A glacier is a large, enduring mass of dense ice that forms on land and moves slowly under its own weight. This movement makes it a dynamic part of the Earth’s surface. Valley glaciers are a specific type of glacier found in high-altitude mountainous regions across the globe. These “rivers of ice” play a significant role in sculpting mountain landscapes, leaving behind unmistakable geological signatures.
Defining Valley Glaciers
Valley glaciers, also known as alpine or mountain glaciers, are constrained by the steep walls of pre-existing stream valleys, which is where they get their name. Unlike vast continental ice sheets that cover entire landmasses, these glaciers are long, narrow streams of ice that flow downhill through a confined path. Their size can range from a few hundred meters to over 100 kilometers in length, depending on the mountain range and climate conditions.
They typically originate in bowl-shaped depressions high on a mountain, called a cirque, before flowing down the valley. The glacier is divided into two main sections: the accumulation zone and the ablation zone. The accumulation zone is the upper part where snowfall exceeds the loss of ice mass. Conversely, the ablation zone is the lower part where ice loss is greater than accumulation.
The line separating these two regions is known as the equilibrium line, where gain and loss are balanced over a year. Valley glaciers are found in mountain ranges on every continent except the Australian mainland, including the Himalayas, Andes, Alps, and the Southern Alps of New Zealand.
The Process of Glacial Formation
The journey from a snowflake to dense glacial ice is a process called densification, requiring a persistent climate where annual snowfall exceeds melt. Snowflakes, which are about 90 percent air, settle and break down into rounded, granular ice grains. This granular snow, having survived at least one summer melt season, is called firn.
The weight of subsequent snow layers compresses the firn, reducing the air space between the grains. In temperate regions, this transformation can take about a year, but in colder, polar regions, it may take a century. As the pressure increases further, the grains recrystallize, and the air spaces become sealed off, trapping air as tiny bubbles.
Once the ice mass reaches a density where the air is trapped as bubbles, it becomes glacial ice. This dense glacial ice has a distinctive blue tint because the compact structure absorbs red light. The required depth and time vary significantly, taking only a few years in wet, warm climates but potentially thousands of years and over 100 meters of burial in cold, dry polar regions.
How Valley Glaciers Move
The movement of a valley glacier is driven by gravity and occurs through two primary mechanisms: internal deformation and basal sliding. Internal deformation, or creep, occurs because ice behaves as a plastic material under the immense pressure of its own weight. This involves the individual ice crystals changing shape and sliding past one another, causing the entire mass to flow downslope.
This deformation is present in all glaciers, but cold-based glaciers, where the ice is frozen to the bedrock, rely almost entirely on this mechanism, resulting in very slow movement. The second mechanism, basal sliding, involves the entire glacier mass sliding over the underlying bedrock. This occurs when the temperature at the base of the glacier is at the pressure melting point, meaning a thin layer of meltwater is created.
This meltwater acts as a lubricant, reducing friction and allowing the glacier to slip faster over the valley floor. Temperate, or warm-based, valley glaciers, such as those in the Alps, often move predominantly by basal sliding, with speeds sometimes reaching several meters per day.
The Landforms They Create
Valley glaciers are agents of erosion and deposition, creating a suite of unique landforms. The most recognizable erosional feature is the U-shaped valley, or glacial trough, which contrasts sharply with the V-shaped valleys carved by rivers. The glacier carves this broad shape through abrasion, where rocks embedded in the ice scrape the bedrock, and plucking, where meltwater seeps into cracks, freezes, and tears out blocks of rock.
High in the mountains, the glacier begins in a cirque, a deep, armchair-shaped hollow with steep walls. When two cirques on opposite sides of a ridge erode toward each other, they form a sharp, knife-edge ridge known as an arête. If three or more cirques surround a mountain peak, their headward erosion creates a distinctive, pyramid-shaped summit called a horn.
The depositional features are composed of till, which is unsorted sediment carried and dropped by the ice. Moraines are ridges of this till, with lateral moraines forming along the sides of the glacier from debris falling off the valley walls. Where two tributary glaciers merge, their inner lateral moraines combine to form a medial moraine running down the center. A terminal moraine marks the furthest point the glacier ever advanced, forming a ridge across the valley floor.