Permafrost Characteristics and an Introduction to Periglacial Processes and Landforms

FROST ACTION (Weathering)

Freeze-thaw cycles (when water freezes - it expands in volume by 9% - exerts pressure) - seasonal and diurnal freeze-thaw activity

FROST ACTION (in ground)

Cryoturbation - expansion and contraction of material in the ground - this results in the churning / mixing of soil / rock and sediment - this can therefore alter horizontal beds of sediment - causing them to twist and fold.

Ice Segregation - freezing through the active layer occurs at unequal rates - pockets of ice start to form as water is drawn / segregated from the sediment towards a 'freezing centre' - this may form an ice lens - which enlarges as more water is drawn in and freezes on it (particularly in finer sediments) - pushing the ground upwards.

Ground Cracking (Ground Contraction) - where ground is already frozen, further temperature decreases causes thermal contraction of the ground sediments and ice (will then allow the formation of ice wedges in the cracks that have opened up as water seeps in from the active layer and freezes and cracks).

Frost Heave - stones move upwards during freeze-thaw cycles due to the processes of frost pull and frost push.

• Frost Pull - as freezing descends from the surface and freezes onto a stones upper surface, the stone is lifted by upward expansion of the freezing sediments;
• Frost Push - as the stone is lifted upward by frost pull, a space is created beneath the stone; due to greater thermal conductivity of the stone ice crystals grow into the space beneath the stone, pushing it upwards.
• As thawing occurs downwards, surface sediments contract and lower, but the stone is kept in position by the ice crystals, the stone is then held in the thawed sediments as the ice crystals thaw and the space beneath the stone is infilled by sediment which has collapsed inwards upon thawing of the ice-crystals)

Movement of the Active Layer

Solifluction / Gelifluction (the process of solifluction in a permafrost zone is known as Gelifluction) - this is where the surface (active) layer melts and becomes mobile as it becomes saturated due to being above an underlying impermeable permafrost layer. Solifluction, basically means, the downward movement of soil. This seidment is then able to move downwards (the rate of downward movement will increase with gradient, although only to a point, as the rate of water draining out of the soil also increases with gradient)

Meltwater

Fluvial Action - during a short spring thaw season - large amounts of meltwater may be present, resulting in flooding over the underlying impermeable layer and resulting in water erosion as well as the formation of braided streams

Wind Action

Aeolian Processes (wind action) - Periglacial areas are characterised by very strong winds (there are no obstacles in the way to break up the wind and there is a good supply of material from glacial outwash areas and periglacial deposits which are loose due to the absence of vegetation) - wind abrasion is therefore effective and fine sediment may be removed by the wind and deposited elsewhere.

Snowbank Processes

Nivation - freeze-thaw weathering and other procesess taking place under a snow bank (this setof processes may include freeze-thaw, solifluction, transport by running water and even chemical weathering).

A SUMMARY TABLE OF PERIGLACIAL PROCESSES & THEIR ASSOCIATED LANDFORM FEATURES

Lesson Resources (Handouts and Extra Links)

Fundamentals of Physical Geography - Periglacial Processes and Landforms

Periglacial Proceses and Landforms - summary notes (University of Regina)