McKnight's Physical Geography

Chapter 19

Glacial Modificationof Terrain

© 2014 Pearson Education, Inc.

Andrew MercerMississippi State University

Modified by AJ Allred for Geography 1000

Salt Lake Community College, Fall 2013

• Snowpack over years turns to ice.

• Ice mass motion under gravity grinds anything in its path.

• Glaciation increases erosion rate on a mountain by at least 10 times that of an un-glaciated mountain.

Types of Glaciers• Continental ice sheets– Exist in non-mountainous areas

– Antarctica and Greenland only two

– Outlet glaciers

OR

• mountain

glaciers

Glaciations Past and Present• Glacial ice volume has varied considerably over the

last few million years

• Evidence left behind allows scientists to determine the chronology of past glaciations

• Pleistocene glaciation– Began at least 2.59 million years ago

– Last major ice retreat occurred only 9000 years ago

– Wisconsin glacial stage marked end

– At peak, one-third of total land covered in ice

• Most recent “ice age” has advanced and retreated many times, like a bulldozer scraping the earth

Glaciations Past and PresentMaximum extent of the Pleistocene glaciation

Glaciations Past and PresentContemporary glaciation

– About 10 percent of total Earth land surface still covered by ice

– 96 percent of the total ice cover is Greenland and Antarctica

– Many scattered, small North American glaciers

– Mt. Timpanogos (shady side) may still have a small glacier – the last one in Utah?

Glaciations Past and PresentClimate change related to contemporary glaciation

– Retreating of polar ice caps

– Shrinking ice caps an indicator of a warming climate

– Antarctic ice shelves breaking

– Higher flow rates of outlet glaciers

Glacier Formation and Movement• Small variation in global temperature easily changes

balance between ice advance and ice retreat

• Snow begins as crystallized water vapor

• Compresses to granular form

• Further compression results in glacial ice

• Glacial ice can be thousands

of feet thick

Glacier Formation and Movement• Glacier “flow” is orderly sliding of ice molecules

• Ice under extreme pressure deforms instead of slipping

• Melt water helps glacier to slide along

• Flow in response to overlying weight

Glacier Formation and MovementGlacier flow versus glacier advance

The Effects of GlaciersErosion by glaciers

Volume and speed determine success of glacial erosion

Erosive power of moving ice is greater than that of water

Glacial plucking – picking up of rock material through refreezing of melt-water

Glacial abrasion – bedrock worn down by rock debris inside glacier

The Effects of GlaciersTransportation by glaciers

– Glaciers can move large rocks

– Typically move sand/silt

– Most rock transported along base of the ice

– Role of flowing water on moving ice, melt streams

– Cracks in ice in which streams run – moulins

[Insert Fig. 19-16 p. 461]

The Effects of GlaciersDeposition by glaciers

– Glaciers move rock and soil from one region to another – much U.S. farm soil came from Canada

– Till – rock debris deposited by moving or melting ice

– Large boulders that are different from surrounding local bedrock, glacial erratics

A glacial erratic

The Effects of GlaciersDeposition by melt water

– Large portion of debris carried by glaciers deposited or re-deposited by melt water

– Sub-glacial streams from glaciers carry sedimentary material

Continental Ice SheetsThe “Finger Lakes” of New York state resulted from heavy amounts of ice forming on streams during an ice age.

As the “ice age” deepened, ice carved stream beds like “bulldozers”.

Later, ice covered

the entire region.

Only in the last several

thousands years has ice

retreated enough to

expose land for farming

and city-building.

Continental Ice SheetsErosion by ice sheets– Heavy ice can even carve or crush large granite peaks or ‘plutons’

– Abrasion on one side – think of scraping like sand paper

– “Plucking” on the other side – think of sticking your tongue on a frozen surface and leaving behind some of your skin when you pull away.

Plucking or pulling off frozen chunks off the back side

Scraping or “abrasion” on the front side

Continental Ice SheetsDeposition by Ice Sheets

Moraines – piles of debris left by the glacial “bull dozer”

Pots and kettles – holes in the ground caused by glacier movement and “plucking”

Drumlins – stripes of debris

left along the glacier’s path

Minnesota

“Land of 10,000 Lakes”

Much of central and northern United States owes farm

land to glacial scraping that delivered soil from Canada.

[Insert Fig. 19-29 p. 467]

Continental Ice SheetsGlacier deposited landscape features

Mountain GlaciersMountain glacier development and flow

– Usually form in sheltered depressions near heads of stream valleys

– Basic landform in glaciated mountains is the cirque, which is a

pocket or scoop mark left

by a glacier moving down

a mountain.

- Some cirques reach

all the way to the

bottom of the mountain

cirque

Mountain Glaciers

Cirque

Mountain Glaciers

“U” shaped valley carved by mountain glacier.

Looks just like “Little Cottonwood Canyon”

Mountain GlaciersErosion in the valleys– Some glaciers never leave cirques

– Principle erosive work is to deepen, steepen, and widen valleys

– U-shaped glacial troughs

Mountain GlaciersDeposition by mountain glaciers

Notice the piles of debris left each time the glacier stopped and backed-up

Mountain GlaciersNotice the moraine (debris piles) at the front edge of where a glacier stopped “bulldozing” and retreated.

The Periglacial EnvironmentMuch of high-latitude regions are still covered by “permafrost” or “gelisols” that remain frozen or partly frozen.

Earth warming is causing such areas to shrink.

Frozen soils become less stable when they thaw and become soggy.

Causes of the Pleistocene Glaciations• Milankovitch cycles – changes in earth “wobble”• Variations in solar output• Variations in carbon dioxide in atmosphere• Changes in continental positions• Atmospheric circulations• Tectonic upheaval – quaking and volcanoes• Are we still in an ice age? <no one really knows>

The start and stop of “ice ages” may have many causes