The tundra biome is a vast, treeless expanse defined by a cold climate and short growing seasons, found predominantly in the high latitudes of the Arctic or at high altitudes globally. The unique, often geometric appearance of this landscape results from the physics of water freezing and thawing in the ground. The cyclical action of freezing and expanding water dominates geological processes, controlling soil movement, drainage, and the surface shape. These forces create a distinctive environment of patterned surfaces and isolated mounds, dictated by temperature and the presence of frozen ground.
The Foundation: Permafrost and the Active Layer
The foundation for most tundra landforms is permafrost, which is ground that remains below 0°C for two or more consecutive years. This layer can extend hundreds of meters deep in the continuous permafrost zone, acting as an impervious, frozen barrier to water drainage. The presence of this frozen layer dictates how water behaves in the ecosystem above it.
Resting atop the permafrost is the “active layer,” the surface zone that thaws during the summer and refreezes each winter. Its depth is highly variable, ranging from 10 centimeters in the High Arctic to several meters in warmer regions. Since meltwater and rain cannot percolate past the frozen permafrost, the active layer often becomes saturated and poorly drained during the summer thaw.
This saturated, seasonally mobile layer drives most tundra landform creation through a process called cryoturbation. As the soil freezes and thaws, the expansion and contraction of ice churns and mixes the soil, leading to surface instability and sediment sorting. This constant subsurface movement and poor drainage are the underlying conditions for nearly all surface features found in the tundra.
Patterned Ground Formations
Continuous freeze-thaw cycles within the active layer create small-scale, geometric features known as patterned ground, including circles, nets, and polygons. These result from ice segregation and frost heave, where the repeated growth of ice lenses forces soil and stones to move. Differential frost heave pushes larger stones upward and outward from the center of a freezing cell.
This sorting mechanism creates “sorted” features, moving coarse materials like pebbles and boulders to the periphery, forming rings or nets around finer-grained soil. Sorted stone circles can range from a few centimeters up to 20 meters in diameter. On flat terrain, these patterns form polygons or nets, but on gentle slopes, the downslope movement of the saturated active layer elongates them into stone stripes.
Another type of patterned ground is the ice-wedge polygon, formed by thermal contraction. During cold winters, the soil contracts and cracks into a polygonal pattern, which meltwater fills in the spring. This water freezes into vertical ice wedges. The wedges expand, forcing the surrounding ground upward, creating raised borders and resulting in low-center polygons. These polygons are typically larger, spanning 5 to 30 meters across, and characterize the continuous permafrost zone.
Dynamic Features: Pingos and Thermokarst
On a larger scale, the tundra features landforms resulting from either massive ice-core growth or large-scale ground collapse. Pingos are prominent ice-cored hills that rise from the flat tundra plain, sometimes reaching 50 meters in height. Their formation depends on the hydrological system and is categorized into two types.
Closed-System Pingos
Closed-system pingos, or hydrostatic pingos, form in areas of continuous permafrost, often where lakes have drained. The exposed lake bed (a talik) begins to freeze. As permafrost advances inward, it traps saturated soil and water below. Immense hydrostatic pressure forces this water to freeze into an expanding ice lens, which domes the surface sediment.
Open-System Pingos
Open-system pingos, or hydraulic pingos, are found in areas of discontinuous permafrost, typically in valley bottoms or sloped terrain. Groundwater flows through unfrozen layers and is forced upward by artesian pressure. When this pressurized water encounters the permafrost layer, it freezes and forms an ice core, causing the surface to heave into a mound.
The opposite of pingo formation is thermokarst, a process where the thawing of ice-rich permafrost leads to the collapse of the ground surface and the creation of irregular topography. When ground ice melts, the loss of volume causes the overlying land to subside, forming depressions, sinkholes, and hummocks. These depressions often fill with water, creating thermokarst lakes (thaw lakes). These lakes accelerate permafrost thaw beneath them through thermal abrasion, where the lake water conducts heat into the surrounding sediment, causing the lake to expand.
Arctic Versus Alpine Tundra
Tundra landforms differ between the high-latitude Arctic and high-altitude mountain regions (Alpine tundra). Arctic tundra is defined by vast, relatively flat plains and continuous permafrost, leading to features like large ice-wedge polygons and widespread closed-system pingos. The low terrain gradient and impermeable permafrost result in widespread poor drainage, creating extensive wetlands and boggy areas during the summer.
Alpine tundra, found above the tree line, is characterized by steeper slopes and variable, often discontinuous permafrost. Due to the steeper topography and better drainage, the most prominent land-shaping process is solifluction. Solifluction is the slow, downslope movement of the saturated active layer over frozen ground or bedrock, creating distinctive lobe-shaped terraces and sheets of soil. While frost-sorted patterned ground occurs, larger, ice-wedge features are less common due to the patchy permafrost and steeper gradient.
The geomorphology of the tundra is a direct expression of the physics of water freezing and expanding. The cryogenic processes that dominate this biome create a dynamic environment where every landform is controlled by the annual cycle of thaw and freeze. As global temperatures rise, the permafrost foundation supporting these features is beginning to thaw, initiating widespread ground subsidence and transforming the landscape. The stability of tundra landforms is linked to the stability of its frozen ground, making them a sensitive barometer of climate change.