Chapter 7.1 Glaciers

Types of Glaciers 15,000 years ago the Earth was 30% covered in ice. It was coming out of an ice age – a period of time when much of Earth’s land is covered in glaciers. A glacier is a thick ice mass that forms over hundreds or thousands of years. Today glaciers cover 10% of the Earth’s land area.

Glaciers form in areas where more snow fall than melts. The snowline is the lowest elevation in an area that remains covered by snow all year. Glaciers slowly move downhill. They are a large force of erosion.

Valley Glaciers Valley glaciers are ice masses that move slowly down valleys that were originally occupied by streams. It is a stream of ice that flows between steep rock walls from a place near the top of the mountain valley.

Ice Sheets Ice Sheets are enormous ice mass that flow in all directions from one or more centers and cover everything but the highest land. This can be called continental ice sheets because they cover large areas where the climate is extremely cold and they are very large compared to valley glaciers. There are 2 ice sheets – one covers 80% of Greenland. The other is the Antarctic Ice Sheet.

How Do Glaciers Move? Glaciers flow in 2 ways: plastic flow and basal slip. Plastic flow is movement within ice. This makes the ice distort and change shape (plasticity). This changing ice exerts pressure on the ice below and that causes it to flow. Basal slip is due to gravity and the entire ice mass slips and slides downhill along the ground. The upper part of a glacier is brittle and called the zone of fracture and this will crack during movement causing crevasses.

Rates of Glacial Movement Different glaciers move at different speeds. Some are extremely slow and some move several meters a day.

Budget of a Glacier Glaciers constantly gain and lose ice. A glacier has a zone of wastage – this is where the glacier loses ice and any new snow to melting. Glaciers can also lose ice when large pieces break off by calving. Calving creates icebergs. The foot of a glacier can advance, retreat, or remain in place. The glacial budget is the balance or lack of balance between accumulation at the upper end of a glacier and loss, or wastage, at the lower end.

Glacial Erosion Ice from the glacier scrapes, scours, and tears rock from valley floors and walls. Glaciers carry rock down hill that also tear up the valley floor.

How Glaciers Erode Glaciers erode in 2 ways: Plucking - when rock breaks up underneath the glacier and the melted water from the glacier goes in the cracks and the water refreezes and further cracks the rock. This will cause rock to break off and become part of the glacier. Abrasion – Glaciers smooth and polish the rocks underneath – the pulverized rock is called rock flour The amount of erosion is controlled by: 1. The rate of glacial movement 2.) thickness of the ice 3) shape, abundance, and hardness of rock fragments in the ice at the base of the glacier, and 4) the type of surface below the glacier.