Glaciers, Glacial Geology, and Environments
by Owen Borville
January 30, 2021
Learning, Geology, Science
A glacier is a large perennial accumulation of ice, snow, rock, sediment and liquid water originating on land. Valley glaciers form in mountainous regions at high altitudes and fill stream valleys. The movement downslope is confined by the paths of the valley. Valley glaciers move down slope under the influence of weight and gravity. Glaciers are dynamic rivers of ice and are classified by their size, location, and thermal regime. A continental glacier is a thick, subcontinental to continental scale accumulation of glacier ice and perennial snow that spreads from a center of accumulation, typically in all directions (also called an ice sheet). Continental glaciers can be found in Greenland and Antarctica.
The glacial budget is the balance of accumulation of ice at the top of a glacier by snowfall in relation to the loss of ice at the bottom of the glacier by melting. The accumulation area is the part of a glacier that is perennially covered with snow or where snow accumulation exceeds melting (also called Névé). The accumulation zone is the area at the top of the glacier where the addition of ice and snow into a glacier system occurs through a variety of processes. These processes include precipitation and wind transportation of snow from an adjacent area into a glacier basin and the head of a glacier. The wastage zone is the lower and warmer altitude area down slope from a valley glacier where the ice melts away faster than it can build up. Ablation refers to all processes by which snow, ice, or water in any form is lost from a glacier including evaporation and melting.
A firn is an intermediate stage in the transformation of snow to glacier ice where snow has been compressed so that no pore space remains between flakes or crystals, a process that takes less than a year. The firn limit is the term used for the boundary where the amount of snow loss from melting and evaporation equals the amount of snow accumulation from snowfall (also called the annual snowline). The firn line (snow line) is the line across the glacier that marks the transition between exposed glacier ice and the snow-covered surface of a glacier. During the summer melt season, this line migrates up the glacier. At the end of the melt season the firn line separates the accumulation zone from the ablation zone.
Glacier flow is the movement of ice in a glacier normally in a downward and outward direction and is caused by the force of gravity. Normal flow rates are in feet per day. Rapid flow rates are 10 or 100 feet per day.
Abrasion is the wearing away of the Earth’s surface by agents of erosion such as wind, water, or ice. Glacial striations (grooves) are lines or scratches on rock that result from the scraping caused by the rock trapped under a glacier and against the bedrock surface as the glacier advances. Plucking or quarrying is the mechanical removal of pieces of rock from a bedrock face that is in contact with glacier ice. Blocks of rock are quarried and prepared for removal by the freezing and thawing of water in cracks, joints, and fractures. The resulting pieces of rock are frozen into the glacier ice and transported. Glacial polish is the smooth, shiny rock surface produced by the movement of a glacier over the surface along with sediment.
Glacial drift is a general term applied to all rock material (including clay, silt, sand, gravel, and boulders) transported by a glacier and deposited directly by the ice, or by running water issued from a glacier. Glacial till is dominantly unsorted and unstratified drift, which is generally unconsolidated and deposited directly by and underneath a glacier without subsequent reworking by meltwater. Stratified drift consists primarily of sand and gravel deposits that were deposited in layers by meltwater streams flowing from retreating glacial ice.
A U-shaped valley has a parabolic or U-shaped cross-section, steep walls, and generally a broad and flat floor that is formed by glacier erosion. A U-shaped valley results when a glacier widens a V-shaped stream valley. A fiord (fjord) is a glacially eroded or modified U-shaped valley that extends below sea level and connects to the ocean. Filled with seawater, depths may reach more than 1,000 feet below sea level. The largest Alaskan fiords are more than 100 miles long and more than five miles wide.
A hanging valley is a former tributary glacier valley that is incised into the upper part of a U-shaped glacier valley and is higher than the floor of the main valley as a result of glacial erosion of the main valley. Hanging valley streams often enter the main valley as waterfalls. A cirque is a bowl-shaped or amphitheater-like depression eroded into the head or the side of a glacier valley. Typically, a cirque has a lip at its lower end. A tarn is a lake that develops in the basin of a cirque, generally after the melting of the glacier. An arête is a jagged, narrow ridge that separates two adjacent glacier valleys or cirques. The ridge frequently resembles the blade of a serrated knife. A horn is a pointed mountain peak, typically pyramidal in shape and bounded by the walls of three or more cirques. Headward erosion has cut prominent faces and ridges into the peak. A peak that has four symmetrical faces is called a Matterhorn.
A crevasse is a crack or series of cracks that open in the surface of a moving glacier in response to differential stresses caused by glacier flow. A bergschrund is a single large crevasse or series of sub-parallel crevasses that develop at the head of a glacier where ice pulls away from the bedrock wall of the cirque against which the glacier accumulated. In the winter, the crevasse fills with snow and in the spring or summer, the crevasse reopens. Bergy Seltzer is a crackling or sizzling sound (similar to that made by soft drinks or seltzer water) made as air bubbles formed at many atmospheres of pressure are released during the melting of glacier ice (also called ice sizzle).
Glacial erratics are rocks of unspecified shape and size transported a significant distance from the original location by a glacier or iceberg and deposited by the melting of the ice. Glacial erratics range from the size of a pebble to larger than a house and usually are of a different composition than the bedrock or sediment on which they are deposited. A roche moutonee is a small asymmetric-shaped hill formed by glacial erosion. The upper sides are rounded and smoothed while the lower sides are rough and broken due to quarrying by the glacier.
Moraines are deposits of drift from a glacier. A lateral moraine is a sediment ridge located on a glacier surface adjacent to the valley walls and extends down glacier to the terminus. Lateral moraines form by the accumulation of rock material falling onto the glacier from the valley wall, rather than by water deposition. An end moraine is an accumulation of rock that forms at the current terminus of a glacier. Terminal moraines are ridge-like accumulations of glacial sediment that form at the furthest point reached by the terminus of an advancing glacier (an end moraine can be a terminal moraine if it is located at the furthest point reached by the glacier). A recessional moraine is a ridge of glacial sediment that forms when the terminus of a retreating glacier remains at or near a single location for a period of time sufficient for a cross-valley accumulation to form. A medial moraine is a sediment ridge located on the exposed ice surface of the glacier away from its valley walls and extending down glacier to the terminus. Medial moraines form by the joining of two lateral moraines when two glaciers merge. A ground moraine is a blanket of glacial till deposited on all of the surfaces over which a glacier moves.
Drumlins are elongated ridges of glacial sediment which are positioned parallel to the direction of glacial flow. Generally the down-glacier end is oval or rounded and the up-glacier end tapers. Rock flour (glacial flour) is fine-grained, silt-sized sediment formed by the mechanical erosion of bedrock at the base and sides of a moving glacier. When the rock flour enters a stream, the stream color turns brown, gray, iridescent blue-green or milky white.
An outwash plain (or sandur) is a broad, low-sloped alluvial plain composed of glacially eroded and sorted sediment (termed outwash) that has been transported by meltwater. The outwash plain begins at the foot of a glacier and may extend for miles. Typically the sediment becomes finer grained with increasing distance from the glacier terminus. Long glacial outwash deposits similar to braided stream deposits are called valley trains. A glacier outburst flood can result from the failure of a glacier ice dam, a glacier sediment dam, or from the melting of glacier ice by a volcanic eruption. These outburst floods are termed Jökulhlaup. A kettle is a depression that forms in an outwash plain or other glacial deposit by the melting of a block of glacier ice that was separated from the retreating glacier margin and subsequently buried by glacier sedimentation. As the buried ice melts, the depression enlarges.
A kame is a stratified sand and gravel deposit that forms in crevasses, cracks, or holes on a stagnant or moving glacier or glacial ice and is deposited by running water. A kame terrace forms between the glacier and the adjacent land surface. Shapes formed include hills, mounds, knobs, hummocks, or ridges. An esker is a meandering, water-deposited, generally steep-sided sediment ridge that forms in a stream channel on or underneath a glacier. The floor of the esker can be bedrock, sediment, or ice. Subsequent melting of the glacier exposes the deposit. Generally composed of stratified sand and gravel, eskers can range from feet to miles in length and may exceed 100 feet in height.
A varve is a pair of sedimentary layers, or a couplet, that forms in an annual cycle as the result of seasonal weather changes. Varve couplets are typically formed in glacial lakes and consist of a coarser grained summer layer formed during open-water conditions, and a finer grained winter layer formed from deposition from suspension during a period of winter ice cover.
Isostasy occurs when heavy loads from glaciers accumulate on the Earth’s surface and the crust or lithosphere of the Earth is depressed. This causes the crust to sink into the mantle and is frequently caused by advances of glaciers. Isostatic rebound occurs when the lithosphere rises or rebounds after layers of rock have been removed by erosion or after a glacier melts. Mountain building also causes isostasy when orogenic activity occurs along with erosion.
Loess is a thick wind-blown deposit (or possibly water transported) of sediment made mostly of silt-sized grains. The composition of loess is a gritty, lightweight, porous material composed of tightly packed grains of quartz, feldspar, mica, and other minerals. Loess can originate from glacial deposits, deserts, dune fields, playa lakes, or volcanic ash. During the Ice Age, glaciers advanced down into the mid-continent of North America, grinding underlying rock into fine powder-like sediment called glacial flour. As temperatures warmed, the glaciers melted and enormous amounts of water and sediment rushed down the Missouri River valley. The sediment was eventually deposited on flood plains downstream, creating huge mud flats.
During the winter season the melt waters would recede, leaving the mud flats exposed. After drying, the fine-grained mud material or silt was picked up and carried by strong winds. These large dust clouds were moved eastward by prevailing westerly winds and were re-deposited over broad areas. Heavier and coarser silt deposited closest to its Missouri River flood plain source formed sharp, high bluffs on the western margin of the loess hills. Finer, lighter silt deposited farther east created gently sloping hills on the eastern margin. This process repeated for thousands of years, building layer upon layer until the loess reached thicknesses of 60 feet or more and became the dominant feature of the terrain.
Loess deposits contain valuable records of paleoclimates, or climates of the past. Loess deposits cover approximately 10 percent of the Earth's surface. Loess deposits can yield valuable information about past wind direction, past wind strength, moisture balance, and degree and type of vegetation cover. Between loess deposits, ancient buried soils are often found that mark periods when loess was not being deposited. These buried soils, called paleosols, provide valuable information about past climate and vegetation in addition to being rich in agricultural value. Loess distribution in North America is found in the Mississippi River Valley, the Great Plains of the United States, and Alaska. Shaanxi, China is also known for loess deposits in a region called the Loess Plateau.
by Owen Borville
January 30, 2021
Learning, Geology, Science
A glacier is a large perennial accumulation of ice, snow, rock, sediment and liquid water originating on land. Valley glaciers form in mountainous regions at high altitudes and fill stream valleys. The movement downslope is confined by the paths of the valley. Valley glaciers move down slope under the influence of weight and gravity. Glaciers are dynamic rivers of ice and are classified by their size, location, and thermal regime. A continental glacier is a thick, subcontinental to continental scale accumulation of glacier ice and perennial snow that spreads from a center of accumulation, typically in all directions (also called an ice sheet). Continental glaciers can be found in Greenland and Antarctica.
The glacial budget is the balance of accumulation of ice at the top of a glacier by snowfall in relation to the loss of ice at the bottom of the glacier by melting. The accumulation area is the part of a glacier that is perennially covered with snow or where snow accumulation exceeds melting (also called Névé). The accumulation zone is the area at the top of the glacier where the addition of ice and snow into a glacier system occurs through a variety of processes. These processes include precipitation and wind transportation of snow from an adjacent area into a glacier basin and the head of a glacier. The wastage zone is the lower and warmer altitude area down slope from a valley glacier where the ice melts away faster than it can build up. Ablation refers to all processes by which snow, ice, or water in any form is lost from a glacier including evaporation and melting.
A firn is an intermediate stage in the transformation of snow to glacier ice where snow has been compressed so that no pore space remains between flakes or crystals, a process that takes less than a year. The firn limit is the term used for the boundary where the amount of snow loss from melting and evaporation equals the amount of snow accumulation from snowfall (also called the annual snowline). The firn line (snow line) is the line across the glacier that marks the transition between exposed glacier ice and the snow-covered surface of a glacier. During the summer melt season, this line migrates up the glacier. At the end of the melt season the firn line separates the accumulation zone from the ablation zone.
Glacier flow is the movement of ice in a glacier normally in a downward and outward direction and is caused by the force of gravity. Normal flow rates are in feet per day. Rapid flow rates are 10 or 100 feet per day.
Abrasion is the wearing away of the Earth’s surface by agents of erosion such as wind, water, or ice. Glacial striations (grooves) are lines or scratches on rock that result from the scraping caused by the rock trapped under a glacier and against the bedrock surface as the glacier advances. Plucking or quarrying is the mechanical removal of pieces of rock from a bedrock face that is in contact with glacier ice. Blocks of rock are quarried and prepared for removal by the freezing and thawing of water in cracks, joints, and fractures. The resulting pieces of rock are frozen into the glacier ice and transported. Glacial polish is the smooth, shiny rock surface produced by the movement of a glacier over the surface along with sediment.
Glacial drift is a general term applied to all rock material (including clay, silt, sand, gravel, and boulders) transported by a glacier and deposited directly by the ice, or by running water issued from a glacier. Glacial till is dominantly unsorted and unstratified drift, which is generally unconsolidated and deposited directly by and underneath a glacier without subsequent reworking by meltwater. Stratified drift consists primarily of sand and gravel deposits that were deposited in layers by meltwater streams flowing from retreating glacial ice.
A U-shaped valley has a parabolic or U-shaped cross-section, steep walls, and generally a broad and flat floor that is formed by glacier erosion. A U-shaped valley results when a glacier widens a V-shaped stream valley. A fiord (fjord) is a glacially eroded or modified U-shaped valley that extends below sea level and connects to the ocean. Filled with seawater, depths may reach more than 1,000 feet below sea level. The largest Alaskan fiords are more than 100 miles long and more than five miles wide.
A hanging valley is a former tributary glacier valley that is incised into the upper part of a U-shaped glacier valley and is higher than the floor of the main valley as a result of glacial erosion of the main valley. Hanging valley streams often enter the main valley as waterfalls. A cirque is a bowl-shaped or amphitheater-like depression eroded into the head or the side of a glacier valley. Typically, a cirque has a lip at its lower end. A tarn is a lake that develops in the basin of a cirque, generally after the melting of the glacier. An arête is a jagged, narrow ridge that separates two adjacent glacier valleys or cirques. The ridge frequently resembles the blade of a serrated knife. A horn is a pointed mountain peak, typically pyramidal in shape and bounded by the walls of three or more cirques. Headward erosion has cut prominent faces and ridges into the peak. A peak that has four symmetrical faces is called a Matterhorn.
A crevasse is a crack or series of cracks that open in the surface of a moving glacier in response to differential stresses caused by glacier flow. A bergschrund is a single large crevasse or series of sub-parallel crevasses that develop at the head of a glacier where ice pulls away from the bedrock wall of the cirque against which the glacier accumulated. In the winter, the crevasse fills with snow and in the spring or summer, the crevasse reopens. Bergy Seltzer is a crackling or sizzling sound (similar to that made by soft drinks or seltzer water) made as air bubbles formed at many atmospheres of pressure are released during the melting of glacier ice (also called ice sizzle).
Glacial erratics are rocks of unspecified shape and size transported a significant distance from the original location by a glacier or iceberg and deposited by the melting of the ice. Glacial erratics range from the size of a pebble to larger than a house and usually are of a different composition than the bedrock or sediment on which they are deposited. A roche moutonee is a small asymmetric-shaped hill formed by glacial erosion. The upper sides are rounded and smoothed while the lower sides are rough and broken due to quarrying by the glacier.
Moraines are deposits of drift from a glacier. A lateral moraine is a sediment ridge located on a glacier surface adjacent to the valley walls and extends down glacier to the terminus. Lateral moraines form by the accumulation of rock material falling onto the glacier from the valley wall, rather than by water deposition. An end moraine is an accumulation of rock that forms at the current terminus of a glacier. Terminal moraines are ridge-like accumulations of glacial sediment that form at the furthest point reached by the terminus of an advancing glacier (an end moraine can be a terminal moraine if it is located at the furthest point reached by the glacier). A recessional moraine is a ridge of glacial sediment that forms when the terminus of a retreating glacier remains at or near a single location for a period of time sufficient for a cross-valley accumulation to form. A medial moraine is a sediment ridge located on the exposed ice surface of the glacier away from its valley walls and extending down glacier to the terminus. Medial moraines form by the joining of two lateral moraines when two glaciers merge. A ground moraine is a blanket of glacial till deposited on all of the surfaces over which a glacier moves.
Drumlins are elongated ridges of glacial sediment which are positioned parallel to the direction of glacial flow. Generally the down-glacier end is oval or rounded and the up-glacier end tapers. Rock flour (glacial flour) is fine-grained, silt-sized sediment formed by the mechanical erosion of bedrock at the base and sides of a moving glacier. When the rock flour enters a stream, the stream color turns brown, gray, iridescent blue-green or milky white.
An outwash plain (or sandur) is a broad, low-sloped alluvial plain composed of glacially eroded and sorted sediment (termed outwash) that has been transported by meltwater. The outwash plain begins at the foot of a glacier and may extend for miles. Typically the sediment becomes finer grained with increasing distance from the glacier terminus. Long glacial outwash deposits similar to braided stream deposits are called valley trains. A glacier outburst flood can result from the failure of a glacier ice dam, a glacier sediment dam, or from the melting of glacier ice by a volcanic eruption. These outburst floods are termed Jökulhlaup. A kettle is a depression that forms in an outwash plain or other glacial deposit by the melting of a block of glacier ice that was separated from the retreating glacier margin and subsequently buried by glacier sedimentation. As the buried ice melts, the depression enlarges.
A kame is a stratified sand and gravel deposit that forms in crevasses, cracks, or holes on a stagnant or moving glacier or glacial ice and is deposited by running water. A kame terrace forms between the glacier and the adjacent land surface. Shapes formed include hills, mounds, knobs, hummocks, or ridges. An esker is a meandering, water-deposited, generally steep-sided sediment ridge that forms in a stream channel on or underneath a glacier. The floor of the esker can be bedrock, sediment, or ice. Subsequent melting of the glacier exposes the deposit. Generally composed of stratified sand and gravel, eskers can range from feet to miles in length and may exceed 100 feet in height.
A varve is a pair of sedimentary layers, or a couplet, that forms in an annual cycle as the result of seasonal weather changes. Varve couplets are typically formed in glacial lakes and consist of a coarser grained summer layer formed during open-water conditions, and a finer grained winter layer formed from deposition from suspension during a period of winter ice cover.
Isostasy occurs when heavy loads from glaciers accumulate on the Earth’s surface and the crust or lithosphere of the Earth is depressed. This causes the crust to sink into the mantle and is frequently caused by advances of glaciers. Isostatic rebound occurs when the lithosphere rises or rebounds after layers of rock have been removed by erosion or after a glacier melts. Mountain building also causes isostasy when orogenic activity occurs along with erosion.
Loess is a thick wind-blown deposit (or possibly water transported) of sediment made mostly of silt-sized grains. The composition of loess is a gritty, lightweight, porous material composed of tightly packed grains of quartz, feldspar, mica, and other minerals. Loess can originate from glacial deposits, deserts, dune fields, playa lakes, or volcanic ash. During the Ice Age, glaciers advanced down into the mid-continent of North America, grinding underlying rock into fine powder-like sediment called glacial flour. As temperatures warmed, the glaciers melted and enormous amounts of water and sediment rushed down the Missouri River valley. The sediment was eventually deposited on flood plains downstream, creating huge mud flats.
During the winter season the melt waters would recede, leaving the mud flats exposed. After drying, the fine-grained mud material or silt was picked up and carried by strong winds. These large dust clouds were moved eastward by prevailing westerly winds and were re-deposited over broad areas. Heavier and coarser silt deposited closest to its Missouri River flood plain source formed sharp, high bluffs on the western margin of the loess hills. Finer, lighter silt deposited farther east created gently sloping hills on the eastern margin. This process repeated for thousands of years, building layer upon layer until the loess reached thicknesses of 60 feet or more and became the dominant feature of the terrain.
Loess deposits contain valuable records of paleoclimates, or climates of the past. Loess deposits cover approximately 10 percent of the Earth's surface. Loess deposits can yield valuable information about past wind direction, past wind strength, moisture balance, and degree and type of vegetation cover. Between loess deposits, ancient buried soils are often found that mark periods when loess was not being deposited. These buried soils, called paleosols, provide valuable information about past climate and vegetation in addition to being rich in agricultural value. Loess distribution in North America is found in the Mississippi River Valley, the Great Plains of the United States, and Alaska. Shaanxi, China is also known for loess deposits in a region called the Loess Plateau.
