A glacier head is the top of a glacier. Although glaciers seem motionless to the observer they are in constant

motion In physics, motion is when an object changes its position with respect to a reference point in a given time. Motion is mathematically described in terms of displacement, distance, velocity, acceleration, speed, and frame of reference to an o ...

and the

terminus Terminus may refer to: Ancient Rome *Terminus (god), a Roman deity who protected boundary markers Transport *Terminal train station or terminus, a railway station serving as an end destination *Bus terminus, a bus station serving as an end des ...

is always either advancing or retreating. Glacier diagram

On a glacier, the accumulation zone is the area above the

firn __NOTOC__ Firn (; from Swiss German "last year's", cognate with ''before'') is partially compacted névé, a type of snow that has been left over from past seasons and has been recrystallized into a substance denser than névé. It is ice that ...

line, where snowfall accumulates and exceeds the losses from

ablation Ablation ( – removal) is the removal or destruction of something from an object by vaporization, chipping, erosion, erosive processes, or by other means. Examples of ablative materials are described below, including spacecraft material for as ...

, (

melting Melting, or fusion, is a physical process that results in the phase transition of a substance from a solid to a liquid. This occurs when the internal energy of the solid increases, typically by the application of heat or pressure, which inc ...

evaporation Evaporation is a type of vaporization that occurs on the Interface (chemistry), surface of a liquid as it changes into the gas phase. A high concentration of the evaporating substance in the surrounding gas significantly slows down evapora ...

, and sublimation). The annual equilibrium line separates the accumulation and

ablation zone Ablation zone or ''ablation area'' refers to the low-altitude area of a glacier or ice sheet below firn with a net loss in ice mass. This loss can result from melting, sublimation, evaporation, ice calving, aeolian processes like blowing snow, ...

annually. The accumulation zone is also defined as the part of a glacier's surface, usually at higher elevations, on which there is net accumulation of snow, which subsequently turns into firn and then glacier ice. Part of the glacier where snow builds up and turns to ice moves outward from there. The glacier head is the highest upslope edge of an alpine glacier or the upslope end of the zone of accumulation. The head of the glacier comes up against a steep bedrock cliff called a

cirque A (; from the Latin word ) is an amphitheatre-like valley formed by Glacier#Erosion, glacial erosion. Alternative names for this landform are corrie (from , meaning a pot or cauldron) and ; ). A cirque may also be a similarly shaped landform a ...

headwall In physical geography and geology, the headwall of a glacier, glacial cirque (landform), cirque is its highest cliff. The term has been more broadly used to describe similar geomorphic features of non-glacial origin consisting of a concave depress ...

Movement and ice loss

The head can come away from the cirque in a downslope movement which can create a large crevasse called a

bergschrund A bergschrund (from the German for ''mountain cleft''; sometimes abbreviated in English to "schrund") is a crevasse that forms where moving glacier ice separates from the stagnant ice or firn above. It is often a serious obstacle for mountaineer ...

. This crevasse can be a major obstacle for mountaineers. The existence of a bergschrund is evidence that ice mass has pulled away from the cirque. A shrinking glacier thins faster near the terminus than near the head, which explains why glaciers retreat up-valley and the glacier head stays in place The speed of erosion or accumulation is partly dependent on a shape factor which is the ratio of the change in thickness at the glacier head to the change in the thickness at the terminus.

Notes

''attribution'':Contains text copied from

Accumulation zone On a glacier, the accumulation zone is the area above the firn line, where snowfall accumulates and exceeds the losses from ablation, ( melting, evaporation, and sublimation). The annual equilibrium line separates the accumulation and ablation ...

and

Bergschrund A bergschrund (from the German for ''mountain cleft''; sometimes abbreviated in English to "schrund") is a crevasse that forms where moving glacier ice separates from the stagnant ice or firn above. It is often a serious obstacle for mountaineer ...

and

Terminus Terminus may refer to: Ancient Rome *Terminus (god), a Roman deity who protected boundary markers Transport *Terminal train station or terminus, a railway station serving as an end destination *Bus terminus, a bus station serving as an end des ...

. Glaciers {{glaciology-stub