Ice Age:

When glaciers covered the Earth

Manny, a depressed woolly mammoth, Sid, a fast-talking sloth, an acorn-crazed squirrel named Scrat and the devilish sabre-toothed tiger named Diego; all these characters would remind you in a trice(instantly) of the animated movie, Ice Age, which shows a period where Earth is dominated by sub-zero temperatures.

This movie could be enjoyed by children like you as well as adults, but did you know that such a phenomenon (remarkable happening) is really associated with Earth?

An ice age is defined as a period of long-term reduction in the temperature of the Earth's climate, which results in an expansion of the continental ice sheets, polar ice sheets and mountain glaciers. Glaciologically, ice age is often used to mean a period of ice sheets in the northern and southern hemispheres.

If you go by this definition, does it mean that we are still in an ice age because the Greenland and Antarctic ice sheets still exist? To get rid of this ambiguity (uncertainty), ice age is used to refer to colder periods with extensive ice sheets over the North American and Eurasian continents, when the last few million years are considered.

In this sense, the most recent ice age ended about 10,000 years ago.

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Major ice ages

It is said that there have been at least four major ice ages in the Earth's past. Interestingly, outside of these periods, the Earth is said to have been free of any ice even in high latitudes, although now we find quite a lot of ice when the entire globe is considered.

The earliest ice age, called the Huronian, is believed to have existed around 2.7 to 2.3 billion years ago during the early Proterozoic Eon.

The earliest well-documented ice age, and probably the most severe ice age of the last billion years, is said to have occurred from 850 to 630 million years ago in the Cryogenian period and may have made Earth look like a 'Snowball Earth' in which permanent ice covered the entire globe. This is said to have ended very rapidly as water vapour returned to Earth's atmosphere.

A minor ice age known as the Andean-Saharan, has occurred around 460 to 430 million years ago, during the Late Ordovician and the Silurian period. There have been extensive polar ice caps at intervals from 350 to 260 million years ago, during the Carboniferous and early Permian Periods, associated with the Karoo Ice Age.

The present ice age began 40 million years ago with the growth of an ice sheet in Antarctica. It had increased during the late Pliocene, around three million years ago, with the spread of ice sheets in the Northern Hemisphere.

Since then, the world has seen cycles of glaciation with ice sheets advancing and retreating on 40,000 and 100,000 year time scales. The most recent glacial period ended about ten thousand years ago.

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Evidence of ice ages

There are three main types of evidence for ice ages - geological, chemical and paleontological.

Geological evidence for ice ages comes in various forms, including rock rubbing and scratching, glacial moraines (mass of stones), drumlins, valley cutting, and the deposition of till and glacial erratics. 'Till' refers to various rocks mixed with ice when glaciers are created, and 'glacial erratics' are rocks which reside in an area which deviate (differ) a lot in size and type of the surrounding rocks.

When glaciers are created, they gradually erase the geological evidence, making it difficult to understand the origin. It has taken some time for the current theory to be worked out because of this.

The chemical evidence mainly consists of variations in the ratios of isotopes in fossils present in sediments and sedimentary rocks, ocean sediment cores, and for the most recent glacial periods, ice cores.

Because water containing heavier isotopes has a higher heat of evaporation, its proportion decreases with colder conditions. This allows a temperature record to be constructed. This is studied to get evidence about ice ages.

For your information, sedimentary rocks are one kind of the three main rock types on Earth, and it contains common rock types such as chalk, limestone, dolomite and sandstone. Furthermore, 'isotope' is a term which appears in Chemistry, which in simple terms, means a form of the several different forms of an element where each has a different atomic mass.

The paleontological evidence or, in other words, evidence gained by studying prehistoric life forms on Earth through analysing fossils, consists of changes in the geographical distribution of fossils. During a glacial period, cold-adapted organisms spread into lower regions, and organisms that prefer warmer conditions become extinct or are pushed off into lower latitudes.

This simple observation can reveal when there have been ice ages. However, this evidence is also difficult to interpret due to a few reasons. Firstly, it requires sequences of rocks which cover a long time-span and a wide range of elevations which easily correlate (connect).

Apart from that, ancient organisms which survive for several million years without change and whose temperature preferences are easily diagnosed have to be identified. Finally, finding the relevant fossils requires a lot of luck.

Compiled by Aravinda Dassanayake