Geology

Abstract

Socotra is a typical example of an ancient continental island, but in biological evolutionary terms, it is more akin to an oceanic island due its relatively long duration of isolation. Post-Lower Miocene uplift with arching, block-faulting and tilting has played a major role in the present structure and morphology of the island. Most parts of Socotra, with the possible exception of the central Hagghier mountains, were intermittently submerged until the Miocene. As a consequence, the Precambrian basement rocks are overlain unconformably by Cretaceous and Tertiary limestones, but are exposed in three main uplift areas, most prominently in the Hagghier mountains. Prior to the Gulf of Aden rifting in the Oligocene, Socotra was probably located adjacent to southern Arabia. The precise time at which the Socotra Archipelago became detached from the African mainland is unclear, but could be of considerable biogeographical significance. The climate history of the Horn of Africa, with Socotra, differed from that of the adjacent regions, because since about the Late Cretaceous, and extending into the Middle Eocene, the evidence suggests that arid conditions persisted there, at least intermittently. During the glacial phases of the Pleistocene, it seems that as the rainforests contracted, “arid corridors” existed between South Africa and the north-east of the continent, including the Horn of Africa, and that repeated plant migrations were taking place in both directions. Such corridors could help explain the present-day disjunct distributions of various taxa.