The tundra is a biome characterized by extremely low temperatures and a short growing season, which severely limits the development of vegetation. This environment, dominated by cold, wind, and frozen ground, results in a unique topography shaped more by subsurface geological processes than by surface erosion. The terrain’s distinct features are a direct consequence of the cyclical freezing and thawing of the ground, which leads to structural instability and a constant churning of the surface material.
Defining the Tundra Landscape
Tundra landscapes are broadly categorized into two primary types. The Arctic Tundra is found at high latitudes, primarily encircling the North Pole across regions in North America, Europe, and Asia. The Alpine Tundra, by contrast, occurs at high altitudes on mountain ranges globally, existing above the continuous tree line where temperatures are too low for forest growth.
The Arctic Tundra is subjected to extremely cold winters, with average temperatures in midsummer remaining cool, often between \(3^{circ}\)C and \(16^{circ}\)C, and low annual precipitation, often falling as snow. Alpine Tundra is generally drier but features steep topography and lacks the expansive, permanently frozen subsurface layer that defines its northern counterpart. High winds are a common factor across both types, contributing to the harsh conditions that prevent the establishment of tall vegetation and expose the ground to mechanical weathering.
The Role of Permafrost
The structure of the Arctic tundra terrain is fundamentally controlled by permafrost, which is ground that has remained at or below \(0^{circ}\)C for at least two consecutive years. This permanently frozen layer can extend to depths of more than 1,450 meters in parts of Siberia. The distribution of this frozen ground is classified by its coverage across the landscape.
Continuous permafrost underlies 90 to 100 percent of the ground, typically in the coldest northern regions, while discontinuous permafrost covers 50 to 90 percent of the area, allowing for some patches of unfrozen ground. Further south, sporadic permafrost is scattered, covering 10 to 50 percent of the landscape, often existing only in sheltered or insulated spots. This frozen layer acts as an impermeable barrier to water infiltration, which has profound implications for the overlying soil.
Above the permafrost lies the “active layer,” a surface layer of soil and organic material that thaws each summer and refreezes every winter. The thickness of this active layer varies significantly, ranging from as little as 10 centimeters in extremely cold areas to several meters in warmer, more southern regions. The annual cycle of freezing and thawing within this layer generates pressures, destabilizing the ground and driving the formation of the tundra’s unique surface features. This continuous freeze-thaw action, known as cryoturbation, sorts and mixes the soil, preventing the formation of deep, fertile soil horizons.
Unique Surface Landforms
The freeze-thaw cycles acting upon the active layer create a variety of topographical features. Patterned ground refers to symmetrical geometric shapes formed on the surface, such as circles, stripes, and polygons. These patterns result from the frost heave mechanism, which pushes stones and larger sediment particles upward and outward from the finer-grained centers during freezing, sorting the material into defined shapes.
Another feature is the pingo, a dome-shaped, ice-cored hill that can rise up to 70 meters in height and span up to 1,000 meters in diameter. Pingos form when hydrostatic pressure forces groundwater to accumulate and freeze beneath the surface, pushing the overlying sediments upward into a mound. These are classified as either open-system, where water is supplied from an outside source, or closed-system, which form in drained lake beds where water is trapped between the permafrost below and the freezing surface layer above.
On slopes, the saturated active layer creeps downhill over the impermeable permafrost, a process called solifluction. This movement creates solifluction lobes, which are tongue-shaped features of soil and debris that accumulate where the slope lessens. These lobes are evidence of the ground’s flow, where the saturated soil acts almost like a viscous fluid moving over the frozen subsurface.
Water and Drainage in Tundra Terrain
The presence of permafrost dictates the movement and retention of water across the tundra surface. Because the frozen ground acts as a barrier, preventing water from draining downward, the soil in the active layer becomes saturated during the summer thaw. This poor drainage leads to the widespread formation of bogs, marshes, and shallow, standing water across the flat Arctic lowlands.
The most dynamic water bodies in this environment are thermokarst lakes, which form when ice-rich permafrost thaws. The melting of ice wedges or ice-rich sediments causes the ground to subside, creating depressions that fill with meltwater. These lakes are highly unstable and can expand rapidly as the water’s heat further melts the surrounding permafrost, or they can drain suddenly if the lake water creates an outflow channel through the permafrost layer.