Prairie School Earth Science Chapter 17: Glaciers Prairie School Earth Science

Glaciers: Moving Ice When you take a handful of snow and squeeze it, you are creating a product that is similar to that of glaciers. Glacier: fast moving, large mass of ice. Snowfield: Where snow starts to accumulate on the snowline. Snowline: the elevation where snow remains throughout the year. Places where the temperature is below freezing of water.

Glaciers As the temperature rises, there are periods of melting and recrystallization. This makes the snow grainy ice (firn) Within the deep layers of the snow and firn, the pressure from the layers on top start to flatten the ice and push the air out from between the grains. This leads to a glacier that starts to move when this buildup of firn and snow starts to move because of gravity and its own weight.

Two Types of Glaciers Alpine Glacier: a narrow, wedged shaped mass of ice that forms in a mountainous region and is confined to a small area because of surrounding topography. Ex: Himalaya mountains, Andes, Alps Continental Glacier: massive sheets of ice that may cover millions of square kilometers, and may be thousands of meters thick. They are not confined by surrounding topography. Also called “ice sheets,” exist only in Greenland and Antarctica. When these melt, could cause and increase in sea level.

Movement of Glaciers Sometimes called “rivers of ice. ”However, glaciers move very differently than rivers. Few centimeters to 1km per year Large crevasses (cracks up to 50 meters deep!) form from unequal ice movement Basal Slip: Glaciers “slip” over a thin layer of water and sediment, which is between the ice and the ground Increase in the pressure from weight of ice decreases the melting point of ice. Ice melts where the glacier touches the ground. Water mixes with sediment and creates a slippery surface, leading to basal slip Internal Plastic Flow: Pressure deforms grains of ice under the glacier. As the grains deform, they slide over each other and cause the glacier to flow slowly. The interior of a glacier moves this way, unlike basal slip where the entire glacier moves. Edges of a glacier moves move slowly than the center because of an increase in friction.

Glacial Erosion There are many different landforms created from glacial erosion and deposition. Glacial Erosion: Like rivers, glaciers are agents of erosion. Rocks carried by the glacier have a smooth appearance to them As the glaciers move through the narrow, V- shaped valley, erosion takes place and the walls get steeper. This also pulls rocks from the floor of the glacier. Cirque: bowl shaped depression Arête: a sharp jagged ridge When several arêtes join together, they form a pyramid peak called a horn.

Glacial Erosion U shaped Valleys: the original V shape eventually forms a U shape after glacial erosion. Geologists can look at an area and decide if there was once a glacier, based on the U shaped valleys. Erosion by Continental Glaciers: leveled landforms, producing smooth and rounded landscape. This is similar to the way that bulldozers level landscape. Scratches on the rock surface from glacier movement may also be seen.

Glacial Deposition Deposition happens when a glacier starts to melt (reaching lower elevation or increase in temperature) As the glacier melts, starts to deposit all the material it was carrying. General term for glacial deposits: glacial drift Large rocks: erratics Unsorted grouping of glacial drift from a melting glacier is called a till. Sorted grouping of sediments is called a stratified drift-deposited in layers by streams or melt water from glacier.

Glacial Deposition Landforms that results when a glacier deposits till are called moraines. Lateral, Medial and terminal moraines The unsorted material left behind beneath a glacier is called ground moraine. Outwash plain: a deposit of stratified drift that lies in front of a terminal moraine and is crossed with melt water streams. Kettles: depressions in outwash plains, fill with water, form kettle lakes Esker: A long, winding ridge of gravel and coarse sand deposited by glacial streams.

Glacial Lakes Lake basins can form where glaciers erode surfaces and leave depressions in the bedrock. Can also form from uneven surface of ground moraine deposited by glaciers. Can form where terminal and lateral moraines block existing streams Example: Great Lakes Glacial erosion widened and deepened existing river valleys and moraines to the south blocked off the ends of these valleys. As ice sheets melted, the water was trapped in the valleys by the moraines and lakes formed

Ice Ages The last ice age happened 4 million years ago and the earliest known is estimated to have happened 800 million years ago. Ice age is a long period of climatic cooling where continents are glaciated repeatedly. Last glacial period, 1/3 of Earth surface covered with glacier, water levels 140 meters lower (extended coastlines), Canada and Alaska under ice A drop in the average temperature around 5 degrees Celsius may be enough to start an ice age. Cooler times=glacier advancement Warmer times=retreating glaciers (interglacial period) Currently we are in an interglacial period

Causes of Glaciation Milankovitch theory: cyclical changes in Earth’s orbit and the tilt of the Earth’s axis occur over thousands of years and this causes climate change. Every 100,000 years the shape of the Earth's orbit changes to become more circular path. Every 41,000 years the tilt changes between 22.2 degrees and 24.5 degrees Procession: A gradual change or “wobble” in the orientation of the Earth’s axis affects the relationship between the Earth’s eccentricity and tilt. These three factors are proposed to affect the distribution of solar energy that reaches earth’s surface.