Chapter 21 Glaciers: The Work of Ice

(1) permafrost

(1) The ground is always frozen in very cold regions where the summer temperature never gets high enough to melt more than a thin surface layer. Perennially frozen soil, or _______, today covers as much as 25 percent of Earth's total land area.

(1) slow (2) Greenland, Antarctica (3) ice shelves (4) Ice caps

*Continental Glaciers* (1) A continental glacier is a thick, ____-moving sheet of ice (sometimes called an ice sheet) that covers a large part of a continent or other large landmass. (2) Today, the world's largest continental glaciers overlie much of _____and ______, covering about 10 percent of Earth's land surface and storing about 75 percent of the world's fresh water. (3) Parts of Antarctica are rimmed by thinner sheets of ice—______ _____—floating on the ocean and attached to the main glacier on land. (4) ___ ____ are the masses of ice that blanket Earth's North and South Poles

(1) rock (2) Glaciers (3) valley, continental

*Ice as a Rock* (1) To a geologist, a block of ice is a _____, a mass of crystalline grains of the mineral ice. (2) _______ are large masses of ice on land that show evidence of being in motion, or of once having moved, under the force of gravity. (3) We divide glaciers, on the basis of their size and shape, into two basic types: _____ glaciers and _____ glaciers.

(1) temperatures (2) snow line (3) precipitation, cold (4) accumulation (5) ablation

* How Glaciers Form: Basic Ingredients- Freezing Cold and Lots of Snow* (1) For a glacier to form,_____ must be low enough to keep snow on the ground year-round (2) ____ _____the elevation above which snow does not completely melt in summer—generally decreases toward the poles, where snow and ice cover the ground year-round even at sea level. (3) The _____ of snow and the formation of glaciers require moisture as well as ____. Moisture-laden winds tend to drop most of their snow on the windward side of a high mountain range, so the leeward side is likely to be dry and unglaciated. (4) The amount of ice added to the glacier annually is its _____. (5) The total amount of ice that a glacier loses annually is called _____. This is composed of 4 mechanisms: Melting, Iceberg calving, sublimation, and wind erosion.

(1) isostasy, continents (2) tillites

*Glacial Cycles and Climate Change and Geo Record of Ancient Glaciation* (1) as a result of _____, only changes in the ice volume on _______directly affect sea level. (2) The Pleistocene glacial cycles were not unique in Earth's history. Since the early part of the twentieth century, we have known from glacial striations and lithified ancient tills, called _____, that glaciers covered parts of the continents several times in the distant geologic past, long before the Pleistocene.

(1) cirque (2) horns, arêtes (3) U-shaped (4) hanging valley (5) fjords

*Glacial Landscapes* (1) A flowing valley glacier carves a series of erosional landforms as it flows from its origin to its lower edge. At the head of the valley, the plucking and tearing action of the ice tends to carve out an amphitheater-like hollow called a _____, usually shaped like half of an inverted cone. (2) With continued erosion, cirques at the heads of adjacent valleys gradually meet to form sharp mountaintops, or _____, and jagged crests called _____along the divide. (3) As the glacier flows down from its cirque, it excavates a new valley or deepens an existing stream valley, creating a characteristic ___-_____ valley. (4) _____ _____—one whose floor lies high above the main valley floor (5) arms of the sea carved out by glaciers, called ______, create the spectacular rugged scenery for which the coasts of Alaska, British Columbia, Norway, and New Zealand are renowned.

(1) movement (2) sea level (3) rock flour (4) striations. (5) roches moutonnées

*Glacial Landscapes* (1) The ______of glaciers is responsible for the immense amount of geologic work done by ice: erosion, transportation, and sedimentation. (2) Ice is a far more efficient agent of erosion than water or wind. If an iceberg in the ocean melts, ___ _____does not change. However, If ice on land melts or slides into the ocean, ___ ____ rises. (3) This grinding action of glaciers on cracked blocks of rock on bedrock, fragments rocks into a great range of sizes, from boulders as big as houses to fine silt- and clay-sized material called ___ ____. (4) As a glacier drags rocks along its base, those rocks scratch or groove the bedrock beneath it. Such abrasions are termed ______. The orientation of these shows us the direction of ice movement—an especially important factor in the study of continental glaciers, which lack obvious valleys. (5) Advancing glacial ice smooths small hills of bedrock— known as ____ ________ ("sheep rocks") for their resemblance to a sheep's back—on their upstream side and plucks them to a rough, steep slope on their downstream side. These contrasting slopes indicate the direction of ice movement.

(1) surge (2) crevasses (3) ice streams

*Glacial movements* (1) A sudden period of fast movement of a valley glacier, called a surge, sometimes occurs after a long period of little movement. (2) The upper parts of a glacier have little pressure on them. At these low pressures, the ice at the surface of the glacier (shallower than about 50 m) behaves as a rigid, brittle solid, cracking as it is dragged along by the plastic flow of the ice below. The cracks, called _____, break up the surface ice into many small and large blocks. (3) glaciers flow rapidly in ___ _____.

(1) drumlins (2) Kames (3) Eskers (4) Kettles (5) varve

*Glacial sedimentation and Sedimentary Landforms* (1) Some continental glacial terrains display prominent landforms called _____: large, streamlined hills of till and bedrock that parallel the direction of ice movement. (2) Deposits of outwash by glacial meltwater take a variety of forms. ____are small hills of sand and gravel created when drift fills a hole in a glacier and is left behind when the glacier recedes. (3) ______ are long, narrow, winding ridges of sand and gravel found in the middle of ground moraines. These are deposited by meltwater streams flowing in tunnels along the bottom of a melting glacier. (4) _____ are hollows or undrained depressions that often have steep sides and may be occupied by ponds or lakes. Modern glaciers, which may leave behind huge isolated blocks of ice in outwash plains as they melt, offer the clue to the origin of these. (5) A _____ is a pair of layers formed in one year by seasonal freezing of the lake surface.

(1) drift (2) till (3) erratics (4) outwash (5)

*Glacial sedimentation and Sedimentary Landforms* (1) The term _____ is now used for all material of glacial origin found anywhere on land or beneath the ocean. (2) Some drift is deposited directly by melting ice. This unstratified and poorly sorted sediment is known as _____, and it may contain all sizes of rock fragments from clay to boulders (3) The large boulders often contained in till are called _____ because of their seemingly random composition, often very different from that of local rocks. (4) Drift that has been picked up and distributed by melt water streams is called______, and it often forms broad sedimentary plains downstream of melting glaciers, known as _____ plains. (5) A moraine is an accumulation of rocky, sandy, and clayey material carried by glacial ice or deposited as till. There are many types of these, each named for its position with respect to the glacier that formed it.

(1) plastic flow (2) basal slip (3) pressure (4) central part, faster (5) -

*How Glaciers Move* (1) The force of gravity exerted on a glacier causes individual crystals of ice to slip tiny distances relative to each other—on the order of a ten-millionth of a millimeter—over short intervals. The sum of many such movements among the enormous number of ice crystals that make up a glacier deforms the whole mass of ice in a process known as _____ _____. (2) The other mechanism of glacial movement is ____ ____, the sliding of a glacier along the boundary between the ice and the ground (3) The melting point of ice decreases as _____ increases, so ice at the base of a glacier, where the weight of the overlying ice is greatest, melts at a lower temperature than ice within the glacier. Think ice skating. (4) The deformation of long vertical tubes pounded deep into a glacier demonstrates that ice near the base flowes more slowly than ice in the center. This type of deformation, in which the ___ ___ of a glacier moves _____ than its sides or its base, is diagnostic of plastic flow. (5)Other valley glaciers have been observed to move at more uniform speeds, sliding as a single unit almost entirely by basal slip along a lubricating layer of meltwater next to the ground. Most often, however, valley glaciers flow by a combination of mechanisms—partly by plastic flow within the mass of ice and partly by basal slip.

(1) glacier (2) downhill (3) plastic flow, basal slip (4) plastic flow, basal slip

*How Glaciers Move* (1) When ice becomes thick enough for gravity to overcome its resistance to movement—normally at least several tens of meters—it starts to flow, and thus becomes a _____. (2) The ice in a glacier moves _____ in the same kind of laminar flow as a slowly flowing stream of water. Unlike the readily observed flow of a stream, however, glacial movement is so slow that the ice seems not to move at all from day to day, giving rise to the expression "moving at a glacial pace." (3) The flow of glaciers occurs in two ways: by ____ ____ and by ____ _____. (4) In ____ _____, the movement occurs as deformation within the glacier. In ____ ____, the glacier slides downslope as a single unit along its base, like a block of ice sliding down a ramp.

(1) glacial budget (2) climate change

*The Glacial Budget: Accumulation Minus Ablation* (1) The relationship between accumulation and ablation, called the ____ ____, determines the growth or shrinkage of a glacier. (2) glacial shrinkage is a good indicator of ____ _____, glacial budgets are now carefully monitored.

(1) End (2) Terminal (3) Lateral (4)Medial (5)Ground

*Types of Glacial Moraines* *Type - Location with Respect to Ice Front - Comments* (1) ______Moraine - At Ice front - After glacier melts, seen as ridge parallel to former ice front. (2) ______Moraine - At ice front Marking farthest advance of ice - Type of end moraine (3) ______moraine - Along edge of glacier where it scrapes side of walls of valley - Heavy sediment load eroded from valley walls; when ice melts, seen as ridge parallel to valley walls. (4) ______moraine - formed as two joined glaciers merge their lateral moraines below junction - Inherits its sediment load from lateral moraines that formed it; forms ridge parallel to valley walls. (5) ______moraine - beneath the ice as a layer of glacial debris - ranges from thin and patchy to a thick blanket of tills.

(1) alpine (2) complete (3) iceberg calving

*Valley Glaciers* (1) Many skiers and mountain climbers are familiar with valley glaciers, sometimes called _____ glaciers. These rivers of ice form in the cold heights of Mountain ranges, where snow accumulates. They then move downslope, either flowing down an existing stream valley or carving out a new one. (2) A valley glacier usually occupies the _____ width of the valley and may bury its floor under hundreds of meters of ice (3) Valley glaciers that flow down coastal mountain ranges at high latitudes may terminate at the ocean's edge, where masses of ice break off and form icebergs—a process called ____ _____.

*6 of 6 (Summary)* -What does the geologic record tell us about past ice ages?

Glacial drift of Pleistocene age is widespread over high-latitude regions that now enjoy temperate climates. This widespread drift is evidence that continental glaciers once expanded far beyond the polar regions. Studies of the geologic ages of glacial deposits on land and in marine sediments show that continental ice sheets advanced and retreated many times during the Pleistocene epoch. The most recent glacial advance, known as the Wisconsin glaciation, covered the northern parts of North America, Europe, and Asia with ice and exposed large areas of continental shelves. During interglacial intervals, sea level rose and submerged the shelves.

*1 of 6 (Summary)* -What are the basic types of glaciers?

Glaciers are divided into two basic types. A *valley glacier* is a river of ice that forms in the cold heights of mountain ranges and moves downslope through a valley. A *continental glacier* is a thick, slow-moving sheet of ice that covers a large part of a continent or other large landmass. Today, continental glaciers cover much of Greenland and Antarctica.

*5 of 6 (Summary)* -How do glaciers shape the landscape?

Glaciers erode bedrock by scraping, plucking, and grinding it into sizes ranging from boulders to fine rock flour. Valley glaciers erode cirques, horns, and arêtes at their heads; excavate U-shaped and hanging valleys; and create fjords by eroding their valleys below sea level at the coast. Glacial ice has both high competence and high capacity, which enable it to carry abundant sediment particles of all sizes. Glaciers transport huge quantities of sediments to the ice front, where melting releases them. The sediments may be deposited directly by the melting ice as till or picked up by meltwater streams and laid down as outwash. Moraines and drumlins are characteristic landforms deposited by ice. Eskers and kettles are formed by meltwater. Permafrost forms where summer temperatures never rise high enough to melt more than a thin surface layer of soil.

*2 of 6 (Summary)* -How do glaciers form?

Glaciers form where climates are cold enough that snow, instead of melting completely in summer, is transformed into ice by recrystallization. As snow accumulates, either at the tops of valley glaciers or at the domed centers of continental glaciers, the ice thickens. Its thickness increases until it becomes so massive that gravity starts to pull it downhill.

*3 of 6 (Summary)* -How do glaciers shrink or grow?

Glaciers lose ice by melting, sublimation, iceberg calving, and wind erosion. The glacial budget is the relationship between ablation (the amount of ice a glacier loses annually) and accumulation. If ablation is balanced by accumulation of new snow and ice in the glacier's upper reaches, the size of the glacier remains constant. If ablation is greater than accumulation, the glacier shrinks; conversely, if accumulation exceeds ablation, the glacier grows.

*4 of 6 (Summary)* -How do glaciers move?

Glaciers move by a combination of plastic flow and basal slip. Plastic flow dominates in very cold regions, where the glacier's base is frozen to the ground. Basal slip is more important in warmer climates, where meltwater at the glacier's base lubricates the ice.

*Intro*

In this chapter, we take a close look at Earth's glaciers, how they form and change over time, and how they leave their marks on Earth's surface by eroding and depositing material as they advance and retreat. We examine the role glaciers play in the climate system and discover what the geologic record of glaciation can tell us about climate change over time.