Continental and Alpine Glacial Landscapes: Processes, Landforms, and Global Extent
Overview of Glacial Types and Global Ice Sheets
Glaciers are categorized into two primary types based on their scale and location: Ice Sheets and Alpine Glaciers. Ice sheets are massive glacial bodies that cover large landmasses, while alpine glaciers are specifically situated within mountain regions, such as the Wind River of Wyoming. In the historical context of the Earth, ice sheets reached their peak extent approximately (Before Present) and have been receding since that time. In the modern era, only two major ice sheets persist on the planet: the Antarctic Ice Sheet and the Greenland Ice Sheet. These ice sheets exhibit significant depth, varying in thickness from several hundreds of feet to more than two miles thick.
The Antarctic Ice Sheet is a monolithic structure covering much of the Antarctic continent. It is a critical component of the global hydrologic cycle, containing approximately of the world’s freshwater. The scale of this ice sheet is so immense that if it were to melt completely, global sea levels would rise by approximately . A notable feature of this ice sheet is its ice shelves, which are floating extensions of the glaciers that reach into the ocean.
The Greenland Ice Sheet is considerably smaller, measuring one-eighth the size of its Antarctic counterpart. It covers the majority of Greenland's surface and contains approximately of the global freshwater supply. A complete melting of the Greenland Ice Sheet would result in a sea-level rise of roughly .
Landforms and Processes of Continental Ice Sheets
Continental ice sheets produce distinct erosional and depositional landforms. One of the primary erosional features is the Shield, which refers to the core bedrock of a continent. These shields consist of very ancient rock; for instance, the Canadian Shield forms the fundamental core of the North American continent. Additionally, glaciers facilitate the creation of Glacial Lakes. During their movement, glaciers erode massive quantities of rock, creating significant depressions in the landscape. When the ice retreats, these depressions fill with water, a process responsible for the formation of the Great Lakes.
Depositional landforms produced by ice sheets include Drumlins, Kettles, and Moraines. A drumlin is described as a smooth, steep-sided, and elliptical-shaped mound, often compared in appearance to "half-buried eggs." These develop beneath glacial ice and align parallel to the direction of the glacier's movement. In the state of Wisconsin, topographic maps reveal extensive drumlin fields characterized by their distinct steep sides. Kettles are formed through a specific four-step sequence: first, a chunk of ice is left behind by a retreating glacier; second, these chunks are buried or covered in glacial outwash; third, the ice eventually melts, leaving behind a depression; finally, the depression fills with water to become a kettle lake.
Moraines are linear ridges composed of glacial drift. Glacial drift itself varies by its transport medium: material laid down directly by ice is unsorted, whereas material laid down by water is sorted. There are two primary types of continental moraines. A Terminal Moraine marks the outermost limit of an ice sheet and provides definitive evidence of the glacier's maximum extent. For example, terminal moraines created Long Island and other New England islands, and the city of Chicago is situated on top of a terminal moraine. A Recessional Moraine forms when the retreat of an ice sheet stalls or when the ice undergoes a minor re-advancement.
Classification and Evolution of Alpine Glaciers
Alpine glaciers, also known as non-continental glaciers, are classified into three main sizes and types based on their position relative to mountain topography. On the highest mountain peaks, one finds Cirque Glaciers, such as the Piegan Glacier in Montana or various cirque glaciers in Alaska. As these glaciers grow, they may extend downward into the valleys of mountainous areas, becoming Valley Glaciers. If a glacier extends further past the mountain valleys onto a broad, open plain, it is classified as a Piedmont Glacier.
The progression of a landscape through glaciation follows a specific temporal sequence. In the Pre-glation phase, the landscape is characterized by rounded ridges, rounded peaks, and V-shaped valleys carved by streams. During Maximum Glaciation, snow accumulates at higher slopes, and the resulting ice moves downslope under the force of gravity, leading to active glacial erosion. In the Post-glaciation phase, the landscape reflects the permanent structural changes caused by the ice.
Erosional Landforms of Alpine Glaciation
Mountain glaciers produce several unique erosional landforms. Glacial Troughs are formed when a mountain glacier fills an existing river valley. The ice widens the valley bottom, transforming a V-shaped stream-carved valley into a U-shaped valley. Related to this are Hanging Valleys, which occur when a tributary side valley sits at a higher elevation than the main valley floor. This creates a valley that "hangs" above the larger one, typically marked by the presence of a scenic waterfall where the higher valley empties into the lower one. When a glacial trough is inundated by seawater, it forms a Fjord—a narrow, steep-sided, and elongated ocean inlet.
Near the tops of mountains, glaciers carve out Cirques, which are bowl-shaped landforms. If multiple cirque glaciers (specifically three or more) erode the rock at a mountain top from different sides, they leave behind a steep-sided, sharp-edged peak known as a Horn. Mt. Whitney in California serves as a primary example of a horn. Furthermore, an Arête is a narrow, jagged ridge that forms as an erosional landform between two adjacent cirque glaciers.
Depositional Landforms in Alpine Regions
In addition to the terminal and recessional moraines common to all glaciers, alpine glaciers produce specific types of depositional ridges called Lateral and Medial Moraines. Rock fragments are often deposited at the margins of mountain glaciers in ridges of debris. Lateral Moraines are ridges of debris located along both sides of a valley glacier. When two valley glaciers join together, their respective inner lateral moraines merge to create a Medial Moraine, which appears as a linear strip of debris running down the center of the combined glacial flow.