Chapter 8 Erosional Forces Section 8-2 Glaciers Note Guide How Glaciers Move and Form Snow remains on the ground year-round in some areas of the world. As snow piles up, the weight increases and it compresses the lower layers into ice. The pressure makes the ice plastic like, so the mass begins to flow and move away from its source. The pressure makes the ice plastic like, so the mass begins to flow and move away from its source. A large mass of ice and snow moving on land under its own weight is a glacier. An Advancing Glacier! A glacier that has retreated! Ice Eroding Rock Glaciers are agents of Erosion. As glaciers move along the surface of the Earth, they pick up eroded material and deposit it somewhere else. When glacial ice melts, water flows into the cracks of rocks. When the water freezes, it expands and fractures the rock. The pieces of rock are lifted out by the ice. This process is called plucking. Ice Eroding Rock Plucking results in boulders, gravel, and sand being added to the bottom of glaciers. Plucked rock fragments and sand at the bottom of a glacier scrape and scour the soil and bedrock on Earths surface and erode the ground. Deep, long, parallel scars called grooves are left behind by rock fragments. Shallower marks on the bedrock are called striations Both of these indicate the direction the glacier moved. Plucking! Striations Ice Depositing Sediment When glaciers begin to melt, they deposit sediment on land. When a glacier melts and shrinks back, it is said to retreat. As it retreats, it leaves a jumble of boulders, sand, silt and clay behind. This is called glacial till. Ice Depositing Sediment Till is also deposited at the end of a glacier when it is not moving. Rocks and soil are moved to the end of the glacier and piles up in a big ridge. This ridge is called a moraine. Moraines are also deposited along the sides of a glacier. Moraine Till Outwash and Till Ice Depositing Sediment In a melting glacier, meltwater forms a stream within the ice. This river carries sand and gravel and deposits them within its channel. When the glacier melts, a winding ridge of sand and gravel is left behind. This is called an esker. Esker! 2 Types of Glaciers Continental glaciers cover 10% of the Earth, mostly near Antarctica and Greenland. The continental glaciers are huge masses of Ice and snow, and are thicker than some Mountain ranges. In the past, continental glaciers covered as much as 28 % of Earth. The periods of widespread glaciations are known as Ice ages. 2 Types of Glaciers Valley glaciers occur today in high mountains where the average temperature is low enough to prevent snow from melting in the summer Evidence that valley glaciers existed in the mountains includes striations or evidence of plucking from the top of a mountain where the glacier was in contact with solid rock. Valley glaciers erode curved, bowl- shaped basins, called cirques into the sides of mountains. 2 Types of Glaciers If two valley glaciers side by side erode a mountain, then a long ridge called an Arte forms between them. If valley glaciers erode a mountain from several directions, a horn might form. 2 Types of Glaciers To tell the difference between valleys that have been eroded by glaciers or those that have been eroded by streams: Valleys that have been eroded by glaciers are U -shaped. Valleys that have been eroded by streams are V - shaped. Glaciers are important because they change the Earths surface and they have left behind sediments that are important resources. Valley Glacier Valley Glacier Erosion Glacier Coverage!