3.1.4.4 Periglacial Landforms Flashcards
Periglacial landscapes aren’t glaciated but what are they still exposed to
E.g
Very cold conditions with intense frost action + permanent,y frozen ground (permafrost)
E.g tundra areas like northern Russia and Canada, Alaska
What’s permafrost
3 types
Soil, rock or sediment that has been permanently frozen for at least 2 consecutive years
Continuous
Discontinuous
Sporadic
Where is continuous permafrost found
In coldest regions, deep into surface layers where there’s hardly any melting of upper most layer
Where does discontinuous permafrost occur
In slightly warmer regions, where ground isn’t frozen as deeply (still 20-30m deep)
Where is sporadic permafrost found
Where mean annual temperatures are around freezing point
What does sporadic permafrost occur in
Isolated spots
Describe an active layer
When summer temps are above freezing, the surface layer thaws from the surface downwards to form an active layer
Why is an active layer often saturated
As meltwater can’t infiltrate downwards as permafrost is impermeable
What’s talik’s
Unfrozen layer beneath/within permafrost
7 periglacial landforms
Patterned ground Ice wedges Pingos Blockfields Solifluction lobes + sheets Terracettes Thermokarst
What can produced patterned ground
What’s the main process
Repeated cycles of freezing + thawing of the active layer
Main proces involved is frost heave
What does frost heave form on the surface
Small domes
What’s frost heave
As the active layer starts to refreeze, ice crystals begin to develop
This increases the soil volume + causes an upward expansion of the soil surface
Why is the soil immediately. Beneath a stone likely to freeze + expand (patterned ground)
As stones in fine-grained material have lower specific heat capacity so heat up + cool down faster than surrounding finer material.
The cold penetrating from the surface passes through the stones faster than through surrounding material
What does the soil beneath a stone do as it freezes + expands before other material
It pushes up the stone until it reaches the surface, meaning a range of features can form
2 patterns formed by frost heave
Circles
Stripes
2 shapes on flat ground
Circles
Polygons
What shape is created by permafrost frost action
Steps
5 patterned ground shapes
Circles Nets Polygons Steps Stripes
How are ice wedges formed
In low temperatures the ground contracts + cracks develop
During summer, meltwater fills these cracks + freezes in the winter to form ice wedges
How do ice wedges increase in size
Repeated freezing + thawing
What does frost heave cause to form which ponds may from between in summer
Narrow surface ridges
What do ice wedges begin as
Cracks less than 5cm
What pattern does the ground produce when it contracts
How big are they
Ice wedge polygons
Larger than stone polygons, 15-30m wide, but can reach 100m
Whats a pingo
Rounded hill up to 90m high and 800m wide
What do pingos often look like
Often green + vegetated on outside with a core of solid ice
Where are pingos thought to form
On the site of a lake gradually infilled with sediment
What may happen to the pingo in the summer
Part of the ice core will melt + centre may collapse
Resembling a volcanic crater that may fill with water
How do ‘closed’ pingos form
- Lake infills with sediment insulating ground below. Liquid water is trapped in talik between lake sediment + permafrost
- Water freezes as climate cools to form ice core. Ice expands due to hydrostatic pressure increase and talik is squeezed. Lake sediment is pushed up to form a pingo
What are blockfields
What are they repeatedly subjected to
Periglacial landscapes with extensive frost-shattered bedrock (broken up angular rock fragments)
Repeated freezing + thawing
What does solifluction involve
Where is it most likely to occur
The downslope movement of rock + solid material due to gravity in a periglacial environment
Occur in areas where summer melting is taking place and there’s a reasonably thick + saturated active layer
What are formed where solifluction occurs on slopes around 1-20*
Describe them
Solifluction sheets
Have a smooth surface + can extend over 100m across a slope
What will form where slopes have a steeper gradient of 10-20*
Describe them
Solifluction lobes
Have a tongue like look that extends downslope + can be up to 50m in width + 5m high
What are terracettes
Small periglacial features often referred to as ‘sheep tracks’ as they look like long, narrow pathways running horizontally alongside a slope
Narrow steps often 20-50cm high and only tens cm wide that run parallel to slopes contours
How do terracettes form
In winter soil freezes + expands
In summer particles melts + goes downwards
Particles shifting upwards + downwards create steps in hillsides
Soil creep usually occurs on the slopes of about 5* and produces them
2 processes that help produce terracettes
Frost heave
Soil creep
What is thermokarst
Depressions in the ground surface resulting from ice melting within permafrost
What can cause thermokarst as well as just temperature change
Human activity warming surface layers of permafrost
