Geology

2. Then follow your instructor’s directions for completing the worksheets.

ACTIVITY 13.2 Mountain Glaciers and

Glacial Landforms

THINK About It | How do glaciers affect landscapes?

ACTIVITY 13.1 Cryosphere Inquiry

THINK About It |

What is the cryosphere, and how do

changes in the cryosphere affect other

parts of the Earth system?

OBJECTIVE Analyze global and regional components of the cryosphere, and then infer how they may change and ways that such change may aff ect other parts of the Earth system.

PROCEDURES

1. Before you begin , do not look up defi nitions and information. Use your current knowledge, and complete the worksheet with your current level of ability. Also, this is what you will need to do the activity:

____ pen ____ Activity 13.1 Worksheets (pp. 347–348 ) and

pencil

2. Complete the worksheet in a way that makes sense to you.

3. After you complete the worksheet , be prepared to discuss your observations and classifi cation with other geologists.

OBJECTIVE Analyze features of landscapes aff ected by continental glaciation and infer how they formed.

PROCEDURES

1. Before you begin , read the Introduction, Glaciers, and Glacial Processes and Landforms. Also, this is what you will need :

____ Activity 13.3 Worksheet (p. 351 ) and pencil

2. Then follow your instructor’s directions for completing the worksheets.

ACTIVITY 13.3 Continental Glaciation of

North America

THINK About It | How do glaciers affect landscapes?

Dynamic Cryosphere The total amount of ice on Earth’s surface is ever- changing due to annual variations in global patterns of air circulation and regional variations in things like ground temperature, ocean surface temperature, and the weather (daily to seasonal conditions of the atmosphere, such as air temperature and humidity, wind, cloud cover, and precipitation). Global and regional amounts of ice are also affected by climate —the set of atmospheric conditions (like air temperature, humidity, wind, and precipitaion) that prevails in a region over decades. A region’s climate is generally determined by measuring the average conditions that exist there over a period of years or the conditons that normally exist in the region at a particular time of year.

Climate Change A region’s climate is based on factors like latitude, altitude, location relative to oceans (moisture sources), and location relative to patterns of global air and ocean circulation. Climate change refers to a significant change in atmospheric conditions of a region or the planet. This can occur due to natural factors like changing patterns of global air circulation, variations in volcanic activity, and changes in solar activity. It can also occur due to human factors like construction of regional urban centers (adding regional sources of heat energy) and deforestation (removing a transpiration source of atmospheric water vapor; adding soot and gases to the atmosphere as the forest is burned).

Glaciers and the Dynamic Cryosphere ■ 331

Map of Regional Variations in the Cryosphere

ICE SHELF: A sheet of ice attached to the land on one side but afloat on the ocean on the other side.

SEA ICE: A sheet of ice that originates from the freezing of seawater.

SEASONAL SNOW: Snow and ice may accumulate here in winter, but it melts over the following summer.

PERMAFROST CONTINUOUS: The ground is permanently frozen over this entire area.

PERMAFROST DISCONTINUOUS: The ground is permanently frozen in isolated patches within this area.

ICE SHEET: A pancake-like mound of ice covering a large part of a continent (more than 50,000 km2).

MOUNTAIN GLACIERS AND ICE CAPS: This area contains permanent patches of ice on mountain sides (cirques), river-like bodies of ice that flow down and away from mountains (valley and piedmont glaciers), and dome-shaped masses of ice and snow that cover the summits of mountains so that no peaks emerge (ice cap).

• South Pole

• North Pole

FIGURE 13.1 Cryosphere components. You can also download a complete world map of cryosphere components from this UNEP

(United Nations Environment Programme) website: http://www.grida.no/graphicslib/detail/the-cryosphere-world-map_e290

Glaciers Glaciers are large ice masses that form on land areas that are cold enough and have enough snowfall to sus- tain them year after year. They form wherever the win- ter accumulation of snow and ice exceeds the summer ablation (also called wastage ). Ablation (wast- age) is the loss of snow and ice by melting and by sublimation to gas (direct change from ice to water vapor, without melting). Accumulation commonly occurs in snowfields —regions of permanent snow cover ( FIGURE  13.2 ).

Glaciers can be divided into two zones, accumulation and ablation ( FIGURE 13.2 ). As snow and ice accumulate in and beneath snowfields of the zone of accumulation , they become compacted and highly recrystallized under their own weight. The ice mass then begins to slide and flow downslope like a very viscous (thick) fluid. If you slowly squeeze a small piece of ice in the jaws of a vise or pair of pliers, then you can observe how it flows. In nature, glacial ice formed in the zone of accumulation flows and slides downhill into the zone of ablation , where it melts or sublimes (undergoes sublimation) faster than new ice can form. The snowline is the boundary between the zones of accumulation and ablation. The bottom end of the glacier is the terminus .

It helps to understand a glacier by viewing it as a river of ice. The “headwater” is the zone of accumula- tion, and the “river mouth” is the terminus. Like a river, glaciers erode (wear away) rocks, transport their load

(tons of rock debris), and deposit their load “down- stream” (down-glacier).

The downslope movement and extreme weight of glaciers cause them to abrade and erode (wear away) rock materials that they encounter. They also pluck rock material by freezing around it and ripping it from bedrock. The rock debris is then incorporated into the glacial ice and transported many kilometers by the glacier. The debris also gives glacial ice extra abrasive power. As the heavy rock-filled ice moves over the land, it scrapes surfaces like a giant sheet of sandpaper. Rock debris falling from valley walls commonly accumulates on the surface of a moving glacier and is transported downslope. Thus, glaciers transport huge quantities of sediment, not only in, but also  on the ice.

When a glacier melts, it appears to retreat up the valley from which it flowed. This is called glacial retreat , even though the ice is simply melting back (rather than moving back up the hill). As melting occurs ( FIGURE 13.3 ), deposits of rocky gravel, sand, silt, and clay accumulate where there once was ice. These deposits collectively are called drift . Drift that accumulates directly from the melting ice is unstratified (unsorted by size) and is called till . However, drift that is transported by the meltwater becomes more rounded, sorted by size, layered, and is called stratified drift . Wind also can transport the sand, silt, and clay particles from drift. This wind-transported sediment can form dunes or loess deposits (wind-deposited, unstratified accumulations of clayey silt).

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332 ■ L A B O R AT O R Y 13

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FIGURE 13.2 Mountain glaciation. This is an ASTER infrared satellite image of a 20-by-20 km area in Alaska. Vegetation appears red, glacial ice is blue, and snow is white. (Image courtesy of NASA/GSFC/METI/ERSDAC/JAROS and U.S./Japan ASTER Science Team.)

There are five main kinds of glaciers based on their size and form.

■ Cirque glaciers —small, semicircular to triangular glaciers that form on the sides of mountains. If they form at the head (up-hill end) of a valley and grow large enough, then they evolve into valley glaciers.

■ Valley glaciers —long glaciers that originate at cirques and flow down stream valleys in the mountains.

■ Piedmont glaciers —mergers of two or more valley glaciers at the foot (break in slope) of a mountain range.

■ Ice sheet —a vast, pancake-shaped ice mound that covers a large portion of a continent and flows independent of the topographic features beneath it and covers an area greater than 50,000 km 2 . The Antarctic Ice Sheet (covering the entire continent of Antarctica) and the Greenland Ice Sheet (covering Greenland) are modern examples.

■ Ice cap —a dome-shaped mass of ice and snow that covers a flat plateau, island, or peaks at the summit of a mountain range and flows outward in all directions from the thickest part of the cap. It is much smaller than an ice sheet.

Glaciers and the Dynamic Cryosphere ■ 333

Glacial Processes and Landforms Glaciated lands are affected by either local to regional “mountain glaciation” or more continent-wide “ continental glaciation.”

Mountain Glaciation Mountain glaciation is characterized by cirque glaciers, valley glaciers, piedmont glaciers, and ice caps. Poorly developed mountain glaciation involves only cirques, but the best-developed mountain glaciation involves all three types. In some cases, valley and piedmont glaciers are so well developed that only the highest peaks and ridges extend above the ice. Ice caps cover even the peaks and ridges. FIGURE  13.2 shows a region with mountain glaciation. Note the extensive snowfield in the zone of accumulation. Snowline is the elevation above which there is permanent snow cover.

Also note that there are many cracks or fissures in the glacial ice of FIGURE 13.2 . At the upper end of the glacier is the large bergschrund (German, “mountain crack”) that separates the flowing ice from the relatively immobile portion of the snowfield. The other cracks are called crevasses —open fissures that form when the velocity of ice flow is variable (such as at bends in valleys). Transverse crevasses are perpendicular to the flow direction, and longitudinal crevasses are aligned parallel with the direction of flow.

FIGURE 13.3 shows the results of mountain glaciation after the glaciers have completely melted. Notice the characteristic landforms, water bodies, and sedimentary deposits. For your convenience, distinctive features of glacial lands are summarized in three figures: erosional features in FIGURE 13.4 , depositional features in FIGURE 13.5 , and water bodies in FIGURE 13.6 . Note that some features are identical in mountain glaciation and continental glaciation, but others are unique to one or the other. Study the descriptions in these three figures and compare them with the visuals in FIGURES 13.2 and  13.3 .

Continental Glaciation During the Pleistocene Epoch, or “Ice Age,” that ended 11,700 years ago, thick ice sheets covered most of Canada, large parts of Alaska, and the northern contiguous United States. These continental glaciers produced a variety of characteristic landforms ( FIGURE  13.7 , FIGURE 13.8 ).

Recognizing and interpreting these landforms is important in conducting work such as regional soil analyses, studies of surface drainage and water supply, and exploration for sources of sand, gravel, and  minerals. The thousands of lakes in the Precambrian Shield area of Canada also are a legacy of this continental glaciation, as are the fertile soils of the north-central United States and south-central Canada.

Snowfield

Bergschrund Arête

Horn

Snowline

Cirque glacier

Medial moraines

Valley glacier

Longitudinal crevasses

ZONE OF

ACCUM ULATION

Snowline

Ground moraine

Transverse crevasses

Cirque glaciers

Lateral moraine

ZONE OF

ABLATION

FIGURE 13.3 Active mountain glaciation, in a hypothetical region. Note the cutaway view of glacial ice, showing flow lines and direction (blue lines and arrows).

334 ■ L A B O R AT O R Y 13

Glacier National Park, Montana Glacier National Park is located on the northern edge of Montana, across the border from Alberta and British Columbia, Canada. Most of the erosional features formed

by glaciation in the park developed during the Wisconsinan glaciation that ended about 11,700 years ago. Today, only small cirque glaciers exist in the park. Thirty-seven of them are named, and nine of those can be observed on the topographic map of part of the park in FIGURE 13.14 .

Cirque

Tarn

Tarn

ArêteHorn

Hanging valley

Paternoster lakes

Medial moraine

Waterfall

Present-day misfit-stream valley

Lateral moraines

Ground moraine

U-shaped valley carved by

valley glacier

FIGURE 13.4 Erosional and depositional features of mountain glaciation. The same region as FIGURE 13.3 , but showing erosion features remaining after total ablation (melting) of glacial ice.

OBJECTIVE Evaluate the use of Nisqually Glacier as a global thermometer for measuring climate change.

PROCEDURES

1. Before you begin , read Nisqually Glacier—A Global Thermometer? Also, this is what you will need :

____ ruler ____ Activity 13.5 Worksheets (p. 353–354 ) and

pencil

2. Then follow your instructor’s directions for completing the worksheets.

ACTIVITY 13.5 Nisqually Glacier Response

to Climate Change

THINK About It | How is the cryosphere affected by climate change?

OBJECTIVE Analyze glacial features in Glacier National Park and infer how glaciers there may change in the future.

PROCEDURES

1. Before you begin , read about Glacial National Park, Montana below. Also, this is what you will need :

____ calculator ____ Activity 13.4 Worksheet (p. 352 ) and pencil

2. Then follow your instructor’s directions for completing the worksheets.

ACTIVITY 13.4 Glacier National Park

Investigation

THINK About It | How do glaciers affect landscapes? How is the cryosphere affected by

climate change?

Glaciers and the Dynamic Cryosphere ■ 335

Nisqually Glacier—A Global Thermometer? Nisqually Glacier is one of many active valley glaciers that occupy the radial drainage of Mt. Rainier—an active volcano located near Seattle, Washington, in the Cascade Range of the western United States. Nisqually Glacier occurs on the southern side of Mt. Rainier and flows south toward the Nisqually River Bridge in FIGURE  13.15 . The position of the glacier’s terminus (downhill end) was first recorded in 1840, and it has been measured and mapped by numerous geologists since that time. The map in FIGURE 13.15 was prepared by the U.S. Geological Survey in 1976 and shows where the terminus of Nisqually Glacier was located at various times from 1840 to 1997. (The 1994, 1997, and 2010 positions were added for this laboratory, based on NHAP aerial photographs and satellite imagery.) Notice how the glacier has more or less retreated up the valley since 1840.

Sea Ice Sea ice is frozen ocean water. The largest masses of sea ice occur in the Arctic Ocean and around the continent of Antarctica ( FIGURE 13.16 ). In both locations, the sea

MOUNTAIN GLACIATION

CONTINENTAL GLACIATION

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EROSIONAL FEATURES OF GLACIATED REGIONS

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