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BY GEORGE: Ice ages
Submitted by George Elliott on Mon, 09/17/2012 - 7:05am.
Within a long-term ice age, individual pulses of cold climate are termed “glacial periods,” or alternatively "glacials" or "glaciations" (or colloquially as "ice age"), and intermittent warm periods are called “interglacials.” Glaciaologically, ice age implies the presence of extensive ice sheets in the northern and southern hemispheres. By this definition, we are still in the ice age that began 2.6 million years ago at the start of the Pleistoncene epoch, because the Greenland and Antarctic ice sheets still exist.
At some periods in history the continents were flat and mostly covered by warm shallow seas, the climate was warm and there was no ice anywhere on Earth. But every 250 million years or so, an ice age begins.
During an ice age, ice forms on the mountains and flows down in frozen rivers called glaciers. They wear away the mountains, grinding out valleys and carrying bits of rock down to the lower land. There the ice spreads out in sheets. Some of it reaches the sea and breaks off as icebergs. Remember that water expands when it freezes into ice. That is why icebergs float. Good thing. If ice sank, than all the oceans would freeze solid and life would be impossible.
Sometimes the ice is thick and covers a lot of land. This is during a glacial period, and it lasts about ten thousand years. At other times most of the ice melts, covering only the tops of mountains and the poles. This is an inter-glacial period, and it lasts about twenty thousand years or more. We are in one of these now. Each ice age is made up of several glacial and interglacials.
A glacial period (or glaciation) is a time during an ice age when the ice gets thicker and glaciers advance down mountains. They typically last tens of thousands of years. The last glacial advance period ended about 15 thousand years ago.
The changes from glacial to interglacial and back again are probably caused by changes in the way the Earth moves round the Sun. This changes the amount of sunlight reaching the Earth. They are often shorter than glacials, lasting only for some thousands of years.
After a few million years the ice age ends, most of the ice melts, and the weather goes back to a warmer state, as we all know we’re definitely in now.
By: George Elliott