Maars, tuff rings, and tuff cones at the Manyara rift escarpment, Tanzania.

Abstract

About 350 maar craters, tuff rings, and tuff cones are spread along the Manyara rift escarpment, and clustered around the Hanang and Kwahara volcanoes in North Tanzania. They lie in the East African Rift, an active extensional setting, where the magma composition is moderately to highly alkaline and carbonate-rich. We present newly-collected depositional characteristic observations, grain size distributions, and texture analyses of tephra deposits from selected maars, tuff rings and tuff cones of alkaline composition in Manyara to illustrate the range of the associated eruption dynamics and eruptive styles. We observed deposits with some evidence for water-magma interaction and deposits without. We attribute this range to varying magma ascent conditions and water availability along intrusive paths. Magma is guided by fractures allowing it to rise fast and to exsolve gas only near the surface. If water was available within a fracture, a moderate phreatomagmatic eruption was triggered and formed a shallow maar. In a dry fracture, the magma was fragmented by gas exsolution near the surface resulting in a tuff ring or a tuff cone. Several tephra deposits show similarities with kimberlite deposits, notably so the spherical and ellipsoidal shape of the juvenile lapilli and the presence of cannonball bombs.