Periglacial Processes and Landforms

Spec mapping (AQA 7037): Paper 1 (Physical), §3.1.4 Glacial systems and landscapes — periglacial processes and landforms: permafrost, the active layer, frost action (heave, shattering, sorting), nivation, solifluction, and the resulting landforms (patterned ground, ice wedges, pingos, thermokarst). This depth lesson treats the periglacial zone as a frost-and-water system in which the seasonal freeze–thaw of an active layer over permafrost drives a distinctive process suite, and it foregrounds the permafrost–carbon feedback as a frontier climate issue. It links to §3.1.1 systems thinking, to the relict head deposits of the coastal lesson (southern England was periglacial in the Pleistocene), and to §3.1.6 Hazards. Assessment objectives: AO1 (process and landform genesis), AO2 (applying permafrost/active-layer reasoning to a named region), AO3 (manipulating active-layer-depth, permafrost-temperature and carbon-flux data).

The organising idea is that periglacial geomorphology is governed by two interacting controls — the permafrost (an impermeable, perennially frozen floor) and the active layer (the surface layer that thaws and refreezes each year). The permafrost traps meltwater in the active layer (it cannot drain away), so the active layer becomes saturated and mobile each summer; the seasonal freeze then segregates ice, heaves the ground and sorts the debris. Almost every periglacial process and landform follows from this active-layer-over-permafrost couplet — so we use it as the spine of the lesson.

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Defining Periglacial Environments

The term "periglacial" literally means "around the glacier" (from Greek peri = around), but the concept has expanded to include any cold, non-glacial environment where frost-related processes dominate.

Characteristics

• Mean annual temperature below 0°C (though some definitions extend to areas where the coldest month averages below −3°C)
• Freeze-thaw cycles are frequent and intense
• Permafrost may or may not be present
• Low precipitation — many periglacial areas are effectively cold deserts
• Sparse vegetation — tundra biome dominates

Global Distribution

Periglacial environments currently cover approximately 25% of the Earth's land surface, including:

• Arctic regions: Northern Canada, Alaska, Siberia, Scandinavia
• Antarctic margins: Areas beyond the ice sheet
• High-altitude regions: Alps, Himalayas, Rockies, Andes (above the tree line)
• Past periglacial environments: During the Pleistocene, much of southern England experienced periglacial conditions

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Permafrost

Permafrost is ground that remains at or below 0°C for at least two consecutive years. It is the defining feature of most periglacial environments.

Types of Permafrost

Type	Characteristics	Distribution	Depth
Continuous	Permafrost underlies > 90% of the area	Highest latitudes (e.g., northern Siberia, Canadian Arctic islands)	Up to 1,500 m (in Siberia)
Discontinuous	Permafrost underlies 50–90% of the area	Slightly lower latitudes; absent under lakes, rivers and warm areas	25–100 m
Sporadic	Permafrost underlies 10–50% of the area	Lower latitudes; only in sheltered, north-facing slopes and shaded areas	10–25 m
Isolated patches	Permafrost underlies < 10% of the area	Margins of the periglacial zone; high-altitude locations	< 10 m

The continuous → discontinuous → sporadic → isolated sequence is essentially a latitudinal (and altitudinal) gradient of decreasing cold, and the percentage cover matters geomorphologically: open-system pingos and taliks, for instance, can only form where permafrost is discontinuous (gaps let groundwater move), while closed-system pingos and the thickest ground ice belong to continuous permafrost. Permafrost thickness also reflects the long-term ground heat balance — up to ~1,500 m in Siberia where intense cold has propagated downward over hundreds of thousands of years (so the deepest permafrost is partly a relict of past glacial cold, not just today's climate, and is thawing only slowly from the top). The southern boundary of continuous permafrost is a sensitive climate indicator that is currently migrating poleward as the Arctic warms — the spatial signature of the thaw discussed below.

The Active Layer

The active layer is the surface layer of ground above the permafrost that thaws each summer and refreezes each winter — the engine room of periglacial geomorphology.

• Depth: typically 0.3 m to 4 m, depending on latitude, altitude, soil type and vegetation cover (vegetation and a thick organic/peat layer insulate the ground, keeping the active layer thin; stripping vegetation deepens it).
• Significance: all periglacial geomorphological processes operate within or because of the active layer.
• It is saturated in summer because meltwater cannot drain through the impermeable permafrost below — this enforced waterlogging is the precondition for solifluction, frost heave and ice-lens growth.

flowchart TD
  SUN[Summer warmth] --> THAW[Active layer thaws<br/>0.3 to 4 m deep]
  THAW --> SAT[Saturated: meltwater trapped<br/>above impermeable permafrost]
  SAT --> SOLI[Loses strength then flows<br/>downslope = SOLIFLUCTION]
  COLD[Winter freeze] --> SEG[Water migrates to freezing front<br/>= ICE LENS / segregation]
  SEG --> HEAVE[Ground heaved up = FROST HEAVE]
  HEAVE --> SORT[Coarse clasts lifted + shoved<br/>= FROST SORTING -> patterned ground]
  PERMA[(PERMAFROST<br/>frozen >= 2 yr<br/>impermeable floor)] --- SAT
  PERMA --- SEG

The Talik

A talik is an area of unfrozen ground within a permafrost region. Taliks can exist:

• Beneath lakes and rivers (where the water prevents freezing)
• Beneath buildings and infrastructure (where heat prevents freezing)
• Between the base of the active layer and the top of deep permafrost (in discontinuous zones)

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Periglacial Processes

1. Frost Heave

Frost heave is the upward displacement of the ground surface caused by the formation of ice lenses within the soil.

Process:

1. As the active layer freezes from the surface downward, water migrates through the soil towards the freezing front by capillary action
2. The water freezes to form an ice lens — a horizontal layer of pure ice within the soil
3. As the ice lens grows, it pushes the soil above it upward
4. Multiple ice lenses can form at different depths, creating significant heaving of the ground surface
5. Upon thawing, the ground subsides unevenly, creating an irregular surface

Effects:

• Lifts stones to the surface (frost sorting)
• Disrupts roads, buildings and pipelines
• Contributes to the formation of patterned ground

The crucial mechanism is ice segregation, not simply the 9% expansion of water freezing in place: as the freezing front advances, the suction it generates draws additional liquid water up through fine, frost-susceptible soils (silts are ideal — coarse sands drain too freely and clays transmit water too slowly), and that water freezes onto the lens, which can therefore grow far thicker than the original pore water. This is why silt-rich soils heave most and why heave can lift the ground by tens of centimetres — far more than 9% of the pore volume. The same segregation process that grows soil ice lenses also drives frost shattering in rock and the growth of pingo and ice-wedge ice, so it is the unifying micro-mechanism behind much of periglacial geomorphology. Engineering on frost-susceptible ground must either remove the silt, prevent water supply, or insulate to stop the freezing front reaching susceptible layers — the practical reason frost heave is a major cold-climate construction hazard.

2. Frost Sorting

Frost sorting is the process by which repeated freeze-thaw cycles separate coarse and fine particles in the soil.

Mechanism:

1. Frost heave preferentially lifts larger particles because they conduct heat more efficiently and freeze first
2. Coarse particles migrate to the surface and move outward
3. Fine particles remain in the centre
4. The result is a sorting of particles by size

3. Frost Shattering (Freeze-Thaw Weathering)

• Water enters cracks and joints in rock
• Upon freezing, water expands by approximately 9%, exerting pressures of up to 2,100 kN/m² (far exceeding the tensile strength of most rock)
• Repeated freeze-thaw cycles shatter the rock into angular fragments
• Most effective where temperatures oscillate frequently around 0 °C and moisture is available — so a maritime periglacial climate with many freeze–thaw crossings can shatter rock faster than a colder but drier continental one, where the temperature seldom crosses 0 °C
• Produces blockfields (felsenmeer), scree (talus) slopes, and contributes to tors (where intense shattering of jointed rock, especially around closely-jointed zones, leaves resistant masses upstanding)

Recent research stresses that simple volumetric expansion (the "9%" mechanism) is only part of the story: ice segregation — the same process that grows ice lenses in soil — also operates in rock, drawing water to growing ice in micro-cracks and prising the rock apart most effectively in a "frost-cracking window" of roughly −3 to −8 °C sustained over time, rather than at brief excursions to 0 °C. The examinable takeaway is that effective frost shattering needs moisture supply and sustained sub-zero conditions, not merely a count of 0 °C crossings — a more sophisticated framing than the textbook "freeze–thaw" caricature.

4. Solifluction

Solifluction is the slow, downslope flow of waterlogged soil over an impermeable surface (usually permafrost).

Process:

1. In summer, the active layer thaws and becomes saturated (meltwater cannot drain through the permafrost below)
2. The waterlogged soil loses internal strength and begins to flow downslope under gravity
3. Rates of movement: typically 0.5 to 5 cm per year, but can exceed 50 cm per year on steep slopes
4. Creates distinctive solifluction lobes and solifluction terraces — tongue-shaped or terrace-shaped features on hillsides

Two mechanisms combine in solifluction proper: gelifluction (flow of the saturated, thawing active layer over impermeable permafrost — the classic periglacial form) and frost creep (the net downslope ratchet produced as frost heave lifts particles perpendicular to the slope and thaw drops them vertically, so each freeze–thaw cycle nudges them downhill). Both depend on the active-layer-over-permafrost couplet: it is the trapped meltwater that destroys the soil's strength. Because saturation, not gradient, is the control, solifluction operates on astonishingly gentle slopes and is one of the most effective mass-movement processes in cold environments — it can mobilise the entire hillside slowly downslope.

Evidence in Southern England (a key synoptic link):

• Head deposits (also called coombe rock in chalk areas) draping many slopes and valley floors in southern England are periglacial solifluction deposits formed during Pleistocene cold stages, when the ground was permafrozen and the active layer flowed.
• These poorly sorted, angular, often crudely-layered deposits — also seen draping coastal cliffs and platforms (lesson 4) — are unambiguous evidence that southern England, beyond the ice limit, experienced full periglacial conditions. They are the clearest demonstration that the periglacial zone of §3.1.4 once extended across lowland Britain, and they directly explain a class of relict coastal landforms — a genuinely synoptic, examiner-rewarded connection.

5. Nivation

• A combination of freeze–thaw weathering, frost creep/solifluction and meltwater removal operating around and beneath a late-lying snowpatch.
• The snowpatch supplies moisture for freeze–thaw at its margins and insulates the ground beneath, creating a microclimate that concentrates weathering at the patch edge; meltwater then evacuates the loosened debris.
• Gradually enlarges a nivation hollow, which — if snow accumulation increases enough for firn and then ice to form — can develop into a corrie (the link to lesson 7). Nivation is thus the transitional process between periglacial and glacial geomorphology, the seed from which corrie glaciation can grow.

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Periglacial Landforms

Ice Wedges

Ice wedges are vertical, wedge-shaped bodies of ice that form in the ground through repeated thermal contraction and infilling.

Formation:

1. In winter, the ground contracts as temperatures fall rapidly below −15°C
2. Thermal contraction cracks form in the frozen ground (similar to cracks in drying mud, but caused by cold contraction)
3. In spring, meltwater or hoar frost fills the cracks
4. The following winter, the ground contracts again, reopening and widening the crack
5. More water enters and freezes, forming a thin vein of ice
6. Over centuries, the ice vein grows into a wedge, typically 1–2 m wide at the top and up to 3–4 m deep
7. Ice wedges join to form a polygonal pattern (ice-wedge polygons, often tens of metres across) visible on the ground surface

Ice wedges are a cumulative, multi-year landform — each winter adds only a thin ice vein, so a metre-wide wedge represents centuries of repeated crack-and-fill, and the wedge therefore records the persistence of severe winter cold. They are also a prime relict indicator: when permafrost thaws, the wedge ice melts and the void fills with collapsed sediment, leaving an ice-wedge cast — a wedge-shaped body of slumped material cutting across the original layering. Fossil ice-wedge casts are widespread in the Pleistocene deposits of lowland England (e.g. in the gravels of East Anglia and the Thames terraces), and their presence is unambiguous evidence that continuous permafrost and intense winter cold once reached southern Britain — a direct, datable link between this lesson and the relict periglacial landscape of §3.1.3/§3.1.4. Ice-wedge polygons come in two forms that themselves carry information: low-centre polygons (raised rims, wet centres — wetter, often active) versus high-centre polygons (degrading, drained — a sign of incipient thaw), so polygon morphology can flag permafrost in trouble.

Patterned Ground

Patterned ground is a collective term for the regular geometric shapes formed on the ground surface in periglacial environments by frost action.

Pattern	Size	Formation Process	Slope
Stone circles	1–3 m diameter	Frost sorting pushes coarse material outward, fine material remains in centre	Flat to gentle (< 3°)
Stone polygons	1–5 m diameter	Ice wedge contraction creates polygonal cracks; frost sorting moves stones to crack edges	Flat (< 2°)
Stone stripes	1–3 m width, many metres long	Stone circles are elongated and aligned downslope by gravity and solifluction	Moderate (7–30°)
Stone nets	Variable	Intermediate between circles and polygons	Gentle

The elegance of patterned ground is the slope control on form, which converts a single sorting process into a continuum of shapes: on flat ground frost sorting acts radially, producing circles/polygons with a fine centre and coarse rim; as slope increases, gravity and solifluction stretch these into nets and then, on steeper ground, stripes aligned downslope. So a hillside can display circles at its flat crest grading into stripes on its flank — a spatial sequence that records the gradient. This makes patterned-ground morphology a readable indicator of slope and process, and (as relict features) evidence that an area once experienced active-layer frost sorting. The sorting mechanism itself is frost heave's by-product: larger clasts are heaved more (they conduct cold deeper and the freezing front grips them first) and, once raised, are shoved laterally as the ground refreezes, migrating away from fine-rich centres toward the cracks and rims.

Pingos

A pingo is a dome-shaped mound of earth-covered ice, rising above the surrounding terrain.

Two types: