Pre-Quaternary Glaciations
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2020-01-01
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Pre-Quaternary glaciations were recognized worldwide and have deeply influenced and conditioned the bioevolutive events and geochemical cycles of the Earth. Geological history has revealed that glaciations during icehouse intervals are typically periods of global sea level falls, while greenhouse intervals are marked by rising sea levels and reduced, or even absent, polar ice sheets. Defining the causes of glaciations is a complex task because the triggers, related to terrestrial and orbital forcing factors, are diverse and sometimes controversial. Glaciations were not common events in most of the Precambrian time, which covers about nine tenths of Earth's history. The glacial rocks of the Meso-Archean Pongola Supergroup and those related to the Huronian glaciation are evidences of ice ages before the Proterozoic “Boring Billion” period. Glaciations in Earth's history became frequent since the beginning of the Cryogenian, suggesting a global cooling of the planet's climate, influenced by a progressively complex biosphere and important changes in geochemical cycles. Ice sheets covered large areas of the planet land surface during three Cryogenian-Ediacaran glaciations, when the Earth was possibly completely covered by ice (“Snowball Earth” hypothesis). Following the long Neoproterozoic icehouse interval, the Earth experienced greenhouse conditions from Cambrian to Middle Devonian, only interrupted by the short-lived widespread late Ordovician glaciation (Hirnantian). The Late Paleozoic Ice Age (LPIA) encompasses glaciations occurred from late Devonian to Permian in many parts of Gondwana. During the Paleozoic, landmasses were concentrated in high southern latitudes forming this supercontinent, and glacial centers migrated following the drift of this huge landmass across the South Pole. The Late Cenozoic Ice Age is the icehouse interval that started at the Eocene-Oligocene boundary and is still ongoing, encompassing all known Quaternary glaciations.
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Encyclopedia of Geology: Volume 1-6, Second Edition, v. 5, p. 475-485.