One of the starngest features of thw world that we inhabit is ice. No other subtance has a solid form which is less dense than the liquid form. To put it another way, ice floats. So, when the temperature drops and the sea begins to freeze, the ice is on top of the ocean , not underneath it. That is good news for life, since the ice creates an insulating layer which prevents the rest of the sea from freezing. In past ice ages, notably one about 700 million years ago, when sea ice stretched to the equator, lofe would have died out had the oceans frozen solid. the effects of ice on land are different. As water expands to become ice, it can shatter a rocky landscape, and a large ice sheet can even push down a whole continent.
Waiting for an ice age ?
Throughout the Earth's history, the temperature has fluctuated many times, perhaps due to changes in the Sun's activity or continental drift. The polar caps that exist today activity or continental of the last ice age, which began 3.25 million years ago. Since then, the northern ice sheet has advanced four times to cover large areas of North America and northen Europe. We may still be in an interglacial period today, waiting for the ice to advance once more.


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