Periglacial landforms Flashcards
Why do peri glacial landforms exist?
bc of climate change before and/or after glacial periods
What do periglacial environments have?
- permafrost overlain by an active layer
- Seasonal temp variatons (above 0 in summer, albeit for a short period)
- Freeze-thaw cycles dominating geomorphic processes (frost heave, frost thaw)
High latitude areas eg Alaska, continental interiors eg Siberia + in high mountains at lower latitudes eg Andes
What is freeze thaw weathering?
Dominant process in periglacial environments due to seasonal fluctuations in temp around freezing.
Freeze-thaw weathering occurs when rocks are porous (contain holes) or permeable (allow water to pass through).
- Water enters cracks in the rock.
- When temperatures drop, the water freezes and expands causing the crack to widen.
- The ice melts and water makes its way deeper into the cracks.
- The process repeats itself until the rock splits entirely.
What is frost heave?
Sub surface process that leads to vertical sorting of material in the active layer. Stones within fine material heat up + cool down faster than their surroundings as they have a lower specific heat capacity.
As temp falls, water beneath stones freezes + expands, pushing stones upwards to the surface.
Ground ice also pushes overlying, finer material upwards, producing a domed surface.
What are periglacial landforms?
Patterned ground, opened and closed pingos
What is patterned ground?
How do they form?
Ground which has surface material distributed across it in a pattern.
- thermal contraction causes cracks in active layer, these cracks are filled with water which freeze and develop ice wedges because they expand.
- as a result of frost heave soil expands upwards on to the surface creating the formation of small domes. stones have lower specific heat capacity than soil -> soil (pocket of water) will freeze faster near stones -> soil increase in volume -> stone forced upwards
- The stones move radially, under gravity, down each domed surface to form a network of stone polygons (1-2m in diameter) .
CXT eg Barrow in Alaska, on slope angles of 3-50 degrees, the larger stones move greater distances downslope + the polygons become elongated into stone garlands.
What are pingos? How are they formed?
Rounded ice cored hills that can be as much as 90m in height + 800m in diameter. Grow at rates of couple cm/year
Open pingos -
1. found in discontinuous permafrost, form in valley bottoms where water from surrounding slopes collects within in an area of a talik
2. when temp drops, hydraulic pressure increases + active layer freezes, talik = trapped in between freezing ice plan + permafrost creating an ice lens
3. talik expands + pushes land above it into a dome.
4. if pressure in dome is too great = will crack + expose ice = melt + leave lake
Eg east Greenland
Closed system pingos -
- develop beneath lake beds when lake water seeps into talik beneath
- As permafrost grows during cold periods, groundwater beneath a lake is trapped by permafrost
- The saturated talik or unfrozen ground is compressed by expanding ice around it + is under hydrostatic pressure.
- When talik eventually freezes it forces up the overlying sediments
- ice then melts as temp rise + a depression ognip is formed as pingo collapses
How does temp effect patterned ground?
Rose at end of periglacial period, often colonised by vegetation, making it hard to find + identify.
How does mass movement effect patterned ground?
Over time, by creep also degrades the frost heaved domes, making landform less obvious.
How does temp effect pingos?
Collapse when temp rises + the ice core thaws. When this happens the top of the dome collapses leavinga circular depression called an ognip.
what is solifluction? (mass movement)
slow downslope flow of saturated soil, often forming lobes of movement and terracettes.
it occurs when summer temp rise -> melts surface layer -> becomes wet as water can’t move anywhere (can’t evaporate or sink bc of permafrost) becomes waterlogged -> saturated surface layer -> excess lubrication -> reduces friction between soil particles -> flows downslope
what is gelifluction? (mass movement)
a periglacial category of solifluction where the downslope sliding movement of seasonally thawed and saturated soil material is facilitated by an impermeable layer of permafrost beneath.
what are Solifluction lobes?
created when the saturated active layer of soil is thawed, usually during the summer months.
Winter freeze-thaw weathering loosens material while summer thaw melts the ice content and then it will flow down the hill. When the gradient changes again and flattens out, the material flow slows and is deposited in a tongue shape.
what are terracettes?
a series of small ridges in the ground, underneath vegetation, on a sloped piece of land.
formed when the active layer thaws and gravity allows material to slump down underneath the vegetation
what is a thermokarst?
an irregular land surface consisting of hills and hollows formed when permafrost thaws.
