Glaciers

A glacier is a thick ice mass that forms over hundreds or thousands of years. It originates on land from the accumulation, compaction, and recrystallization of snow.

A glacier appears to be motionless, but glaciers move very slowly. Glaciers are dynamic weathering and erosional agents that accumulate, transport, and deposit sediment. http://www.youtube.com/watch?v=njTjfJcAsBg http://www.youtube.com/watch?v=g4kha1RY-LM&NR=1&feature=fvwp http://www.youtube.com/watch?v=89sOW-FzolI

Types of Glaciers Valley or alpine glaciers Commonly originating from mountain glaciers or ice fields, these glaciers spill down valleys 2. Piedmont glaciers Piedmont glaciers occur when steep valley glaciers spill into relatively flat plains, where they spread out into bulb-like lobes.

3. Ice sheets Found only in Antarctica and Greenland, ice sheets are enormous continental masses of glacial ice and snow expanding over 50,000 square kilometers. 4. Ice caps Ice caps are miniature ice sheets, covering less than 50,000 square kilometers. They form primarily in polar and sub-polar regions that are relatively flat and high in elevation.

Parts of a Glacier Glaciers form in areas where more snow falls in winter than can melt during the summer. Glaciers are constantly gaining and losing ice. Snow accumulation and ice formation occur in the zone of accumulation. Here the addition of snow thickens the glacier and promotes movement.

Beyond the zone of accumulation, is the zone of wastage (or zone of ablation). Here there is a net loss to the glacier as all of the snow from the previous winter melts, as does some of the glacial ice. Mass of the glacier can also be lost through sublimation – the process of a solid turning directly into a gas.

In addition to melting, glaciers also waste as large pieces of ice break off the front of a glacier in a process called calving. Where glaciers reach the sea, calving creates icebergs.

Glacial Erosion Glaciers erode tremendous volumes of rock primarily in two ways: 1. As a glacier flows over a fractured bedrock surface, it loosens and lifts blocks of rock, incorporates them into the ice, and carries them off. This process, known as plucking, occurs when meltwater penetrates the cracks and joints along the rock floor of the glacier and refreezes. fl As the water expands, it exerts tremendous leverage that pries the rock loose. In this manner, sediment of all sizes becomes part of the glacier’s load.

2. The second major erosional process happens due to abrasion 2. The second major erosional process happens due to abrasion. As the ice with its load of rock fragments moves along, it acts as a giant rasp or file and grinds the surface below as well as the rocks within the ice. The pulverized rock produced by the glacier is called rock flour. This sediment is then carried away by the meltwater. Long scratches and grooves called glacial striations may be gouged into the bedrock passed over by the glacier.

Features of Valley Glaciers As a glacier moves down a valley once occupied by a stream, the ice modifies the valley by widening, deepening, and straightening it, so that what was once a narrow V-shaped valley is transformed into a U-shaped glacial trough.

Yosemite National Park, California After the ice of the glacier has receded, the valleys of tributary glaciers are left standing above the main trough and are termed hanging valleys. Rivers flowing through hanging valleys may produce spectacular waterfalls. This occurs because the ice in the tributary glaciers is not as thick as the ice in the main glacier in the valley, so the tributary glaciers do not cut the valley as deeply as the glacial trough. Yosemite National Park, California

A cirque is a bowl-shaped depression with very steep sides that forms at the head of a mountain glacier. Cirques form from cold-climate weathering processes including frost wedging and plucking.

After the glacier has melted, the cirque basin in often occupied by a small lake called a tarn. Glacier National Park

The enlargement of cirques by plucking and frost action can produce other features such as arêtes, horns, and fjords. An arête is a thin, almost knife-like, ridge of rock which is typically formed when two glaciers erode parallel U-shaped valleys.

Matterhorn, Switzerland A horn is a sharp, pyramid-like peak formed when cirques, that are adjacent to one another, carve back into the headwall of the mountainside. Matterhorn, Switzerland Glacier National Park Grand Tetons, Wyoming

Formation of glacial horns, arêtes, and cirques Notice how multiple glaciers must be in an area in order for a horn or arête to form.

A fjord is a long, narrow inlet to a sea with steep sides or cliffs, created in a valley carved by glacial activity. Tracy Arm Fjord with Sawyer glacier in Alaska. Notice the calving of icebergs from the glacier.

Glacial Deposits Glaciers pick up and transport a huge load of debris as they slowly advance across the land. Ultimately, these materials are deposited when the ice melts. In regions where glacial sediment is deposited, it can play a truly significant role in forming the physical landscape. Drift is the term used for all sediments of glacial origin.

There are two types of glacial drift: Materials deposited directly by the glacier, which are known as till. Till is deposited as glacial ice melts and drops its load of rock fragments. Unlike moving water and wind, ice cannot sort the sediment it carries; therefore, deposits of till are characteristically unsorted mixtures of many particle sizes. Sediments deposited by glacial meltwater are called stratified drift. Stratified drift is sorted according to size and weight of the fragments by the water. Stratified drift is usually sand and gravel because the water cannot carry larger rocks.

Layers or ridges of drift are called moraines. The sides of a valley glacier accumulate large quantities of debris from the valley walls. When the glacier wastes away, these materials are left as ridges, called lateral moraines, along the side of the valley. The end moraine forms at the end of the glacier where large quantities of rock debris create a ridge of till. If the glacier in recessing, the end moraine is called the recessional moraine. The farthest advance of the glacier is called the terminal moraine.

A medial moraine forms when two valley glaciers join to form a single ice stream. Till that was once carried along the edges of each glacier joins to form a single stripe of debris within the newly enlarged glacier. As the glacier recedes, a layer of till is laid down, forming a gently undulating surface of ground moraine. The ground moraine lies between the terminal and end moraine.

As the end moraines are forming, meltwater emerges from the ice in rapidly moving streams. Often they carry a substantial bed load. As the water leaves the glacier, it rapidly loses velocity and much of the bed load is dropped. A broad, ramp-like surface of stratified drift is built adjacent to the downstream edge of most end moraines. When the feature is formed with an ice sheet, it is called an outwash plain, when it is formed with a mountain glacier, it is called a valley train.

When kettles fill with water, they become kettle lakes. Often end moraines, outwash plains, and valley trains are pockmarked with basins or depressions known as kettles. Kettles form when blocks of stagnant or dead ice become buried in drift and melt, leaving pits in the glacial sediment. When kettles fill with water, they become kettle lakes. Walden Pond in Massachusetts is a kettle lake.

In the ground moraine, drumlins are long, asymmetrical hills, eskers are deposits made by streams flowing in tunnels beneath the ice near the terminus of a glacier, and kames are steep sided hills formed in depressions near the terminus of a glacier.

Drumlin Esker Kame

Mendenhall Glacier Juneau, Alaska Mendenhall Glacier is a glacier about 12 miles (19 km) long located in Mendenhall Valley, about 12 miles (19 km) from downtown Juneau in the southeast area of the U.S. state of Alaska.

Sawyer Glacier in Tracy Arm Fjord Alaska Tracy Arm is a fjord in Alaska that is located about 45 miles (72 km) south of Juneau. During the summer, the fjord has considerable floating ice from the Sawyer glacier ranging from the size of a three-story building to hand-size pieces. During the most recent glaciated period, this fjord filled with active glaciers.