13 Deserts and Glaciers

At first glance, deserts and glaciers may seem like opposites—one conjures images of endless heat and aridity, while the other evokes thoughts of frozen, icy expanses. However, both deserts and glaciers are part of a larger conversation about extreme environments shaped by similar geological forces. These landscapes are defined by scarcity, either of water or warmth, and both are profoundly influenced by processes of erosion, deposition, and weathering.

Deserts, which cover about one-third of the Earth’s land surface, are characterized by their dry conditions, where evaporation exceeds precipitation. Although glaciers are composed of frozen water, they can be considered “frozen deserts” because they too exist in regions where precipitation is low, albeit in solid form. In both environments, the absence or freezing of water leads to unique landforms and geologic features. Understanding these landscapes not only enriches our appreciation of the diversity of Earth’s environments but also highlights their vulnerability to climate change.

This chapter is divided into two parts; the first half explores the characteristics, formation, and geological processes of deserts, while the second half delves into the features and dynamics of glaciers.

13.1 The Origin of Deserts

Approximately 30% of the Earth’s terrestrial surface consists of deserts, which are defined as locations of low precipitation. While temperature extremes are often associated with deserts, they do not define them. Deserts exhibit extreme temperatures because of the lack of moisture in the atmosphere, including low humidity and scarce cloud cover. Without cloud cover, the Earth’s surface absorbs more of the Sun’s energy during the day and emits more heat at night.

Deserts are not randomly located on the Earth’s surface. Many deserts are located at latitudes between 15° and 30° in both hemispheres and at both the North and South Poles, created by prevailing wind circulation in the atmosphere. Sinking, dry air currents occurring at 30° north and south of the equator produce trade wind deserts like the African Sahara and Australian Outback.

Another type of desert is found in the rain shadow created from prevailing winds blowing over mountain ranges. As the wind drives air up and over mountains, atmospheric moisture is released as snow or rain. Atmospheric pressure is lower at higher elevations, causing the moisture-laden air to cool. Cool air holds less moisture than hot air, and precipitation occurs as the wind rises up the mountain. After releasing its moisture on the windward side of the mountains, the dry air descends on the leeward, or downwind, side of the mountains to create an arid region with little precipitation called a rain shadow. Examples of rain-shadow deserts include the Western Interior Desert of North America and Atacama Desert of Chile, which is the Earth’s driest warm desert.

Finally, polar deserts, such as vast areas of the Antarctic and Arctic, are created from sinking cold air that is too cold to hold much moisture. Although they are covered with ice and snow, these deserts have very low average annual precipitation. As a result, Antarctica is Earth’s driest continent.

13.1.1 Atmospheric Circulation

Geographic location, atmospheric circulation, and the Earth’s rotation are the primary causal factors of deserts. Solar energy converted to heat is the engine that drives the circulation of air in the atmosphere and water in the oceans. The strength of the circulation is determined by how much energy is absorbed by the Earth’s surface, which in turn is dependent on the average position of the Sun relative to the Earth. In other words, the Earth is heated unevenly depending on latitude and angle of incidence. Latitude is a line circling the Earth parallel to the equator and is measured in degrees. The equator is 0°, and the North and South Poles are 90° N and 90° S, respectively (see Figure 13.4 for a diagram of generalized atmospheric circulation on Earth). Angle of incidence is the angle made by a ray of sunlight shining on the Earth’s surface. Tropical zones are located near the equator, where the latitude and angle of incidence are close to 0°, and receive high amounts of solar energy. The poles, which have latitudes and angles of incidence approaching 90°, receive little or almost no energy.

The figure shows the generalized air circulation within the atmosphere. Three cells of circulating air span the space between the equator and poles in both hemispheres, the Hadley cell, the Ferrel or midlatitude cell, and the polar cell. In the Hadley cell, located over the tropics and closest to the equatorial belt, the Sun heats the air and causes it to rise. The rising air cools and releases its contained moisture as tropical rain. The rising dried air spreads away from the equator and toward the North and South Poles, where it collides with dry air in the Ferrel cell. The combined dry air sinks back to the Earth at 30° latitude. This sinking drier air creates belts of predominantly high pressure at approximately 30° north and south of the equator, called the horse latitudes. Arid zones between 15° and 30° north and south of the equator thus exist, within which desert conditions predominate. The descending air flowing north and south in the Hadley and Ferrel cells also creates prevailing winds called trade winds near the equator and westerlies in the temperate zone. Note the arrows indicating general directions of winds in these zones.

Other deserts, like the Great Basin Desert that covers parts of Utah and Nevada, owe at least part of their origin to other atmospheric phenomena. The Great Basin Desert, while somewhat affected by sinking air effects from global circulation, is a rain-shadow desert. As westerly moist air from the Pacific rises over the Sierra Nevadas and other mountains, it cools and loses moisture as condensation and precipitation on the upwind, rainy side of the mountains.

One of the driest places on Earth is the Atacama Desert of northern Chile. The Atacama Desert occupies a strip of land along Chile’s coast just north of latitude 30°S, at the southern edge of the trade-wind belt. The desert lies west of the Andes Mountains, in the rain shadow created by prevailing trade winds blowing west. As this warm moist air crossing the Amazon basin meets the eastern edge of the mountains, it rises, cools, and precipitates much of its water out as rain. Once over the mountains, the cool, dry air descends onto the Atacama Desert. Onshore winds from the Pacific are cooled by the Peru (Humboldt) ocean current. This super-cooled air holds almost no moisture and, with these three factors, some locations in the Atacama Desert have received no measured precipitation for several years. This desert is the driest nonpolar location on Earth.

Notice in Figure 13.7 that the polar regions are also areas of predominantly high pressure created by descending cold dry air, the polar cells. As with the other cells, cold air, which holds much less moisture than warm air, descends to create polar deserts. This is why historically, land near the North and South Poles has always been so dry.

13.1.2 Coriolis Effect

The Earth rotates toward the east, where the sun rises. Think of spinning a weight on a string around your head. The speed of the weight depends on the length of the string. The speed of an object on the rotating Earth depends on its horizontal distance from the Earth’s axis of rotation. Higher latitudes are a smaller distance from the Earth’s rotational axis and therefore do not travel as fast eastward as lower latitudes that are closer to the equator. When a fluid like air or water moves from a lower latitude to a higher latitude, the fluid maintains its momentum from moving at a higher speed, so it will travel relatively faster eastward than the Earth beneath at the higher latitudes. This factor causes deflection of movements that occur in north-south directions.

Another factor in the Coriolis effect also causes deflection of east-west movement due to the angle between the centripetal effect of Earth’s spin and gravity pulling toward the Earth’s center (see Figure 13.9). This produces a net deflection toward the equator. The total Coriolis deflection on a mass moving in any direction on the rotating Earth results from a combination of these two factors.

Since each hemisphere has three atmospheric cells moving respectively north and south relative to the Earth beneath them, the Coriolis effect deflects these moving air masses to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The Coriolis effect also deflects moving masses of water in the ocean currents.

For example, in the Northern Hemisphere Hadley cell, the lower altitude air currents are flowing south toward the equator. These are deflected to the right (or west) by the Coriolis effect. This deflected air generates the prevailing trade winds that European sailors used to cross the Atlantic Ocean and reach South America and the Caribbean Islands in their tall ships. This air movement is mirrored in the Hadley cell in the Southern Hemisphere; the lower-altitude air current flowing toward the equator is deflected to the left, creating trade winds that blow to the northwest.

In the northern Ferrel, or midlatitude, cell, surface air currents flow from the horse latitudes (latitude 30°) toward the North Pole, and the Coriolis effect deflects them toward the east or to the right, producing the zone of westerly winds. In the Southern Hemisphere Ferrel cell, the poleward flowing surface air is deflected to the left and flows southeast, creating the Southern Hemisphere westerlies.

Another Coriolis-generated deflection produces the polar cells. At 60° north and south latitude, relatively warmer rising air flows poleward, cooling and converging at the poles where it sinks in the polar high. This sinking dry air creates the polar deserts, the driest deserts on Earth. The persistence of ice and snow is a result of cold temperatures at these dry locations.

The Coriolis effect operates on all motions on the Earth. Artillerymen must take the Coriolis effect into account on ballistic trajectories when making long-distance targeting calculations. Geologists note how its effect on air and oceanic currents creates deserts in designated zones around the Earth as well as creating the surface currents in the ocean. The Coriolis effect causes the ocean gyres to turn clockwise in the Northern Hemisphere and counterclockwise in the Southern. It also affects weather by creating high-altitude, polar jet streams that sometimes push lobes of cold arctic air into the temperate zone, down to as far as latitude 30° from the usual 60°. It also causes low-pressure systems and intense tropical storms to rotate counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.

13.2 Desert Processes

13.2.1 Desert Weathering and Erosion

Weathering takes place in desert climates by the same means as other climates, only at a slower rate. While higher temperatures typically spur faster chemical weathering, water is the main agent of weathering, and lack of water slows both mechanical and chemical weathering. Low precipitation levels also mean less runoff as well as ice wedging. When precipitation does occur in the desert, it is often heavy and may result in flash floods in which a lot of material may be dislodged and moved quickly.

One unique weathering product in deserts is desert varnish. Also known as desert patina or rock rust, this is thin dark-brown layers of clay minerals and iron and manganese oxides that form on very stable surfaces within arid environments. The exact way this material forms is still unknown, though cosmogenic and biologic mechanisms have been proposed.

While water is still the dominant agent of erosion in most desert environments, wind is a notable agent of weathering and erosion in many deserts. This includes suspended sediment traveling in haboobs, or large dust storms, that frequent deserts. Deposits of windblown dust are called loesses. Loess deposits cover wide areas of the Midwestern United States, much of it from rock flour that melted out of the ice sheets during the last ice age. Loess was also blown from desert regions in the West. Possessing lower energy than water, wind transport nevertheless moves sand, silt, and dust. As noted in Chapter 11, the load carried by a fluid (air is a fluidlike water) is distributed among bedload and suspended load. As with water, in wind these components depend on wind velocity.

Sand-sized material moves by a process called saltation in which sand grains are lifted into the moving air and carried a short distance, where they drop and splash into the surface, dislodging other sand grains that are then carried a short distance and splash, dislodging still others.

Since saltating sand grains are constantly impacting other sand grains, windblown sand grains are commonly quite well rounded, with frosted surfaces. Saltation is a cascading effect of sand movement, creating a zone of windblown sand up to a meter or so above the ground. This zone of saltating sand is a powerful erosive agent in which bedrock features are effectively sandblasted. The fine-grained suspended load is effectively sorted from the sand near the surface, carrying the silt and dust into haboobs. Wind is thus an effective sorting agent separating sand and dust-sized (≤70 µm) particles (see Chapter 5). When wind velocity is high enough to slide or roll materials along the surface, the process is called creep.

One extreme version of sediment movement was shrouded in mystery for years: sliding stones. Also called sailing stones and sliding rocks, these are large boulders that move along flat surfaces in deserts, leaving trails. This includes the famous example of the Racetrack Playa in Death Valley National Park, California. For years, scientists and enthusiasts attempted to explain their movement, with few definitive results. In recent years, several experimental and observational studies have confirmed that the stones, embedded in thin layers of ice, are propelled by friction from high winds. These studies include measurements of actual movement, as well as recreations of the conditions, with resulting movement in the lab.

The zone of saltating sand is an effective agent of erosion through sand abrasion. A bedrock outcrop which has such a sandblasted shape is called a yardang. Rocks and boulders lying on the surface may be blasted and polished by saltating sand. When predominant wind directions shift, multiple sandblasted and polished faces may appear. Such wind-abraded desert rocks are called ventifacts.

In places with sand and silt accumulations, clumps of vegetation often anchor sediment on the desert surface. Yet, winds may be sufficient to remove materials not anchored by vegetation. The bowl-shaped depression remaining on the surface is called a blowout.

13.2.2 Desert Landforms

In the American Southwest, as streams emerge into the valleys from the adjacent mountains, they create desert landforms called alluvial fans. When a stream emerges from the narrow canyon, the flow is no longer constrained by the canyon walls and spreads out. At the lower slope angle, the water slows down and drops its coarser load. As the channel fills with this conglomeratic material, the stream is deflected around it. This deposited material deflects the stream into a system of radial distributary channels in a process similar to a delta’s formation by a river entering a body of water. This process develops a system of radial distributaries and constructs a fan-shaped alluvial fan.

Alluvial fans continue to grow and may eventually coalesce with neighboring fans to form an apron of alluvium along the mountain front called a bajada.

As the mountains erode away and their sediment accumulates first in alluvial fans, then bajadas, the mountains eventually are buried in their own erosional debris. Such buried mountain remnants are called inselbergs, “island mountains,” as first described by the German geologist Wilhelm Bornhardt (1864–1946).

Where the desert valley is an enclosed basin—i.e., streams entering it do not drain out—the water is removed by evaporation and a dry lake bed called a playa is formed.

Playas are among the flattest of all landforms. Such a dry lake bed may cover a large area and be filled after a heavy thunderstorm to only a few inches deep. Playa lakes and desert streams that contain water only after rainstorms are called intermittent or ephemeral. Because of intense thunderstorms, the volume of water transported by ephemeral drainage in arid environments can be substantial during a short period of time. Desert soil structures lack organic matter that promotes infiltration by absorbing water. Instead of percolating into the soil, the runoff compacts the ground surface, making the soil hydrophobic (i.e., water-repellant). Because of this hardpan surface, ephemeral streams may gather water across large areas, suddenly filling with water from storms many miles away.

High-volume ephemeral flows, called flash floods, may move as sheet flows or sheetwash; they can also be channeled through normally dry arroyos or canyons. Flash floods are a major factor in desert deposition. Dry channels can fill quickly with ephemeral drainage, creating a mass of water and debris that charges down the arroyo, even overflowing the banks. Flash floods pose a serious hazard for desert travelers because the storm activity feeding the runoff may be miles away. People hiking or camping in arroyos that have been bone dry for months or years have been swept away by sudden flash floods.

13.2.3 Sand

The popular concept of a typical desert is a broad expanse of sand. Geologically, deserts that are defined by a lack of water and arid regions resembling seas of sand belong to the category of desert called an erg. An erg consists of fine-grained, loose sand grains, often blown by wind, or aeolian forces, into dunes. Probably the best known erg is the Rub’ al Khali, which means empty quarter, of the Arabian Peninsula. Ergs are also found in the Great Sand Dunes National Park (Colorado), Little Sahara Recreation Area (Utah), White Sands National Monument (New Mexico), and parts of Death Valley National Park (California). Ergs are not restricted to deserts but may form anywhere there is a substantial supply of sand, including as far north as 60° N in Saskatchewan, Canada, in the Athabasca Sand Dunes Provincial Park. Coastal ergs exist along lakes and oceans as well, and examples are found in Oregon, Michigan, and Indiana.

An internal cross section of a sand dune shows a feature called cross-bedding. As wind blows up the windward side of the dune, it carries sand to the dune crest, depositing layers of sand parallel to the windward (or “stoss”) side. The sand builds up the crest of the dune and pours over the top until the leeward (downwind or slip) face of the dune reaches the angle of repose, the maximum angle which will support the slip face. Dunes are unstable features and move as the sand erodes from the stoss side and continues to drop down the leeward side, covering previous stoss and slip-face layers and creating the cross-beds. Mostly, these are reworked over and over again, but occasionally, the features are preserved in a depression, then lithified. Shifting wind directions and abundant sand sources create chaotic patterns of cross-beds like those seen in Zion National Park of Utah.

In the Mesozoic Era, Utah was covered by a series of ergs, with the thickest being in southern Utah, which lithified into sandstone (see Chapter 5). Perhaps the best known of these sandstone formations is the Navajo Sandstone of Jurassic age. This sandstone formation consists of dramatic cliffs and spires in Zion National Park and covers a large part of the Colorado Plateau. In Arches National Park, a later series of sand dunes covered the Navajo Sandstone and lithified to become the Entrada Formation also during the Jurassic. Erosion of overlying layers exposed fins of the underlying Entrada Sandstone and carved out weaker parts of the fins forming the arches.

As the cements that hold the grains together in these modern sand cliffs disintegrate and the freed grains gather at the base of the cliffs and move down the washes, sand grains may be recycled and redeposited. These Mesozoic sand ergs may represent ancient quartz sands recycled many times from igneous origins in the early Precambrian, just passing now through another cycle of erosion and deposition. An example of this is Coral Pink Sand Dunes State Park in southwestern Utah, which contains sand that is being eroded from the Navajo Sandstone to form new dunes.

13.2.4 Dune Types

Dunes are complex features formed by a combination of wind direction and sand supply, in some cases interacting with vegetation. There are several types of dunes representing variables of wind direction, sand supply, and vegetative anchoring. Crescent-shaped barchan dunes form where sand supply is limited and there is a fairly constant wind direction. Barchans move downwind and develop a crescent shape with wings on either side of a dune crest. Barchans are known to actually move over homes, even towns.

Longitudinal dunes, or linear dunes, form where sand supply is greater and the wind blows a dominant direction in a back-and-forth manner. They may form ridges tens of meters high lined up with the predominant wind directions.

Parabolic dunes form where vegetation anchors parts of the sand and unanchored parts blow out. Parabolic dune shape may be similar to barchan dunes but usually reversed, and it is determined more by the anchoring vegetation than a strict parabolic form.

Star dunes form where the wind direction is variable in all directions. Sand supply can range from limited to quite abundant. It is the variation in wind direction that forms the star.

13.3 The Great Basin and the Basin and Range

The Great Basin is the largest area of interior drainage in North America, meaning there is no outlet to the ocean and all precipitation remains in the basin or is evaporated. It covers western Utah, most of Nevada, and extends into southeastern California, southern Oregon, and southern Idaho. Because there is no outlet to the ocean, streams in the Great Basin deliver runoff to lakes and playas within the basin. A subregion within the Great Basin is the Basin and Range, which extends from the Wasatch Front in Utah west across Nevada to the Sierra Nevada Mountains of California. The basins and ranges referred to in the name are horsts and grabens, formed by normal fault blocks from crustal extension, as discussed in Chapters 2 and 9. The lithosphere of the entire area has stretched by a factor of about 2, meaning from end to end, the distance has doubled over the past 30 million years or so. Valleys without outlets form individual basins, each of which is filled with alluvial sediments leading into playa depositional environments. During the recent ice age, the climate was more humid, and while glaciers were forming in some of the mountains, pluvial lakes formed over large areas. During the ice age, valleys in much of western Utah and eastern Nevada were covered by Lake Bonneville. As the climate became arid after the ice age, Lake Bonneville dried, leaving as a remnant the Great Salt Lake in Utah.

The desert of the Basin and Range extends from about 35° to near 40° and results from a rain shadow effect created by westerly winds from the Pacific rising and cooling over the Sierras, becoming depleted of moisture by precipitation on the western side. The result is relatively dry air descending across Nevada and western Utah.

A journey from the Wasatch Front southwest to the Pacific Ocean will show stages of desert landscape evolution from the fault block mountains of Utah with sharp peaks and alluvial fans at the mouths of canyons, through landscapes in Southern Nevada with bajadas along the mountain fronts, to the landscapes in the Mojave Desert of California with subdued inselbergs sticking up through a sea of coalesced bajadas. These landscapes illustrate the evolutionary stages of desert landscape development.

13.3.1 Desertification

When previously arable land suitable for agriculture transforms into desert, this process is called desertification. Plants and humus-rich soil (see Chapter 5) promote groundwater infiltration and water retention. When an area becomes more arid due to changing environmental conditions, the plants and soil become less effective in retaining water, creating a positive feedback loop of desertification. This self-reinforcing loop spirals into increasingly arid conditions and further enlarges the desert regions.

Desertification may be caused by human activities, such as unsustainable crop-cultivation practices, overgrazing by livestock, overuse of groundwater, and global climate change. Human-caused desertification is a serious worldwide problem. The world map figure above shows what areas are most vulnerable to desertification. Note the red and orange areas in the Western and Midwestern regions of the United States, which also cover large areas of arable land used for raising food crops and animals. The creation of the Dust Bowl in the 1930s (see Chapter 5) is a classic example of a high-vulnerability region impacted by human-caused desertification. As demonstrated in the Dust Bowl, conflicts may arise between agricultural practices and conservation measures. Mitigating desertification while allowing farmers to make a survivable living requires public and individual education to create community support and understanding of sustainable agriculture alternatives.

13.4 Glaciers

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 of 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.

13.4.1 Glacier Formation

Glaciers form when repeated annual snowfall accumulates as 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 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.

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 1,500 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.

Ice cap glaciers are smaller versions of ice sheets that cover less than 50,000 km², 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 Mount Olympus on the Olympic Peninsula in the state of Washington.

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.

13.5 Glacier Dynamics

13.5.1 Glacier Movement

As the ice accumulates, it begins to flow downward under its own weight. In 1948, glaciologists installed hollow vertical rods in 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.

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, with the other 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.

13.5.2 Glacial Budget

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 in which the ice balance grows 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.

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 sea levels rising more quickly 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 in a process called calving, becoming floating icebergs. A fjord is a steep-walled valley flooded with seawater. 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.

13.6 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 eighteenth- and nineteenth-century geologists in Europe, the terminology applied to glaciers and glacial features contains many terms derived from European languages.

13.6.1 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 boulders that have been plucked and pried from the bedrock. As the glaciers slide across the bedrock, they grind the 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.

Alpine glaciers produce a variety of unique erosional landforms, such as U-shaped valleys, arêtes, cirques, tarns, horns, cols, 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.

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 as it erodes the ends of side ridges, a steep triangle-shaped cliff is formed called a truncated spur.

13.6.2 Depositional Glacial Landforms

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

Glacial ice carries a lot of sediment, which is called till when deposited by a melting glacier. 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 always flows downslope, every glacier has moraines built up at its terminus, even if it is 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 by retreating glaciers pausing in their retreats. 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.

In addition to moraines, glaciers leave behind other depositional landforms. Silt, sand, and gravel produced by the intense grinding process are carried by streams of water and deposited in 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.

Drumlins are common in continental glacial areas of Germany, New York, and Wisconsin, where they are typically 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, 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 a glacier 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.

13.6.3 Glacial Lakes

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.

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 look milky due to finely ground material called rock flour suspended in the water.

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.

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 2,000 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.

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

Pluvial lakes form in humid environments that experience low temperatures and high precipitation. During the last glaciation, the climate of most of the Western United States 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, while Bonneville occupied much of western Utah and eastern Nevada. Figure 13.58 illustrates the tremendous size of Lake Bonneville. 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 were then filled by precipitation.

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.

13.7.1 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.

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 that 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 the Milankovitch 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 glacial-interglacial Milankovitch cycle from 2.5 to 1 million years ago, followed by another longer cycle of about 100,000 years from 1 million years ago to today (see the Wikipedia page on Milankovitch cycles).

13.7.2 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 because of thermal expansion of seawater. 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.

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.

The isostatic-rebound map in Figure 13.61 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 occurs 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.

Summary

Approximately 30% of Earth’s surface is arid lands shaped by factors such as latitude, atmospheric circulation, and terrain. Deserts between 15° and 30° latitude are formed by descending air masses in the atmosphere’s major cells, including the Sahara and deserts of the Middle East. Other deserts, like those in western North America and the Atacama in South America, form in rain shadows behind mountain ranges. Dry, descending air also creates polar deserts at the poles.

Atmospheric circulation, involving the Hadley, Ferrel, and polar cells, drives desert formation. Warm air rises in the tropics, dries out, and descends in the arid zones of both hemispheres. The Earth’s rotation deflects air masses, producing prevailing winds like the trade winds and westerlies. Complex interactions of latitude, rain shadows, and ocean currents make the Atacama Desert the driest place on Earth.

In deserts, weathering occurs slowly due to low moisture, with unique features like desert varnish and wind-driven erosion. Wind transports sand, shaping landforms like dunes and eroded bedrock features. Poor land management can accelerate desertification, a global problem that degrades once-productive land into desert.

Glaciers form when snowfall exceeds melting, compressing snow into ice. They can be classified into alpine glaciers in valleys, ice sheets covering continents, and ice caps over smaller areas at high elevations. As glaciers grow, they flow under their own weight, with the upper brittle ice cracking into crevasses, while the lower plastic zone bends and flows. Rock debris, known as moraine, gets carried along, grinding against bedrock and causing significant erosion.

Glaciers have a budget that balances accumulation and ablation. If more ice accumulates than melts, the glacier advances; if melting exceeds accumulation, the glacier recedes, a trend seen globally due to warming. Glaciers shape landscapes by carving U-shaped valleys and depositing features like moraines, drumlins, and kettle lakes. Their meltwater forms outwash plains and can create massive floods, as seen with ice age lakes like Lake Missoula, which caused widespread erosion in areas like the Channeled Scablands.