At the end of this chapter, students should be able to:
- Differentiate the different types of glaciers and contrast them with sea ice.
- Describe how glaciers move, erode, and create landforms.
- Identify glacial erosional and depositional landforms and interpret their origin.
- Describe the history and causes of past glaciations and their relationship to climate and sea-level changes.
The hydrosphere, liquid water, is the single most important agent of erosion and deposition. The cryosphere, the solid state of water in the form of ice also has its own unique erosional and depositional features. Large accumulations of year-round ice on the land surface are called glaciers. Masses of ice floating on the ocean as sea ice or icebergs are not glaciers, although they may have had their origin in glaciers.
Glaciers cover about 10% of the surface of the Earth, and are powerful erosional agents that sculpt the planet’s surface. Glaciers form when more snow accumulates over a long span of time than melts and eventually turns into ice. This usually occurs in mountainous areas that have both cold temperatures and high precipitation. But snow can also accumulate and turn into ice in extremely cold low lying areas such as Greenland and Antarctica. This chapter focuses on types of glaciers, how glaciers function, erosional and depositional landforms created by glaciers, and how glaciers are connected to past climates and modern day climate change.
14.1 Types of Glaciers
There are two general types of glaciers – alpine glaciers and ice sheets. Alpine glaciers form in mountainous areas either at high elevations or near cool and wet coastal areas. A common type of alpine glacier is a valley glacier which is confined to a long, narrow valley located in mountainous areas especially at higher latitudes (closer to either the north or south pole). Most alpine glaciers are located in the major mountain ranges of the world such as the Andes, Rockies, Alps, and Himalayas.
The other major glacier type is ice sheets (also called a continental glaciers). These are thick accumulations of ice that occupy a large geographical area. The main ice sheets on the earth today are located on Greenland and Antarctica. The Greenland Ice Sheet has an extensive surface area and thickness up to 3,300 meters (10,800 feet or two miles) and has a volume estimated at nearly 3 million cubic kilometers (~102 billion cubic feet) .
The Antarctic Ice Sheet is much larger and covers almost the entire continent. The thickest parts of this massive ice sheet are over 4,000 meters thick (>13,000 feet or 2.5 miles) and its weight depresses the Antarctic bedrock to below sea level in many places beneath the ice . The Antarctic Ice Sheet contains the most ice as illustrated by the figure below comparing cross-sectional views of both ice sheets.
14.2 Glacier Formation and Movement
Glaciers form when accumulating snow compresses into firn and eventually turns into ice. In some cases, perennial snow accumulates on the ground and lasts all year. This makes a snow field and not a glacier since it is a thin accumulation of snow. Snow and glacial ice actually has a fair amount of void space (porosity) that traps air. As the snow settles, compacts, and bonds with underlying snow, the amount of void space diminishes. When the snow gets buried by more snow, it compacts into granular firn (or névé) with less air and it begins to resemble ice more than snow. Continual burial, compression, and recrystallization make the firn more dense and ice like. Eventually, the accumulated snow turns fully to ice, however, small air pockets remain trapped in the ice and form a record of the past atmosphere.
As the ice accumulates it begins to flow downward under its own weight. An early study of glacier movement conducted in 1948 on the Jungfraufirn glacier in the Alps installed hollow vertical rods in the ice and measured the tilt over two years. The study found that the top part was fairly rigid and the bottom part flowed internally. A P-T diagram of ice shows that ice actually melts under pressure (one of the unique properties of water) so ice at the base of a typical glacier is actually melting. About half of the overall glacial movement was from sliding on a film of meltwater along the bedrock surface and half from internal flow . These studies show that the ice near the surface (about the upper 165 feet [50 meters] depending on location, temperature, and flow rate) is rigid and brittle . This upper zone is the brittle zone, the portion of the ice in which ice breaks when it moves forming large cracks along the top of a glacier called crevasses. These crevasses can be covered and hidden by a snow bridge and thus are a hazard for glacier travelers.
Below the brittle zone, there is so much weight of the overlying ice (typically exceeding 100 kilopascals-approximately 100,000 times atmospheric pressure) that it no longer breaks when force is applied to it but rather it bends or flows. This is the plastic zone and ice flows within this zone. The plastic zone represents the great majority of the ice of a glacier and often contains a fair amount of sediment from as large as boulders and as small as silt and clay which act as grinding agents. The bottom of the plastic zone slides and grinds across the bedrock surface and represents the zone of erosion.
14.3 Glacial Budget
Glaciers gain mass during the winter as snow accumulates. During summer the snow melts. The glacier is like a bank account, if more money is coming in (snow accumulating in winter) than going out (snow melting in summer), then the bank account grows. The glacial budget works in a similar way. The glacial budget describes how ice accumulates and melts on a glacier which ultimately determines whether a glacier advances or retreats. The balance of accumulating ice (zone of accumulation) is weighed against melting ice (zone of melting or zone of ablation), and whichever is greater determines whether the glacier will advance or retreat. In the zone of accumulation, the rate of annual snowfall is greater than the rate of melting. In other words, not all of the snow that falls each winter melts during the following summer, and the ice surface is always covered with snow. In the zone of melting or ablation, more ice melts then accumulates as snow during the year. The equilibrium line (or snowline, also called the firnline) marks the boundary between the zones of accumulation and ablation. Below the equilibrium line, in the zone of melting, bare ice is exposed because last winter’s snow has all melted; above that line, the ice is still mostly covered with snow from last winter. The position of the equilibrium line changes from year to year as a function of the balance between snow accumulation in the winter and snowmelt during the summer. More winter snow and less summer melting obviously favors the advance of the equilibrium line (and of the glacier’s leading edge (or terminus), but of these two variables, it is the summer melt that matters most to a glacier’s budget. Cool summers promote glacial advance and warm summers promote glacial retreat .
If warmer summers promote glacial retreat, then overall climate warming over many decades and centuries causes glacier to melt and retreat significantly. Since global climate has been warming due to human burning of fossil fuels , this warming is likely causing the ice sheets to melt (or lose mass) at an increasing rate over years and decades rather than over centuries . This means that as time goes on, the glaciers are melting faster and contributing more to rising sea-level than expected based on previous history.
When ice sheets start to melt, such as those in Antarctica and Greenland, they increase their flow into the ocean eventually creating floating ice sheets. The edges of the glacier or its extension as floating ice break off in a process called calving. In cases like these, the end of the glacier in the fjord may retreat but it will also lose thickness or deflate . A fjord is a narrow ocean-flooded valley with steep walls that was carved by a recent glacier. The retreating glacier or glaciers may add to sea level, and this increased sea level can also add to the the flooding of the former glacially-carved valleys. Glacial retreat and deflation is well illustrated in the 2009 TED Talk by James Balog.
14.4 Glacial Landforms
Glacial landforms are of two kinds, erosional and depositional landforms. Erosional landforms are formed by removing material. The internal pressure and movement within glacial ice causes some melting and glaciers slide over bedrock on a thin film of water. Glacial ice also contain a large amount of sediment such as sand, gravel, and boulders. Together, the movement plucks off bedrock and grinds the bedrock producing a polished surface and a fine sediment called rock flour and other poorly-sorted sediments. Whereas depositional landforms are formed when the ice melts and retreats and leaves behind these sediments with distinct shapes and compositions.
14.4.1 Erosional Glacial Landforms
Both alpine and continental glaciers erode bedrock and create erosional landforms. Glacier are heavy masses of abrasive ice that grind over the surface. Elongated grooves usually about ¼ inch wide (½ centimeter) are created by fragments of rock embedded in the ice at the base of a glacier scraping along the bedrock surface called glacial striations. In addition, rock flour as fine grit in the ice can polish a hard granite or quartzite bedrock to a smooth surface called glacial polish. This figure illustrates these glacial landforms.
Following is a description of erosional landforms produced by alpine glaciers. Since glaciers are typically much wider than rivers of similar length, and since they tend to erode more at their bases than their sides, they produce broad valleys that have relatively flat bottoms and steep sides with a distinctive “U” shape . Little Cottonwood Canyon near Salt Lake City, Utah was occupied by a large glacier that extended down to the mouth of the canyon and into Salt Lake Valley . Today, that canyon is the location of many erosional landforms including the U-shaped valley, as well as polished and striated rock surfaces. In contrast, river-carved canyons have a V-shaped profile when viewed in cross-section. Big Cottonwood Canyon, its neighbor to the north, has that V-shape in its lower parts, indicating that its glacier did not extend clear to its mouth.
When two U-shaped valleys are adjacent to each other, the ridge between them can be carved into a sharp ridge called an arête. Since glaciers erode a broad valley, the arêtes are left behind with steep walls separating them. At the head of a glacially carved valley is a a bowl-shaped feautre called a cirque representing where the head of the glacier is eroding against the mountain by plucking rock away from it. After the glacier is gone, in the bowl at the bottom of the cirque there may be a lake called a tarn. Headward cirque erosion by three or more mountain glaciers produce horns, which are steep-sided, spire-shaped mountains with pronounced cirques on three or more sides. Low points along arêtes or between horns (also mountain passes) are called cols. When a smaller tributary glacier intersects a larger trunk glacier, the smaller glacier erodes down less. Therefore, once the ice has been removed, the tributary valley is left as a hanging valley, sometimes with a waterfall. Where the trunk glacier straightens and widens the former V-shaped valley and erodes the ends of side ridges, the result is a steep triangle-shaped cliff called a truncated spur.
14.4.2 Depositional Glacial Landforms
Sediment is deposited by glaciers in both alpine and continental environments. Since ice is responsible for a large amount of erosion, there is a lot of sediment in glacial ice. When sediment is left behind by a melting glacier it is called till and it is characteristically poorly sorted with grain sizes ranging from clay and silt to subrounded pebbles and boulders. Many of the depositional landforms described in this section are composed of till. Lithified rocks of this type are sometimes referred to as tillites, though that implies an interpretation of glacial origin. A more objective and descriptive term is diamictite, meaning a rock of very different clast sizes.
Material carried by the glacier is called moraine, which is an accumulation of glacial till produced by the grinding and erosive effects of a glacier. In valley glaciers, moraine also includes material falling on the sides of the glacier from the valley walls. The glacier acts like a conveyor belt, carrying sediment inside and on the ice and depositing it at the end and sides of the glacier. Because ice in the glacier is always flowing downslope, the deposited moraines build up even if the end of the glacier doesn’t advance. The type of moraine depends on its location. A terminal moraine is a ridge of unsorted till at the maximum extent of a glacier or the farthest extent of a glacier . When glaciers retreat in episodes, they may leave behind additional deposits called recessional moraines. The recessional moraines look similar to terminal moraines, but are formed when the glacier retreat pauses. Moraines located along the side of a glacier are called lateral moraines and mostly represent material that fell on the sides of the glacier from mass wasting of the valley walls. When two tributary glaciers meet, the two lateral moraines combine to form a medial moraine. Behind the terminal and recessional moraines is a veneer (or thin sheet) of till on top of bedrock called the till sheet (or ground moraine).
In addition to moraines, as glaciers melt they leave behind other depositional landforms. The intense grinding process creates a lot of silt. Streams of water melting from the glacier carry this silt (along with sand and gravel) and deposit it in front of the glacier in an area called an outwash plain or sandur. In addition, when glaciers retreat, they may leave behind large boulders of a type of rock that doesn’t match the local bedrock. These are called glacial erratics. When continental glaciers melt, large blocks of ice can be left behind to melt within the impermeable till and can create a depression called a kettle that can be later filled with surface water as a kettle lake. As glaciers melt, the meltwater flows over the ice surface until it descends into cravasses, perhaps finding channels among the crevasses or continuing to the base of the glacier into channels along the bottom. Such streams located under continental glaciers carry sediment in a sinuous channel within or under the ice, similar to a river. When the ice recedes, the sediment remains as a long sinuous ridge known as an esker. Meltwater descending down through the ice or along the margins of the ice may deposit mounds of sediment that remain as hills called kames.
Also common in continental glacial areas of New York state and Wisconsin are drumlins. A drumlin is an elongated asymmetrical hill with its steepest side pointing upstream to the flow of ice and streamlined side (low angle side) pointing in the direction the ice is flowing.
Drumlins can occur in great numbers in drumlin fields. The origin of drumlins is still debated and some leading ideas are incremental accumulation of till under the glacier, large catastrophic meltwater floods located under the glacier, and surface deformation by the weight of the overlying glacial ice .
14.4.3 Glacial Lakes
Lakes are common features in alpine glacial environments. A lake that is confined to a glacial cirque is known as a tarn such as Silver Lake near Brighton Ski resort located in Big Cottonwood Canyon or Avalanche Lake in Glacier National Park. Tarns are common in areas of alpine glaciation because the thick ice that carves out a cirque also typically hollows out a depression in bedrock that after the glacier is gone, fills with precipitated water.
Lakes that occupy long glacially carved depressions are known as finger lakes. .
In areas of continental glaciation, the crust is depressed isostatically by crustal loading from the weight of thick glacial ice. Basins are formed along the edges of continental glaciers (except for those that cover entire continents like Antarctica and Greenland), and these basins fill with glacial meltwater forming proglacial lakes. The classic example of a proglacial lake is Lake Agassiz, located in mostly in Manitoba, Canada, with Lake Winnipeg serving as the remnant of the lake. Many such lakes, some of them huge, existed at various times along the southern edge of the Laurentide Ice Sheet.
Other proglacial lakes formed when glaciers dammed rivers causing the valley to be flooded. The classic example is Glacial Lake Missoula, which formed in Idaho and Montana when the Clark Fork River was dammed by the ice sheet. During the latter part of the last glaciation about 18,000 years ago, the ice holding back Lake Missoula was breached enough to allow some of the lake water to start flowing out, which escalated into a massive and rapid outflow (over days to weeks) during which much of the volume of the lake drained along the valley of the Columbia River to the Pacific Ocean. It is estimated that this process of massive flooding happened at least 25 times over that span, and in many cases, the rate of outflow was equivalent to the discharge of all of Earth’s current rivers combined.
The landscape produced by these massive floods is preserved in the Channelled Scablands of Idaho, Washington, and Oregon .
During the last glaciation, most of the western United States had a cooler and wetter climate. Due to less evaporation and more precipitation many large lakes formed in the basins of the Basin and Range Province called pluvial lakes. Two of the largest pluvial lakes were Lake Bonneville and Lake Lahontan. Lake Bonneville occupied much of western Utah and eastern Nevada (Figure from Miller et al 2013) and filled Salt Lake Valley, which is densely urbanized today, with water hundreds of feet deep. During the last glaciation, the level of the lake fluctuated many times leaving several pronounced old shorelines in the western portion of Utah including Salt Lake Valley. Lake level peaked around 18,000 years ago and spilled over Red Rock Pass in Idaho into the Snake River causing rapid erosion and a very large flood that rapidly lowered the lake level and scoured land in Pocatello Valley, Snake River Plain, and Twin Falls Idaho. The flood ultimately flowed into the Columbia River across part of the scablands area and had an incredible discharge of about 4,750 km3 which is about the volume of Lake Michigan. This would be as if one of the Great Lakes completely emptied within days. Lake Lahontan existed at about the same time mostly in northwestern Nevada.
14.5 Ice Age Glaciations
A glaciation (or ice age) is when the Earth’s climate is cold enough that large ice sheets grow on continents. There have been four major, well documented glaciations in Earth’s history: one during the Archean-early Proterozoic (~2.5 billion years ago), another in late Proterozoic (~700 million years ago), another in the Pennsylvanian (323 to 300 million years ago), and the most recent Pliocene-Quaternary glaciation (Chapter 15). A minor glaciation that occurred around 440 million years ago in modern-day Africa is also mentioned by some authors. The best studied glaciation is, of course, the most recent. The Pliocene-Quaternary glaciation is a series of many glacial cycles, possibly 18, during the last 2.5 million years. There is especially strong evidence for eight glacial advances within the last 420,000 years as recorded in the Antarctic ice core record (Figure). The last of these, known in popular media as “The Ice Age” but known by geologists as the last glacial maximum, reached its height between 26,500 and 19,000 years ago . This infographic illustrates the glacial and climate changes since the then.
14.5.1 Causes of Glaciations
Why do glaciations occur? The causes include both long-term and short-term factors. In the geologic sense, long-term means a scale of 10’s to 100’s of millions of years and short-term means a 100 to 200,000 year scale. Ideas about long-term causes of glaciations over geologic time include the positioning of continents near poles by plate tectonics and the Wilson Cycle, and changes in ocean circulation due to re-positioning of the continents such as the closing of the Panama Strait. Short-term factors are more recognizable for the most recent Pliocene-Quaternary Glaciation and are most relevant to today’s anthropogenic climate change.
Short-term causes of glacial fluctuations are attributed to cycles in Earth-Sun relations due to variations in the earth’s orbit called Milankovitch Cycles. These cycles affect the amount of incoming solar radiation, and changes in carbon dioxide in the atmosphere. During the Cenozoic, carbon dioxide levels have steadily decreased (Figure) causing a gradual climatic cooling . As the climate cooled, the effects of the Milankovitch Cycles began to influence climate with regular cycles of warming and cooling. Milankovitch Cycles are three orbital changes named after the Serbian astronomer Milutan Milankovitch. The three orbital changes are the wobbling of Earth’s axis called precession with a span of 21,000 years, the angle of Earth’s axis called obliquity with a span of about 41,000 years, and variations of the distance from the sun in Earth’s orbit around the sun referred to as eccentricity with a span of 93,000 years . These orbital changes created a glacial-interglacial cycle of 41,000 years from 2.5 to 1.0 million years ago and a longer cycle of ~100,000 years from 1.0 million years ago to today (Figure). The combination of these three Milankovitch Cycles changes the angle at which the sun’s energy strikes the surface of the earth near the poles and the amount of energy (insolation) received by Earth (Figure). As the climate cooled during the Cenozoic Era, the subtle changes in energy received by the planet were expressed as a warmer and cooler climate cycle, thus the glacial-interglacial cycles.
14.5.2 Sea-Level Change and Isostatic Rebound
Since glaciers are ice located on land (not floating in the ocean), when glaciers melt and retreat two things happen—sea-level rises globally and the land rises locally due to isostatic rebound. Melting glacial water runs off into the ocean and sea-level worldwide will rise. For example, since the last glacial maximum about 19,000 years ago sea-level has risen about 400 feet (125 meters) . An overall global change in sea level is called eustatic sea-level change. More water in the ocean causes a eustatic sea-level rise. Another important factor causing eustatic sea-level rise is thermal expansion. According to basic physics, thermal expansion occurs when a solid, liquid, or gas expands in volume when the temperature increases (Figure). Watch this 30 second video demonstrating thermal expansion with the classic brass ball and ring experiment. About ⅔ of the eustatic sea-level rise during the last interglacial span was the result of thermal expansion .
However, tectonics and isostatic rebound can move the land up and down. The change of sea level as it relates to a more local continental landscape is called relative sea-level change. Relative sea-level change includes both the vertical movement of eustatic sea-level and the vertical movement of land, so that the sea-level change is measured relative to the land. Therefore, if the land rises a lot and sea-level rises only a little, then the sea-level would appear to go down.
The lithosphere can move vertically as a result of two main processes, tectonics and isostatic rebound. Tectonic uplift occurs when tectonic plates collide as discussed in the Plate Tectonics chapter. Isostasy describes the equilibrium that exists for the earth’s lithosphere, where denser lithosphere “sinks” lower on top of the asthenosphere and less dense lithosphere “floats” higher on the asthenosphere. Isostatic rebound is when some weight is removed from the continental lithosphere causing it to “float” higher on the asthenosphere. Erosion can remove this weight very slowly or the relatively rapid removal of glaciers can remove a lot of weight in a short amount of time. Melting glaciers removes weight from the continental lithosphere causing it to rise or “rebound” from being previously depressed. Most glacial isostatic rebound is occurring where glaciers recently melted (19,000 years ago) such as Canada and Scandinavia. Glacial isostatic rebound causes the relative sea-level to fall or rise less quickly as seen from the land. Isostatic rebound also occurred in Utah when the water from Lake Bonneville was removed . Isostatic rebound is still taking place wherever Ice Age ice or water bodies were present on continental surfaces. Its effects can be seen in terraces forming on river floodplains that cross these areas.