P7 Flashcards
There are two main sources of meltwater from glaciers
- : surface melting and basal melting.
Surface melting
- contributes most of the supply and peaks in late summer; it is the only source of meltwater for cold basal glaciers.
- Supraglacial surface streams form running along the top of the ice, especially in the ablation zone.
- These supraglacial channels are often very fast flowing and may plunge down into the ice either through a crevasse or, more commonly, via a moulin (a cylindrical, vertical tunnel rather like a pothole in limestone landscapes), becoming englacial streams.
- As meltwater moves through a glacier it may refreeze or contribute to further melting and reach the subglacial supply, depending on the temperature of the ice inside the glacier.
Basal melting
- occurs if the temperature of the ice at the base of a glacier is at pressure melting point (in a warm-based glacier).
- The basal meltwater flows under hydrostatic pressure beneath the glacier and can excavate subglacial tunnels by cutting through the bedrock.
- The meltwater streams eventually emerge from subglacial tunnels at the glacier snout via portals (caves).
Processes of fluvio-glacial erosion and deposition
- It is important to remember that away from the glacier, outlet streams behave similarly to normal streams, although their discharge, sediment loads and the lack of vegetation lead to some variations in operation.
- Within the glacier, however, fluvio-glacial streams operate very differently because of the high pressure and velocity of flow.
- This causes the erosion of underlying bedrock by abrasion, cavitation and chemical means beneath the glacier ice, and can also lead to intense erosion by meltwater streams as they exit the glacier snout.
- The ablation rates are very high during deglaciation, and many of the meltwater streams have very high discharges leading to powerful erosion.
When meltwater deposits material subglacially, englacially and supraglacially, the material is referred to as an
ice-contact fluvio-glacial deposit.
Where the fluvio-glacial material is deposited at or beyond the ice margin, by streams coming out of the snout, it is known as
outwash or proglacial.
Characteristics of fluvio-glacial deposits
In comparison to glacial deposits (tills), fluvio-glacial deposits tend to be:
- generally smaller than glacial till as meltwater streams; although having seasonally high discharge, they still have less energy than large valley glaciers so they generally carry finer material
- generally smoother and rounder through water contact and attrition
- sorted horizontally, especially in the case of outwash deposition, with the largest material found up-valley or nearer the glacier snout, and progressively finer material down-valley, due to the sequential nature of deposition mechanisms
- stratified vertically with distinctive layers that reflect either seasonal or annual sediment accumulation.
In contrast, glacial till is classed as a
diamicton, being angular, poorly sorted and non-stratified.
A further distinction can be made between ice-contact fluvio-glacial deposits and outwash deposits.
Outwash deposits experience more attrition, causing clasts to become more rounded, and the material is better sorted horizontally.
Three main zones of outwash deposition extend from the front of the glacier, and the characteristics change through these zones, as shown in Table 6.4.
Note that varved deposits form in meltwater lakes along or beyond a glacier margin (see page 87).
Ice contact landforms of fluvioglacial deposition
Proglacial landforms of fluvioglacial deposition
Fluvio-glacial landforms
- Subglacial meltwater can excavate large meltwater channels.
- These can cut across contours as the direction of the meltwater flow is controlled by the hydrostatic pressure gradient.
- Subglacial meltwater can even flow uphill, so these channels can have a ‘humped’ long profile.
- Examples of these meltwater channels can be found in many parts of the UK including the Gwaun Valley in North Pembrokeshire.
Ice-contact features - eskers
- Eskers result from subglacial meltwater deposition.
- They are sinuous ridges of relatively coarse sand and gravel deposited by meltwater flowing through tunnels, sometimes englacially, but normally subglacially.
- Eskers came in a variety of sizes.
- A small esker can be found in the UK at Wark on the River Tweed; it is about 1 km long, 40 m wide and about 20 m high.
Eskers are thought to occur when
- a subglacial or englacial channel becomes obstructed, leading to deposition of material upstream from the blockage.
- The ice needs to be stagnant for englacial eskers to form, otherwise the material would be reworked by glacier ice movement.
- Another possible mode of formation is where a delta of fluvio-glacial material extends outward, perpendicular to the ice margin, taking on an elongated form under conditions of rapid ice retreat.