ascension is a geologically young formation, the tip of an undersea volcano which rose above the waves only a million years ago. although volcanic activity is mainly associated with the mid-atlantic ridge plate boundary 80 km to the west, ascension also displays some features which are commonly attributed to "hotspot" volcanism. such volcanism is typically assumed to arise from a deep mantle thermal plume from the core-mantle boundary. alternatively it may result from minor deformations of the oceanic crust that cause extension and permit magma to rise passively up from the asthenosphere. ascension last erupted about 500 years ago. due to the low rainfall and geologically recent eruptions, its soil consists mostly of clinker.