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A glaciation (a created composite term meaning Glacial Period, referring to the Period or Era of, as well as the process of High Glacial Activity), often called an ice age, is a geological phenomenon in which massive ice sheets form in the Arctic and Antarctic and advance toward the equator. Conversely, the term interglacial or Interglacial Period, such as the current era, is used to denote the absence of large-scale glaciation on a global scale — i.e., a non-Ice Age. Interglacials are, in general, shorter than glacial epochs.

Contents 1 What glaciation means 2 Named glacial periods 3 Newer means of detecting glaciations 4 The term, 'ice age' is used in two ways 5 See also

Glaciations are characterized by cool, wet climates and thick ice sheets extending from each pole. Mountain or alpine glaciers in otherwise unglaciated areas expand and extend to lower elevations even in the lowest of latitudes. Sea levels drop due to the presence of large volumes of water above sea level in the icecaps. There is evidence that ocean circulation patterns are disrupted by glaciations. Since the earth has significant continental glaciation in the Arctic and Antarctic, we are currently in a glacial minimum of a glaciation. Such a period between glacial maxima is known as an "interglacial". The current one is the Flandrian.

In general, the Earth seems to have been ice-free even in high latitudes except during relatively rare glacial maximums such as the one from which we emerged 10 to 15 thousand years ago.

The causes of glaciations have been much debated ever since the phenomenon was clearly identified in the 17th century. Modern theories tend to revolve around periodic oscillations in the Earth's orbit, hypothesized periodic changes in solar output, changes in atmospheric circulation due to mountain orogenies, and/or the effects of continental masses drifting into polar regions where Antarctica currently resides.

Known periods of glaciation include the Huronian (2400 Ma - 2100 Ma), Cryogenian (formerly known as Sturtian-Varangian) (850 Ma - 635 Ma), Andean-Saharan (450 Ma - 420 Ma), Karoo (360 Ma - 260 Ma), Cenozoic (30 Ma - Present). These can be further divided by location and time (e.g.: the names Riss (180,000 - 130,000 years bp) and Würm (70,000 - 10,000 years bp) refer specifically to glaciation at the headwaters of the Danube/Rhine rivers.) Not every year in each interval was a time of complete or even partial glaciation. The best-studied glaciation, that of the recent past, appears to have taken place in at least four separate ice incursions and retreats. Unfortunately, the scouring action of each glaciation tends to remove most of the evidence of prior ice sheets almost completely, except in regions where the later sheet doesn't achieve full coverage. It is probable that glacial periods other than those above have been overlooked because of the paucity of exposed rocks from high latitudes from older periods. The Varanger glaciation was especially severe and may have extended to the Equator. This has led to a recent "Snowball Earth" hypothesis - that the Earth froze over completely in the late Proterozoic, then thawed very rapidly as trapped water and carbon dioxide were returned to Earth's atmosphere. An alternative hypothesis, sometimes called Slushball Earth, maintains that the Equator at least was ice-free.

The problem of the removal of evidence of earlier glacials by the later ones has been overcome to some extent by drilling cores of ice and of benthic mud. The ice of an ice cap contains bubbles of the atmosphere of the time when it was trapped. In so far as a high proportion of carbon dioxide represents an interglacial, it is possible to analyze the gas in the bubbles and see when the warmer periods were.

On the other hand, ice is solid water. A water molecule includes an oxygen atom but some isotopes of oxygen are more readily incorporated into ice crystals than others. Thus, when these are locked up in ice caps, they are rarer in the remaining waters of the world. This is detectable in the layers of mud laid down each year in the ocean depths. Therefore, oxygen isotope variation in the mud varies with time and with the extent of the process of glaciation. As far as they go, the three sources of information, the traditional, the carbon dioxide proportion and the oxygen isotope tend to corroborate each other though the newer ones appear to show more detailed fluctuation in glaciation.

The term ice age is used to refer either to a single glaciation or to an entire period of repeated glaciations such as the recent 30 million years of the Cenozoic era, especially the Pleistocene glaciations.

• Glacial history of Minnesota
• Timeline of glaciation This includes a graph drawn from carbon dioxide concentrations in ice cores. Its oxygen isotope graph has a much longer time scale.