The earth has been free of permanent ice sheets for most of its history, and there have been nine glacial periods over the past 3 billion years. The earth is now in the Late Cenozoic Ice Age, which began around 33.9 million years ago, when ice began to build up in the Antarctic. Its latest phase is the Quaternary glaciation, which started around 2.58 million years ago and is still ongoing. Within ice ages, there exist periods of more severe glacial conditions and more temperate conditions, referred to as glacial periods and interglacial periods, respectively. Earth is currently in such an interglacial period of the Quaternary glaciation, with the Last Glacial Period of the Quaternary having ended around 11,700 years ago. The current interglacial is known as the Holocene epoch. Based on climate proxies, paleoclimatologists study the different climate states originating from glaciation.
Known ice ages
Descriptions
The third ice age, and possibly most severe, is estimated to have occurred from around 717 to 660 million years ago, during the Cryogenian Period, and it has been suggested that it produced a second "Snowball Earth", i.e. a period during which Earth was completely covered in ice. It has also been suggested that the end of that second cold period was responsible for the subsequent Cambrian explosion, a time of rapid diversification of multi-cellular life during the Cambrian Period. The hypothesis is still controversial, though is gaining credence among researchers, as evidence in its favour has mounted. The Hirnantian glaciation lasted from around 460 to 420 million years ago, and then the Late Paleozoic icehouse from around 360 to 255 million years ago. The Late Cenozoic Ice Age has seen extensive ice sheets in Antarctica for the last 33.9 million years. During the last 3 million years, ice sheets have also developed on the northern hemisphere. That phase is known as the Quaternary glaciation, and was marked by more or less extensive glaciation. They first appeared with a dominant frequency of 41,000 years, but after the Mid-Pleistocene Transition that changed to high-amplitude cycles, with an average period of 100,000 years.
Nomenclature of Quaternary glacial cycles Whereas the first 30 million years of the Late Cenozoic Ice Age mostly involved Antarctica, the Quaternary has seen numerous ice sheets extending over parts of Europe and North America that are currently populated and easily accessible. Early geologists therefore named apparent sequences of glacial and interglacial periods of the Quaternary Ice Age after characteristic geological features, and these names varied from region to region. The marine record preserves all the past glaciations; the land-based evidence is less complete because successive glaciations may wipe out evidence of their predecessors. Ice cores from continental ice accumulations also provide a complete record, but do not go as far back in time as marine data. Pollen data from lakes and bogs as well as loess profiles provided important land-based correlation data. The names system has mostly been phased out by professionals. It is now more common for researchers to refer to the periods by their marine isotopic stage number. For example, there are five Pleistocene glacial/interglacial cycles recorded in marine sediments during the last half million years, but only three classic glacials were originally recognized on land during that period (Mindel, Riss, and Würm). Land-based evidence works acceptably well back as far as MIS 6 (see Marine isotope stages, Stages), but it has been difficult to coordinate stages using just land-based evidence before that. Hence, the "names" system is incomplete and the land-based identifications of ice ages previous to that are somewhat conjectural. Nonetheless, land based data is essentially useful in discussing landforms, and correlating the known marine isotopic stage with them.
Historical nomenclature in the Alps Biber (2.6–1.8 Mya, Gelasian) Biber-Danube interglacial (not in use) Danube (1.8–1 Mya, Calabrian) Danube-Gunz interglacial (not in use) Günz (1–0.4 Mya, MIS 21 – MIS 11 ?) Günz-Haslach interglacial (not in use) Haslach (seldom used) Haslach-Mindel interglacial (not in use) Mindel (MIS 12?, MIS 10) Mindel-Riss interglacial (MIS 9) Riss (MIS 8-6) Riss-Würm interglacial (MIS 5e) Würm (MIS 5d-2)
Historical nomenclature in Great Britain and Ireland Bramertonian Stage Baventian Stage/Pre-Pastonian Pastonian Stage Beestonian stage Cromerian Stage (MIS 21-13 ?) Anglian Stage (MIS 12, perhaps also MIS 10 ?) Hoxnian Stage (MIS 11, perhaps also MIS 9 ?) Wolstonian Stage (MIS 8–6, perhaps also MIS 10–9 ?) Ipswichian interglacial (MIS 5e) Devensian glaciation (MIS 5d-2) Flandrian interglacial (MIS 1)
Historical nomenclature in Northern Europe Pre-Tiglian Tiglian interglacial Eburonian Waalian interglacial Menapian glacial stage Bavelian Cromerian complex (MIS 21-13 ?) Elster glaciation (MIS 12) Holstein interglacial (MIS 11) Saale glaciation (MIS 10-6) Eem interglacial (MIS 5e) Weichsel glaciation (MIS 5d-2)
Historical nomenclature in North America Nebraskan glaciation (replaced by Pre-Illinoian in modern scientific literature) Aftonian interglacial (replaced by Pre-Illinoian in modern scientific literature) Kansan glaciation (replaced by Pre-Illinoian in modern scientific literature) Yarmouthian (stage) (replaced by Pre-Illinoian in modern scientific literature) Illinoian stage (MIS 6) Sangamonian (MIS 5e, sometimes also 5d-5a) Wisconsin glaciation (MIS 4-2, sometimes also 5d-5a)
Historical nomenclature in South America Caracoles (Río Frío) glaciation Río Llico (Colegual) glaciation Santa María (Casma) glaciation Valdivia interglacial (MIS 5e) Llanquihue glaciation (at least MIS 4-2)
Uncertain correlations It has proved difficult to correlate the traditional regional names with the global marine and ice core sequences. The indexes of MIS often identify several distinct glaciations that overlap in time with a single traditional regional glaciation. Some modern authors use the traditional regional glacial names to identify such a sequence of glaciations, whereas others replace the word "glaciation" with "complex" to refer to a continuous period of time that also includes warmer stages. As shown in the table below, it is only during the last 200,000-300,000 years that the time resolution of the traditional nomenclature allow for clear correspondence with MIS indexes. In particular there has been a lot of controversy regarding the glaciations MIS 10 and MIS 12, and their correspondence to the Elster and Mindel glaciations of Europe.
See also Brunhes–Matuyama reversal – Most recent geomagnetic reversal event (780,000 years ago) Geologic time scale – System that relates geologic strata to time Glacial history of Minnesota Glacial period – Geological era Ice age – Period of long-term reduction in temperature of Earth's surface and atmosphere Last Glacial Period – Period of major glaciations of the Northern Hemisphere (115,000–11,700 years ago) Thermal history of Earth Geologic temperature record – Very long term changes in Earth's temperature List of periods and events in climate history
Footnotes
References
External links The dictionary definition of glaciation at Wiktionary
Aber, J.S. (2006). "Regional Glaciation of Kansas and Nebraska". Emporia KS: Emporia State University. Work Group on Geospatial Analysis of Glaciated Environments (GAGE) (2000). "Pre-Wisconsin Glaciation of Central North America". Emporia KS: INQUA Commission on Glaciation, Emporia State University. Archived from the original on May 13, 2008. Subcommission on Quaternary Stratigraphy (2011). "Global correlation tables for the Quaternary". Cambridge UK: Department of Geography, University of Cambridge. Gibbard, P.L.; Boreham, S.; Cohen, K.M.; Moscariello, A. (2011). "Global chronostratigraphical correlation table for the last 2.7 million years v. 2011". Cambridge UK: Subcommission on Quaternary Stratigraphy, Department of Geography, University of Cambridge. Hambrey, M.J.; Harland, W.B., eds. (1981). Earth's pre-Pleistocene glacial record. Cambridge University Press. Archived from the original on October 11, 2006. 1004 + xv pp. (book downloadable as series of PDF files) Silva, P.G.; Zazo, C; Bardají, T.; Baena, J.; Lario, J.; Rosas, A.; Van der Made (2009). "Tabla Cronoestratigráfica del Cuaternario AEQUA V.2" (PDF). Departamento de Geología, Universidad de Salamanca, Spain: Asociación Española para el Estudio del Cuaternario (AEQUA). Archived from the original (PDF 3.6 Mb) on 2012-06-26. (Correlation Chart of European Quaternary and cultural stages and fossils)