The Vanished Detritus of Deeply Denuded Islands.
Many volcanic islands, now deeply denuded in irregular forms, give clear indication of their initial conical form in the outward slant of their marginal lava-beds. It is in such cases a comparatively simple matter to reconstruct their original cone, VW (fig. 5), and to estimate the volume of detritus that has been removed in reducing the island to its present maturely denuded form, RM. Even if no submergence be assumed, the volume of detritus that has been carried away from so much of the initial volcanic mass as is now above sea-level is, as noted above, vastly greater than the volume of the lagoon waters, G, on all the reef-encircled islands that I have seen. How has this great volume of detritus been disposed of?
Let the island be supposed to have been formerly more emerged than now, and let it stand still with respect to sea-level, SC, during a period of deep dissection. Under these conditions the detritus washed out from its valleys would soon completely overwhelm any fringing reef that might by chance be established on its shores, and the waves would then cut cliffs, CL, all around its circuit, as is now the case on Réunion. This consideration alone is sufficient to discredit Murray's theory of outgrowing reefs on still-standing islands. Moreover, if the island stand still, cliff-cutting will continue and no opportunity for barrier-reef formation will be allowed. Under what conditions, then, is the formation of barrier reefs permitted?
An apparent escape from the difficulty of accounting for the vanished detritus around a still-standing island is found in changes of ocean-level during the Glacial period; for the detritus discharged while the ocean stood at a lower level than now would be deposited on the lower slopes of the island, and when the ocean rose again a barrier reef might grow up with it. But during the discharge of the detritus reefs could not flourish, and waves would then cut the island-shores back in cliffs; and if cliff-cutting endured through the time required to excavate the valleys now drowned in embayments the cliffs would surely be high enough to be still visible after the ocean has resumed its normal level. Hence the amount of submergence thus provided is insufficient for the needs of the problem. Moreover, all volcanic islands the eruptional growth of which was completed earlier than the beginning of the Glacial period should have had cliffs cut around their margin in pre-Glacial time, and some trace of these cliffs should now be found. Another supposition must therefore be made, as follows:—
If an island, VW (fig. 6), with sea-level originally at NV, does not stand still, it must subside to a great depth, NS, if no cliffs are to be cut around
its margin and if the larger part of its discharged detritus is to be deposited in the lagoon, G, of an upgrowing barrier reef, B; but in this case the early stages of subsidence must be so rapid, in order to provide sufficient lagoon-space for the deposition of detritus, that the upgrowth of a reef could hardly keep pace with it. It is not likely that the numerous barrier reefs of to-day have all survived so threatening a danger: hence a slower rate of early subsidence must be postulated.
Let the island, therefore, stand almost or quite still during a considerable period after its eruptive growth ceases. In this case the detritus supplied by the erosion of deep valleys, CY (fig. 5), and by the abrasion of high cliffs, CL, will be swept off shore in large amount, D, by vigorous waves, unimpeded by a barrier reef; then, if intermittent subsidence begin, placing sea-level at TE, the further discharge of detritus will be detained in the embayed valleys, E, and reef-upgrowth may begin. But, as under these conditions strong cliff-cutting will have accompanied the erosion of deep valleys, a considerable measure of subsidence, placing sea-level at UV, will be eventually necessary to submerge the cliff-tops, L, if they are not seen to-day. Whether this supposition represents the actual history of reef-encircled islands or not, it certainly provides a more reasonable condition for reef-growth than any other supposition here considered.
Various combinations of diverse conditions may be imagined. For example, the succession of events may be as follows: (1) Moderate cliff-cutting during a still-stand period before reefs are developed; (2) moderate submergence and reef-upgrowth; (3) a second still-stand period, resulting in the smothering of reefs by outwashed detritus, and renewal of cliff-cutting; (4) further subsidence and renewed reef-growth. Tahiti seems now to be approaching the third phase of this succession, for, if the present still-stand that is attested by the alluvial lowland around the island border endures as long as the earlier reefless period of valley and cliff-cutting, the lagoon will be overfilled, the smothered reefs will be abraded, and a new attack will be made by the waves on the cliffs at a higher level than before.
In any event, the only way of developing a barrier reef around a deeply dissected and non-cliffed volcanic island seems to be either to allow it to subside rapidly to a great depth while its reef is growing up, or to allow it to subside to a less depth after strong cliffs have been cut around its shore. And inasmuch as Réunion, Tutuila and the Marquesas, and Tahiti exemplify the second of these alternatives, the first alternative is regarded as the less probable of the two.
Many more observations of reef-encircled islands are needed before the questions here raised can be settled; and the observations must evidently be directed much more to the islands than to the reefs around them. The various possibilities here outlined, and as many others as can be invented,
should be critically reviewed by the observer while he is still on the ground, in order that he may give conscious attention to the details which are confirmatory of or contradictory to the different suppositions. The absence of records regarding significant details in many accounts of reef-encircled islands makes it impossible to use them in a settlement of the questions at issue.