2.7 - Glacial Meltwater
The role of meltwater in glacial systems
Meltwater is the water produced when glaciers melt, and it plays a vital part in shaping landscapes even as the ice retreats.
How meltwater contributes to glacial processes
- It carries sediment through the system, helping to redistribute materials across large areas.
- It lubricates the base of the glacier, making it easier for the ice to slide forward as a whole.
- It enhances erosion by supporting actions like plucking and abrasion.
Key terms in meltwater processes
- Fluvioglacial - Refers to erosion or deposition caused by flowing meltwater from glaciers.
- Proglacial - Describes the area immediately in front of a glacier's edge.
Pathways of meltwater movement through glaciers
Main types of meltwater flows
- Supraglacial flows - These occur on the glacier's surface, mainly in the ablation zone. The water often follows winding paths and can seep into the ice through crevasses.
- Englacial flows - Meltwater moves through the internal structure of the glacier, typically via tunnels. It can also descend vertically through moulins.
- Subglacial flows - These take place beneath the glacier, channelled under pressure over the bedrock. The water emerges at the glacier's snout and flows into the proglacial area.
Characteristics of fluvioglacial deposits
Key features of fluvioglacial deposits
- Stratification - Deposits form in distinct layers due to seasonal changes in meltwater flow and sediment supply, creating a stratified structure.
- Sorting and grading - Materials are organised by size, with larger particles deposited near the glacier's snout. Finer sediments are carried further away from the glacier.
- Imbrication - Rocks are arranged in overlapping patterns aligned with the flow direction. The longest axis of each rock points parallel to the current, and larger stones dip upstream, resembling a series of fallen dominoes.
Landforms formed in contact with melting ice
Formation of eskers
Eskers are elongated ridges of sand and gravel that trace the paths of former subglacial streams. As meltwater flows through tunnels beneath the ice, it deposits sorted and stratified sediments. When the glacier retreats and the tunnels collapse, these deposits remain as winding ridges, often extending for hundreds of kilometres and reaching heights of up to 50 metres.
Formation of kames
Kames appear as stratified mounds of sorted sand and gravel on valley floors near the glacier's snout. They form when supraglacial streams pool in ice depressions, slowing down and dropping their load in layers. As the ice melts away, the accumulated debris is left behind on the ground.
Formation of kame terraces
Kame terraces are piles of deposits along valley walls, created by meltwater streams flowing between the glacier's edge and the valley side. These features resemble lateral moraines but are distinguished by their sorted, layered composition. Heavier gravel settles at the base, with finer sediments on top.
Proglacial landforms created by meltwater
Meltwater channels
Meltwater channels are deep, wide troughs carved by powerful streams, often following existing river paths. Their size reflects the high volume and energy of glacial meltwater, which can erode significantly. After retreat, these channels may contain braided streams – interwoven channels formed when sediment islands divert the flow – especially if the water carries a lot of debris.
Sandurs (outwash plains)
Sandurs, also called outwash plains, are broad sheets of sediment spread in front of a former glacier snout. Meltwater emerging from the ice deposits gravel first (as the heaviest material), followed by sand and then clay furthest away, creating sorted layers.
Kettle holes and kettle lakes
Kettle holes are depressions formed when isolated ice blocks left by a retreating glacier melt, causing surrounding sediment to collapse into the void. These can fill with water to become kettle lakes, often found on sandurs where meltwater buries the ice before it fully melts.
Proglacial lakes and varves
Proglacial lakes develop when terminal moraines block meltwater streams, acting as natural dams. As streams enter the lake, they slow and deposit sediment in deltas. Each annual layer of this sediment is a varve, with thickness varying based on meltwater volume – thicker in high-melt years.