A glacier is a massive, slow-moving body of ice formed from compacted snow that survives year-round. These structures are dynamic systems that flow downhill or spread outward under the influence of gravity and their own immense weight. This constant movement causes the ice mass to behave like a very thick, viscous fluid, transporting rock and sediment. Because glaciers are constantly in motion and losing mass at their lowest elevations, they have a distinct, measurable point where the ice finally ceases to exist.
The Glacier Terminus and Its Names
The scientific name for the very end of a glacier, which marks its lowest extent, is the terminus. This location is where the ice flow is balanced by the rate of ice loss, defining the glacier’s boundary. Glaciologists often use less formal alternatives, referring to it as the glacier’s snout or toe.
The position of the terminus is not fixed but reflects the glacier’s mass balance—the difference between ice gained and ice lost. If the glacier gains more mass than it loses, the terminus will advance, moving further down the valley. Conversely, if the rate of ice loss exceeds the rate of flow, the terminus will appear to retreat, moving back up the valley. Tracking the shifting location of the terminus is a primary method scientists use to monitor the health and movement of a glacier.
The Role of the Ablation Zone
The location of the terminus is determined by the ablation zone, the low-altitude area where the glacier experiences a net loss in ice mass. This contrasts with the higher-altitude accumulation zone, where mass gain from snowfall exceeds loss. The boundary separating these two areas is called the equilibrium line, where accumulation and ablation are equal.
The physical processes that cause mass loss are collectively called ablation. These processes include surface melting and sublimation (the direct transformation of ice into water vapor). For glaciers that end in a body of water, such as a lake or the ocean, a significant form of ablation is calving, where large chunks of ice break off the face of the glacier. The terminus is the lower, physical edge of this ablation zone, marking the furthest point the flowing ice can reach before it is completely melted or broken away. In a land-terminating glacier, surface meltwater is often the dominant form of mass loss.
Landscape Features Formed at the End
The constant action of the terminus, which grinds and deposits material, creates distinctive landscape features. The glacier acts like a conveyor belt, transporting unsorted rock, clay, and boulders, collectively known as glacial till, to its end. When the terminus remains in a relatively stable position, this deposited material piles up into a ridge called a terminal moraine.
These terminal moraines mark the maximum extent of the glacier’s advance. The material forming the moraine is often a mix of fine sediment and massive boulders, pushed forward by the force of the ice. Moraines can act as natural dams, trapping meltwater to form large bodies of water known as proglacial lakes.
The melting ice at the terminus generates vast quantities of water that flow away from the glacier. This meltwater carries sediment away and deposits it in a wide, flat area called an outwash plain, or sandur. Unlike the unsorted till of the moraine, the sediment in the outwash plain is sorted by water, with the heaviest gravels deposited first and the finer sands carried further away.
Sometimes, as a glacier retreats, large blocks of ice become detached and buried within the deposited material. When these buried ice blocks eventually melt, they leave behind depressions that fill with water, creating small, circular bodies of water called kettle lakes. The unique forms created at the end of the ice provide a geological record of the glacier’s past size and movement.