Glacial Geology: Ice Sheets Glacial Processes and Landscape Formation
Glacial Geology: Ice Sheets Glacial Processes and Landscape Formation
Glacial geology examines the processes by which glaciers and ice sheets erode, transport, and deposit sediment, and the landforms that result from glaciation. Glaciers are powerful geological agents that have shaped vast areas of the Earth’s surface, particularly during the ice ages of the Pleistocene epoch when ice sheets covered up to thirty percent of the land surface. The evidence of past glaciation is preserved in landforms and deposits that record the extent and dynamics of ancient ice sheets. Understanding glacial geology is essential for interpreting past climate change, managing water resources in glaciated regions, and understanding the response of modern ice sheets to climate change. This guide explores the formation and movement of glaciers, the processes of glacial erosion and deposition, and the characteristic landforms of glaciated landscapes.
Glacier Formation and Movement
Glaciers form when snow accumulates faster than it melts over many years. The accumulated snow compresses under its own weight, transforming first into granular firn and then into dense glacial ice. When the ice becomes thick enough, it begins to flow under its own weight. The flow of glacial ice occurs through internal deformation, where ice crystals slide past each other, and basal sliding, where the glacier slides over its bed on a film of meltwater.
Glaciers are classified by their size and setting. Valley glaciers flow within mountain valleys, while ice sheets, which are much larger, cover vast areas and flow outward from their centers. The Greenland and Antarctic ice sheets are the largest ice masses on Earth, containing over ninety-nine percent of the world’s glacial ice. Ice caps are smaller versions of ice sheets, and ice shelves are floating extensions of ice sheets that extend over the ocean.
Glacial Erosion
Glaciers erode the underlying bedrock through two main processes. Abrasion occurs when rocks frozen into the base of the glacier scrape against the bedrock, like sandpaper. The direction of glacial abrasion is recorded by striations, parallel scratches on bedrock surfaces. The size and depth of striations indicate the coarseness of the debris carried by the glacier.
Plucking occurs when glacial ice freezes onto bedrock and pulls blocks of rock away as the glacier moves. Plucking is most effective on the down-glacier sides of bedrock obstacles, producing streamlined forms called roches moutonnées that have a gentle up-glacier slope and a steep, quarried down-glacier slope. The combination of abrasion and plucking produces characteristic landforms including U-shaped valleys, fjords, and cirques.
Glacial Deposition
Glaciers transport vast quantities of sediment ranging from clay-sized particles to enormous boulders. This sediment, called till, is deposited directly by the ice without sorting by water. Till is characteristically unsorted, with a wide range of grain sizes. The composition of till reflects the bedrock over which the glacier passed.
Glacial deposits are classified by their mode of deposition. Lodgment till is plastered onto the bed beneath a moving glacier. Ablation till is deposited from melting ice at the glacier surface. Flow till is reworked by flowing water on the glacier surface. The characteristics of till provide information about glacial dynamics and the geology of the source area.
Glacial Landforms
Glacial erosion produces distinctive landforms. Cirques are amphitheater-shaped depressions at the heads of glacial valleys, formed by the combination of glacial erosion and frost wedging. Arêtes are sharp ridges separating adjacent glacial valleys. Horns are pyramidal peaks where multiple cirques have eroded a mountain from several sides. Glacial troughs are U-shaped valleys with steep walls and flat floors.
Depositional landforms include moraines, ridges of till deposited at the margins of glaciers. Terminal moraines mark the maximum extent of glacial advance. Lateral moraines form along the sides of valley glaciers. Medial moraines form where two glaciers merge. Ground moraine is the gently undulating blanket of till left beneath a retreating glacier.
Glaciofluvial Processes
Meltwater from glaciers sorts and deposits sediment in distinctive ways. Eskers are sinuous ridges of sand and gravel deposited by meltwater streams flowing in tunnels within or beneath glacial ice. Kames are mounds of stratified sediment deposited by meltwater. Outwash plains are broad, gently sloping surfaces of sorted sediment deposited by meltwater streams beyond the glacier margin.
Glacial lakes form where meltwater is impounded by ice or moraines. Varves, annual layers of sediment in glacial lakes, provide detailed records of past climate. The silt deposited in glacial lakes, called rock flour, is produced by glacial abrasion of bedrock and gives glacial lakes their characteristic turquoise color.
The Ice Ages
The Pleistocene epoch, from about two point six million to eleven thousand seven hundred years ago, was characterized by repeated glaciations separated by warmer interglacial periods. During the last glacial maximum, about twenty thousand years ago, ice sheets covered much of North America, northern Europe, and Asia. Sea level was about one hundred twenty meters lower than today because so much water was stored in ice sheets.
The causes of ice ages include changes in Earth’s orbit, known as Milankovitch cycles, which affect the distribution of solar radiation; changes in atmospheric carbon dioxide concentrations; and changes in ocean circulation. The current interglacial, the Holocene, has been unusually stable in climate, allowing human civilization to develop and flourish.
Frequently Asked Questions
How much of Earth is covered by glaciers today? About ten percent of Earth’s land surface is currently covered by glaciers, primarily the Greenland and Antarctic ice sheets. During the last glacial maximum, about thirty percent was covered.
Are glaciers currently retreating? Most glaciers worldwide are retreating due to climate change. The retreat is accelerating in many regions, including the Alps, Himalayas, and Alaska. The Greenland and Antarctic ice sheets are losing mass at accelerating rates.
What is a glacial erratic? A glacial erratic is a boulder that was transported by a glacier and deposited in an area with different bedrock geology. Erratics can be enormous, weighing thousands of tons, and provide evidence of the direction and extent of glacial flow.
How do scientists study ancient glaciers? Scientists study ancient glaciers through the landforms and deposits they left behind, including moraines, striations, and till. The analysis of glacial sediments and landforms allows reconstruction of past ice extent and dynamics.
Conclusion
Glacial geology reveals the powerful role of ice in shaping Earth’s surface. Glaciers have eroded mountains, carved valleys, and deposited sediments across vast areas, leaving a landscape that records the dynamics of past ice sheets. As modern glaciers retreat in response to climate change, understanding glacial processes becomes increasingly important for predicting sea level rise, managing water resources, and interpreting the geological legacy of the ice ages.