Geology of Saint Helena

Volcanism on the island ceased seven million years ago, leading to long-running erosion and the formation of deep, v-shaped valleys along with steep coastal cliffs.

Some geochemists have observed significant trace element variations between South Atlantic islands and proposed that deep-ocean sediments may have mixed into the magma which formed Saint Helena.

The breccia and basalt lava flows of the Northeastern Volcanic Center formed due to undersea eruptions 14 to 11 million years ago, in the Miocene.

[2] The isotopic composition of volcanic rocks in Saint Helena and nearby Ascension Island and Tristan da Cunha are very different from the Mid-Atlantic Ridge basalts.

Some geochemists have proposed that small amounts of 1.5 to 2 billion year old pelagic sediments contaminated the magma source of the island, as a means of explaining the differing trace element ratios relative to Tristan da Cunha.

The Barn is a capping of younger lavas upon weaker rocks. The Barn features cliffs on the side that faces the sea. It overlooks the pyroclasts and weak flows of Turk's Cap Valley to the south.