The tundra biome is a vast, treeless expanse with exceptionally cold climates and brief growing seasons. Found in Earth’s polar regions like the Arctic, it extends across parts of North America, Europe, and Asia. Alpine tundra also exists at high mountain elevations globally, where similar harsh conditions prevail. This cold, arid landscape supports specialized flora and fauna.
The Role of Permafrost and Frost Action
The distinctive terrain of tundra regions is shaped by two geological forces: permafrost and frost action. Permafrost is ground that remains completely frozen for at least two consecutive years. This layer can extend hundreds of meters deep, acting as a rigid, impermeable base.
Above this frozen foundation lies the active layer, a superficial zone of soil that undergoes seasonal thawing and refreezing. This layer thaws during the brief summer months, allowing for water infiltration and biological activity, before refreezing in winter. The depth of this active layer varies greatly, ranging from a few centimeters to over a meter, depending on local conditions like vegetation cover and snow depth.
Within this active layer, repeated freezing and thawing of water, known as frost action, modifies the ground. As water within soil pores freezes, it expands by approximately 9% in volume, exerting pressure on surrounding soil particles. This expansion pushes soil and rocks upward, a process termed frost heaving, which can lift objects from the ground over time.
Cold can also lead to frost cracking, where the ground contracts and fractures due to temperature drops. These cracks can extend deep into the active layer and even into the upper permafrost, forming polygonal patterns on the surface. Water can then fill these cracks, freezing and expanding, widening them over successive cycles.
Distinctive Features of the Tundra Landscape
The continuous interplay of permafrost and frost action sculpts the tundra into a landscape with unique formations. One widespread phenomenon is patterned ground, characterized by geometric shapes. These patterns, which include circles, polygons, and stripes, are formed as frost heaving sorts soil particles and rocks. Larger stones are pushed to the edges of the polygons or circles, while finer sediments accumulate in the centers, creating distinct borders that can be several meters across.
Pingos represent another feature, isolated, conical hills rising from the flat tundra. These ice-cored mounds can reach heights exceeding 50 meters and diameters of several hundred meters at their base, making them significant landmarks. They form when pressurized groundwater beneath the permafrost, or within a thawed zone, is forced upward. This causes the water to freeze into a large, growing ice lens that pushes the overlying soil into a dome shape, creating the characteristic hill.
Thermokarst describes a landscape marked by irregular depressions, pits, and lakes, formed by the thawing of ice-rich permafrost. As the ground ice melts, the unsupported overlying soil collapses, creating these characteristic subsidence features. The size and shape of thermokarst features vary widely, from small, shallow depressions only a few meters wide to extensive, water-filled basins known as thermokarst lakes that can span hundreds of meters, altering drainage patterns significantly.
Sloping tundra terrains display solifluction lobes, tongue-shaped masses of saturated soil slowly moving downslope. This process occurs when the active layer becomes saturated with meltwater during the summer, reducing its shear strength and making it fluid. The water-logged soil then flows sluggishly over the impermeable, frozen permafrost layer beneath, creating distinct, wavy terraces or lobes on the hillside. These features can range from a few meters to tens of meters in length, indicating the slow, continuous movement of material across the landscape, driven by gravity and the presence of the frozen sublayer.
Effects of a Warming Climate on Tundra Terrain
A warming global climate is altering the appearance and stability of tundra landscapes. Rising temperatures are causing permafrost to thaw at an accelerated rate. This widespread thawing destabilizes the ground, leading to changes in terrain.
A direct consequence of permafrost thaw is thermokarst expansion. As ground ice melts, the resulting subsidence creates new depressions and expands existing thermokarst lakes, transforming stable land into a more fragmented and watery landscape. This process can lead to erosion along coastlines and riverbanks, especially where ice-rich permafrost is exposed to warmer water.
Pingos, relying on surrounding permafrost stability, are also vulnerable to warming. Thawing permafrost encapsulating their ice core can cause the ice lens to melt, leading to pingo collapse and the formation of a central depression or pond.
Increased meltwater in the active layer due to warmer summers intensifies solifluction. This leads to more frequent, faster downslope movement of saturated soil, contributing to landscape instability and altering drainage patterns.