A glacier is a persistent body of dense ice that forms on land and moves under the weight of its own mass. Glaciers originate where the accumulation of snow over many years exceeds the rate of melting and sublimation. These masses are primarily categorized into two major types based on their size, location, and relationship to the surrounding topography: Continental and Alpine.
The Primary Distinction: Continental vs. Alpine Glaciers
The fundamental difference between these two glacier types lies in their scale and the degree to which they are constrained by the landscape. Continental glaciers are vast, unconfined ice masses that flow over the top of the land, largely ignoring underlying terrain features. Their movement is driven by the sheer thickness of the ice sheet, which causes the ice to spread outward in all directions. Alpine glaciers, conversely, are much smaller and are strictly controlled by the mountains and valleys in which they form. Their flow is governed by gravity, pulling the ice down the slopes and through existing drainage channels.
Characteristics of Continental Glaciers
Continental glaciers are immense sheets of ice, known as ice sheets or ice caps, that cover expansive areas of land. An ice sheet is defined as an ice mass covering more than 50,000 square kilometers; modern examples include the Antarctic and Greenland Ice Sheets. These massive glaciers hold approximately 99% of all glacial ice on Earth and represent 68% of the planet’s freshwater.
The flow of a continental glacier is radial, moving horizontally outward from a central, dome-shaped high point where the ice is thickest. This movement is independent of the underlying topography, allowing the glacier to overwhelm and bury surface features. Smaller continental masses, called ice caps, cover less than 50,000 square kilometers, such as the Vatnajökull ice cap in Iceland. When these glaciers move, they smooth the landscape through abrasion and deposition, leaving behind distinctive landforms like drumlins and moraines.
Characteristics of Alpine Glaciers
Alpine glaciers, sometimes called mountain or valley glaciers, originate at high altitudes in mountainous regions across the globe. They are significantly smaller than continental glaciers and are characterized by their linear, downhill flow. They begin in bowl-shaped depressions called cirques, where snow accumulates and compacts. As the ice mass grows, it spills out of the cirque and flows down the mountain, often occupying a pre-existing stream valley.
The movement of an alpine glacier is highly constrained by the mountain topography, creating profound erosional features. The weight and grinding action of the ice transform the landscape, carving out steep-sided, U-shaped valleys distinct from V-shaped canyons cut by rivers. Other landforms specific to this type include sharp ridges called arêtes and pyramid-like mountain peaks known as horns. Common subtypes include valley glaciers, which fill a single valley, and cirque glaciers, which remain confined to their high-altitude hollows.
The Mechanism of Glacier Formation and Flow
Both continental and alpine glaciers begin with the persistent accumulation of snow that survives the summer melt season. As new snow layers bury the old, pressure squeezes air out of the flakes, causing them to recrystallize into denser, granular ice known as firn. With continued compression, the firn transforms into solid, dense glacial ice. This process, known as firnification, can take a year in temperate regions or up to a century in colder environments.
Once the ice mass is thick enough, the glacier begins to move through two main mechanisms. The first is internal deformation, where pressure causes the ice crystals to change shape and slide past each other, a process known as creep. The second is basal slip, which occurs when a layer of meltwater at the base of the glacier lubricates the contact point, allowing the entire ice mass to slide over the bedrock.