Glacial/Periglacial landscapes

Question 1

What are the 3 types of glacial motion

Answer 1

1. internal deformation (where creep can accommodate applied stress in the ice, faults and folds develop)
2. Basal sliding
3. Substrate deformation

Question 2

describe basal sliding

Answer 2

occurs in warm -based glaciers resting on bedrock

melt water from glacier acts as lubricant → caused by pressured melt water or small water-saturated sediments → gives the glacier a smooth surface on which to move as opposed to a harsh surface that slows the speed of sliding

Question 3

What is pressure melting?

Answer 3

melting point depends on temperature AND pressure

• takes place at the base of a glacier

ice melts at 0° at 1 atmosphere

• Below 4 km ice the pressure is 35 atmospheres so ice melts at sub zero temperatures
• Melting of a 2000m thick ice sheet is -1.6° rather than 0

Question 4

Describe cold and warm based glaciers

Answer 4

Warm:

• ice at pressure melting point except near surface that cools in winter
• thin ice
• fast moving
• summer melting

Cold:

• considerable portion of ice below a pressure melting point
• thick ice
• slow movement
• no summer melting

now recognised that warm and cold ice may occur within the same glacier or ice sheet

Question 5

What is the thermal regime

Answer 5

to do with warm and cold bases
Glacial thermal regime is one of the most important factors in determining subglacial processes.
- controls patterns of erosion and deposition within the ice

Regelation: melting under pressure then refreezing when pressure is released (striking an object and moving past it)

Question 6

Describe plucking

Answer 6

• occurs in lee side cavities when rock fragments are frozen to glacier base during regelation and quarried as the glacier slide
• bedrock is fractured then entrained in glacier
• favoured by thin and fast flowing ice as it encourages extensive separation of ice from its bed to create subglacial cavities

Question 7

Describe abrasion

Answer 7

• “sandpapering” effect
• clasts are dragged across the rock substrate by basal sliding
  the clasts:
• cut continuous scratches - STRITATIONS
• move in stick-slip manner - CHATTERMARKS

Question 8

(Land forms of glacial erosion)

Small scale: Roche Moutonnee

Answer 8

• small hills that are modified by glacial action
• from tens to hundreds of meters long
• good pointer to direction of past ice flow if used in conjunction with striations, grooves etc.

plucking at stoss end abrasion at lee

Question 9

(Land forms of glacial erosion)

Large scale: Troughs (U-shaped valley)

Answer 9

• warm based glaciers re-shape valley cross-profiles by concentrating erosion on valley sides then valley floor
• irregular long-profile

2 kinds:

• Glaciated valley - lies above sea level
• Fjords lies below sea level and is a glaciated valley drowned by the sea

Question 10

(Land forms of glacial erosion)

Large scale: Cirque (Corrie)

Answer 10

• arm- chair hollows
• deep rock basin wit steep head wall at its back, residual lip at front often contains a lake
• lip can be buried under terminal moraine

Question 11

Describe:

Lodgement Till
Melt-out Till
Subglacial mel-out till
Supraglacial melt-out till

Answer 11

Till = sediment deposited directly from glacier ice
lodgement - subglacial material thats deposited by the actively moving glacier. DRUMLINS formed from this material

melt-out - released by melting of stagnant or slowly moving debris-rich glacier ice

subglacial - melting of debris-rich ice at bottom of glacier

supraglacial - melting of ice on glacier surface

Question 12

(Land forms of glacial deposition)

Terminal moraine

Answer 12

• material piles up at snout of glacier to form high ridge, tens of meters high across valley
• represents the furthest extent of a glaciers advance

Question 13

(Land forms of glacial deposition)

Lateral moraine

Answer 13

• forms at edge of glacier
• mostly scree that’s fallen off valley sides
• when ice melts it leaves a slight ridge on the side of a valley

Question 14

(Land forms of stagnant ice deposition)

Kames and Kame terraces

Answer 14

melt water ponds between stagnant ice and slope → sediment deposited by flowing water or into ice marginal lakes → ice disappears and the sedimentary land form remains as steep-sided, flat- topped surface.

Question 15

Give examples of periglacial processes and their associated land forms

Answer 15

Frost cracking → ice wedges and ice wedge casts

Cryoturbation → Patterned ground

Solifluction → Solifluction sheets and lobes

Question 16

Give the thermal characteristics of permafrost

Answer 16

Active layer - ground subject to seasonal freezing

Permafrost table - upper surface of permanently frozen ground

Permafrost base - lower surface of permanently frozen ground

Talik - Any pocket of unfrozen ground within the soil

Question 17

Describe frost cracking and ice wedge clasts

Answer 17

Frost cracking caused by shrinkage of permanently frozen ground and expanding near ground surface →space is infilled by sediment (ice or sand)

• paleoclimatic indicators
• express themselves in the surface as sorted circles or ice wedge polygons

Question 18

Explain cryostatic pressure

Answer 18

pressure on talik by frozen ground eventually results in wet sediment being forced upwards

• patterned ground : sorted circles on level surfaces and stone stripes on slopes

Question 19

Explain solifluction

Answer 19

• Produces lobes and terraces in soil cover

- slow down slope creep or flow of soil saturated by thawing ground ice

Question 20

What is needed for solifluction to occur

Answer 20

• Frost susceptible soils, i.e a significant proportion of silt size suction on freezing leads to ice segregation
• Moderate slope angles (8-36 in Scotland)
• Seasonal freezing and thawing to depths of decimeters