Glaciation - Unit 6 Flashcards
What is permafrost?
Frozen ground that remains frozen all year for atleast 2 consecutive years
What percentage of the Earth’s landscape contains permafrost?
25% of exposed land
What does the mean annual ground temperature need to be for permafrost to occur?
Below 0ºC
How deep can permafrost be?
1500m
What is the active layer?
Top layer of soil that thaws in summer and freezes in autumn
What 3 ways can ground ice exist?
1- Pore ice
2- Needle ice
3- ice lenses
Where does pore ice develop?
In pore spaces between soil/sediment particles where liquid water can accumulate and freeze
What does needle ice consist of?
Narrow ice silvers that are up to several cm long
Where does needle ice form?
In moist soils when temperatures drop below freezing overnight
When do ice lenses form?
When moisture, mixed within soil or rock, accumulates in a localised zone. They usually run parallel to the surface
What are ice wedges?
- Downward narrowing masses of ice
- They are 2-3m wide and 10m deep
Describe the formation of ice wedges
- Crack forms in ground during winter
- Temp increases in summer so ice melts and fills crack
- Temp decreases in winter again so water refreezes and ice expands
- Process continues each year, causing ice wedge to continue increasing in size
What is patterned ground?
The surface of periglacial areas, characterised by presence of ground materials arranged in symmetrical and geometric shapes
Describe the formation of patterned ground
- Ground freezes from surface downwards, causing ground to expand
- Moisture in ground begins to freeze & rise, forming an ice lens
- As ice lens expands, it pushes stones above it as they have a lower SHC than the ice lens
- Stones are raised due to cryostatic pressure (pressure exerted when ice expands) = frost heave
- Mound formed
- When ice thaws, regolith (loose material) falls into the gap beneath stones, preventing it from dropping back
- Process repeats until stones are raised high enough to break through surface of ground
- Once on surface of the mound, stones slip, slide or roll to the base to form patterns
What is regolith?
Loose material covering solid rock
What is frost heave?
Upward dislocation of soil and rocks by the freezing and expansion of soil water
What are pingos?
- Ice-covered hills
- They are 3-70m high and 30-1000m wide
Describe the formation of a closed system pingo
- Lakes form in summer and freeze in winter
- Unfrozen lake water and sediment beneath ice acts as an insulator for underlying ground
- Prevents ground freezing, results in talik
- During prolonged periods of cold, permafrost will advance & encroach an overlying talik
7- Liquid contained within talik will freeze, causing an ice lens
8- As ice lens grows, it will exert cryostatic pressure & rise upwards, displacing overlying lake
9- The sediment that was once at the bottom of the lake now covers the raised ice lens
What is a thermokarst?
A land surface characterised by very irregular surfaces of marshy hollows and small hummocks formed as permafrost thaws
What is a thermakarst lake?
A body of freshwater formed in a depression by meltwater from thawing permafrost
What is frost weathering?
A collective term for several mechanical weathering processes induced by stresses created by the freezing of water into ice
What is a blockfield?
A surface covered by large, angular rocks. They are a product of freeze thaw weathering, e.g. Snowdonia
What is a scree slope?
A slope of loose rock debris at the base of a steep incline or cliff
Describe the formation of a scree slope
- FTW occurs
- Weathered materials fall to bottom of slope through mass movement
- Over time, debris will accumulate at the base of the slope angle
How does a protalus rampart form?
- Forms in a similar way as a scree slope but shape is different
- Scree lands on the ice & slides down the base of the ice
- Accumulates in the mound
- When ice melts, this accumulation of scree is left
What does solifluction mean?
Soil flow
What are head deposits?
Materials that have been weathered and then subject to mass movement downslope by solifluction
Describe the process of soil creep and the formation of terracettes
- Active layer freezes & expands upwards parallel to the slope
- As it thaws, gravity pulls it vertically downslope, forming terracettes
Describe the formation of dry valleys
- Meltwater is released at end of a glaciated period
- Permafrost is impermeable so warter doesn’t infiltrate
- Huge quantities of meltwater carve out deep, steep, V-shaped valleys
- When permafrost thaws, meltwater infiltrates, leaving behind a dry valley
Describe the formation of loess plateaux
- During autumn and winter, melting is reduced
- Flow of meltwater down rivers is greatly reduced
- Large parts of formerly submerged floodplains dry out and are exposed to wind
What is the periglacial action of wind?
Loess deposits
What is periglacial action of water?
Dry valleys
