unit four Flashcards
(37 cards)
where can rock debris be transported or entrained in a glacier ? (4)
- the surface (supraglacial debris)
- within the glacier (englacial debris)
- at the base of the glacier (subglacial debris)
- at the front of a glacier (proglacial debris)
describe supraglacial debris
found on the surface of the glacier, this is where frost shattered, avalanche and other debris fall onto the glacier, the material is angular and unsorted
describe subglacial debris
found at the base of a glacier, small debris can be entrained by basal ice freezing, while large boulders can be entrained by deformational flow and transported by traction. some materials can be plucked where regelation takes place around the bedrock
describe englacial debris
found within the glacier, they occur as ice moves forming crevasses where debris can fall into the cracks or be carried by meltwater and trapped within the glacier
describe erratics
different rock types to the bedrock they sit on.
why is there so much subglacial debris found at the snout of a glacier?
it is in the zone of ablation so it always has a negative mass balance. outputs are larger than inputs and so more debris are visible as the ice will constantly be reducing, exposing debris on the surface
what flow type does ice that flows over an increasingly steep gradient have?
extensional flow due to internal stress. this can result in supraglacial debris falling down crevasses and becoming englacial debris
what flow type does ice that flow over a decreasing gradient have?
compressional flow due to internal stress, resulting in sub/englacial material being moved towards the surface becoming supra-glacial debris
how do supraglacial debris become englacial debris?
via extensional flow, they fall into crevasses and become englacial debris
how do englacial debris become supraglacial debris?
via compressional flow, they move towards the surface due to internal stress
What are deposits deposited directly by a glacier called?
Glacial deposits
What are deposits deposited by glacial meltwater called?
Fluvioglacial deposits
What are some examples of landforms formed by glacial deposits?
Drumlins, terminal moraines, recessional moraines, lateral moraines, medial moraines, push moraines
What are some examples of landforms formed by glacial meltwater (fluvioglacial)?
Eskers, Kames, Kame terraces, kettle holes, kettle lakes, varves
Where can fluvioglacial (glacier meltwater) landforms be found?
Landscapes called sandur‘s or outwash planes
Where can glacial ice landforms be found?
Till planes
Describe what glacial deposits tend to be composed of
Unsorted, unstratified debris containing rock fragments of all shapes and sizes ranging from boulders to small clay sized particles, they’re called glacial till or sometimes boulder clay
Where is lodgement till formed?
Subglacially
How is lodgement till formed?
It’s plastered onto the underlying surface by the glacier above, where friction between the debris and the bed is greater than the drag produced by the ice moving over it. This is common where there is a large amount of subglacial debris or slow moving ice
Describe lodgement till
A mix of rounded boulders in a matrix of sand, mud, and clay. Due to the pressure involved this till tends to be compacted and then often forms drumlins
Describe how ablation till is formed
This material is deposited from supra-glacial, englacial and sub glacial debris due to the process of ablation. Meltwater may remove some of the finer clay leaving only the larger ones
Describe ablation till
poorly stratified, compacted and unsorted till that is often more angular in nature than lodgement till
Where is ablation till mainly formed?
At the snout of a glacier
What is deformation till?
When a glacier re-advances over an area of previously deposited till, the sediment can be folded or faulted.
