Geology of Socotra

The oldest rocks on Socotra date from the Precambrian, with a minimum age of approximately 800 Ma.[12] These comprise metasedimentary rocks, primarily schist and gneiss, formed under the conditions of amphibolite facies metamorphism.[13] Also part of the island's basement are several types of igneous rock. Plutonic granites were formed from intrusions of magma from beneath the surface,[10] while andesite, dacite and rhyolite were deposited by lava flows 60–70 metres (200–230 ft) thick. Breccia and tuff were the result of more recent, and more explosive, volcanism.[12] These periods of volcanic activity have not been precisely dated, but the resulting rocks are cut by the granite and gabbro of the Hagghier Mountains, whose formation has been established as Precambrian.[13][14]

The Hagghier Mountains represent the largest exposure of basement rocks on the island. Other Precambrian outcrops exist at Ras Momi and Ras Shu'ub, at the extreme east and west of the island, respectively, and in the Qalansiyah valley in the northwest. The neighboring islands of Abd al Kuri and Samhah also have exposed Precambrian basements.[12]

After the Precambrian, the area that is now Socotra underwent a long period of peneplanation, eroding the existing surface without depositing new layers, resulting in a substantial nonconformity.[13] Paleozoic rocks are nearly unknown from Socotra, although one potassium–argon dating of a sample of shale from near Hadibu to about 400 Ma suggests an origin in the Devonian.[10] In the late 1970s, some sedimentary rock on the island was ascribed to the Permo-Carboniferous (about 300 Ma) based on biostratigraphy;[10] later authors demonstrated that the index fossils involved supported more recent dating.[12]

Prior to the 1970s, it was believed that the oldest rocks above the nonconformity were deposited in the Cretaceous.[13] However, in the late 1990s, a 320 m (1,050 ft) thick layer of Triassic material underneath 110 metres (360 ft) of Jurassic deposits was described. These strata, located in a narrow band on the east-southeast coast of the island around Ras Falanj, include the sediments that had been erroneously assigned to the Paleozoic.[12]

The earliest Triassic stratum (about 250 Ma) is a sandstone, associated with less common finer-grained material, indicative of the deposits of a braided river.[10] This transitions into limestone deposited in a shallow water marine environment approximately 240–220 Ma.[15]

Following the deposition of Triassic limestone, the region now comprising Socotra experienced a period of uplift and erosion, so 190–180 Ma Jurassic marine sandstones sit unconformably on the older rock. These sandstones show considerable cross-bedding associated with deposition in an environment with strong tidal currents.[12] Later in the period, sandstone deposits alternate with siltstone and limestone, some of which include abundant coral fossils, evidence of the shallowing of the marine environment and the development of coral reefs during the Jurassic.[12][15]

A marine transgression flooded the region in the Cretaceous, leading to the deposition of shallow-marine limestones and siliclastic offshore sediments.[14]

Limestone, which now forms cliffs, deposited in the Eocene in an offshore shelf environment, followed by Oligocene and Miocene calcareous deposits. [10]