14 Glaciers

Learning Objectives

By the end of this chapter, students should be able to:

  • Differentiate the different types of glaciers and contrast them with sea icebergs.
  • Describe how glaciers form, move, and create landforms.
  • Describe glacial budget; describe the zones of accumulation, equilibrium, and melting.
  • Identify glacial erosional and depositional landforms and interpret their origin; describe glacial lakes.
  • Describe the history and causes of past glaciations and their relationship to climate, sea-level changes, and isostatic rebound.

The Earth’s cryosphere, or ice, has a unique set of erosional and depositional features compared to its hydrosphere, or liquid water. This ice exists primarily in two forms, glaciers and icebergs. Glaciers are large accumulations of ice that exist year-round on the land surface. In contrast, masses ice floating on the ocean are icebergs, although they may have had their origin in glaciers.

Glaciers cover about 10% of the Earth’s surface and are powerful erosional agents that sculpt the planet’s surface. These enormous masses of ice usually form in mountainous areas that experience cold temperatures and high precipitation. Glaciers also occur in low lying areas such as Greenland and Antarctica that remain extremely cold year-round.

14.1 Glacier Formation

A long sheet of ice filling an alpine valley with parallel lines of sediment running lengthwise along the ice. In the background is a thick deposit of ice higher up in a mountain cirque.
Figure 14.1: Glacier in the Bernese Alps.

Glaciers form when repeated annual snowfall accumulates deep layers of snow that are not completely melted in the summer. Thus there is an accumulation of snow that builds up into deep layers. Perennial snow is a snow accumulation that lasts all year. A thin accumulation of perennial snow is a snow field. Over repeated seasons of perennial snow, the snow settles, compacts, and bonds with underlying layers. The amount of void space between the snow grains diminishes. As the old snow gets buried by more new snow, the older snow layers compact into firn, or névé, a granular mass of ice crystals. As the firn continues to be buried, compressed, and recrystallizes, the void spaces become smaller and the ice becomes less porous, eventually turning into glacier ice. Solid glacial ice still retains a fair amount of void space and that traps air. These small air pockets provide records of the past atmosphere composition.

There are three general types of glaciers: alpine or valley glaciers, ice sheets, and ice caps. Most alpine glaciers are located in the world’s major mountain ranges such as the Andes, Rockies, Alps, and Himalayas, usually occupying long, narrow valleys. Alpine glaciers may also form at lower elevations in areas that receive high annual precipitation such as the Olympic Peninsula in Washington state.

Aerial photo of a broad expanse of white snow and ice. There are numerous waterways off the right side of the ice sheet.
Figure 14.2: Greenland ice sheet.
Map of Greenland showing the thickness of the ice sheet that covers nearly the entire country. The ice is thickest near the center of Greenland with a maximum thickness of 3,205 meters and gets thinner outward toward the coasts.
Figure 14.3: Thickness of Greenland ice sheet in meters.

Ice sheets, also called continental glaciers, form across millions of square kilometers of land and are thousands of meters thick. Earth’s largest ice sheets are located on Greenland and Antarctica. The Greenland Ice Sheet is the largest ice mass in the Northern Hemisphere with an extensive surface area of over 2 million sq km (1,242,700 sq mi) and an average thickness of up to 1500 meters (5,000 ft, almost a mile).

The Antarctic Ice Sheet is even larger and covers almost the entire continent. The thickest parts of the Antarctic ice sheet are over 4,000 meters thick (>13,000 ft or 2.5 mi). Its weight depresses the Antarctic bedrock to below sea level in many places. The cross-sectional diagram comparing the Greenland and Antarctica ice sheets illustrates the size difference between the two.

Antarctic ice sheet is very wide (roughly 6,000km). Greenland ice sheet is narrower (roughly 1,000 km). Both ice sheets have roughly the same height. Crust of each ice sheet remains at or below sea level.
Figure 14.4: Cross-sectional view of both Greenland and Antarctic ice sheets drawn to scale for size comparison.

Ice cap glaciers are smaller versions of ice sheets that cover less than 50,000 km2 and usually occupy higher elevations and may cover tops of mountains. There are several ice caps on Iceland. A small ice cap called Snow Dome is near Mt. Olympus on the Olympic Peninsula in the state of Washington.

Two photos: the left photo shows the side profile of a rocky mountain that is covered with an ice cap. The right photo is an aerial view of a white ice cap covering a large area of land.
Figure 14.5: Snow Dome ice cap near Mt. Olympus, Washington (left) and Vatnajökull ice cap in Iceland (right).
Map centered over the North Pole, showing the maximum extent of the Laurentide ice sheet. The ice sheet covers Greenland, Canada, the northern United States, northern Europe, and northern Asia.
Figure 14.6: Maximum extent of Laurentide ice sheet.

The figure shows the size of the ancient Laurentide Ice Sheet in the Northern Hemisphere. This ice sheet was present during the last glacial maximum event, also known as the last Ice Age.

sheet[/pb_glossary] was present during the last glacial maximum eventalso known as the last Ice Age.

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14.2 Glacier Movement

Two photos: the left photo shows deep cracks in a sheet of glacial ice and the right photo shows a person stepping over a deep crack in a sheet of glacial ice.
Figure 14.7: Glacial crevasses (left) and Cravasse on the Easton Glacier in the North Cascades (right).

As the ice accumulates, it begins to flow downward under its own weight. In 1948, glaciologists installed hollow vertical rods in into the Jungfraufirn Glacier in the Swiss Alps to measure changes in its movement over two years. This study showed that the ice at the surface was fairly rigid and ice within the glacier was actually flowing downhill. The cross-sectional diagram of an alpine (valley) glacier shows that the rate of ice movement is slow near the bottom, and fastest in the middle with the top ice being carried along on the ice below.

Cross sectional diagram of a glacier on a slope with elevation on the vertical axis. The glacier flows downhill at different rates with red numbers showing the stress on the ice in kilopascals and arrows showing the relative extent of deformation. In general, the stress increases deeper below the glacier surface and deforms most along the base of the glacier.
Figure 14.8: Cross-section of a valley glacier showing stress (red numbers) increase with depth under the ice. The ice will deform and flow where the stress is greater than 100 kilopascals, and the relative extent of that deformation is depicted by the red arrows. Down slope movement is shown with blue arrows. The upper ice above the red dashed line does not flow but is pushed along en masse.

One of the unique properties of ice is that it melts under pressure. About half of the overall glacial movement was from sliding on a film of meltwater along the bedrock surface and half from internal flow. Ice near the surface of the glacier is rigid and brittle to a depth of about 50 m (165 ft). In this brittle zone, large ice cracks called crevasses form on the glacier’s moving surface. These crevasses can be covered and hidden by a snow bridge and are a hazard for glacier travelers.

Below the brittle zone, the pressure typically exceeds 100 kilopascals (kPa), which is approximately 100,000 times atmospheric pressure. Under this applied force, the ice no longer breaks, but rather it bends or flows in a zone called the plastic zone. This plastic zone represents the great majority of glacier ice. The plastic zone contains a fair amount of sediment of various grades from boulders to silt and clay. As the bottom of the glacier slides and grinds across the bedrock surface, these sediments act as grinding agents and create a zone of significant erosion.

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14.3 Glacial Budget

A cross sectional diagram of an alpine glacier on a slope with internal flow lines shown as blue arrows that point downhill parallel to the slope. The zone of accumulation and zone of melting are separated by a vertical equilibrium line, with the zone of accumulation being upslope and the zone of ablation downslope.
Figure 14.9: Cross-sectional view of an alpine glacier showing internal flow lines, zone of accumulation, snow line, and zone of melting.

A glacial budget is like a bank account, with the ice being the existing balance. If there is more income (snow accumulating in winter) than expense (snow and ice melting in summer), then the glacial budget shows growth. A positive or negative balance of ice in the overall glacial budget determines whether a glacier advances or retreats, respectively. The area where the ice balance is growing is called the zone of accumulation. The area where ice balance is shrinking is called the zone of ablation.

The diagram shows these two zones and the equilibrium line. In the zone of accumulation, the snow accumulation rate exceeds the snow melting rate and the ice surface is always covered with snow. The equilibrium line, also called the snowline or firnline, marks the boundary between the zones of accumulation and ablation. Below the equilibrium line in the zone of ablation, the melting rate exceeds snow accumulation leaving the bare ice surface exposed. The position of the firnline changes during the season and from year to year as a reflection of a positive or negative ice balance in the glacial budget. Of the two variables affecting a glacier‘s budget, winter accumulation and summer melt, summer melt matters most to a glacier’s budget. Cool summers promote glacial advance and warm summers promote glacial retreat.

Water-filled valley with steep side walls.
Figure 14.10: Fjord.

If a handful of warmer summers promote glacial retreat, then global climate warming over decades and centuries will accelerate glacial melting and retreat even faster. Global warming due to human burning of fossil fuels is causing the ice sheets to lose in years, an amount of mass that would normally take centuries. Current glacial melting is contributing to rising sea-levels faster than expected based on previous history.

As the Antarctica and Greenland ice sheets melt during global warming, they become thinner or deflate. The edges of the ice sheets break off and fall into the ocean, a process called calving, becoming floating icebergs. A fjord is a steep-walled valley flooded with sea water. The narrow shape of a fjord has been carved out by a glacier during a cooler climate period. During a warming trend, glacial meltwater may raise the sea level in fjords and flood formerly dry valleys. Glacial retreat and deflation are well illustrated in the 2009 TED Talk “Time-lapse proof of extreme ice loss” by James Balog.

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Video 14.1: Time-lapse proof of extreme ice loss, by James Balog.

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14.4 Glacial Landforms

Both alpine and continental glaciers create two categories of landforms: erosional and depositional. Erosional landforms are formed by the removal of material. Depositional landforms are formed by the addition of material. Because glaciers were first studied by 18th and 19th century geologists in Europe, the terminology applied to glaciers and glacial features contains many terms derived from European languages.

14.4.1 Erosional Glacial Landforms

Erosional landforms are created when moving masses of glacial ice slide and grind over bedrock. Glacial ice contains large amounts of poorly sorted sand, gravel, and bouldersthat have been plucked and pried from the bedrock. As the glaciers slide across the bedrock, they grind these sediments into a fine powder called rock flour. Rock flour acts as fine grit that polishes the surface of the bedrock to a smooth finish called glacial polish. Larger rock fragments scrape over the surface creating elongated grooves called glacial striations.

Two photos: the left photo shows a big, smooth rock with parallel linear grooves and the right photo shows smoothed, polished rock with parallel linear grooves abraded.
Figure 14.11: Glacial striations on granite in Whistler, Canada (left) and Glacial striations in Mt. Rainier National Park (right).
A U-shaped valley with steep ridges on the sides and a wide curved valley at the base.
Figure 14.12: The U-shape of the Little Cottonwood Canyon, Utah, as it enters into the Salt Lake Valley.

Alpine glaciers produce a variety of unique erosional landforms, such as U-shaped valleys, arêtes, cirques, tarns, hornscols, hanging valleys, and truncated spurs. In contrast, stream-carved canyons have a V-shaped profile when viewed in cross-section. Glacial erosion transforms a former V-shaped stream valley into a U-shaped one. Glaciers are typically wider than streams of similar length, and since glaciers tend to erode both at their bases and their sides, they erode V-shaped valleys into relatively flat-bottomed broad valleys with steep sides and a distinctive “U” shape. As seen in the images, Little Cottonwood Canyon near Salt Lake City, Utah was occupied by an Ice Age glacier that extended down to the mouth of the canyon and into Lake Bonneville. Today, that U-shaped valley hosts many erosional landforms, including polished and striated rock surfaces. In contrast, Big Cottonwood Canyon to the north of Little Cottonwood Canyon has retained the V-shape in its lower portion, indicating that its glacier did not extend clear to its mouth, but was confined to its upper portion.

Animated GIF of the formation of a glacial valley: the valley begins as a V-shaped valley with a river running through it. As ice forms on the mountains and fills the valley, the glacier then carves away at the base of the valley, changing it into a U-shaped valley. At the end of the animation, the ice melts away and leaves behind a U-shaped glacial valley with a few rivers and a lake at the base.
Figure 14.13: Formation of a glacial valley. Glaciers change the shape of the valley from a “V” shape to a “U”.

When glaciers carve two U-shaped valleys adjacent to each other, the ridge between them tends to be sharpened into a sawtooth feature called an arête. At the head of a glacially carved valley is a bowl-shaped feature called a cirque. The cirque represents where the head of the glacier eroded the mountain by plucking rock away from it and the weight of the thick ice eroded out a bowl. After the glacier is gone, the bowl at the bottom of the cirque often fills with precipitation and is occupied by a lake, called a tarn. When three or more mountain glaciers erode headward at their cirques, they produce horns, steep-sided, spire-shaped mountains. Low points along arêtes or between horns are mountain passes termed cols. Where a smaller tributary glacier flows into a larger trunk glacier, the smaller glacier cuts down less. Once the ice has gone, the tributary valley is left as a hanging valley, sometimes with a waterfall plunging into the main valley. As the trunk glacier straightens and widens a V-shaped valley and erodes the ends of side ridges, a steep triangle-shaped cliff is formed called a truncated spur.

Three photos: the left photo shows a steep rocky mountain that has an amphitheatre-like valley carved out of the top of the mountain, with a lake at the base. The middle photo shows a steep and pointy rocky mountain top. The right photo shows a steep rocky mountain with a U-shaped valley halfway up the slope; a large waterfall flows out of the mid-height valley onto the valley below.
Figure 14.14: Cirque with Upper Thornton Lake in the North Cascades National Park, Washington (left). An example of a horn, Kinnerly Peak, Glacier National Park, Montana (center). Bridalveil Falls in Yosemite National Park, California (right) is a good example of a hanging valley.

14.4.2 Depositional Glacial Landforms

A large boulder, dark in color, with smaller lighter colored clasts of a range of sizes inside of it.
Figure 14.15: Boulder of diamictite of the Mineral Fork Formation, Antelope Island, Utah, United States.

Depositional landforms and materials are produced from deposits left behind by a retreating glacier. All glacial deposits are called drift. These include till, tillitesdiamictites, terminal moraines, recessional moraines, lateral morainesmedial moraines, ground moraines, silt, outwash plains, glacial erraticskettleskettle lakes, crevasses, eskers, kames, and drumlins.

Glacial ice carries a lot of sediment, which when deposited by a melting glacier is called till. Till is poorly sorted with grain sizes ranging from clay and silt to pebbles and boulders. These clasts may be striated. Many depositional landforms are composed of till. The term tillite refers to lithified rock having glacial origins. Diamictite refers to a lithified rock that contains a wide range of clast sizes; this includes glacial till but is a more objective and descriptive term for any rock with a wide range of clast sizes.

Moraines are mounded deposits consisting of glacial till carried in the glacial ice and rock fragments dislodged by mass wasting from the U-shaped valley walls. The glacier acts like a conveyor belt, carrying and depositing sediment at the end of and along the sides of theice flow. Because the ice in the glacier is always flowing downslope, all glaciers have moraines build up at their terminus, even those not advancing.

Moraines are classified by their location with respect to the glacier. A terminal moraine is a ridge of till located at the end or terminus of the glacier. Recessional moraines are left as glaciers retreat and there are pauses in the retreat. Lateral moraines accumulate along the sides of the glacier from material mass wasted from the valley walls. When two tributary glaciers merge, the two lateral moraines combine to form a medial moraine running down the center of the combined glacier. Ground moraine is a veneer of till left on the land as the glacier melts.

Two photos: the left photo shows a long sheet of ice at the base of mountains with parallel lines of sediment running lengthwise along the ice, resembling an ice road. The right photo shows two long sheets of ice at the base of mountains that meet in a Y-shape; both sheets have parallel lines of sediment running lengthwise along the ice, resembling an ice road.
Figure 14.16: Lateral moraines of Kaskawulsh Glacier within Kluane National Park in the Canadian territory of Yukon (left) and Medial moraines where tributary glaciers meet. At least seven tributary glaciers from upstream have joined to form the trunk glacier flowing on out of the upper left of the picture (right).

In addition to morainesglaciers leave behind other depositional landforms. Silt, sand, and gravel produced by the intense grinding process are carried by streams of water and depositedin front of the glacier in an area called the outwash plain. Retreating glaciers may leave behind large boulders that don’t match the local bedrock. These are called glacial erratics. When continental glaciers retreat, they can leave behind large blocks of ice within the till. These ice blocks melt and create a depression in the till called a kettle. If the depression later fills with water, it is called a kettle lake.

If meltwater flowing over the ice surface descends into crevasses in the ice, it may find a channel and continue to flow in sinuous channels within or at the base of the glacier. Within or under continental glaciers, these streams carry sediments. When the ice recedes, the accumulated sediment is deposited as a long sinuous ridge known as an esker. Meltwater descending down through the ice or over the margins of the ice may deposit mounds of till in hills called kames.

Aerial photograph of a small town surrounded by green landscape; numerous elongated asymmetrical teardrop-shaped hills are seen surrounding the town, which is also located on a small ridge.
Figure 14.17: A small group of Ice Age drumlins in Germany.

Drumlins are common in continentalglacial areas of Germany, New York, and Wisconsin, where they typically are found in fields with great numbers. A drumlin is an elongated asymmetrical teardrop-shaped hill reflecting ice movement with its steepest side pointing upstream to the flow of ice and its streamlined or low-angled side pointing downstream in the direction of ice movement.

Glacial scientists debate the origins of drumlins. A leading idea is that drumlins are created from accumulated till being compressed and sculpted under aglacier that retreated then advanced again over its own ground moraine. Another idea is that meltwater catastrophically flooded under the glacier and carved the till into these streamlined mounds. Still another proposes that the weight of the overlying ice statically deformed the underlying till.

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14.4.3 Glacial Lakes

A rocky mountainous area with a circular bowl-shaped valley filled with a lake.
Figure 14.18: Tarn in a cirque.

Glacial lakes are commonly found in alpine environments. A lake confined within a glacial cirque is called a tarn. A tarn forms when the depression in the cirque fills with precipitation after the ice is gone. Examples of tarns include Silver Lake near Brighton Ski resort in Big Cottonwood Canyon, Utah and Avalanche Lake in Glacier National Park, Montana.

Series of elongate lakes between moraines within an alpine glacial valley.
Figure 14.19: Paternoster lakes.

When recessional moraines create a series of isolated basins in a glaciated valley, the resulting chain of lakes is called paternoster lakes.

Lakes filled by glacial meltwater often looks milky due to finely ground material called rock flour suspended in the water.

Satellite view of a series of long, finger-shaped lakes next to each other over a large area.
Figure 14.20: Satellite view of Finger Lakes region of New York.

Long, glacially carved depressions filled with water are known as finger lakes. Proglacial lakes form along the edges of all the largest continental ice sheets, such as Antarctica and Greenland. The crust is depressed isostatically by the overlying ice sheet and these basins fill with glacial meltwater. Many such lakes, some of them huge, existed at various times along the southern edge of the Laurentide Ice Sheet. Lake Agassiz, Manitoba, Canada, is a classic example of a proglacial lake. Lake Winnipeg serves as the remnant of a much larger proglacial lake.

Map showing a large lake covering southeastern Manitoba, northwestern Ontario, northern Minnesota, eastern North Dakota, and Saskatchewan.
Figure 14.21: Extent of Lake Agassiz.

Other proglacial lakes were formed when glaciers dammed rivers and flooded the river valley. A classic example is Lake Missoula, which formed when a lobe of the Laurentide ice sheet blocked the Clark Fork River about 18,000 years ago. Over about 2000 years the ice dam holding back Lake Missoula failed several times. During each breach, the lake emptied across parts of eastern Washington, Oregon, and Idaho into the Columbia River Valley and eventually the Pacific Ocean. After each breach, the dam reformed and the lake refilled. Each breach produced a catastrophic flood over a few days. Scientists estimate that this cycle of ice dam, proglacial lake, and torrential massive flooding happened at least 25 times over a span of 20 centuries. The rate of each outflow is believed to have equaled the combined discharge of all of Earth’s current rivers combined.

Aerial photo of a barren and pockmarked terrain.
Figure 14.22: View of Channeled Scablands in central Washington showing huge potholes and massive erosion.

The landscape produced by these massive floods is preserved in the Channeled Scablands of Idaho, Washington, and Oregon.

Map of the western United States showing present day lakes, Pleistocene lakes, and the Pleistocene ice sheet; Lake Bonneville is a Pleistocene lake in northwestern Utah and arrows show that it floods toward the northwest, along the Snake River, until it reaches the Scablands of eastern Washington; the modern Great Salt Lake is in the northern part of Lake Bonneville and is much smaller than Lake Bonneville. The ice sheet is located in Canada and the northern United States; it also floods toward the Scablands. Other smaller Pleistocene lakes are scattered across the western U.S.
Figure 14.23: Pluvial lakes in the western United States.
Aerial view of the five great lakes (Superior, Huron, Michigan, Erie, Ontario) that occupy basins left by the ice sheet in the Ice Age.
Figure 14.24: The Great Lakes.

Pluvial lakes form in humid environments that experience low temperatures and high precipitation. During the last glaciation, most of the western United States’ climate was cooler and more humid than today. Under these low-evaporation conditions, many large lakes, called pluvial lakes, formed in the basins of the Basin and Range Province. Two of the largest were Lake Bonneville and Lake Lahontan. Lake Lahontan was in northwestern Nevada. The figure illustrates the tremendous size of Lake Bonneville, which occupied much of western Utah and into eastern Nevada. The lake level fluctuated greatly over the centuries leaving several pronounced old shorelines marked by wave-cut terraces. These old shorelines can be seen on mountain slopes throughout the western portion of Utah, including the Salt Lake Valley, indicating that the now heavily urbanized valley was once filled with hundreds of feet of water. Lake Bonneville’s level peaked around 18,000 years ago when a breach occurred at Red Rock Pass in Idaho and water spilled into the Snake River. The flooding rapidly lowered the lake level and scoured the Idaho landscape across the Pocatello Valley, the Snake River Plain, and Twin Falls. The floodwaters ultimately flowed into the Columbia River across part of the scablands area at an incredible discharge rate of about 4,750 cu km/sec (1,140 cumi/sec). For comparison, this discharge rate would drain the volume of Lake Michigan completely dry within a few days.

The five Great Lakes in North America’s upper Midwest are proglacial lakes that originated during the last ice age. The lake basins were originally carved by the encroaching continental ice sheet. The basins were later exposed as the ice retreated about 14,000 years ago and filled by precipitation.

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14.5 Ice Age Glaciations

A glaciation (or ice age) occurs when the Earth’s climate becomes cold enough that continental ice sheets expand, covering large areas of land. Four major, well-documented glaciations have occurred in Earth’s history: one during the Archean-early Proterozoic Eon, ~2.5 billion years ago; another in late Proterozoic Eon, ~700 million years ago; another in the Pennsylvanian, 323 to 300 million years ago, and most recently during the Pliocene-Pleistocene epochs starting 2.5 million years ago (chapter 8). Some scientists also recognize a minor glaciation around 440 million years ago in Africa.

The best-studied glaciation is, of course, the most recent. This infographic illustrates the glacial and climate changes over the last 20,000 years, ending with those caused by human actions since the Industrial Revolution. The Pliocene-Pleistocene glaciation was a series of several glacial cycles, possibly 18 in total. Antarctic ice-core records exhibit especially strong evidence for eight glacial advances occurring within the last 420,000 years. The last of these is known in popular media as “The Ice Age,” but geologists refer to it as the Last Glacial Maximum. The glacial advance reached its maximum between 26,500 and 19,000 years ago.

14.5.1 Causes of Glaciations

Glaciations occur due to both long-term and short-term factors. In the geologic sense, long-term means a scale of tens to hundreds of millions of years and short-term means a scale of hundreds to thousands of years.

Long-term causes include plate tectonics breaking up the supercontinents (see Wilson Cycle, chapter 2), moving land masses to high latitudes near the north or south poles, and changing ocean circulation. For example, the closing of the Panama Strait and isolation of the Pacific and Atlantic oceans may have triggered a change in precipitation cycles, which combined with a cooling climate to help expand the ice sheets.

A graph of atmospheric CO2 levels over time. The vertical axis shows Benthic O-18 in per mil, decreasing upward; the horizontal axis shows the time from 65 to 0 million years ago. During the Cenozoic Era, carbon dioxide levels steadily decreased from a maximum in the Paleocene, causing the climate to gradually cool. By the Pliocene, ice sheets began to form. There are short-term cycles of warming and cooling within the larger glaciation event.
Figure 14.25: Atmospheric CO2 has declined during the Cenozoic from a maximum in the Paleocene–Eocene up to the Industrial Revolution.

Short-term causes of glacial fluctuations are attributed to the cycles in the Earth’s rotational-axis and to variations in the earth’s orbit around the Sun which affect the distance between Earth and the Sun. Called Milankovitch Cycles, these cycles affect the amount of incoming solar radiation, causing short-term cycles of warming and cooling.

During the Cenozoic Era, carbon dioxide levels steadily decreased from a maximum in the Paleocene, causing the climate to gradually cool. By the Pliocene, ice sheets began to form. The effects of theMilankovitch Cycles created short-term cycles of warming and cooling within the larger glaciation event.

Milankovitch Cycles are three orbital changes named after the Serbian astronomer Milutan Milankovitch. The three orbital changes are called precession, obliquity, and eccentricity. Precession is the wobbling of Earth’s axis with a period of about 21,000 years; obliquity is changes in the angle of Earth’s axis with a period of about 41,000 years; and eccentricity is variations in the Earth’s orbit around the sun leading to changes in distance from the sun with a period of 93,000 years. These orbital changes created a 41,000-year-long glacialinterglacial Milankovitch Cycle from 2.5 to 1.0 million years ago, followed by another longer cycle of about 100,000 years from 1.0 million years ago to today (see Milankovitch Cycles).

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Watch the video to see summaries of the ice agesincluding their characteristics and causes.

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Video 14.2: Ice ages and climate cycles.

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14.5.2 Sea-Level Change and Isostatic Rebound

When glaciers melt and retreat, two things happen: water runs off into the ocean causing sea levels to rise worldwide, and the land, released from its heavy covering of ice, rises due to isostatic rebound. Since the Last Glacial Maximum about 19,000 years ago, sea-level has risen about 125 m (400 ft). A global change in sea level is called eustatic sea-level change. During a warming trend, sea-level rises due to more water being added to the ocean and also thermal expansion of sea water. About half of the Earth’s eustatic sea-level rise during the last century has been the result of glaciers melting and about half due to thermal expansion. Thermal expansion describes how a solid, liquid, or gas expands in volume with an increase in temperature. This 30 second video demonstrates thermal expansion with the classic brass ball and ring experiment.

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Video 14.3: Thermal expansion.

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Relative sea-level change includes vertical movement of both eustatic sea-level and continents on tectonic plates. In other words, sea-level change is measured relative to land elevation. For example, if the land rises a lot and sea-level rises only a little, then the relative sea-level would appear to drop.

Continents sitting on the lithosphere can move vertically upward as a result of two main processes, tectonic uplift and isostatic rebound. Tectonic uplift occurs when tectonic plates collide (see chapter 2). Isostatic rebound describes the upward movement of lithospheric crust sitting on top of the asthenospheric layer below it. Continental crust bearing the weight of continental ice sinks into the asthenosphere displacing it. After the ice sheet melts away, the asthenosphere flows back in and continental crust floats back upward. Erosion can also create isostatic rebound by removing large masses like mountains and transporting the sediment away (think of the Mesozoic removal of the Alleghanian Mountains and the uplift of the Appalachian plateau; chapter 8), albeit this process occurs more slowly than relatively rapid glacier melting.

World map, color coded by vertical crustal motions in mm per year. The highest rebound rate is 18.0 mm per year and high motions are indicated the blue-to-purple zones (top end of the scale). The lowest rebound rate is negative 6.0 mm per year, indicating isostatic lowering; these are orange-to-red zones (bottom end of the scale). Most glacial isostatic rebound is occurring where continental ice sheets rapidly melted about 19,000 years ago, such as in Canada, Scandinavia, and western Antarctica, and most isostatic lowering is occurring in the northern Atlantic Ocean and Arctic Ocean, with small spots of lowering off the coast of western Antarctica.
Figure 14.26: Rate of isostatic rebound.

The isostatic-rebound map below shows rates of vertical crustal movement worldwide. The highest rebound rate is indicated by the blue-to-purple zones (top end of the scale). The orange-to-red zones (bottom end of the scale) surrounding the high-rebound zones indicate isostatic lowering as adjustments in displaced subcrustal material have taken place.

Most glacial isostatic rebound is occurring where continental ice sheets rapidly melted about 19,000 years ago, such as in Canada and Scandinavia. Its effects can be seen wherever Ice Age ice or water bodies are or were present on continental surfaces and in terraces on river floodplains that cross these areas. Isostatic rebound occurred in Utah when the water from Lake Bonneville drained away. North America’s Great Lakes also exhibit emergent coastline features caused by isostatic rebound since the continental ice sheet retreated.

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Glaciers form when average annual snowfall exceeds melting and snow compresses into glacial ice. There are three types of glaciers, alpine or valley glaciers that occupy valleys, ice sheets that cover continental areas, and ice caps that cover smaller areas usually at higher elevations. As the ice accumulates, it begins to flow downslope and outward under its own weight. Glacial ice is divided into two zones, the upper rigid or brittle zone where the ice cracks into crevasses and the lower plastic zone where under the pressure of overlying ice, the ice bends and flows exhibiting ductile behavior. Rock material that falls onto the ice by mass wasting or is plucked and carried by the ice is called moraine and acts as grinding agents against the bedrock creating significant erosion.

Glaciers have a budget of income and expense. The zone of income for the glacier is called the Zone of Accumulation, where snow is converted into firn then ice by compression and recrystallization, and the zone of expense called the Zone of Ablation, where ice melts or sublimes away. The line separating these two zones latest in the year is the Equilibrium or Firn Line and can be seen on the glacier separating bare ice from snow covered ice. If the glacial budget is balanced, even though the ice continues to flow downslope, the end or terminus of the glacier remains in a stable position. If income is greater than expense, the position of the terminus moves downslope. If expense is greater than income, a circumstance now affecting glaciers and ice sheets worldwide due to global warming, the terminus recedes. If this situation continues, the glaciers will disappear. An average of cooler summers affects the stability or growth of glaciers more than higher snowfall. As the Greenland and Antarctic ice sheets flow seaward, the edges calve off forming icebergs.

Glaciers create two kinds of landforms, erosional and depositional. Alpine glaciers carve U-shaped valleys and moraine carried in the ice polishes and grooves or striates the bedrock. Other landscape features produced by erosion include horns, arête ridges, cirques, hanging valleys, cols, and truncated spurs. Cirques may contain eroded basins that are occupied by post glacial lakes called tarns. Depositional features result from deposits left by retreating ice called drift. These include till, and moraine deposits (terminal, recessional, lateral, medial, and ground), eskers, kames, kettles and kettle lakes, erratics, and drumlins. A series of recessional moraines in glaciated valleys may create basins that are later filled with water to become paternoster lakes. Glacial meltwater carries fine grained sediment onto the outwash plain. Lakes containing glacial meltwater are milky in color from suspended finely ground rock flour. Ice Age climate was more humid and precipitation that did not become glacier ice filled regional depressions to become pluvial lakes. Examples of pluvial lakes include Lake Missoula dammed behind an ice sheet lobe and Lake Bonneville in Utah whose shoreline remnants can be seen on mountainsides. Repeated breaching of the ice lobe allowed Lake Missoula to rapidly drain causing massive floods that scoured the Channeled Scablands of Idaho, Washington, and Oregon.

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URLs Listed Within This Chapter

Infographic: Infographic. “A Timeline of Earth’s Average Temperature since the Last Ice Age Glaciation” xkcd https://xkcd.com/1732

Milankovitch cycles: https://en.wikipedia.org/wiki/Milankovitch_cycles

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Figure References

Figure 14.1: Glacier in the Bernese Alps. Dirk Beyer. 2005. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Grosser_Aletschgletscher_3178.JPG

Figure 14.2: Greenland ice sheet. Hannes Grobe. 1995. CC BY-SA 2.5. https://commons.wikimedia.org/wiki/File:Greenland-ice_sheet_hg.jpg

Figure 14.3: Thickness of Greenland ice sheet in meters. Eric Gaba. 2011. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Greenland_ice_sheet_AMSL_thickness_map-en.svg

Figure 14.4: Cross-sectional view of both Greenland and Antarctic ice sheets drawn to scale for size comparison. Kindred Grey. 2022. CC BY 4.0. Adapted from Steve Earle (CC BY 4.0). https://opengeology.org/textbook/14-glaciers/14-2_steve-earle_antarctic-greenland-2-300×128/

Figure 14.5: Snow Dome ice cap near Mt. Olympus, Washington (left) and Vatnajökull ice cap in Iceland (right). Mount Olympus Washington by United States National Park Service, 2004 (Public domain, https://commons.wikimedia.org/wiki/File:Mount_Olympus_Washington.jpg). Vatnajökull by NASA, 2004 (Public domain, https://commons.wikimedia.org/wiki/File:Vatnaj%C3%B6kull.jpeg).

Figure 14.6: Maximum extent of Laurentide ice sheet. USGS. 2005. Public domain. https://commons.wikimedia.org/wiki/File:Pleistocene_north_ice_map.jpg

Figure 14.7: Glacial crevasses (left) and Cravasse on the Easton Glacier in the North Cascades (right). chevron crevasse by Bethan Davies, 2015 (CC BY-NC-SA 3.0, https://www.antarcticglaciers.org/glacier-processes/structural-glaciology/chevron-crevasse/). Glaciereaston by Mauri S. Pelto, 2005 (Public domain, https://en.wikipedia.org/wiki/File:Glaciereaston.jpg).

Figure 14.8: Cross-section of a valley glacier showing stress (red numbers) increase with depth under the ice. Kindred Grey. 2022. CC BY 4.0. Adapted from Steve Earle (CC BY 4.0). https://opengeology.org/textbook/14-glaciers/14-2_steve-earle_ice-flow-and-stress/

Figure 14.9: Cross-sectional view of an alpine glacier showing internal flow lines, zone of accumulation, snow line, and zone of melting. Kindred Grey. 2022. CC BY 4.0. Adapted from Steven Earle (CC BY 4.0). https://opentextbc.ca/geology/chapter/16-2-how-glaciers-work/

Figure 14.10: Fjord. Frédéric de Goldschmidt. 2007. CC BY-SA 3.0. https://sco.wikipedia.org/wiki/File:Geirangerfjord_(6-2007).jpg

Figure 14.11: Glacial striations on granite in Whistler, Canada (left) and Glacial striations in Mt. Rainier National Park (right). Glacial striations by Amezcackle, 2003 (Public domain, https://en.m.wikipedia.org/wiki/File:Glacial_striations.JPG). Glacial striation 21149 by Walter Siegmund, 2007 (CC BY-SA 3.0, https://commons.wikimedia.org/wiki/File:Glacial_striation_21149.JPG).

Figure 14.12: The U-shape of the Little Cottonwood Canyon, Utah, as it enters into the Salt Lake Valley. Wilson44691. 2008. Public domain. https://commons.wikimedia.org/wiki/File:UshapedValleyUT.JPG

Figure 14.13: Formation of a glacial valley. Cecilia Bernal. 2015. CC BY-SA 4.0. https://commons.wikimedia.org/wiki/File:Glacier_Valley_formation-_Formaci%C3%B3n_Valle_glaciar.gif

Figure 14.14: Cirque with Upper Thornton Lake in the North Cascades National Park, Washington (left). Thornton Lakes 25929 by Walter Siegmund, 2007 (CC BY-SA 3.0, https://commons.wikimedia.org/wiki/File:Thornton_Lakes_25929.JPG). Kinnerly Peak by USGS, 1982 (Public domain, https://uk.wikipedia.org/wiki/%D0%A4%D0%B0%D0%B9%D0%BB:Kinnerly_Peak.jpg). Closeup of Bridalveil Fall seen from Tunnel View in Yosemite NP by Daniel Mayer, 2003 (CC BY-SA 1.0, https://commons.wikimedia.org/wiki/File:Closeup_of_Bridalveil_Fall_seen_from_Tunnel_View_in_Yosemite_NP.JPG).

Figure 14.15: Boulder of diamictite of the Mineral Fork Formation, Antelope Island, Utah, United States. Jstuby. 2002. Public domain. https://commons.wikimedia.org/wiki/File:Diamictite_Mineral_Fork.JPG

Figure 14.16: Lateral moraines of Kaskawulsh Glacier within Kluane National Park in the Canadian territory of Yukon (left) and Medial moraines where tributary glaciers meet. Kluane Icefield 1 by Steffen Schreyer, 2005 (CC BY-SA 2.0 DE, https://commons.wikimedia.org/wiki/File:Kluane_Icefield_1.jpg). Nuussuaq-peninsula-moraines by Algkalv, 2010 (CC BY-SA 3.0, https://commons.wikimedia.org/wiki/File:Nuussuaq-peninsula-moraines.jpg).

Figure 14.17: A small group of Ice Age drumlins in Germany. Martin Groll. 2009. CC BY 3.0 DE. https://commons.wikimedia.org/wiki/File:Drumlin_1789.jpg

Figure 14.18: Tarn in a cirque. Jrmichae. 2012. CC BY-SA 4.0. https://commons.wikimedia.org/wiki/File:Verdi_Leak_in_The_Ruby_Mountains.JPG

Figure 14.19: Paternoster lakes. wetwebwork. 2007. CC BY-SA 2.0. https://en.wikipedia.org/wiki/File:View_from_Forester_Pass.jpg

Figure 14.20: Satellite view of Finger Lakes region of New York. NASA. 2004. Public domain. https://commons.wikimedia.org/wiki/File:New_York%27s_Finger_Lakes.jpg

Figure 14.21: Extent of Lake Agassiz. USGS. 1895. Public domain. https://commons.wikimedia.org/wiki/File:Agassiz.jpg

Figure 14.22: View of Channeled Scablands in central Washington showing huge potholes and massive erosion. DKRKaynor. 2019. CC BY-SA 4.0. https://commons.wikimedia.org/wiki/File:Channeled_Scablands.jpg

Figure 14.23: Pluvial lakes in the western United States. Fallschirmjäger. 2013. CC BY-SA 3.0. https://en.wikipedia.org/wiki/File:Lake_bonneville_map.svg

Figure 14.24: The Great Lakes. SeaWiFS Project, NASA/Goddard Space Flight Center, and ORBIMAGE. 2000. Public domain. https://commons.wikimedia.org/wiki/File:Great_Lakes_from_space.jpg

Figure 14.25: Atomospheric CO2 has declined during the Cenozoic from a maximum in the Paleocene–Eocene up to the Industrial Revolution. Robert A. Rohde. 2005. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:65_Myr_Climate_Change.png

Figure 14.26: Rate of isostatic rebound. NASA. 2010. Public domain. https://commons.wikimedia.org/wiki/File:PGR_Paulson2007_Rate_of_Lithospheric_Uplift_due_to_post-glacial_rebound.png



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