Controls on Island morphologic evolution

Islands are interesting geomorphic features because they possess a well defined base level. Non-volcanic islands, in particular, are the product of rifting of a small continental fragment such that they start their geomorphic evolution as a more or less elevated flat plateau. After tens of millions of years of evolution, some islands, such as Madagascar, still present a Pi-shaped form composed of large watersheds on top of a central plateau surrounded by smaller ones that connect the plateau to the coastline. Other islands, such as Sri Lanka have a more conical or Lambda-shape composed of a radial distribution of basins connecting the island summit to the coastline. Here we investigate the conditions that lead to the transformation of an initially plateau-shape island to either a Pi- or Lambda-shape by using a Landscape Evolution Model solving the Stream Power Law and talking into account flexural isostasy.

We find that to maintain a Pi-shape, an island must fulfil two criteria: firstly, its extent must be sufficiently large in comparison with the underlying effective elastic thickness (EET), and, secondly, it must be subjected to limited erosion. We introduce a morphometric index that allows to discriminate between the two types of morphologies and show how it evolves through time as a function of both EET and erodibility.

Constraining the time evolution of the morphology of an island is important to study the evolution of its bio-diversity. Our finding implies that the micro-endemism that characterises Madagascar is linked to the strength of the underlying lithosphere.