13.2 Glacial Erosion and Deposition
6 min read•july 30, 2024
Glaciers are powerful sculptors of Earth's surface, carving and shaping landscapes through erosion and deposition. As massive ice bodies move, they grind away rock, creating unique landforms like U-shaped valleys and cirques. These processes leave lasting marks on our planet.
Glacial activity also deposits vast amounts of sediment, forming features like and . Understanding these processes helps us interpret past climates and predict future changes. Glacial landforms tell stories of Earth's icy past and ongoing transformations.
Glacial Erosion Processes
Abrasion, Plucking, and Quarrying
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- occurs when rocks and debris embedded in the base of a glacier scrape and grind against the underlying bedrock
- Abrasion smooths and polishes the bedrock surface (, )
- is the process by which glaciers freeze to and lift loose rocks from the underlying bedrock
- Plucked rocks are incorporated into the ice and transported by the glacier
- Plucking can create a rough, jagged appearance on the bedrock surface
- involves the removal of large blocks of bedrock by the combined action of abrasion, plucking, and pressure release
- Pressure release occurs when the weight of the overlying ice is removed, causing the bedrock to fracture and break apart
- Quarrying can create large, angular blocks of bedrock (joint blocks, )
Factors Influencing Erosion Rates
- The rate of glacial erosion depends on factors such as , velocity, and the presence of
- Thicker ice and higher velocities generally lead to more intense erosion
- Meltwater can enhance erosion by lubricating the base of the glacier and transporting sediment
- The resistance of the bedrock to erosion also plays a role in determining the intensity of glacial erosion
- Softer, more easily weathered rocks (shale, limestone) are more susceptible to erosion than harder, more resistant rocks (granite, quartzite)
- The presence of preexisting weaknesses in the bedrock, such as joints and faults, can facilitate plucking and quarrying
- Glaciers can exploit these weaknesses, leading to more efficient erosion and the formation of distinctive landforms (cirques, horns)
Formation of Erosional Features
Cirques, Arêtes, and U-Shaped Valleys
- Cirques are bowl-shaped depressions carved into mountainsides by the erosive action of glaciers
- Cirques often have a steep headwall and a flat or overdeepened floor, where a tarn (glacial lake) may form after the glacier retreats
- The Matterhorn in the Alps is a classic example of a peak surrounded by cirques
- Arêtes are sharp, narrow ridges that form between two adjacent glacial cirques
- Arêtes are created by the erosion of the walls, leaving a knife-edge ridge
- The Sawtooth Ridge in the Sierra Nevada (California) is an example of a series of arêtes
- U-shaped valleys, also known as glacial troughs, are characterized by steep, parallel walls and a flat or rounded bottom
- U-shaped valleys result from the erosive power of glaciers, which can carve deep into the landscape
- Yosemite Valley (California) and the Lauterbrunnen Valley (Switzerland) are well-known examples of U-shaped valleys
Hanging Valleys and Other Features
- Hanging valleys are tributary valleys that are left "hanging" above the main glacial valley
- Hanging valleys form due to the more rapid erosion of the main valley by a larger, more powerful glacier
- Waterfalls often form where hanging valleys meet the main valley (Yosemite Falls, Bridal Veil Falls)
- Roches moutonnées are asymmetrical bedrock hills created by glacial erosion
- The upstream side is gently sloping and smoothed by abrasion, while the downstream side is steep and rough due to plucking
- Roches moutonnées are named for their resemblance to sheeps' backs (mouton is French for sheep)
- Striations and grooves are linear scratches and gouges on bedrock surfaces caused by glacial abrasion
- Striations and grooves can indicate the direction of ice flow and are useful for reconstructing past glacial movements
- These features are often visible on exposed bedrock in glaciated regions (Canadian Shield, Scandinavian mountains)
Glacial Deposits
Moraines and Till
- Moraines are accumulations of glacial debris () deposited by glaciers
- Terminal moraines form at the end of a glacier, marking its maximum extent (Long Island, New York)
- Lateral moraines form along the sides of a glacier, composed of debris fallen from the valley walls
- Medial moraines form when two glaciers converge, and their lateral moraines join to create a deposit running down the center of the combined glacier
- Ground moraines are a layer of till deposited across the valley floor as the glacier retreats (Midwest US, central Europe)
- Till is the unsorted sediment deposited directly by glacial ice
- Till consists of a mixture of clay, silt, sand, gravel, and boulders
- The composition of till reflects the bedrock and sediments eroded and transported by the glacier
- are large boulders transported by glaciers and deposited far from their origin
- Erratics often rest on bedrock of a different composition, indicating their glacial transport
- The Madison Boulder in New Hampshire is a famous example of a glacial erratic
Glaciofluvial Deposits
- refers to the sediments deposited by meltwater streams flowing from the glacier
- Outwash is sorted and stratified, with larger particles (gravel, sand) deposited closer to the glacier and finer particles (silt, clay) carried further downstream
- Outwash plains are broad, flat areas of glaciofluvial deposits that form in front of a glacier (Knik River , Alaska)
- are long, sinuous ridges of sand and gravel deposited by meltwater streams flowing within or beneath a glacier
- Eskers can range from a few meters to several kilometers in length and are often used as natural roadbeds (Denali Highway, Alaska)
- are mound-like hills of stratified sand and gravel deposited by meltwater in depressions on the surface of a glacier or at its margin
- Kames often form in association with kettles, which are depressions left by the melting of buried ice blocks (Kame and Kettle Topography, Wisconsin)
Glacial Impact on Landscapes
Landscape Alteration and Unique Features
- Glacial erosion can significantly alter pre-existing landscapes, creating unique features like cirques, arêtes, and U-shaped valleys
- The erosive power of glaciers can expose resistant bedrock, creating features like roches moutonnées and striations on the rock surface
- Glacial erosion can also create , which are basins eroded below the surrounding landscape (Great Lakes, fjords)
- Glacial deposition creates a variety of landforms, such as moraines, drumlins, and eskers
- These depositional features contribute to the complexity and diversity of post-glacial landscapes
- Drumlins are elongated, streamlined hills formed by glacial deposition and shaping (Boston Harbor Islands, Massachusetts)
- The sediments deposited by glaciers, particularly glacial outwash, can influence soil composition and fertility in the affected regions
- Glacial soils are often rich in nutrients due to the grinding and mixing of bedrock materials
- , windblown silt derived from glacial outwash, forms highly fertile soils in many regions (Mississippi River Valley, US; Loess Plateau, China)
Hydrological and Climatic Implications
- Glacial erosion and deposition can reroute drainage systems and create lakes
- form when buried ice blocks melt, leaving depressions that fill with water (Kettle Moraine State Forest, Wisconsin)
- can create large, deep lakes in bedrock basins (Finger Lakes, New York; Great Lakes, North America)
- Glaciers can modify river patterns and shape the hydrological characteristics of a landscape
- Meltwater from glaciers can contribute to river flow and influence the seasonal discharge patterns of streams and rivers
- Glacial sediments can alter the course and behavior of rivers, leading to the formation of braided or meandering patterns
- The study of glacial landforms and deposits () provides valuable insights into past climates and the extent of glaciation
- The distribution and characteristics of glacial features can be used to reconstruct the timing, extent, and dynamics of past glaciations
- Glacial deposits, such as moraines and outwash plains, can be dated using techniques like radiocarbon dating and cosmogenic nuclide dating to establish a chronology of glacial events
- Understanding the history of glaciation is crucial for predicting future climate change and its potential impacts on landscapes and ecosystems
Key Terms to Review (30)
Arête: An arête is a sharp ridge that forms between two glacial valleys or cirques, resulting from the erosion of surrounding rock by glaciers. This distinctive landform is characterized by its steep, narrow profile and is often seen in mountainous regions shaped by glacial activity. The formation of an arête illustrates the powerful influence of glacial processes on landscape development.
Cirque: A cirque is a bowl-shaped depression formed by glacial erosion, typically found at the head of a valley. These features are created through the combination of freeze-thaw weathering and the intense scraping action of glaciers as they move. Cirques often serve as the starting point for glaciers and can lead to the formation of other landforms such as arêtes and horns, showcasing the power of glacial processes in shaping the landscape.
Drumlins: Drumlins are elongated, teardrop-shaped hills formed by the movement of glacial ice, typically found in clusters or fields. These landforms are created from the accumulation of glacial till, shaped by the flow of ice over the underlying sediment. Drumlins can provide insights into the direction of ice flow and are significant features in understanding glacial erosion and deposition processes.
Erratics: Erratics are large boulders or rock fragments that have been transported from their original location by glacial activity and deposited in an area where the underlying bedrock is different. These geological features provide key evidence of past glacial movements and the dynamics of glacial erosion and deposition, showcasing how glaciers can carry and relocate massive amounts of material over considerable distances.
Eskers: Eskers are long, winding ridges of sediment that are formed by glacial meltwater flowing beneath a glacier. They consist of sand and gravel deposits that accumulate in subglacial channels, eventually becoming exposed as the ice retreats. These features provide important insights into glacial processes and the dynamics of past ice sheets.
Glacial Abrasion: Glacial abrasion is the process by which glaciers grind against the underlying bedrock, causing erosion through the mechanical action of embedded debris within the ice. This process results in the smoothing and polishing of rock surfaces, creating distinctive landforms and sedimentary features associated with glacial landscapes. The degree of abrasion is influenced by factors such as the thickness of the ice, the velocity of glacier movement, and the type and size of sediments carried by the glacier.
Glacial Flow: Glacial flow refers to the movement of ice within a glacier, driven by gravity and the pressure of overlying ice. This process is essential in shaping the landscape, as it leads to both glacial erosion, where material is worn away from the earth's surface, and glacial deposition, where sediment is dropped as the glacier melts or retreats. Understanding glacial flow helps to explain how glaciers can transport large amounts of debris over long distances and significantly alter landforms.
Glacial Geomorphology: Glacial geomorphology is the study of landforms and processes created by glaciers, including their movement, erosion, and deposition. It examines how glaciers shape the landscape through mechanisms like plucking and abrasion, which sculpt valleys and create unique features such as moraines, drumlins, and fjords. This field helps understand the past climate conditions and the ongoing impact of glaciers on Earth's surface.
Glacial Outwash: Glacial outwash refers to the sediment and debris that is transported and deposited by meltwater streams flowing from a glacier. This material is typically composed of sand, gravel, and silt, which are carried away from the glacier as it retreats. The outwash is often found in broad, flat plains known as outwash plains, and plays a crucial role in shaping the landscape through processes of erosion and deposition.
Glacial plucking: Glacial plucking is a process where glaciers erode the landscape by lifting and removing rock fragments from the bedrock as they move. This occurs when meltwater penetrates cracks in the rock, freezing and bonding with it. As the glacier advances, it exerts pressure that pulls the rock fragments free, allowing them to be carried away, which contributes significantly to the shaping of valleys and mountains.
Glacial Quarrying: Glacial quarrying is the process by which glaciers erode and remove large amounts of rock and sediment from the landscape as they advance and retreat. This occurs primarily through the mechanical action of the glacier, where embedded debris, such as rocks and boulders, scrape against the bedrock beneath the ice, effectively carving out features like U-shaped valleys and fjords. The process plays a critical role in shaping landforms and contributes to the overall understanding of glacial erosion and deposition.
Glacial Scouring: Glacial scouring is a process of erosion that occurs when glaciers move over the land, scraping and polishing the underlying bedrock. This powerful action removes debris and sediments while creating distinctive landforms, such as striations and polished surfaces, showcasing the glacier's movement. It plays a crucial role in shaping landscapes, particularly in glaciated regions, by carving out valleys and altering topography.
Gps monitoring: GPS monitoring refers to the use of Global Positioning System technology to track the location and movement of objects, such as glaciers, in real-time. This technology allows scientists to gather precise data on glacial movement, which is essential for understanding glacial dynamics, erosion, and deposition processes.
Grooves: Grooves are elongated, linear depressions or scratches that are formed in the earth's surface by the movement of glacial ice. These features provide crucial evidence of past glacial activity and indicate the direction of glacier flow. As glaciers move, they can carve out these grooves in the bedrock beneath them, creating distinctive patterns that help geologists understand the history of glaciation in a region.
Hanging Valley: A hanging valley is a small valley that is perched above the level of the main valley into which it feeds, often created by glacial erosion. This geological feature typically results from the different rates of erosion between the main valley and its tributary, leading to a pronounced elevation difference. Hanging valleys are often marked by steep cliffs and can give rise to spectacular waterfalls when streams flow over the edge into the lower valley.
Ice thickness: Ice thickness refers to the vertical measurement of ice layers, particularly in glaciers and ice sheets, that plays a crucial role in understanding glacial dynamics and the processes of erosion and deposition. Variations in ice thickness can significantly influence the flow of glaciers, the rate of melting, and the landscape changes resulting from glacial activity. In this context, ice thickness affects how glaciers erode bedrock and transport sediments, shaping the surrounding environment over time.
Kames: Kames are irregularly shaped hills or mounds of glacial sediment that are formed by the accumulation of debris left behind by melting glaciers. They are typically composed of sand, gravel, and other materials, resulting from both glacial erosion and deposition processes. These features provide insight into the history of glacial movement and the landscape evolution during and after glaciation.
Kettle lakes: Kettle lakes are depressions or holes left behind by melting glaciers that fill with water, creating lakes. These features typically form when chunks of ice break off from a glacier and become buried in sediment, and as the ice melts, it leaves a basin that collects rainwater and runoff. Kettle lakes are often found in areas previously covered by ice sheets and are a testament to glacial processes of erosion and deposition.
Last glacial maximum: The last glacial maximum refers to the period during the last Ice Age, around 26,500 years ago, when ice sheets were at their greatest extent, covering large parts of North America, Europe, and Asia. This significant climatic event shaped landscapes through glacial erosion and deposition processes, leading to the formation of various landforms and sediment deposits that are crucial for understanding Earth's geological history.
Loess: Loess is a fine, wind-blown sediment composed mostly of silt-sized particles, which can form extensive deposits across landscapes. This material is primarily created through the weathering of rocks and the action of glaciers, leading to rich, fertile soils that support agriculture in many regions. Its unique properties allow it to accumulate in thick layers, often resulting in steep bluffs and cliffs when eroded.
Meltwater: Meltwater is the water released when ice and snow melt, typically from glaciers and ice sheets. This meltwater plays a critical role in shaping the landscape, influencing erosion and deposition processes, and contributing to the hydrological cycle as it flows into rivers and lakes.
Moraines: Moraines are accumulations of debris, primarily composed of rocks and sediment, that have been transported and deposited by glaciers. They form distinctive landforms that indicate past glacial activity, showcasing the movement and melting of glaciers across landscapes. These features play a critical role in understanding geomorphic processes and the history of glacial erosion and deposition.
Outwash Plain: An outwash plain is a flat area formed by the deposition of sediments carried away from a glacier by meltwater. This landscape features layered deposits of sand and gravel, often resulting in a well-drained area that contrasts sharply with the surrounding terrain. Outwash plains are typically found at the leading edge of glaciers, where streams and rivers carry the sediment away from the ice, contributing to the unique topography and ecological characteristics of these regions.
Overdeepenings: Overdeepenings are depressions or basins formed in the bedrock due to the intense erosion caused by glaciers. As glaciers move, they can carve out areas of the landscape more deeply than their surrounding terrain, leading to these distinctive features. Overdeepenings often become significant lakes or other water bodies once the glaciers retreat, showcasing the power of glacial erosion and its lasting impacts on the geography of an area.
Pleistocene Glaciation: Pleistocene glaciation refers to the period during the Quaternary Ice Age, roughly 2.6 million to about 11,700 years ago, characterized by extensive glacial coverage across much of the Earth. This era was marked by repeated glacial advances and retreats, significantly shaping the landscape through processes of erosion and deposition that transformed continents and influenced ecosystems.
Remote sensing: Remote sensing is the process of acquiring information about an object or phenomenon without making physical contact, primarily using satellite or aerial imagery. This technique plays a crucial role in understanding and monitoring various physical processes and resources on Earth, providing data that can be analyzed for applications in multiple disciplines.
Roches moutonnées: Roches moutonnées are distinctively shaped rock formations created by glacial erosion, characterized by a smooth, rounded upstream side and a steeper, jagged downstream side. These formations serve as important indicators of past glacial movement and the directional flow of ice. They illustrate how glaciers sculpt the landscape as they advance and retreat, effectively demonstrating the process of glacial erosion.
Striations: Striations are parallel grooves or scratches found on rock surfaces, formed by the movement of glaciers as they scrape and drag debris across the landscape. These markings provide crucial evidence of glacial activity, helping to reveal the direction of ice flow and the intensity of erosion. Striations are often a key indicator in understanding the history of glacial erosion and deposition in a given area.
Till: Till is a type of unsorted glacial sediment that is deposited directly by the ice of a glacier. It consists of a mixture of clay, silt, sand, gravel, and boulders, reflecting the various materials that the glacier has eroded and transported. As glaciers advance and retreat, they leave behind this heterogeneous deposit, which plays a significant role in shaping the landscape and influencing soil formation in glaciated regions.
U-shaped valley: A u-shaped valley is a glacially eroded valley characterized by its distinct 'U' shape, with steep sides and a broad, flat floor. These valleys are formed by the powerful movement of glaciers that carve through mountainous terrain, shaping the landscape over time. The unique formation of u-shaped valleys indicates significant glacial activity and serves as a key indicator of past climatic conditions.