Geophysics and Geology of an Explosion Crater in the Ethiopian Rift Valley * D. A. EMILIA B. J. LAST C. A. WOOD F. M. DAKIN Bendix Field Engineering Corp., Box 1569, Grand Junction, CO 81501, USA Geophysical Observatory, Addis Ababa University, Box 1176, Ethiopia Dept. o[ Geology, Brown University, Providence, RI 02912, USA School of Earth Sciences, Macquarie University, North Ryde, NS 2113, Australia ** ABSTRACT Geophysical and geological studies of an Ethiopian maar, Haro Maja, demonstrate that its eruptive history is mo~'e complex than surface geology alone suggests. The crater is 750 m by 1000 m in diameter and varies in depth from 70 m to ll0 m. A strong magnetic anomaly is caused by a central basaltic mound, but a broader crater-wide anomaly is best modelled by a 50 m thick frozen lava lake. 30 m below the crater floor. The central mound was not erupted directly onto the lava lake, but was extruded onto top of the sedimentary infill after a quiescent deposJtional interval. Electrical resistivity measurements further indicate that other basaltic intrusions failed to reach the sur- face during that eruptive period. I NTRODUCTION The Ethiopian Rift Valley contains many large central-vent volcanoes and numerous cinder cones and lava fields, attesting to recent, extensive volcanic activity. Although detailed geologic stud- ies of individual volcanoes have been published (THRALL, 1975; Dt PAOLA, 1972; COLE, 1969; GraS0N, 1967), geophysical exploration has been limited to regional gravity surveys such as those of GOUIN and MOI-IR(1964), SEARI.E and GOUIN (1972), * Work done while the authors were at Addis Ababa University, Ethiopia. ** Temporary address. and MAKRIS et at. (1972). The purpose of the present investigation was to use detailed geophysical measurements in conjunction with a thorough geologic field study to determine the subsurface structure and history of Haro Maja, an explosion crater (maar) in the Ethiopian Rift Valley. Similar studies have been carried out at only a few maars anywhere in the world (e.g., Wo0o, 1974; KIM, 1968) and consequently there is no general, clear picture of their subsurface struc- ture or geologic history. GEOLOGY OF HARO MAJA CRATER Haro Maja (Fig. 1) is one in a group of at least thirteen explosion craters located near the town of Debre Zeit, 45 km southeast of Addis Ababa. These craters are described by Mona (1961, 1967), although he was not able to visit Haro Maja itself. The craters are aligned roughly in a NE-SW direction, along the ill-defined western margin of the Ethio- pian Rift Valley, with the exceptions ot Haro Maja and its neighbor Kilole, which are offset about 7 km to the east. The bedrock is predominantly rhyolite and welded tuff, both of which are pierced by later eruptions forming basaltic cinder cones and lava flows. In several instances, explosion crater rim deposits are draped over cinder cones, indicating temporal variations in the style of eruption from closely spaced vents. Bull. Volca~ol., Vol. 40-3, 1976-77