Geology of Tristan da Cunha

Geological map of Tristan da Cunha
Map is somewhat simplified and modified after Baker et al. (1964).

Tristan, and its neighbouring islands, lies about 400 km east of the Mid-Atlantic Ridge. The volcanic activity is unrelated to the Mid-Atlantic Ridge; rather, it is due to a hotspot.

The base of the volcano is at 3700 m below sea-level, and the summit is at almost 2100 m above sea-level. Marine erosion has formed the high, sheer cliffs (the Main Cliffs) that surround most of the island. Above the Main Cliffs is a more gently sloping region between 600 and 1000 m (the Base). Rising from the Base is the steep central cone (the Peak) which has a small summit crater.

The Peak predominantly is composed of pyroclastic deposits erupted from the central vent. The Base and Main Cliffs predominantly are composed of thin basaltic lava flows, commonly separated by thin pyroclastic layers. There are over 30 cinder cones on the flanks of the main volcano, many of which have produced small lava flows. The October 1961 eruption was preceded by earthquake swarms and rock falls from the Main Cliffs, then lava was erupted on the plain immediately east of the settlement. The growing lava mound breached and lava flows were erupted toward the coast. As the eruption waned, an elongate lava dome grew and sealed the vent.

Inaccessible Island, 35 km southwest of Tristan, is the relic of an older volcanic cone. Most of the island is composed of basaltic lava flows, but the southwestern part of Inaccessible has numerous trachyte domes and flows.

Nigtingale Island, and nearby Middle and Stoltenhoff Islands, are 34 km south-southwest of Tristan. Nightingale mostly is composed of trachyte domes and flows, with some pyroclastic deposits. Middle Island is entirely composed of pyroclastic deposits (intruded by dykes), whereas Stoltenhoff Island is entirely composed of trachyte.

If you want some information about the comparitive composition of rocks from Ascension, St. Helena, and Tristan, and what it all means, go here. [Warning - esoteric but interesting!]

 Tristan page

Page maintained by Barry Weaver