- Define the term permafrost and distinguish between the three types.
Permafrost is defined as frozen ground that has been frozen for at least two consecutive summers. For permafrost to develop the mean annual temperatures needs to be at most -4°C and it exists of three types. The first of these is continuous permafrost, which occurs where the upper limit of the permafrost remains at ground surface throughout the year and there are no unfrozen patches. Continuous permafrost is only found in the Arctic circle and is commonly over 300m in depth. Discontinuous contains noticeable unfrozen ground called talik within the permafrost. It also tend to be less deep, between 10 and 50 meters deep. The surface shows a significant depth of melting in the summer, forming an active layer. The last type is sporadic permafrost which occurs where there is more talik than permafrost. The mean annual temperature in these areas may be around 0°C - Describe and explain the variety and location of periglacial landforms.
There are a number of periglacial landforms caused by permafrost. One of the largest, as a single object, would be pingos. These are rounded ice-cored hills that can reach up to 90 meters high. They are formed by ground ice which develops in the winter months as temperatures fall. There are two types of pingos open and closed system. Open pingos are formed in valley bottoms, where water is collected from surrounding slopes under the force of gravity. This water then freezes and expands forcing the overlying surface material to dome upwards. This kind of pingo is commonly found in Greenland. However there are other kinds of pingos a good example of which are found in the Mackenzie delta in Canada. This kind of pingo is formed beneath a lake where the supply of water is from the immediate area. Permafrost grows in the winter season and the ground water below the lake is trapped by the permafrost below and the frozen lake above. The saturated talik is compressed by the expanding ice around it and when it freezes it pushes the overlying material up. When the ice core in the pingos melts the top of the dome collapses leaving a rampart surrounding a circular depression called an ognip.