Chapter 15-1 GlaciersChapter 15-1 GlaciersPages 318-337Pages 318-337

What is a Glacier?What is a Glacier?

• ¾ of earth’s fresh water is frozen in glaciers.

• A Glacier is –• A large mass of compacted snow and ice

that moves under the force of gravity

Where glaciers formWhere glaciers form• Form in areas always covered

by snow• Snow builds up from year to

year• Glaciers may occur in any part

of the world– Closer to the equator, the

higher the elevation• The lowest elevation at which

a layer of permanent snow occurs in summer is called the SNOW LINE

How Glaciers formHow Glaciers form

• Mountain basins above the snow line are filled with snow year round.

• In these snowfields, buried snow becomes compressed and recrystallizes into a rough, granular material called FIRN or NEVE.

FirnFirn

• Resembles the ice of a packed snowball

• Weight of the firn may turn the lower layers into solid ice. This ice may begin to move outward or downward due to the overlying weight of the firn above – this now is a glacier.

Types of glaciersTypes of glaciers• Valley glacier

– Moves within valley walls

• Continental glacier

– Covers a large portion of a continent

Valley glaciersValley glaciers

• Forms in the upper valleys of the world’s mountain ranges where snowfall exceeds snowmelt

• Under the snow, the firn changes into ice

• The ice stays in the valley and slowly begins to move downhill due to gravity

• Valley glaciers are also known as alpine glaciers

• Valley glaciers occur on all continents except for Australia

• Valley glaciers may very small (2km) to very large (100km) and hundreds of meters thick– Large valley glaciers

are found in Alaska and Himalayas

– Glacier National Park has many remnant valley glaciers

Continental GlaciersContinental Glaciers

• In polar regions, large areas are covered in thick masses of ice– Only mountain peaks are visible called nunataks

• Are circular or oval in shape– Greenland = 1.7 million km2 and 3 km thick– Antartica = 13.7 million km2 and 4 km thick

• Largest C. G. is found in Antartica

• A glacier less than 50,000 km2 is sometimes called an icecap– Iceland, Baffin Island,

SpitsbergenIce cap in 1979 and 2003

Glacial Movement and Glacial Movement and ErosionErosion

Ch 15-2Ch 15-2

Pages 321-325Pages 321-325

How do glaciers move?How do glaciers move?

• Some glaciers move a few centimeters a day, while some move several meters in a week or month?

• Move faster after snowy winter, in summer and on steep slopes

• Friction differences exist between valley floor and valley walls – why?

• A typical glacier will move more rapidly at its surface and center than at its sides.

• The heavy weight of the ice above can melt the ice at the base. – This water or slushy layer reduces the friction

and may increase the movement of the glacier. This is called basal slip.

• Basal slip occurs at the base of the glacier, but does not account for the rest of the glacier’s movement.

• Basal slip does not occur in extreme cold

Plastic flowPlastic flow

• In the interior of a glacier, the grains of ice may deform, or change shape, due to the pressure of the snow and ice above.

• Near the bottom of the glacier, the grains are almost flat and can allow grains to slip past one another to create forward movement.

crevassescrevasses

• When glaciers move over a steep downward change of slope, the ice is rigid and crevasses or deep surface cracks form.

• Are rarely larger than 50 meters deep

How do we study glacial How do we study glacial movement?movement?

• Scientists will study glacial movement by placing a row of stakes across a glacier to study its movement.

• Also use satellite data to track glacial movement and positioning over time.

Ice frontIce front

• The ice front occurs where the glacier’s ice melts as fast as it moves.

• A glacier continually is moving forward due to gravity

IcebergsIcebergs

• Where the snow line is close to sea level, glaciers meet the sea.

• As the glacier moves into the sea, large blocks of ice break off - this is called calving

• Occurs in Antartica, northern Alaska, Siberia

How Glaciers Cause ErosionHow Glaciers Cause Erosion

• Glaciers are very powerful agents of erosion.

• Remove loose rock from valley walls (from fine powder to house-size boulders) and transport this material until it is deposited.

• Some material may fall on top of glacier, become apart of the glacier, or be dragged beneath the glacier.

• When this unsorted and unstratified rock material is deposited by a retreating, or melting glacier, it is called TILL

• The accumulation of till is called a MORAINE

• With Valley glaciers, material may be deposited on the sides of the glacier – forming a lateral moraine

• If two valley glaciers meet to form a single glacier, the lateral moraines may combine to form a medial (middle) moraine.

• When clay and silt particles are mixed by a glacier by the crushing of rock below the glacier, it forms ROCK FLOUR

• When this rock flour mixes with water, it takes on a milky white appearance and this meltwater is called GLACIAL MILK

• When rocks pieces frozen into the glacier cut away at existing bedrock on the valley floor and walls, it may leave behind long parallel scratches called STRIATIONS– Striations show

the general direction of the glacier

• Glaciers may also remove large sections of softer bedrock material from the valley walls through process called PLUCKING– This is caused by

melting and refreezing if the glacier along this material, and it tends to pluck the softer bedrock from the valley wall.

Effects of ErosionEffects of Erosion• Glaciers may leave

behind characteristic formations:

1. Roches Moutonnees – is an outcropping of bedrock that looks like a flock of sheep from a distance

2. Valley glaciers often erode a river valley from a typical V-shaped valley into the classic U-shaped glacial valley.

• 3. Main valley glaciers are larger than their tributary glaciers, and erode the valley more as well.– Forms a deeper u-

shaped main glacial valley than the tributary valleys, this is called HANGING VALLEYS

• When a stream is present in the tributary valley, and it now falls into the deeper main valley, this is called a HANGING VALLEY WATERFALL

• At the head of a former valley glacier is a semi-circular basin called a CIRQUE

• THIS CIRQUE MAY FILL UP WITH WATER AND FORM A SMALL LAKE CALLED A CIRQUE LAKE

• When two cirques are formed next to each other, the divide between the two may be narrow and sharp. This divide is called an ARETE

• If three or more cirques meet, they will form a pyramid-shaped peak called a HORN

• Valley glaciers tend to make mountain peaks more jagged and pronounced, while continental glaciers tend to smooth off mountain peaks and leave them polished and rounded.