Glaciation

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Glaciation is an extended period of time during which glaciers are present and active. It also refers to all the processes that form glaciers and that are at work within a glacier. A glacier is a land-based mass of highly compacted ice that moves downward and outward under its own weight due to gravity. Glaciers may be large enough to cover a continent or small enough to fill a mountain valley and periods of glaciation can last hundreds, thousands, or millions of years.

A glacier is formed by a series of processes that begins with accumulation. Accumulation occurs when the buildup of snow and ice through snowfall, avalanching, or wind transport during cold months greatly exceeds the loss through melting or sublimation (the direct conversion of a solid to a gas) in warmer months. As snow accumulates and deepens, its weight causes increased pressure that converts snowflakes first into granular snow and eventually into dense ice granules. Continual and sustained accumulation, compaction, melting, and refreezing eventually create a very dense mass of inter-locking ice with about 10% void space. As a comparison, freshly fallen snow contains about 80% void space. The formation of new glacial ice takes several decades.

A mass of accumulated snow and ice is not strictly considered a glacier until it begins to move downhill. Glacial ice will begin to move when it becomes too thick and heavy to hold its position against gravity. The instant that glacial ice will begin to move depends on the steepness of the ground, the ice thickness, and the ice temperature. In mountainous regions a glacier will start to creep when it reaches a thickness of 65.5–131 ft (20–40 m). Glaciers usually move very slowly, less than one meter a day, but can move up to 164 ft (50 m) a day.

Although a glacier is constantly creeping downhill, the front edge of it may appear to remain in the same place, or even retreat uphill. This is because at the same time a glacier is moving, it may be growing or shrinking. A glacier fluctuates depending on the rate of the accumulation of new ice versus the rate of ablation, or the loss of ice due to melting, sublimation, and wind erosion. A glacier will appear to advance if accumulation exceeds ablation and it will recede if ablation is greater than accumulation. If the two are equal, the glacier will appear to remain stationary. When a glacier advances far enough to reach a body of water, large chunks may break off and fall in into the water. This is called calving and is the source of icebergs.

Throughout geologic time, Earth has experienced major periods of glaciation when glaciers covered large portions of Earth's surface for up to many millions of years. In the last 500,000 years, four major periods of glaciation have occurred. During these times, ice sheets several kilometers thick covered as much as 30% of the global land surface. This type of glaciation, which extends over vast areas of lowlands and mountains, is known as continental glaciation. The most recent major glaciation ended about 10,000 years ago.

Today, continental glaciation occurs only at the polar regions, mostly in Greenland and Antarctica. Active glaciation in other parts of the world exists in mountainous regions at high altitudes and is called alpine glaciation. The causes of glaciation are not completely understood, but major periods of glaciation are attributed to decreases in the amount of sunlight the earth receives due to very long-term cyclic variations in Earth's rotation and angle of orbit.

Glacial Landforms