Glaciers

GlaciersGlaciers

Chapter 15 Chapter 15

Topic 15.1What is a Glacier?

Much of Earth’s fresh water is frozen in valley glaciers or continental glaciers. Glaciers form above the snowline from compressed snow called firn. The snow line elevation is high in areas close to the equator and gets lower in areas farther from the equator. Formation of glaciers requires adequate snowfall and low temperatures. The two main types of glaciers are valley glaciers and continental ice sheets.

Topic 15.2Glacial Movement and Erosion

Gravity causes glaciers to move. The ice front of a glacier may advance or retreat depending on conditions. If a glacier’s ice front is near the sea, icebergs may break off. As glaciers move, they erode the underlying surface, leaving behind characteristic erosional features. Glaciers move by basal slip and internal plastic flow. Glaciers erode the valleys through which they flow, producing characteristic landforms.

Topic 15.3Glacial Deposits

When glaciers retreat, the leave behind deposits. Till is unsorted, unstratified material deposited directly by ice. Outwash is sorted, stratified material deposited by meltwater from the ice. When glaciers melt, they deposit their sediments, creating features that can be readily identifiable. Glaciers may form lake basins by eroding the land or by depositing sediments.

Topic 15.4Ice Ages

Earth has experienced a number of periods of widespread glaciation. Evidence for such ice ages comes from erosional and depositional features. Scientists have identified a number of possible causes for glaciation. During an ice age, glacial advances alternate with glacial retreats. The Milankovitch theory is the most commonly accepted theory of the cause of the ice ages.

Key Termscalving cirquecontinental glacier crevassedrumlin erraticvalley glacier firnglacial valley glacierice age kameinternal plastic flow kettlemoraine outwashstriation snowlineMilankovitch theory tilloutwash plain esker

Topic 15.1What is a Glacier?

About 75% of Earth’s freshwater is frozen as glaciers. Natural forces compact snow to make enormous masses of moving ice.

glacier

A large mass of ice and snow that exists year-round and moves under the influence of gravity.

They have carved some of the most spectacular features on the surface of the earth. They form in areas that are always covered by snow. Layer after layer

Snow lineThe lowest elevation that permanent snow reaches in summer.

The elevation of the snowline varies from place to place. At the equator, it is 5,500 m above sea level. Near the poles, it is at sea level. Snowfields are formed by the accumulation of snow and ice above the snowline. They cover most of the land in the polar regions and the tops of some mountains at lower latitudes.

firn

Partially compacted and refrozen snow which has yet to become a glacier.

In the deepest layers of accumulating snow, the pressure of the overlying ice becomes so great that the ice grains flatten and the air between the firn is squeezed out. It loses its white color and becomes a bight steel-blue color. When the mass becomes large enough, it begins to move downslope due to gravity. Where are temperatures are low and snowfall is high, glacial formation is greatest.

There are two main types of glaciers:

1. Valley 2. Continental

Valley glacierA long, slow moving, wedge shaped glacier that moves within valley walls.

They develop in high mountain regions and flow down slope due to the force of gravity. They are also known as alpine glaciers, after the Alps in south-central Europe. They vary in size.

continental glacierA large sheet of ice that covers a large part of a continent.

Today, they are found only in Greenland and Antarctica. The Antarctic ice sheet is 1 ½ times the size of the United States. In some places it is more than 4,000 m thick. 90% of Greenland is covered by its ice sheet. It is about the size of Mexico and is 3,000 m thick. Scientists estimate that if melted, the ice would release enough water to raise sea level by 60 m if melted.

Homework

Read Textbook pages 318 - 320.

Complete Regents Questions

Topic 15.2Glacial Movement and

ErosionGlaciers are sometimes called rivers of ice. Continental glaciers may move lass than a centimeter a day, while valley glaciers can move several meters in a day. As they move, they erode and deposit earth materials. However, they move very differently than water does.

Examine the seasonal migration of snow cover.

http://www.classzone.com/books/earth_science/terc/content/visualizations/es1501/es1501page01.cfm?chapter_no=visualization






They move in two ways:

1. Basal slip

2. Internal plastic flow

Basal slip

Movement at the base of the glacier.

The water from the melted ice acts like a lubricant between the ice and the ground. This allows the ice to slip on a thin layer of water. The ice can push against a rock causing some of the ice to melt, letting it flow around the barrier. In extremely cold places, basal slip does not occur.

internal plastic flowGrains of ice deform, or permanently change shape and slip past each other to create forward movement. This movement is usually out from the glacier’s center.

The speed of internal plastic flow is not the same at all parts of the glacier. Speed is determined by the slope and thickness of the ice. It is faster near the surface than at the base. Friction caused by contact with the rock below causes the speed to slow. The center also moves faster than the sides.

Because of the pressure from above, the inside of the glacier moves by internal plastic flow. The base moves due to slip, while the surface remains brittle. This tension and compression causes cracks and buckles.

crevasses

A great fissure or crack in a glacier.

They can be more than 30 m deep

Ice sheets move outward from the center in all directions. It can even move out over the ocean forming an ice shelf. The rise and fall of tides causes large blocks to break off. These chunks are called icebergs. Remember that only the top third of an iceberg is above the surface.

Most glaciers extend below the snowline. As a glacier moves down-slope, ice and snow melt away due to warmer temperatures causing it to thin and melt. It tends to be the thinnest where it melts and moves the fastest.

Ice front

The end of a glacier.

As long as the rate of movement and melt are equal, the ice front is stationary. Where the snowline is close to sea level, glaciers reach the sea. As they extend out over the ocean, great blocks of ice break off and fall into the sea. This process is called calving. In Antarctica, the snowline is at sea level. Blocks of ice the size of Connecticut are not uncommon.

Glaciers are powerful agents of erosion. As they move, they carve out the terrain over which the move. The glacial processes begin at the upper end of the valley where a valley glacier forms. As it wedges its way through a valley, it breaks off rock making the valley steeper. Particles can be as small as a fine powder or as large as a house. The particles can ride on top of the glacier or can be dragged along the bottom of the glacier.

Observe how glaciers erode bedrock surfaces.







tillUnsorted and unstratified rock material deposited dirtectly by glacial ice.

Landforms made from glacial till are called moraines.

moraine

A deposit of till left behind when a glacier retreats.

The soil of a ground moraine is very rocky. Long Island is a terminal moraine. They are ridges of unsorted rock material on the ground or on the glacier.

Streams flowing from melted ice are called meltwater. Melting occurs in a glacier during the summer months. Streams of meltwater flow from the edges, the surface, and beneath the glacier. Because of the presence of small rock particles, the meltwater often looks like milk.

Glaciers weather and erode bedrock when they drag rocks with them, cutting and scraping. Smaller particles smooth and polish.

striationsLong, parallel scratches left on rocks and bedrock by glacial movement.

They show the direction of movement of the ice, often creating long parallel grooves in the landscape.

cirque

A semicircular basin found at the head of a glacial valley formed by a valley glacier.

Sharp jagged ridges form between the cirques.

arête

A sharp divide that separates adjoining cirques.

When several arêtes are joined together they form a pyramid-like peak called a horn.

Solid rock over which a glacier moves polishes it by scraping the surface with fragments picked up as the glacier moves. The scraping action picks up even more rock fragments. They will eventually be rounded smooth. The edge of the rock facing the direction from the which the glacier came will be rounded and smooth. The back side will be jagged and steep.

roche moutonnée

In French this means “sheep rock,” because the hill looks like the back of a sheep.

Valley before the glacier passes

Valley after the glacier passes

Small tributary glaciers will flow into

a main valley glacier.

hanging valley

The landscape created by a massive continental ice sheet is very different from the sharp rugged features eroded by a valley glacier. Continental ice sheets flatten mountains producing relatively smooth, rounded landscapes.

Existing valleys may be gouged out or deepened. Rock surfaces are smoothed and rounded. Rock surfaces are gouged. The scratches or grooves run parallel to the direction of glacial movement.

Homework


Complete Regents Questions

Topic 15.3Glacial Deposits

A valley glacier melts when the glacier moves to a lower elevation or when climate changes. When the glacier melts, all of the material accumulated in the ice is deposited. Glacial deposits are easy to spot.

outwash

When a glacier melts, its rock load remains in nearly the same place as in the glacier.

erratic

The composition of an erratic is different from the bedrock they sit on.

drumlin

The long axis of the drumlins are parallel to the direction of glacial movement.

Outwash plain

An outwash plain will be crossed by many meltwater streams.

kettle

A bowl-like hollow found in deposits of glacial outwash; formed when a large block of ice was left behind when the glacier melted.

kame A small, cone-shaped hill of stratified sand and gravel formed at a glacial front by meltwater carrying sediment off the glacier’s surface.

esker A long, winding ridge formed when sand and gravel fill meltwater tunnels beneath a glacier.Many extend hundreds of

kilometers and look like raised winding roadways.

Glaciers often form lake basins by eroding out surface areas, leaving depressions in the bedrock and deepening existing valleys. Thousands of lakes were formed in this way.

Topic 15.4Ice Ages

An ice age is also referred to as a glacial period. The time between glacial periods is known as an interglacial period. Several major glacial periods have occurred throughout our geologic history. The earliest was 600 million years ago. The most recent one was 114,000 years ago. The last advance of the last ice sheet started to retreat only 11,000 years ago.

Ice ages begin with a long-term drop in the earth’s average temperature. An average temperature drop of only 5º, and an increase in snowfall sets the stage for an ice age. Ice sheets advance during periods of cold, and retreat during warm periods.

During the last glacial period, glaciers covered 1/3 of the earth’s surface. Most glaciation was located in North America and Eurasia. In some parts of North America it was several kilometers thick. So much water was locked up in ice that sea level was 140 m lower than it is today.

Observe the retreat of ice sheets from North America.







Many theories have been proposed to account for ice ages. Gradual cooling and increased precipitation brought on the expansion of glaciers. The theories also explain why they retreated.

Milankovitch theory

This is the theory most widely accepted by scientists. According to Milankovitch, three kinds of periodic changes occur in the way the earth moves around the sun.

1. The shape of the orbit. It varies from nearly circular to elongated and back to circular every 100,000 years.

2. The earth tilts on its axis. Over a period of 41,000 years, the tilt ranges from 21.5 to 24.5 degrees.

3. Circular motion, or precession, of the earth’s axis. The axis traces a complete circle every 26,000 years.

Observe changes in Earth's orbit that contribute to climate change.







Other factors include:1. Changes in

atmospheric carbon dioxide levels.

2. Changes in the positions of Earth’s landmasses.

3. Changes in the amount of solar energy reaching Earth’s surface.

Evidence from the ocean floor, in the form of shells, supports this theory. The ratio of two isotopes of oxygen in the sea water can show if a sediment was laid down during an ice age.

There are other theories, however, most of them state that the ice ages resulted from a change in the amount of solar energy reaching the earth’s surface.

Another theory states that tectonic plate motion prevents warm water from reaching the poles.

Continental glaciers cause enormous changes to the Earth’s surface. In North America, the most recent ice age gouged out the Great Lakes, the St. Lawrence River valley, and the Finger Lakes and Lake Champlain in New York State. More than 1 million years ago, where the lakes now lie there were river valleys that flowed through easily eroded sandstone and shales. The ice widened and deepened them. Around 14,000 years ago, the ice started to melt.

The weight of three kilometers of ice depressed the surface of the land about a kilometer, causing the land to tilt to the north. Water pooled in front of the ice and eventually drained to the south into the Mississippi River. As the ice sheet continued to melt, the water levels rose forming the Mohawk, Hudson and Susquehanna rivers. About 7,000 years ago, the ice sheet melted past Lakes Erie, Ontario, and Michigan. As the ice retreated, the land rebounded, forcing the water flow to change direction, creating Niagara Falls and causing the water to drain through the St. Lawrence River.

Homework


Complete Regents Questions.

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Glaciers