Understanding Earth Chapter 21: GLACIERS The Work of Ice Grotzinger • Jordan Understanding Earth Sixth Edition Chapter 21: GLACIERS The Work of Ice © 2011 by W. H. Freeman and Company

Chapter 21 Glaciers: The Work of Ice

About Glaciers and the Work of Ice Glacial erosion creates an enormous amount of sedimentary debris. Ice covers only 10% of Earth now, but 21,000 years ago, the coverage was 30%. The landscapes of vast areas of many continents have been sculpted by the flow of ice in glaciers.

Lecture Outline Ice as a rock 2. How glaciers form 3. How glaciers move 4. Glacial landscapes 5. Glacial cycles and climate change

● Could be viewed as any kind of rock ● igneous – formed from a liquid 1. Ice as a Rock ● Could be viewed as any kind of rock ● igneous – formed from a liquid ● sedimentary – formed in layers ● metamorphic – transformed by pressure and recrystallization

Ice as a Rock: Ice Under Microscope

● Formation of glacial ice ● falls as snow and ice 1. Ice as a Rock ● Formation of glacial ice ● falls as snow and ice ● recrystallizes over time to ice ● with pressure of overlying ice, ice under pressure starts to flow like a very viscous fluid

● Peculiar properties of the rock ice 1. Ice as a Rock ● Peculiar properties of the rock ice ● freezes at very low temperature compared to other rocks ● ice is less dense than the liquid it comes from (water) ● ice is much weaker than most other rocks

● valley glaciers (alpine glaciers) 1. Ice as a Rock ● Types of glaciers ● valley glaciers (alpine glaciers) ● continental glaciers (ice sheets) ● ice shelves and ice caps

Ice as a Rock: A Valley Glacier near Juneau, Alaska

Ice as a Rock: The Ice Cap of Greenland

Ice as a Rock: The Ice Cap of Antarctica

● Basic ingredients: cold and snow ● accumulation and ablation 2. How Glaciers Form ● Basic ingredients: cold and snow ● accumulation and ablation ● Glacial movement (glacial budget) ● advance or recession

How Glaciers Form: The Evolution of Snow into Glacial Ice

● What causes glaciers to advance ● accumulation > ablation 2. How Glaciers Form ● What causes glaciers to advance ● accumulation > ablation ● What causes glaciers to recede ● ablation > accumulation

How Glaciers Form: The Glacial Budget

How Glaciers Form: Glacial Recession Qori Kalis Glacier, Peru, 2004 Qori Kalis Glacier, Peru, 1978

● Factors causing accumulation ● more snow falls than melts 2. How Glaciers Form ● Factors causing accumulation ● more snow falls than melts ● Factors causing ablation ● melting, iceberg calving, sublimation, and wind erosion

How Glaciers Form: Iceberg Calving St. Elias National Park, Alaska

● How thick must ice be to flow? ● several 10s of meters 3. How Glaciers Move ● How thick must ice be to flow? ● several 10s of meters ● How does the glacier ice move? ● laminar flow; downslope

● Mechanisms of glacial ice flow ● plastic flow ● basal slip 3. How Glaciers Move ● Mechanisms of glacial ice flow ● plastic flow ● basal slip

3. How Glaciers Move overall movement of plastic flow

3. How Glaciers Move ice crystals overall movement of plastic flow

3. How Glaciers Move ice crystals overall movement of plastic flow The flow of a glacier is accomplished by small slips along the microscopic planes of ice crystals.

3. How Glaciers Move

In cold regions, the ice at the base of a glacier is frozen to the bedrock.

In cold regions, the ice at the base of a glacier is frozen to the bedrock.

In cold regions, the ice at the base of a glacier is frozen to the bedrock. The rate of movement decreases toward the base.

If one drives a set of stakes deep into the glacier...

If one drives a set of stakes deep into the glacier... …the stakes in the center will have moved farther. This indicates faster movement in the center and top of the glacier.

In temperate regions, the pressure of the overlying ice melts water at the glacier’s base. liquid water

In temperate regions, the pressure of the overlying ice melts water at the glacier’s base. liquid water

The layer of water acts as a lubricant. In temperate regions, the pressure of the overlying ice melts water at the glacier’s base. liquid water The layer of water acts as a lubricant.

3. How Glaciers Move

3. How Glaciers Move In continental glaciers, the ice moves down and out from the thickest section.

● Flow in valley glaciers ● plastic (laminar) flow – 3. How Glaciers Move ● Flow in valley glaciers ● plastic (laminar) flow – central part of the glacier moves faster ● basal slip – uniform movement or surges ● crevasses form on surface

3. How Glaciers Move formation of crevasses

Crevasses form where a glacier moves over steps in bedrock topography…

Crevasses form where a glacier moves over steps in bedrock topography… …and where it curves around topography.

● Earth’s ice cap (Antarctica) ● continental glaciers flowing 3. How Glaciers Move ● Earth’s ice cap (Antarctica) ● continental glaciers flowing away from center of land mass ● plastic flow > basal slip ● highest flow in center of ice

How Glaciers Move: Flow Lines Lambert Glacier, Antarctica

How Glaciers Move: Flow Lines on Glacier Lambert Glacier, Antarctica

● Earth’s ice cap (Antarctica) ● ice streams flow within ice cap 3. How Glaciers Move ● Earth’s ice cap (Antarctica) ● ice streams flow within ice cap ● ice streams have basal slip ● ice cap leads to ice shelf ● icebergs snap off ice cap and float away from ice shelf

How Glaciers Move: Collapse of the Larson Ice Shelf, Antarctica, March 2002

How Glaciers Move: Icebergs Floating

Thought questions for this chapter One of the dangers of exploring glaciers is the possibility of falling into a crevasse. What topographic features of a valley glacier or its surroundings would you use to infer that you were on a part of the glacier that was badly crevassed? The density of ice (0.92 g/cm3) is less than that of water (1.0 g/cm3), which is why icebergs float. Using the principle of isostasy, compute the fraction of an iceberg’s mass that floats above the sea surface?

Thought questions for this chapter Evidence from bore holes drilled into the ice shows that there is liquid water at the base of some glaciers. What kinds of glaciers might have liquid water at the base? What factors might be responsible for the melting of ice at the bottom of these glaciers?

● Glacial erosion and its landforms ● glacial sediment: poorly sorted 4. Glacial Landscapes ● Glacial erosion and its landforms ● glacial sediment: poorly sorted mix of boulders to clay (rock flour) ● scratches and grooves on bedrock (striations)

Glacial Landscapes: Glacial Polish and Striations on Bedrock, Glacier Bay National Park, Alaska

● Glacial erosion and its landforms ● smooth bedrock hills (roches 4. Glacial Landscapes ● Glacial erosion and its landforms ● smooth bedrock hills (roches moutonées) ● amphitheater-shaped bowls on mountain peaks (cirques) ● knife-like ridges between glacial valleys (arêtes)

● Glacial erosion and its landforms ● U-shaped (glacial) valley 4. Glacial Landscapes ● Glacial erosion and its landforms ● U-shaped (glacial) valley ● U-shaped tributary valleys (hanging valleys) ● submerged U-shaped valleys (fjords) ● sculpted peaks (horns)

Glacial Landscapes: Formation of the Roche Moutonée

Glacial Landscapes: Roche Moutonée The Beehive, Sand Beach, Acadia National Park, Maine

Formation of Glacial Erosion Features 4. Glacial Landscapes: Formation of Glacial Erosion Features

Glacial Landscapes: A Horn The Matterhorn, Switzerland

● Glacial sedimentation and landforms 4. Glacial Landscapes ● Glacial sedimentation and landforms ● glacial drift (all glacial sediment) ● glacial till (from melting ice) ● erratics – glacial boulders in till ● outwash  glacial sediments redeposited by glacial meltwater

Glacial Landscapes: Formation of Till and Outwash Deposits

Glacial Landscapes: Formation of Various Moraine Features

● Glacial sedimentation and landforms ● ice-laid deposits ● morraine 4. Glacial Landscapes ● Glacial sedimentation and landforms ● ice-laid deposits ● morraine ● drumlins ● water-laid deposits ● kames, eskers, and kettles ● lake varves

Glacial Landscapes: Moraine (Till) Sierra Nevada Mountains, California

Glacial Landscapes: Till with Kettles Coteau des Prairies, South Dakota

4. Glacial Landscapes

● Permafrost - perennially frozen soil ● soil and sediment 4. Glacial Landscapes ● Permafrost - perennially frozen soil ● soil and sediment ● aggregates of ice crystals ● layers ● wedges ● irregular masses

Glacial Landscapes: Permafrost of the World

Thought questions for this chapter Some parts of a glacier contain much sediment, others very little. What accounts for the difference? Contrast the kinds of till that you would expect to find in two glaciated areas: one a terrain of granitic and metamorphic rocks; the other a terrain of soft shales and loosely cemented sands? What geological evidence would you search for if you wanted to know the direction of ancient glacial movements across the Canadian Shield?

Thought questions for this chapter You are walking over a winding ridge of glacial drift. What evidence will you look for to discover whether you are on an esker or an end moraine?

● The Wisconsin glaciation ● most recent glaciation of the 5. Glacial Cycles and Climate Change ● The Wisconsin glaciation ● most recent glaciation of the Pleistocene ice ages ● reached its maximum about 21,000 to 18,000 years ago ● a global event with profound implications

Glaciations and the Climate System: The Pleistocene Glacial World, as it Was 18,000 Years Ago

● Wisconsin sea-level change ● lower sea level due to ice 5. Glacial Cycles and Climate Change ● Wisconsin sea-level change ● lower sea level due to ice covering the land ● more continental dry land ● related climate changes

● Geological record of Pleistocene glaciations 5. Glacial Cycles and Climate Change ● Geological record of Pleistocene glaciations ● isostasy and sea-level change ● climate changes ● Milankovitch cycles

● Geological record older ice ages ● Permian-Pennsylvanian tillies 5. Glacial Cycles and Climate Change ● Geological record older ice ages ● Permian-Pennsylvanian tillies ● Ordovician tillites ● Precambrian tillites (2,400 and 750-600 million years old)

Glacial Cycles and Climate change: Permian-Pennsylvanian glaciation of the southern supercontinent PACIFIC OCEAN Africa TETHYS OCEAN South America India Australia Antarctica

Glacial Cycles and Climate Change: Permian-Pennsylvanian tillites and their distribution today ATLANTIC OCEAN Africa India PACIFIC OCEAN South America INDIAN OCEAN Australia

Glacial Cycles and Climate Change: Permian Tills on Bedrock with Striations South Africa glacial till glacial striations on bedrock

Glacial Cycles and Climate Change: Permian Glacial Lake Sediments with Dropstones lake sediments South Africa glacial dropstones

● Time of the “snowball” Earth ● global record of tillites 750 to 5. Glacial Cycles and Climate Change ● Time of the “snowball” Earth ● global record of tillites 750 to 600 million years old ● ice may have covered Earth from each pole to the equator ● implications for life on Earth

Glacial Cycles and Climate Change: Progressive Freezing of the “Snowball” Earth

Thought questions for this chapter You live in New Orleans, not far from the mouth of the Mississippi River. What might be your first indication that the world is entering a new glacial age? Some geologists think that one result of continued global warming could be the shrinkage and collapse of the West Antarctic ice sheet. How might this affect the populations of North America and Europe?

Key terms and concepts Accumulation Basal slip Cirque Ablation Accumulation Basal slip Cirque Continental glacier Crevasse Drift Drumlin Esker Fjord Glacier Hanging valley Iceberg calving Ice shelf Ice stream

Key terms and concepts Moraine Outwash Permafrost Plastic flow Iceberg calving Kettle Moraine Outwash Permafrost Plastic flow Striation Surge Till Tillite U-shaped valley Valley glacier Varve