3.1.4.3 - Glacier types, movement and weathering
Bed temperature and the stresses within ice determine how a glacier moves and erodes. This lesson links thermal conditions, weathering and contrasting forms of movement.
Warm-based and cold-based glaciers
The thermal condition at the bed matters more than a simple 'polar versus mountain' label. At a warm-based bed, ice is at the local pressure-melting point and liquid water can be present. At a cold-based bed, ice is below that point and is normally frozen to its substrate. A polythermal glacier contains different thermal zones; classification can change across one glacier and through time.
A glacier develops where accumulated snow survives, compacts and thickens into moving ice. Thick ice insulates the bed from cold surface air; geothermal heat, friction and deformation can warm the basal zone, while meltwater can carry heat into the ice. Severe cooling, limited surface melting and conductive heat loss can maintain a cold bed. The pressure-melting point is slightly below 0°C under pressure, so 'below zero' alone does not establish a frozen bed.
Vatnajökull is a temperate ice cap, providing an active warm-ice example. Some cold-based polar glaciers preserve older surfaces beneath them because limited basal sliding reduces debris movement across bedrock. However, a cold-based glacier can still move by internal deformation and carry sediment. Its surface appearance or latitude alone does not prove that its entire bed is frozen or that no erosion ever occurs.
Frost action and nivation supply debris
Frost action breaks or displaces rock where water, suitable temperatures and cracks interact. Water freezing in confined joints can exert pressure; repeated freezing and thawing widens weaknesses. Ice segregation can also draw water towards growing ice and fracture rock. Effective frost weathering therefore needs moisture and favourable thermal conditions; continuously very cold, dry rock is not necessarily weathering fastest.
On valley walls, loosened fragments fall onto glacier surfaces and supply angular debris. On ice-free ground, repeated frost heave raises material during freezing, while thaw allows settling and movement. Weathering produces debris; erosion removes and transports it. Confusing these stages hides the sediment pathway.
Nivation is the combined activity around persistent snow patches, rather than one unique erosion force. Meltwater, frost weathering near patch edges and movement of saturated debris can enlarge a shallow hollow. Snow cover can insulate its centre while the margins experience strong temperature changes. A favourable hollow may later collect enough snow to initiate a glacier, but a nivation hollow is not already a glacier-eroded corrie. At relict Cwm Idwal, contemporary frost weathering can modify slopes without recreating the ice flow that excavated its large hollow.
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