Black Spherules, Allan Hills

A dust band discovered by Harvey et al. (1998) 5 km west of the western edge of the Allan Hills is composed of an unusual assemblage of dark and rounded particles. These spherical particles contain strongly zoned euhedral olivine crystals in a matrix of iron-rich glass 

A final point to be made is that the spherules must have been deposited on the surface of the East Antarctic ice sheet, which means that East Antarctica was glaciated at the time (i.e., two to three million years ago). Therefore, the terrestrial age of the meteorite-ablation spherules is consistent with the evidence that Cenozoic volcanoes in the Transantarctic Mountains (e.g., Mt. Early and Sheridan Bluff) erupted through glacial ice from which it follows that the glaciation of East Antarctica started during the late Oligocene/early Miocene perhaps as early as 25 million years ago. However, none of the ice cores that have been drilled in East or West Antarctica have encountered ice that is as old as the meteorite ablation spherules in the icefield near the Allan Hills. 

The East Antarctic ice sheet can only be described in terms of superlatives. It is the largest and thickest body of ice in the world, it reaches the highest elevation above sea level of any continental ice sheet, and it is the coldest area on the Earth. The East Antarctic ice sheet has stored most of the fresh water on the Earth because the ice formed from water that has passed through the distillation process of the hydrologic cycle. The ice sheet began to form during the late Oligocene and now covers almost the entire area of the East Antarctic craton. 

The ice forms from snow that is deposited in the central region of the continent where it is converted to firn and then to ice by recrystallization in response to the weight of the overlying layers of snow. The ice moves from the zone of accumulation to the coast of East Antarctica and toward the Transantarctic Mountains, which force it to flow through narrow 

As the ice approaches the Transantarctic Mountains, it is forced to flow over rough terrain consisting of the flat-topped hills and ridges of the western foothills of the Transantarctic Mountains. The resulting structural deformation of the ice causes sediment-laden ice from the base of the ice sheet to be deflected to the surface where the clasts and fine-grained sediment accumulate as supraglacial moraines. 

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The ice fields adjacent to the Allan Hills, where a large number of meteorite specimens have been collected, also contain tephra layers. The outcrop patterns of these layers indicate that the ice was extensively deformed as it flowed over and around the subglacial foothills of the Transantarctic Mountains. A small deposit of black spherules embedded in the ice near the Allan Hills was attributed to the passage through the atmosphere of a stony meteorite. This deposit reminds us that the East Antarctic ice sheet contains extraterrestrial particles and specimens of meteorites in addition to terrigenous dust and volcanic ash. 

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