to differ little, as far as
relates to the actual surface of the reef.  An atoll differs from an
encircling barrier-reef only in the absence of land within its central
expanse; and a barrier-reef differs from a fringing-reef, in being placed
at a much greater distance from the land with reference to the probable
inclination of its submarine foundation, and in the presence of a deep-water
lagoon-like space or moat within the reef.  In the fourth chapter the
growing powers of the reef-constructing polypifers were discussed; and it
was shown, that they cannot flourish beneath a very limited depth.  In
accordance with this limit, there is no difficulty respecting the
foundations on which fringing-reefs are based; whereas, with barrier-reefs
and atolls, there is a great apparent difficulty on this head; in
barrier-reefs from the improbability of the rock of the coast or of banks of
sediment extending, in every instance, so far seaward within the required
depth;--and in atolls, from the immensity of the spaces over which they are
interspersed, and the apparent necessity for believing that they are all
supported on mountain-summits, which although rising very near to the
surface-level of the sea, in no one instance emerge above it.  To escape
this latter most improbable admission, which implies the existence of
submarine chains of mountains of almost the same height, extending over
areas of many thousand square miles, there is but one alternative; namely,
the prolonged subsidence of the foundations, on which the atolls were
primarily based, together with the upward growth of the reef-constructing
corals.  On this view every difficulty vanishes; fringing reefs are thus
converted into barrier-reefs; and barrier-reefs, when encircling islands,
are thus converted into atolls, the instant the last pinnacle of land sinks
beneath the surface of the ocean.

Thus the ordinary forms and certain peculiarities in the structure of
atolls and barrier-reefs can be explained;--namely, the wall-like structure
on their inner sides, the basin or ring-like shape both of the marginal and
central reefs in the Maldiva atolls--the union of some atolls as if by a
ribbon--the apparent disseverment of others--and the occurrence, in atolls
as well as in barrier-reefs, of portions of reef, and of the whole of some
reefs, in a dead and submerged state, but retaining the outline of living
reefs.  Thus can be explained the existence of breaches through barrier-reefs
in front of valleys, though separated from them by a wide space of
deep water; thus, also, the ordinary outline of groups of atolls and the
relative forms of the separate atolls one to another; thus can be explained
the proximity of the two kinds of reefs formed during subsidence, and their
separation from the spaces where fringing-reefs abound.  On searching for
other evidence of the movements supposed by our theory, we find marks of
change in atolls and in barrier-reefs, and of subterranean disturbances
under them; but from the nature of things, it is scarcely possible to
detect any direct proofs of subsidence, although some appearances are
strongly in favour of it.  On the fringed coasts, however, the presence of
upraised marine bodies of a recent epoch, plainly show, that these coasts,
instead of having remained stationary, which is all that can be directly
inferred from our theory, have generally been elevated.

Finally, when the two great types of structure, namely barrier-reefs and
atolls on the one hand, and fringing-reefs on the other, were laid down in
colours on our map, a magnificent and harmonious picture of the movements,
which the crust of the earth has within a late period undergone, is
presented to us.  We there see vast areas rising, with volcanic matter
every now and then bursting forth through the vents or fissures with which
they are traversed.  We see other wide spaces slowly sinking without any
volcanic outburst, and we may feel sure, that this sinking must have been
immense in amount as well as in area, thus to have buried over the broad
face of the ocean every one of those mountains, above which atolls now
stand like monuments, marking the place of their former existence.
Reflecting how powerful an agent with respect to denudation, and
consequently to the nature and thickness of the deposits in accumulation,
the sea must ever be, when acting for prolonged periods on the land, during
either its slow emergence or subsidence; reflecting, also, on the final
effects of these movements in the interchange of land and ocean-water on
the climate of the earth, and on the distribution of organic beings, I may
be permitted to hope, that the conclusions derived from the study of
coral-formations, originally attempted merely to explain their peculiar
forms, may be thought worthy of the attention of geologists.

Related articles by Zemanta

• Volcanoes fingered in oceanic mass extinction
• Reason found for massive ocean oxygen loss that formed Caribbean