Geology of Ascension Island other volcanic islands, which only have a few mafic and felsic endmembers such as basalt or rhyolite, Ascension has a full-range from some of the most mafic to most felsic. This includes basalt, hawaiite, mugearite, benmoreite,trachyte and rhyolite. Potassium-argon dating indicates that the oldest rocks exposed above the water are one million year old rhyolites. Geologists have divided the island's mafic and felsic rocks into different named sequences and sub-divided mafic rocks based on their ratio of zirconium and niobium. The Sister's Peak region in northwest Ascension has intermediate mafic lava flows, scoria cones and mafic ash fall deposits near
Geology of Ascension Island same time as the Sister's Peak region 829,000 years ago. Bentmoreite scoria cones record evidence of mafic volcanism, although older rhyolite and trachye flows, domes and pyroclastic deposits point to felsic eruptions as well. The Central Felsic Complex (CFC), centers on Green Mountain and Middleton Ridge and is older than the Eastern Felsic Complex. Here, ash is typically welded together and breccia is laden with rhyolite and obsidian fragments. Middleton Ridge formed more than one million years ago and is accompanied trachyte and rhyolite flows. By comparison, Green Mountain formed as recently as 395,000 years ago with trachyte flows and