Recent geological and geophysical research indicates the presence of three volcanic systems in the Hengill--Grensdalur area and progressive westerly migration of the accretionary plate boundary. The Hengill system comprises the active Hengill central volcano and a NNE trending fissure swarm that is the present locus of crustal accretion in the area. Extinct volcanic system are identified with the mountain Hromundartindur and the Grensdalur area and associated NNE trending zones. The Grensdalur system was formerly the locus of accretion in the area. A widespread geothermal area encompasses the whole area and is fueled by at least three distinct heat sources associated with the three volcanic systems. A tomographic study of the upper 5 km of crust, using local earthquakes, imaged three bodies with velocities up to 15% higher than the average background velocities and volumes of several tens of cubic kilometers. One of these underlies the Grensdalur volcano, and a second underlies the Olkelduhals area within the Hromundartindur system. These are interpreted as intrusions that are the solidified magma reservoirs of their respective volcanic systems and the heat sources of those parts of the geothermal area above them. The third high-velocity body underlies the extinct basalt shield Husmuli, which is not associated with geothermal resources. That body is interpreted as a cold intrusion that is the frozen magma conduit that fed the surface eruptive site. A low-velocity body with a volume of a few cubic kilometers was imaged in the depth range 2--4 km beneath the northern part of the presently active Hengill central volcano. This volume may contain partial melt and represents the heat source, or part of the heat source, fueling the Hengill field. Heat balance calculations show that subsurface magmatism continued in the extinct Grensdalur volcano long after volcanic activity and crustal accretion migrated from it to the Hengill system. ¿ American Geophysical Union 1989