Cold environments 1
PLUCKING - a process of glacial erosion by which a glacier freezes around a rock on a valley, and subsequent movement of the ice causes the rock to be pulled away with it.
E.G. Roche Moutonnee (has a smooth side & rough side. - rough side is due to plucking as glacier travelled over the resistant rock)
Calving - The process of ablation by which small masses of
ice break from an ice sheet or glacial. This can produce
icebergs if the edge of the ice sheet extends into the sea.
Sublimation - Involves a direct change of state from solid to
a gas without becoming a liquid.
Ablation - A collective loss of water from an ice
sheet or glacier. This loss can take a variety of
forms. It is more dominant in the lower parts of a
glacier and in summer (due to warmer temps)
Accumulation - The net gain in an ice mass.
This gain can take place in a number of ways: precipitation,
refreezing of meltwater; avalanches etc.
It is more dominant in the upper part of the glacier and in
winter (due to lower temps) and where high altitudes.
GLACIAL BUDGET - the net balance/change in the ice mass of a glacier.
ACCUMULATION > ABLATION - POSTIVE NET BALANCE
ABLATION > ACCUMULATION - NEGATIVE NET BALANCE
ZONE OF ACCUMULATION - the area on the glacier where accumulation exceeds ablation over 1 year
ZONE OF ABLATION - the area on the glacier where ablation exceeds accumulation over 1 year
Ice Age - An extended period of time where the
Earth experienced a cycle of very cold phases
(known as glacials) interspersed with warmer
phases (known as interglacials).
The most recent ice age is called the Pleistocene
Ice Age, began 2 million years ago.
Glacial - A very cold phase where global temps
dropped by approx. 5 degrees resulting in ice
covering 30% of the Earth's surface.
The most recent advance was the Devensian.
Interglacial - A warm phase where temperatures
increased to present levels or above which only
lasts tens of thousands of years.
The most recent retreat was the Ipswichian.
Cold (polar) glaciers - Where the base
temperature is well below 0 degrees and so
remains frozen to the bedrock. Found in polar
regions. e.g. Greenland, Antarctica.
Warm (temperate) glaciers - The base
temperature is around 0 degrees so it's not frozen
to the bedrock. Found in temperate climates
where there are obvious seasonal differences in
temperatures. E.g. The Alps (Europe)
Glacial erosion processes
Glacial abrasion - Eroding of the valley floors and sides by the scraping & scouring of rock fragments as they are dragged along by the glacier as it moves.
Plucking - Where glacial ice forms around fractured rock at the base or sides of the glacier. As the ice attatches to both the glacier and rock fragments, when the glacier moves it causes the rocks to be pulled away with it.
Freeze thaw weathering - A process of mechanical weathering in rocks which contain joints or cracks where the temps fluctutate around 0 degrees. When it's warm water will enter joints & then freeze when temps fall below 0 degrees. As the water freezes it expands by 9% which exerts pressure in the joint causing it to widen. This is a repeated processes of melting & freezing. Can lead to breaking of the rock.
Glacial deposition processes
Ablation Till - Material that is left by the glacier as it retreats. It is the scattering of angular, unsorted debris which has 'melted out' of the ice as the snout retreats up the valley. (If the snout remained stationary then the would build up to form a moraine)
Lodgement Till - A form of ground moraine which is deposited under a glacier/ice sheet. It can result in a drumlin forming.
Erratics - A rock which has been transported by a glacier or an ice sheet and deposited in an area of different geology to that of its source. An erratic can be an indication of the direct of the ice sheet. E.g. Norber Moor erratics, North Yorkshire.
Till - Is an unsorted mixture of clay, sand, gravel &
boulders which are deposited directly from the
moving ice. Till deposits can vary considerably in
sediment structure, orientation and size -
reflecting the different processes involved in their
Ablation till - Material which is left behind by the
glacier as it retreats. It's a scattering of angular,
unsorted debris which has melted out of the ice
as the snout retreats up the valley.
Lodgement till (ground moraine) - Subglacial
material deposited by the actively moving glacier.
Form of moraine which is deposited under the
glacier. The rock particles are laid down under
the pressure of the slowly moving ice when the
base melts. The till stones may display some
orientation with the long axis in the direction of ice
Describing glacial till deposited directly by the ice:
- Unsorted, Not stratified, Not graded & Angular (not rounded)
Moraine are ridges of debris that develop along the margins of glaciers in response to the dumping of sediment that has been transported to the front and edge.
End moraine - Moraine found at the snout of the glacier. (Umbrella term for 3 main types: recessional, terminal & push moraine)
Lateral moraines - Along the valley sides. Formed from debris drived from frost shattering of valley sides and carried along the edges of the glacier.
Medial moraines - Found in the centre of a valley amd results from the merging of two lateral moraines where 2 glaciers have joined. Orientation of the till is in the direction of the glacial advance.