« iepriekšējāTurpināt »
THE CORAL REEFS OF TUTUILA,
SAMOA THE preparation of a detailed chart-not yet published-of Tutuila, Samoa, by the U. S. Hydrographic Office, and the studies made by various scientific specialists invited to the island by Dr. A. G. Mayor, director of the department of marine biology of the Carnegie Institution of Washington, have added greatly to the knowledge of that remote possession of ours in recent years. The chart, on a scale of about 1: 50,000, shows the mountainous volcanic island to be surrounded by an extensive submarine bank, from one to three miles wide, somewhat shallower near its inner and outer margins than along an intermediate belt, where soundings of 60 fathoms occur. The shallower parts of the bank are interpreted as submerged fringing and barrier reefs, which are supposed to rest on a wave-cut platform now lying between 60 and 70 fathoms below sea level by reason of island subsidence. The present shores of the island are embayed and are bordered by well developed fringing reefs.
Dr. Mayor's latest Carnegie report contains a condensed statement by R. T. Chamberlin, entitled “The geological interpretation of the coral reefs of Tutuila, Samoa," the result of three weeks' observation there in July, 1920, from which the following extracts are taken:
The island of Tutuila is a volcanic pile whose slopes have been attacked by the sea until a broad wave-cut platform, 2 miles in width, has come to surround the island. This broad shelf of planation, originally cut in the volcanic rocks not far below the sea level, now lies at least (though probably not much more than) 400 feet below sealovel. ... On the outer margin of the wave-cut platform, corals commenced to build a barrier reef, while a fringing reef grew outward from the shore. . . . Subsequently the island became progressively submerged. . . . Tutuila, therefore,
is consistent with the Darwin-Dana coral-reef hy. pothesis to the extent that a submergence of 400 feet has occurred since the corals began to form the old barrier reef; but in other respects it does not fit the requirements of that hypothesis, inasmuch as the barrier reef, instead of being built up several thousand feet from the slopes of a sinking island, is found to be rooted on a broad, wave-cut platform.
Dr. Mayor comments on Chamberlin's statement in part as follows:
Professor R. T. Chamberlin, who made a special study of the relation between the reefs and the volcanic shores of the island, ... finds that the ancient barrier and fringing reefs which once surrounded the island and are now drowned grew upon a platform which had been cut by the sea and afterwards submerged and not upon the unaltered slopes of the island. Thus the DarwinDana theory does not apply to Tutuila.
tions from his “ Structure and Origin of Coral Reefs ” (1842) will make clear.
In the first place, Darwin nowhere asserted that barrier reefs must be “built up several thousand feet from the slopes of a sinking island," or that they could not be built up from a “broad, wave-cut platform," as Chamberlin implies, or that they must grow up from “ the unaltered slopes of an island," as Mayor assumes. All that Darwin's theory of barrier reefs and atolls demands is that a foundation of any form shall subside slowly enough for the reef to grow upward and maintain its surface at sea level. The form of the foundation is immaterial. It is true that the typical island profile which Darwin drew in two figures (pp. 98, 100), to represent a subsiding foundation on which a fringing reef would be transformed into a barrier reef and a barrier reef into an atoll, showed an island of a particular form, as graphic illustrations always must; but as this profile was modeled upon that of the island of Bolabola, a deeply denuded member of the Society group, it effectually disposes of Mayor's assumption that Darwin thought reefs grew up from altered slopes of an island."
It is true that Darwin nowhere wrote anything about the denudation of Bolabola, but he was perfectly familiar with the fact that the slopes of volcanic islands are altered by erosion and abrasion. His geological philosophy was somewhat primitive, for he thought that many volcanic islands had been uplifted after their conical form had been produced by eruption, and that during the resulting emergence the sea cut valleys in the island slopes; it was, indeed, by this process that he accounted for the repeated breaching of certain original “basaltic rings," composed of outward dipping lava beds, and their conversion into a circuit of separated hills, such as characterize the islands of “ St. Jago" in the Cape Verde group, St. Helena, and Mauritius. He also knew that “ deep arms of the sea . penetrate nearly to the heart of some [reef) encircled islands," Raiatea in the Society group being mentioned as one of them; and the depressions occupied by such sea arms were surely understood
Chamberlin's summary concerning the origin of the reefs is excellent as far as it goes, and it is to be presumed that if he publishes a fuller account of his results he will then supplement the present brief statement with an explanation of the conditions which determined that Tutuila should be for a time reeffree and therefore exposed to abrasion before it became reef-encircled, and with a description of the high cliffs that must have risen at the back of the now submerged 2-mile platform and of their relation to the recently cut cliffs the base of which is close to actual sea level.
But excellent as the present summary is with respect to the reefs of Tutuila, neither the passage above quoted from it nor the passage quoted from Mayor's comment upon it does justice to Darwin's theory of coral reefs; for in so far as the quoted passages imply that the submerged barrier reef of Tutuila does not exemplify the “ Darwin-Dana” theory, they hold good only for an imperfect, indeed an incorrect conception of that theory. As a matter of fact the Tutuila reefs, both submerged and at present sea level, exemplify certain special phases of Darwin's theory in a remarkable manner, as the following cita
to be alterations from the original form of Tutuila; and that he gave this possibility little the islands. Hence there is no warrant what- consideration, not because such a reef would ever for thinking that Darwin's theory de- not grow upward into a true barrier if the mands the growth of reefs on unaltered vol- platform subsided, but only because he found canic slopes.
no examples of it. He wrote: The particular kind of alteration caused by
It will, perhaps, occur to some, that the actual the abrasion of circum-insular platforms was reefs formed of coral are not of great thickness, very properly not shown in his type diagram, but that before their first growth, the coasts because, so far as Darwin's observation and of these encircled [non-subsiding] islands were reading went, no barrier reefs were known deeply eaten into, and a broad but shallow subto have grown up from foundations of that marine ledge thus left, on the edge of which the kind. He knew full well, however, that plat
coral grow; but if this had been the case, the forms might be abraded and that reefs might
shore would have been invariably bounded by lofty grow upon them; but he believed that, unless
cliffs, and not have sloped down to the lagoon subsidence occu
channel, as it does in many instances (49). curred, such reefs would differ from ordinary barrier reefs in having shallow Certain volcanic islands that Darwin had lagoons behind them, as will be shown below. seen in the Atlantic, before he was concerned
Various passages in his book make it with the origin of coral reefs, had made him clear enough that no particular form of reef
familiar with the visible occurrence of seafoundation was regarded as essential. Any- cut cliffs; and the “broad but shallow subthing on which a reef might begin its growth marine ledge” that must extend forward from would suffice. For example, Darwin wrote: the base of the cliffs was apparently familiar “If the rim of a (non-subsiding] crater af- by inference. Thus he described St. Helena forded a basis at the proper depth, I am far
as surrounded by enormous cliffs, in many from denying that a reef like a perfectly char
parts between 1,000 and 2,000 feet in height," acterized atoll might not be formed; some
and added that “ the swell of the Atlantic such, perhaps, exist; but I can not believe in ocean has obviously been the active power in the possibility of the greater number having forming these cliffs.” In various other reefthus originated” (89). And again: “A bank
free islands he recognized "the prodigious either of rock or of hardened sediment, level amount of degradation, by the slow action of with the surface of the sea, and fringed with
the sea, which their originally sloping coasts living coral, would ... by subsidence be con
must have suffered, when they are worn back, verted immediately into an atoll, without pass
as is so often the case, into grand precipices.” 2 ing, as in the case of a reef fringing the shore He does not explicitly announce the contrast of an island, through the intermediate form between the "grand precipices” of volcanic of a barrier reef” (101). Evidently, the prime
islands that are not defended by encircling element in Darwin's theory of barrier reefs
reefs, and the moderate slopes that lead “ down and atolls was subsidence; no particular form
to the lagoon channel” in nearly all reefof the foundation on which reef growth be
encircled islands; but he knew and correctly gins was assumed, except for purposes of
described both classes of islands. graphic illustration. Such illustration always
In view of all this it is manifest enough involves definite profiles; but the more gen
that, if Darwin had at hand the facts now eral statements of the text show that definite known about Tutuila, he would have said, in profiles are not required.
effect: Moreover, a careful reading of Darwin's Tutuila is an actual island which must formerly book will discover that he clearly conceived have been deeply eaten into" by the sea, and the possibility of a reef growing up from the which must then have been surrounded by a outer margin of an abraded platform, as now
"broad but shallow submarine ledge” backed by appears to have been actually the case 2 (Geological Observations,' 1844, 91, 128.
"lofty cliffs”; yet the very fact that most other consistent with the context. In any case, Darbarrier reef islands are not “bounded by lofty win clearly knew that a platform could be cliffs" but “slope down to the lagoon channel" abraded around a volcanic island and that shows that they have not been “deeply eaten
such a platform must be backed by cliffs; into”; or if they have been then the resulting
and he further believed that, if a reef grew cliffs have been completely submerged by later
up on the margin of the platform, the lagoon subsidence.
thus enclosed would not have the depth of His general scheme of upgrowing reefs on
most barrier-reef lagoons; but that if the subsiding foundations therefore takes in with
abraded island subsided and the reef grew out any difficulty the special case of an island
higher, the depth that is usually found in around which a platform had been abraded.
barrier-reef lagoons would thereupon be proGood reasons may be given for believing
duced. According to the present understandthat the peculiar case of completely submerged
ing of the coral-reef problem, it is precisely platform-back cliffs, just alluded to, is a very
the occurrence of such subsidence that puts a probable one; for wave-cut platforms and
stop to further abrasion by making reef-growth cliffs presumably occur as normal features in
on a platform margin possible; but Darwin an early, pre-reef stage of young volcanic
did not detect this point, nor did he see that islands; and their rarity to-day is best ex
the opportunity for abrasion of platforms plained by the strong subsidence of the islands
around volcanic islands in the coral seas is best since the platforms were cut; but the discus
provided, as above mentioned, when the islands sion of this question would lead away from
are young and high, with simple, non-embayed the matter here under consideration.
margins, so that a large amount of detritus Another passage from Darwin's book, di
shall be washed down from their steep slopes rectly following the one above quoted about
to the shore, where its accumulation in beaches the possibility of reefs growing on the coast
inhibits coral growth and permits abrasion. of an island that has been deeply eaten into
Indeed, this explanation of the condition un.by the sea, is pertinent here, as it explicitly
der which the abrasion of a platform may considers the growth of a reef upon a plat
occur is not mentioned even in Chamberlin's form margin and the depth of the resulting
summary, though its omission there may be lagoon:
due rather to the conciseness of the summary On this view,8 moreover, the cause of a reef than to a rejection of the explanation. The springing up at such a great distance from the
explanation has, however, a considerable theo(non-subsiding] land, leaving a deep and broad
retical importance in giving reasonable conmoat within, remains altogether unexplained.
sideration to an early pre-reef stage of island Or otherwise phrased: If a reef sprang
development that has been generally overup from the outer margin of a broad platform, looked ; * and it was in view of this explanacut by waves around a still-standing island, tion that the common occurrence of completely the enclosed lagoon could not be so broad submerged platform-back cliffs was above sugand deep as barrier-reef lagoons usually gested as probable in barrier-reef islands; but are, unless subsidence had occurred along with the platforms associated with these submerged reef growth. The quoted statement is not so cliffs need not have been nearly so broad as clear as Darwin's writing generally is, but the the submerged platform of Tutuila. modified phrasing here suggested is believed It may be added that the opportunity for to represent his fuller meaning; it is certainly platform and cliff cutting on Tutuila can not
be advisedly ascribed to the inhibition of 8 A footnote in Darwin's book at this point
coral growth by the lowered temperature of reads: “The Rev. D. Tyerman and Mr. Bennett
the lowered Glacial ocean, as is postulated ... have briefly suggested this explanation of the origin of the encircling reefs of the Society 4“Clift Islands in the Coral Seas,” Proc. Nat. islands,"
Acad Sci., II., 1916, 283–288.
in the Glacial-control theory of coral reefs; If it be true that the submerged barrier for if the Tutuila platform had been cut to a reef of Tutuila was formed on a subsiding width of a mile or two in volcanic rock under platform of marine abrasion, one or two miles such conditions, similar platforms should have in width, the cliffs at the back of the platform been cut around other volcanic islands, and should have been 1,000 feet or more in height. the tops of the platform-back cliffs should be Hence the upper part of their faces ought visible to-day above normal sea level; but as still to be visible after a subsidence of some a matter of fact such partly submerged cliffs, 400 feet; and it should therefore be on the or plunging cliffs as they may be called, have now submerged part of the cliff faces that the not been often detected; besides Tutuila, the present fringing reefs of Tutuila have been other best known examples are Tahiti and the formed. Mayor's accounts of Tutuila tell, Marquesas islands, as will be further told however, of narrow platforms backed by steep below.
cliffs a few hundred feet in height that have To return to Darwin's text: a further ex- been cut close to present sea level since the amination of it discovers a remarkably close submergence of the barrier-reef platform. It parallel to the actual condition of Tutuila, as would therefore seem that these new cliffs the following statement will show. The must have been cut in the slanting faces of Tutuila barrier reef is now drowned; its suc- the earlier and greater cliffs after their partial cessor is a fringing reef on the marginal slopes submergence. This relation of the two sets of the abraded island; and these slopes are, of cliffs has not been mentioned, as far as I according to Mayor, steeper than the sides have learned, by any observer on Tutuila; of the valleys by which the island is dissected. it is a “fier” of my own, based on the diNow in view of the association of fringing mensions of the new cliffs and platforms as reefs with rising or stationary coasts in Dar- reported by Mayor. The relation of the height win's theory—as it is ordinarily quoted—it of these cliffs to the breadth of the platforms might be thought that the occurrence of the at their base suggests that the inclination of Tutuila fringing reef around a subsided island the preexisting spur-end surfaces in which contradicted his views. But that such is not the new cliffs have been cut was much steeper the case is made clear by this prophetic sen- than the ordinary radial slope of the spurs on tence:
a dissected volcanic island, but not steeper If during the prolonged subsidence of a shore than the precipitous descent which the earlier
an old barrier reef were destroyed and sub- cut, spur-end sea cliffs might have had at the merged, and new reefs became attached to the back of their two- or three-mile platform; and land, these would necessarily at first belong to as the cliffs at the back of so wide a platform the fringing class (124).
must have had some such height as 1,000 feet, That is precisely the case at Tutuila. Evi- the upper part of their slanting faces should dently, it is immaterial whether the “old bar- be still visible as plunging cliffs after a 400rier reef” here mentioned had been formed
foot subsidence. Furthermore, the idea that by upgrowth from the slopes of a non-abraded, the new cliffs of Tutuila are cut in the earlier subsiding island, or by upgrowth from the ones gains some support from photographs of margin of a platform on an island that sub- Tutuila by Mayor, and from photographs of sided after the plaform had been abraded. the Marquesas islands by Iddings; for these Darwin's suggested explanation is excellent; islands appear to resemble Tutuila in many it was only because he found no examples of respects, although their submerged platforms, fringing reefs thus produced that he did not the presence of which is indicated by a few pursue the suggestion further; but fringing soundings in front of their plunging cliffs, are reefs of this kind abound in the Philippine lands," Proc. Nat. Acad. Sci., IV., 1918, 197–204. Islands.
7 The islands and coral reefs of Fiji,” Geogr. 6" The fringing reefs of the Philippine Is- Journal, IV., 1920; see p. 218.