The Semien Mountains were formed prior to the creation of the Rift Valley, from lava outpourings
between 40 and 25 million years ago during the Oligocene period. The volcano is believed to have
spread over more than 5000 m2 and resulted in a thick sequence of basaltic lava some 3,000-3,500 m
thick that was deposited on Precambrian crystalline basement. The major part of the Semien Mountains
consists of remnants of a Hawaiian-type shield volcano.