a2.3gq3 glacial and quaternary geology lecture 3 highland active-ice features

A2.3GQ3 Glacial and Quaternary Geology

LECTURE 3

HIGHLAND ACTIVE-ICE FEATURES

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SUMMARY

The highland valley landsystem Landsystem contrasts

– Supraglacial debris supply– Climate– Ice dynamics

Frontal and lateral deposits– Debris-mantled glaciers– Clean glaciers

Coupling to the proglacial system

THE HIGHLAND VALLEY LANDSYSTEM

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Glaciated valley assemblage

The highland valley landsystem develops when the glacier is contained within rock walls.

The depositional pattern is thus often controlled by the available area of the valley floor.

The defining feature is the the contribution of rockwall debris to the sediment load.

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The original model was developed by Boulton & Eyles (1979) and made no distinctions between different mountain settings.

Subsequent work by Benn et al. (2005) indicated a contrast between ‘clean’ highland glaciers and those with debris-covered ablation zones.

This contrast has significant effects on the exact nature of the landsystem.

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LANDSYSTEM CONTRASTS

Supraglacial debris supply

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The supply and nature of the rockwall debris is identified as a major control on the development of the landsystem.

Significant factors include:– the rate of supply by weathering, landsliding etc– the general size of the debris particles– presence of clays

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These are determined by the bedrock lithology and local climate

– crystalline vs fissile (shale/slate) contrast– elevation or precipation contrast.

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Glacier d’Argentière, FrancePhoto: M.A.Paul

12Rockwall debris - Himalaya

LANDSYSTEM CONTRASTS

Climate

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The clean vs dirty balance is also controlled by the input of snow relative to debris.

– High relative input leads to the development of clean ablation zones

– Low relative input leads to the development of debris-mantled ablation zones

The presence of a thick debris cover decreases the rate of ablation. Thus the ablation area will be larger relative to the accumulation area on debris-covered glaciers.

LANDSYSTEM CONTRASTS

Ice dynamics

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Clean vs dirty glacier snouts show a distinctive contrast in their response to changes in ice dynamics

Clean snouts show numerous frontal moraines superimposed on largely subglacial bedforms

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These include:

– ice-push and ice-squeeze features, such as annual moraines that arise from the winter advance into water-saturated debris

– push/readvance (oscillation) moraines, due to decadal changes in mass balance

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BreidamerkurjökullIcelandPhoto: M.A.Paul

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Dirty snouts create multiple frontal moraines, often of great size.

These may be tectonically stacked, nested or superimposed.

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Feegletscher, SwitzerlandPhoto: M.A.Paul

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FRONTAL AND LATERAL DEPOSITS

Debris-mantled glaciers

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The principal landforms are the large latero-frontal moraines and the collapse features associated with them.

deposited by sliding and/or dumping.

not ice cored, not pushed (at first).

rockwall debris is usually angular, lacks subglacial rounding and faceting.

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Sublacial landforms may exist but are often concealed beneath supraglacial debris.

There will be some reworking of debris by fluvial activity but ablation rates/area are slow (due to debris insulation) thus spatial density of reworking is limited.

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FRONTAL AND LATERAL DEPOSITS

Clean glaciers

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The principal landforms are subglacial in origin and reflect the glacier dynamics.

Supraglacial debris is limited and forms scattered cones and debris trains, perhaps associated with e.g. defined medial moraines.

31Glacier des Boissons: France

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BuchananisenSpitsbergenPhoto: M.A.Paul

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SolheimajökullIcelandPhoto: J.W.Merritt

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Composite push ridgesBreidamerkurjökullPhoto: M.A.Paul

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Some authors have suggested that the widespread ‘hummocky moraine’ of the Scottish highlands is an active-ice feature from interacting recessional ridges (Bennett & Boulton) or from local thrusting (Hambrey et al.) This implies clean ice.

Others (Sissons) believe it to be supraglacial and indicative of passive downwasting. This implies a supraglacial debris cover.

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Probable push ridgesScottish HighlandsPhoto: J.W.Merritt

38Hummocky moraine, Glen Torridon

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COUPLING TO THE PROGLACIAL SYSTEM

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The proglacial system is the interlinked system of rivers and lakes by which meltwater is removed from the glacier.

This system also carries sediment by both water and mass flow and creates characteristic landforms and sediment bodies.

Meltwater streams can be powerfully erosive and create systems of channels, which may or may not be interconnected.

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An issue related to the clean/dirty ice model is the extent to which sediment discharge is coupled to the proglacial outwash system.

Large moraines often prevent drainage and create proglacial lakes. These act as sediment traps.

This causes the deposition of deltas and lake sediments. The meltwater streams leaving the area carry relatively little sediment.

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By contrast, clean glaciers lose much of their debris into the meltwater system since there is no way of trapping it.

This creates large spreads of fluvioglacial sediments as outwash terraces (sandar) which consist of overlapping fans.

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Over a period of time the degree of coupling may change, due to the destruction of moraine ridges and the consequent reorganisation of the local drainage system.

Several examples are provided by glaciers in western Spitsbergen during the past 100 years or so.

48Elisabreen: SpitsbergenNorsk Polarinstitutt photo

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SUMMARY

The highland valley landsystem Landsystem contrasts

– Supraglacial debris supply– Climate– Ice dynamics

Frontal and lateral deposits– Debris-mantled glaciers– Clean glaciers

Coupling to the proglacial system

THE END

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