Chapter 17 Vocabulary

Chapter 17 Vocabulary

A glacier is a large mass of moving ice. There are two types of glaciers: alpine, which are confined to a small area by surrounding topography, and continental, which cover millions of square miles, such as Greenland and Antarctica. The amount of snowfall (mostly at higher elevations) must be greater than the amount of melting (mostly at lower elevations) or the glacier will disappear. Due to global warming there is an alarming trend of glaciers rapidly disappearing. The area where snow remains on the ground all year is called a snowfield. The elevation above which snow and ice never melt is called the snowline. Cycles of partially melting and refreezing produce a grainy ice called firn.

A glacier is in constant motion. Snow is accumulating at higher elevations, and melting at lower elevations. Where the glacier touches the land surface it partially melts and mixes with the bedrock and soil. This lubricates the surface and allows the glacier to slide downslope. This process is called basal slip. The pressure from overlying layers of ice and snow allow the glacier to flow slowly and is called internal plastic flow. The surface of the glacier remains brittle and results in deep cracks called crevasses. Sometimes parts of glaciers move off of landmasses and huge chunks of ice break off forming icebergs. Icebergs are hazards for ships because most of the iceberg is underwater.

Glaciers create landforms both by erosion and deposition. Landforms created by erosion include cirques, arêtes, and horns. A cirque is a rounded bowl-shaped depression where the glacier starts. A jagged mountain edge between two cirques is an arête. Where several arêtes come together they form a pyramid-shaped mountain called a horn. While rivers cut narrow V-shaped valleys, a glacier flattens out the valley floor and removes the lower part of the valley walls, making them steeper, and giving the valley a U-shape. Glaciers did not make it this far south, but U-shaped glacial valleys are easily seen in New England. Where smaller tributary valleys enter the main valley from the sides, and at higher elevations due to ice in the major valley, they form hanging valleys. The tremendous weight of the glacier polishes and cuts grooves in the bedrock as it passes over it. Sometimes it rounds a rock formation into the same shape wind forms a sand dune – with the side the glacier came from having a less steep slope. This formation is called a roches moutonn´ees. Glaciers can also mold the eroded material on the surface into linear, tear-shaped ridges of till called drumlins.

Glaciers also create landforms by depositing material. Large boulders can be carried hundreds of miles by the ice sheet and deposited far away from the original mountain they came from. These boulders are called erratics. A glacier deposits material in two ways: the ice sheet can erode and deposit material, and the meltwaters of the glacier can deposit material. When the ice sheet deposits material, it is an unsorted (big and little stuff all mixed up) deposit called till. A ridge of till left when the glacier remains at one location for a while is called a moraine. When the meltwaters of a glacier deposit sediment, you get stratified drift; meaning the large material was deposited first, then the medium-sized material, and finally the small material. Formations of stratified deposits are called outwash plains. Sometimes a large chunk of ice is buried, then melts, forming a depression called a kettle. When the glacier retreats from an area, it leaves behind the deposit from the meltwaters. This ridge of material is called an esker.

Glaciers also carve out huge depressions that form lakes. The Great Lakes were produced by a massive continental glacier. The finger lakes in upstate New York were formed by glaciers. Sometimes a cirque or kettle will fill with the glaciers meltwaters and become a lake.

A long period of climatic cooling is called an ice age. The last ice age was about four million years ago. The warmer periods between ice ages are called interglacial periods. One theory that states the cause of ice ages is changes in Earth’s orbit and axis of rotation is the Milankovitch theory.