‘Periglacial’ describes a landscape that undergoes seasonal freezing and thawing, typically on the fringes of past and present glaciated regions.
The landscapes are governed by specific depositional and erosional processes, and therefore produce completely unique landforms [1].
A dominant characteristic of periglacial landscapes is the presence of permafrost – ground that is frozen all year round for over two consecutive years. However, permafrost is rapidly shrinking in the Northern Hemisphere by 60,000-230,000 km2 per decade [2,3].
The uppermost section of permafrost is called the active layer. It undergoes annual freezing and thawing, and is partly responsible for the formation of many periglacial landforms. This is because several periglacial processes are initiated in the active layer, including nivation [4,5], mass movements (even on gentle slopes [6]), and frost heave [7,8].
Studying these landscapes is very important, as they represent huge areas of relatively unstable, unsafe ground that also have the potential to release substantial volumes of carbon [9,10]. They may even inform us of past and present (peri)glacial processes on other planets! [11,12]
Landforms
Blockfields
A combination of chemical and mechanical weathering can fracture bedrock below the active layer. Over time, these processes produce an uneven bouldery landscape known as a blockfield, which is revealed once ice or permafrost disappears.
Blockfields potentially began forming up to 23 million years ago during the Neogene, when chemical weathering initiated the slow process of eroding jointed bedrock [13].
Ice Wedges
Ice wedges are vertically-oriented wedge-shaped growths of ice that occur near the surface of permafrost. They form where a temperature differential in the permafrost causes the ground to crack, allowing water to enter, refreeze, and expand.
Ice wedges only ever form in periglacial environments. This makes them valuable for identifying former periglacial landscapes [16] and studying past prevailing winter climatic conditions [17].
Patterned Ground
The ‘ordered’ shapes produced by the organisation of sediment and stones are collectively known as ‘patterned ground’. These landforms include polygonal shapes, stone circles and stripes, and labyrinths.
Each pattern is produced during the repeated freezing and thawing of the active layer. Initial freezing separates and sorts solid from stones at/ near the ground surface, whilst subsequent thawing once again redistributes these materials into new orientations. This cycle can operate on decadal to centennial timescales to produce such landforms.
This process is influenced by the pre-existing internal structure of the sediments [19], as well as hillslope angle, which governs the formation of stone stripes and circles [20,21].
Pingos
The Innuit word ‘pingo’ here describes tall, typically circular mounds of former lake sediments in periglacial environments. Over 95% of all pingos are located in the continuous permafrost regions of Arctic North America and northern Asia, where they total >10,000 individuals [22].
Pingos form when growing permafrost uplifts unfrozen sediments (i.e. frost heave) beneath the surface of a draining lake, ultimately creating a stable, predominantly ice-cored mound where the lake was deepest [23].
This process can repeat itself over thousands of years to produce a landscape of multiple buried pingo complexes [24].
Solifluction Lobes
Soils can become highly saturated with moisture when permafrost prevents water percolating deep into the ground. This allows material to start flowing (solifluction) downhill in a lobate structure.
These solifluction lobes will flow until they reach a natural barrier (e.g. a knick point) or melting permafrost finally allows water to percolate away from the lobe [25]. Vegetation cover may also stabilise a lobe, ceasing its growth [26].
Terracettes
Wide, stepped hillslopes that have formed naturally in periglacial environments are known as terracettes.
It is generally agreed that terracettes form when soil creep processes and periglacial freeze-thaw cycles interact, creating regular step features on >20° slopes [27,28]. Some suggest that people and livestock amplify these features when they are used as walkways [29].
Thermokast
‘Thermokarst’ landforms are produced when permafrost and ice-rich ground masses thaw. A variety of landforms are characteristic of thermokarst landscapes, but some of the key features are described below.
Thermokarst lakes can form as water ponds on the surface of thawing permafrost. They can grow remarkably quickly as a response to warming climate and environmental factors such as forest fires [30].
Similarly, thermokarst bogs form as water ponds on ice-rich peat. These bogs are poorly-drained by fluvial activity and groundwater, providing a unique habitat for stale-water plants [31].
Mass movement landforms are also common in thermokarst landscapes. Thaw slumps and active layer detachments slides are both usually triggered by enhanced thawing and mechanical erosion in the active layer, which is typical of thermokarst environments [6,31].
About the Author
Hi I am Alex Clark, I am a PhD student currently working on a project that combines high-resolution geomorphological mapping with new chronological evidence to reconstruct palaeo-ice dynamics in NE Ireland during the Late Glacial (~18,000-15,000 years ago). I am primarily interested in past ice-climate interactions, digital mapping (GIS), and tephrochronology, but enjoy exploring other aspects of geography and the environmental sciences in my research.
This was very well written and useful! Great work!
A letter “r” is probably missing in the subtitle “Thermokast”.
Very fruitful