Arusha National Park lies on the eastern edge of the Great Rift Valley, which is part of a fault in the earth's crust that stretches 8,000 kilometers (4,971 miles) from Turkey to the mouth of the Zambezi River in Mozambique. The Rift Valley was formed 20 million years ago and amidst this terrestial turmoil a small subsidiary vent opened at the eastern foot of the present Mt. Meru and Ngurdoto was born.
Molten rock was forced to the earth's surface by superheated steam and ejected, slowly building up a cone around the vent and imprisoning gases from the earth's core. When the pressure of these gases built up, a vast explosion occurred and a crater was formed. Repeated activity of this kind increased the size of the crater.
Ngurdoto volcano is now extinct but from the pear-shape of the present day crater it seems that towards the end of its activity there were in fact two cones lying very close together. When the molten rock below the cones withdrew to deeper levels, the cones were left without support and then collapsed forming the present crater, or as it should more correctly be called, caldera.
Mt. Meru on the other hand is a dormant volcano. The crater wall was ruptured by a series of violent explosions a quarter of a million years ago. These explosions may have been caused by blocking of the vent or water from a crater lake seeping in. This caused the whole eastern wall of the crater to be blown away and a mass of water, mud, rocks and lava cascaded down the eastern side of the mountain. The Momela Lakes were formed by depressions in the drying mud. Since then repeated volcanic activity has built the ash cone into the shape it is today and the sporadic eruptions have streaked the sides of the mountain with lava. The most recent evidence of activity can be seen on the northwestern side of the ash cone where a small lava flow occurred just over 100 years ago.
At around this time in 1876, Count Teleki, a Hungarian, was the first European to visit the Momela area. He commented on the vast number of hippos and rhinos he saw, but the latter are now sadly no longer in the Park. In 1907 the Trappe family moved to Momela to farm, using large areas of the present Park as a cattle ranch. The elder Mrs. Trappe, who was the first woman to become a professional hunter in East Africa, voluntarily set aside a large part of the Momela estate for a game sanctuary. When the Park was established in 1960 the farm was incorporated into it.
Originally the Park was known as Ngurdoto Crater National Park, but in 1967 Mt. Meru was included and the name changed to Arusha National Park.