Mount Kilimanjaro is a stratovolcano that formed as the plates below it dropped and porous basalt rock magma erupted through the surface of the ocean. The magma and other debris formed layers with each eruption, eventually rising high enough to bring Mount Kilimanjaro to its height of 19,341 feet.
Mount Kilimanjaro emerged from fault movement along part of the Great Rift Valley. The movement occurs in a subduction zone, an area where the oceanic crust descends below the continental crust. As the oceanic slab sinks, magma rises through the porous rock, aiding in the descent of the crust. As magma rises past the crust, it enters an area of the rock known as the mantle, where it pools and adds some of the mantle rock and gases to its composition. Eventually, the built-up magma reaches a high enough pressure that it explodes violently, in what geologists refer to as an explosive eruption. For Mount Kilimanjaro, this process likely began around a million years ago. Repetition of this process eventually formed the three peaks of Kilimanjaro, known as Mawenzi, Shira and Kibo. The last major eruption from this process occurred between 150,000 to 200,000 years ago. Kibo is classified as a dormant volcano, while Mawenzi and Shira are classified as extinct.