Chapter 3 Erosion and Deposition Section 4 Glaciers

Kinds of Glaciers Geologists define a glacier as any large mass of ice that moves slowly over land. There are two kinds of glaciers – valley glaciers and continental glaciers.

Kinds of 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 mountain keep these glaciers from spreading out in all directions.

Kinds of Glaciers A continental glacier is a glacier that covers much of a continent or large island. They are MUCH larger than valley glaciers because they can spread out and currently cover about 10% of Earth’s land (mainly in Antarctica and Greenland)

Ice Ages Ice ages have happened many times in the past, when continental glaciers have covered large parts of Earth’s surface. The most recent ice age ended only 10,000 years ago (it lasted for almost 9 million years).

How Glaciers Form and Move 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 (about 100 to 120 feet), gravity begins to pull the glacier downhill.

How Glaciers Form and Move Valley glaciers flow at a rate of a few centimeters to a few meters a day. A surging glacier can flow as much as 6 km a year. Continental glaciers flow in all directions and spread out like pancake batter in a pan.

Glacial Erosion The glacier’s weight will cause rocks to break apart, then the rock fragments will freeze to the bottom of the glacier and be carried away when it moves. This process is called plucking. Many rocks remain of the bottom of the glacier and the glacier drags them across the land. This process, called abrasion, gouges and scratches the bedrock.

Glacial Erosion

Glacial Deposition When a glacier melts, it deposits the sediment it eroded from the land, creating various landforms. The mixture of sediments that a glacier deposits directly on the surface is called till. The till deposited at the edges of a glacier forms a ridge called a moraine.

Glacial Deposition Another feature left in glacial sediments are prairie potholes, which are shallow depressions in till that were formed by flowing water as the glacier melted. Prairie potholes are currently only filled with water part of the year, but act as a watering hole, nesting ground, and sanctuary for many species.

Glacial Deposition

Glacial Deposition Retreating (or melting) glaciers also create features called kettles, which are small depressions that form when a chunk of ice is left in glacial till. If the ice melts and fills the kettle with water, we get a kettle lake.

Glacial Deposition As the ice of the last ice age moved over Michigan, it scooped out loose sediment and soft rock, forming broad, deep basins. The Great Lakes formed when these basins filled with water.