How Glaciers Form and Move
Geologists define a glacier as any large mass of ice that moves slowly over land. There are two types of glaciers. The kinds of glaciers are continental glaciers, and valley glaciers.
A continental glacier is a glacier that covers much of a continent, or a large island. They can spread out over millions of square kilometers. Today, continental glaciers cover about 10% of Earth's land. They cover Antarctica and most of Greenland. In places, the glacier covering Antarctica is over 3 kilometers thick. Continental glaciers can flow in all directions as they move across the land. Continental glaciers spread out much like pancake batter in a frying pan does.
Many times in the past, continental glaciers have covered more, or larger, parts of Earth's surface. These times are known as ice ages. About 2.5 million years ago, continental glaciers covered about 1/3 of Earth's land. The glaciers advanced and retreated, or melted back, several times. They finally retreated about 10,000 years ago.
A valley glacier is a long, narrow glacier that forms when snow and ice build up in a mountain valley. The sides of mountains keep these glaciers from spreading out in all directions. Instead, they usually move down valleys that have already been cut by rivers. Valley glaciers are found on many high mountains. Although they are much smaller than continental glaciers, valley glaciers can be tens of kilometers long.
High in mountain valleys, temperatures seldom rise above freezing. Snow builds up year after year. As the weight of the snow rises, the snow on the bottom gets compacted into ice. Glaciers can only form in an area where more snow falls than melts. Once the snow and ice reaches a depth more than 30 to 40 meters, gravity will start to pull the glacier downhill.
Valley glaciers flow at a rate of a few centimeters up to a few meters per day. Although, sometimes a valley glacier can slide down more quickly in a process called a surge. A surging glacier can move up to as much as 6 kilometers per year.
How Glaciers Shape the Land
When a glacier moves, it changes the land beneath it. Glaciers are slow, but they are a major force of erosion. The two processes by which glaciers erode the land are plucking and abrasion.
As a glacier flows over the land, it picks up rocks in a process called plucking. Beneath a glacier, the weight of the ice can break rocks apart. These pieces of rock now freeze to the bottom of the glacier. As the glacier moves, it carries the rocks with it. This process can move even huge boulders.
Many rocks remain on the bottom of the glacier, and the glacier drags them across the land. This process is called abrasion. Abrasion is the process that gouges and scratches the bedrock.
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, which creates a variety of landforms. These 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. Till is made up from particles of many different sizes. Some of the materials found in till are clay, silt, sand, gravel, and boulders.
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. Long Island in New York is a terminal moraine from the continental glaciers of the last ice age.
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. This type of lake is common in areas, such as Minnesota, that were covered in ice.