The nature and distribution of permafrost in periglacial areas

Periglacial environments are regions near glacial areas, often characterised by extreme cold and the presence of permafrost, which is ground that remains frozen year-round. This unique feature significantly shapes the landscape and processes in these areas.

Characteristics and types of permafrost

• Definition and structure - Permafrost refers to soil or rock that stays permanently frozen, with a surface layer known as the active layer that thaws during warmer months.
• Global coverage - Approximately 20-25% of the Earth's land surface is covered by permafrost, found mainly in polar and high-altitude regions.
• Continuous permafrost - Occurs where all ground is frozen, requiring a mean annual temperature below -5°C.
• Discontinuous permafrost - Found in patches, forming where the mean annual temperature is below 0°C for at least two consecutive years.
• Impact of thawing - When temperatures rise, the active layer can thaw, leading to mass movement of soil and altering the landscape.

Key periglacial processes such as solifluction and freeze-thaw action

Periglacial areas experience unique processes due to the freeze-thaw cycles and the presence of permafrost. These processes drive significant changes in the landscape by moving soil and shaping surface features.

Solifluction and its effects

• Process description - Solifluction occurs when the active layer of permafrost melts in summer, creating waterlogged soil that cannot drain due to the impermeable frozen layer beneath.
• Movement mechanics - This heavy, saturated soil flows downslope, especially where there is a gradient, creating distinct formations.
• Resulting formations - On gentle slopes, solifluction forms lobe shapes as some soil moves faster than surrounding areas; on steeper slopes, it stretches into tongue-like features.

Freeze-thaw action and frost creep

• Expansion and contraction - Water within soil expands upon freezing, pushing soil particles upwards perpendicular to the slope.
• Downslope movement - As the ground thaws, these particles settle vertically downwards, resulting in a net movement further down the slope.
• Outcome of the process - This gradual shift, known as frost creep, contributes to the slow downslope migration of soil in periglacial regions.

Formation of ice wedges and patterned ground

The intense cold and seasonal temperature changes in periglacial environments create unique surface features through frost activity. Ice wedges and patterned ground are prominent examples of such formations.

Development of ice wedges

• Initial cracking - During harsh winters, extreme cold causes the ground to contract, forming cracks in the permafrost through a process called frost contraction.
• Meltwater infiltration - In spring, the active layer thaws, and meltwater fills these cracks; since the deeper permafrost remains frozen, this water refreezes within the cracks.
• Growth over time - Repeated frost contraction can reopen the same cracks in subsequent years, allowing more water to enter and freeze, thus enlarging the ice wedges.
• Structural impact - These ice-filled cracks, known as ice wedges, widen with each cycle, creating deep fissures in the permafrost.

Creation of patterned ground

• Surface stone arrangements - Frost activity can organise stones on the ground into circles, polygons, or stripes, forming what is known as patterned ground.
• Frost heave mechanism - Water beneath stones freezes and expands, lifting the stones to the surface; they then roll to the edges of the resulting mounds, creating circular patterns.
• Slope influence - On sloped terrain, stones roll further downhill, forming linear or striped patterns instead of circles.
• Polygon formation - Frost contraction also cracks the ground into polygonal shapes, with stones often filling these cracks to outline the patterns on the surface.

Distinctive periglacial landforms like pingos and thermokarst

Periglacial environments host a variety of unique landforms resulting from the interaction of ice, water, and soil under extreme cold conditions. These landforms range from conical hills to collapsed, marshy landscapes.

Formation of pingos

• General structure - Pingos are conical hills with an ice core, reaching heights of up to 80 m and widths of around 500 m.
• Open-system pingos - Develop in areas of discontinuous permafrost where groundwater is forced upwards through unfrozen gaps, collecting and freezing into an ice core that pushes the surface ground upwards.
• Closed-system pingos - Form in continuous permafrost regions beneath a dried-up lake; as permafrost encroaches, unfrozen ground beneath becomes isolated, and trapped water freezes, creating an ice core that elevates the surface.

Formation of blockfields

• Formation process - Blockfields consist of vast areas of loose, angular rocks formed through frost shattering, where repeated freeze-thaw cycles break down bedrock in situ.
• Landscape feature - These expanses of fragmented rock dominate high-altitude or polar periglacial zones, creating a rugged, barren appearance.

Formation of terracettes

• Formation process - Terracettes are step-like features on slopes, created when vegetation or other obstacles interrupt the downslope movement of soil via frost creep or other mass movement processes.
• Resulting shape - Soil accumulates behind these obstructions, forming small, flat terraces that resemble a series of steps across the slope.

Formation of thermokarst landscapes

• Formation process - Thermokarst landscapes emerge when subsurface ice, such as in pingos, melts due to rising temperatures, causing the ground above to collapse.
• Surface impact - This collapse creates depressions or holes that fill with water, resulting in an uneven, marshy terrain characteristic of degrading permafrost regions.