São Tomé and Príncipe both formed within the past 30 million years due to volcanic activity in deep water along the Cameroon line.
Long-running interactions with seawater and different eruption periods have generated a wide variety of different igneous and volcanic rocks on the islands with complex mineral assemblages.
On the island of São Tomé itself, a large stratovolcano rises from abyssal plain a total of 5000 meters, protruding two kilometers above sea level.
Although the magma that formed São Tomé was originally sourced from the mantle, fractional crystallization in contact with seawater resulted in magnetite, apatite, plagioclase, hornblende, augite and olivine.
The island is built on a base of palagonite breccia, which includes pieces of basalt that was part of a tholeiitic magma series which developed deep below water.