GLACIERS
- HOW GLACIERS FORM AND MOVE: Geologists define a glacier as any large mass of ice that moves slowly over land. There are two kinds of glaciers- continental glaciers and valley glaciers.
- CONTINENTAL GLACIERS: A continental glacier is a glacier that cover much of a continent or a large island. They spread out over millions of square kilometers. Continental glaciers cover 10% of earth's land. In places, the glacier is over 3 kilometers thick.
- Many times in the past, continental glaciers have covered larger parts of Earth's surface. Theses times are called the ice age. About 2.5 million years ago continental glaciers covered about 1/3 of Earth's land.
- VALLEY GLACIERS: A valley glacier is a long, narrow glacier that forms when snow and ice build up high in a mountain valley. The sides of the mountains keep the glaciers from spreading out in all directions.They usually move down valleys that have already been cut by rivers. The glaciers are found in many high mountains. Valley glaciers can be up to 10's of kilometers long.
- The weight of more and more snow compacting together or building up creates the bottom to turn into ice. Glaciers can form only in an area where more snow falls than melts. Once the depth of snow and ice reaches more than 30 to 40 meters, gravity begins to pill the glacier downhill.
- Valley glaciers flow at a rate of a few centimeters to a few meters per day. Sometimes a valley glacier slides down more quickly in what is called a surge. A surging glacier can flow as much as 6 kilometers a year.
- HOW GLACIERS SHAPE THE LAND: The movement of a glacier changes the land beneath it. Although glaciers work slowly, they are major force of erosion. The two processes by which glaciers erode the land are plucking and abrasion.
- GLACIAL EROSION: As glacier flows over land, it picks up rocks in a process called plucking. Beneath a glacier, the weight of the ice can break rocks apart. These rock fragments freeze to the bottom of the glacier. When the glacier moves it carries the rocks with it. Plucking can even move huge boulders.
- Many rocks remain on the bottom of the glacier, and the glacier drags them across the land. This process is called abrasion, gouges, and scratches the bedrock.
- GLACIAL DEPOSITION: A glacier gathers a huge amount of rock and soil as it erodes the land in its path. When a glacier melts, it deposits the sediment it eroded from the land, creating various landforms. Theses landforms remain for thousands of years after the glacier has melted. The mixture of sediments that a glacier deposits directly on the surface is called till. This is made of particles of many different sizes. Clay, silt, sand, gravel, and boulders can all be found in till.
- The till deposited at the edges of a glacier forms a ridge called a moraine. A terminal moraine is the ridge of till at the farthest point reached by a glacier.
- Retreating glaciers also create features called kettles. A kettle is a small depression that forms when a chunk of ice is left in glacial till. When the ice melts, the kettle remains. The continental glacier of the last ice age left behind many kettles. Kettles often fill with water, forming small ponds or lakes called kettle lakes. Such lakes are common in areas, such as Minnesota, that were covered with ice.