A glacier is a large, enduring mass of dense ice that forms on land and moves under its own weight. These ice bodies develop where snow accumulation exceeds snowmelt over many years, allowing layers of snow to compact and transform into ice. Valley glaciers, also known as alpine glaciers, are a specific type of glacier confined within mountain valleys. They originate in high-altitude basins and flow downwards, conforming to the existing topography, much like a slow-moving river of ice.
How Valley Glaciers Form
The formation of a valley glacier begins with persistent snowfall in high mountain regions where temperatures remain low enough to prevent complete melting. As new snow falls, it buries older layers, initiating a transformation process called firnification. The weight of overlying snow compacts the snowflakes, squeezing out air and causing them to recrystallize into denser, granular ice known as firn. This firn continues to compact and recrystallize over decades or even centuries, eventually becoming solid glacial ice.
This process occurs in bowl-shaped depressions on mountain sides called cirques. Once enough ice has formed, its mass causes it to begin flowing downhill. The continuous accumulation of snow in the upper reaches, combined with cold temperatures that limit melting, sustains the glacier’s growth and movement.
The Dynamics of Glacier Movement
Valley glaciers are not static; they are constantly flowing. This movement occurs through two primary mechanisms: internal deformation and basal slip. Internal deformation, also referred to as creep, involves ice crystals within the glacier sliding past each other and deforming under the pressure of the overlying ice. This microscopic movement allows the entire ice mass to flow downslope, similar to a highly viscous fluid.
Glaciers also move by basal slip, where the entire body of ice slides over the bedrock or underlying sediment. This sliding is facilitated by a thin layer of meltwater that forms at the glacier’s base. This meltwater can be generated by pressure melting or by surface meltwater percolating through cracks to the base. Factors such as ice thickness, slope steepness, and ice temperature influence glacier movement speed, with warmer glaciers and steeper slopes leading to faster flow.
Sculpting the Landscape
As valley glaciers move, they reshape the landscapes they traverse through erosion and deposition. One of the most distinctive features created by these glaciers are U-shaped valleys, also known as glacial troughs. Unlike river valleys, which are V-shaped, glaciers widen and deepen valleys, giving them a U-profile with steep sides and a broad, flat floor. This occurs because the glacier erodes across the entire valley floor, not just the stream channel.
Other erosional landforms include cirques, the bowl-shaped amphitheaters at the head of glacial valleys. When two cirques on opposite sides of a mountain erode back into each other, they can form a sharp, knife-edge ridge called an arête. If three or more cirques erode into a single peak, they can carve out a pointed mountain summit known as a horn. Smaller tributary glaciers erode less deeply than main glaciers, resulting in hanging valleys, which are elevated valleys that meet the main U-shaped valley partway up its side, often marked by waterfalls.
Beyond erosion, glaciers also leave behind depositional features. As a glacier moves, it picks up and transports unsorted rock and sediment. When the ice melts, this material is deposited, forming features like moraines. Moraines are ridges or mounds of till, the unsorted glacial sediment, which can accumulate along the sides (lateral moraines) or at the terminus (end or terminal moraines) of the glacier, marking its past extent. These landforms provide evidence of the impact of valley glaciers on Earth’s surface.