Mount Meru, situated east of the Great Rift Valley and about 40 km southwest of Kilimanjaro in northern Tanzania's Arusha National Park, is considered an active volcano and is the country's second highest mountain. It is also considered the fourth highest mountain in all of Africa by some (after Kilimanjaro, Mount Kenya, and the Rwenzoris - Known as the Mountains of the Moon). 500,000+ years ago.
Mount Meru erupted in a tremendous explosion that destroyed it's cone shape and resulted in a horseshoe crater with the eastern side removed. The resulting mountain has its summit on the west side with it's inner walls rising over 1,500m from the crater floor, making them among the tallest cliffs in Africa. In the past 100 years, eruptions have been reported as the Ash Cone continues to build inside the crater. The first ascent is still in dispute and credited to either Carl Uhlig in 1901 or Fritz Jaeger in 1904.