Volcanism, tectonism, sedimentation, and the paleoanthropological record in the Ethiopian Rift system

The Ethiopian Rift System consists of basins that are in different stages of evolution. Some of the rift-related basins in southwestern Ethiopia are half-grabens that have not evolved to symmetrical rifts since the initiation of rifting here in the middle Miocene. These basins contain fossiliferous Pliocene–Pleistocene volcaniclastic sediments and volcanic rocks and have been occupied by early hominid populations. The Afar and the Main Ethiopian Rifts are symmetrical, with both margins fully developed. Several paleoanthropological localities, ranging in age from the Quaternary to the Pliocene, were discovered within these rift basins. The discovery of Australopithecus afarensis (the “Lucy” species) at Hadar and Ardipithecus ramidus and Australopithecus garhi in the Middle Awash makes the region the most prolific early hominid area in the world. Many of the known Pliocene–Pleistocene paleoanthropological localities that have given us information about our ancestors are concentrated in the East African Rift System. This is not a coincidence, because the volcanic and tectonic activities that were responsible for the formation of the rift basins and coeval sedimentation created ideal environments for the proliferation of life and the preservation of faunal and floral remains. Volcanic and tectonic activities created plateaus and mountains; most of the sediments in the basins were derived from these topographic highs located within and outside the rift valleys. Volcaniclastic sediments and volcanic ash were responsible for the quick burial and preservation of fossils during diagenesis. Diagenetic processes involving silicification, calcification, zeolitization, feldspathization, clay formation, and pedogenesis all played roles in fossil preservation in the volcaniclastic sediments. Volcanic rocks interbedded with the fossiliferous sediments also provide temporal information about geologic processes, faunal evolution, paleoenvironment, and early hominid behavior and lithic technology.