Two Islands in Comparison
Instead of presenting here a summary of the geomorphological differences that exist between geologically ancient and young substrates, as described and compared in the examples presented in Sect. 7, one might end with the comparison of two major regions,
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Two Islands in Comparison
Instead of presenting here a summary of the geomorphological differences that exist between geologically ancient and young substrates, as described and compared in the examples presented in Sect. 7, one might end with the comparison of two major regions, such as Sri Lanka and New Guinea. For both islands excellent recent monographs are available, however (Bremer er al.1981 and Laffler 1977). Therefore the summarizing comparison will be presented in a nutshell: in restriction to two tiny ocean islands. Of course the complexity of tropical landforms cannot be fully reduced to the analysis of two small islands. On the other hand significant traits of relief formation, namely rapid weathering, valley cutting and slope retreat in young rocks on the first island, and the role of highly resistant lithology on the second one, can be identified and highlighted as if being placed under a magnifying-glass. The first island is Bora-Bora, situated in French Polynesia. It is the eroded remnant of a Pliocene, relatively large and steep shield volcano, as can be reconstructed from the circumference of its coral reefs. In addition to the emptied caldera (SE of Vaitape) some valley heads widened by slope retreat can be identified (Fig. 115). Because of subsidence and/or glacio-eustatic sea-level rise the valley floors have been transformed into marine bays, and the interfluves have been reduced to spits and tiny islands.
A. Wirthmann, Geomorphology of the Tropics © Springer-Verlag Berlin Heidelberg 2000
8 Two Islands in Comparison
272
151"45'W
BORA-BORA
PACIFIC-
16"30' S
3 km !
OCEAN
P. O.
Fig. 115. The Island of Bora Bora, French Polynesia. A former shield volcano now extremely worn down by erosion. Deep and broad valleys have been drowned by subsidence and eustatic sea-level movements, as has also happened to the major part of the much reduced interfluves. (Redrawn from ORSTOM 1993)
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55°14' E
SILH OUETTE
4°20' S
2 km
P. O.
Fig. 116. The Island of Silhouette, Seychelles. Little affected by valley incision, this is a minuscule case of Gondwana continent morphology. (Redrawn from Seychelles Traveller's Map, Macmillan)
The other island, Silhouette, is part of the granitic core of the Seychelles, a minute splinter of continental crust that was left behind as a group of inselbergs when the Indian Ocean came into being. The island's rocks are the solidified contents of a subvolcanic magma chamber of Early Tertiary age, the volcanic superstructure of which has long since been eroded. They are massive, poorly jointed, full of veins and largely, but to very different degrees, resistant to weathering. The major part of the island, which from the distance looks like
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8 Two Islands in Comparison
a hardly differentiated cone (Fig. 116), consists of syenite, which often pierces the soil cover or even the rainforest in unweathered blocks or rather large rocky outcrops. The broad depression around Grand Barbe, on the other hand, is formed in micro granite
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