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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 boulders that 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. 

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

 

14.4.2 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 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 the ice 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. 

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 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 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[12, 13]. 

14.4.3 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 looks 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 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. 

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, 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 cu mi/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 [16]. 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 [10, 17]. 

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. 

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 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 [19]. These orbital changes created a 41,000–year-long glacial–interglacial 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). 

Watch the video to see summaries of the ice ages, including their characteristics and causes. 

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 [21, 22]. 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 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 [23]. North America’s Great Lakes also exhibit emergent coastline features caused by isostatic rebound since the continental ice sheet retreated. 

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Summary

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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References