periglacial processes and landforms

• periglacial areas contain permafrost - permanently frozen ground with a top layer that can melt in the summer (called the active layer) 20-25% of the earths land surface is permafrost

• areas of permafrost can be continuous (all he ground is frozen) or discontinuous (only patches of the ground are frozen)

• for discontinuous permafrost to form the mean annual temperature needs to be below zero degrees celsius for at least 2 years. for continuous permafrost to form the mean annual temperature needs to be below minus 5 degrees celsius.

• the layer of permafrost is impermeable (water cant flow through it) if the the temperature gets above zero degrees celsius in the summer, the active layer melts but the water cant go anywhere. this means tat the active layer gets waterlogged and will easily flow wherever there's a gradient. this gradient is called solifluction.

• when temperatures drop very low in winter the ground contracts and cracks from in the permafrost. this is called frost contraction

• when temperatures increase in spring the active layer thaws and the melt water seeps into the cracks

• the permafrost layer is still frozen, so the water freezes in the cracks - the ice filled cracks formed in this way are called ice wedges

• frost contraction in following years can re-open cracks in the same place, splitting the ice wedges. more water seeps in and freezes, widening the ice wedge. the ice wedge gets bigger each time it happens

• water freezing in the ground can make humps on the surface

• when the active layer freezes in winter, the ice forms a kind of lens shape

• in fine-grained soil (like silt or clay) the ice lifts (heaves) up the surface layers of soil. this is called frost heave

• ice lenses also form underneath stones because stones lost heat faster than the soil around them, so when temperatures drop its colder beneath the stones

• as the ice lenses expand, they push the stones upwards towards the surface of the ground. the ice lenses underneath the stones stop the stones from slipping back down. if the ice thaws, fine material ills the space were the ice was so the stones don't fall down. eventually the stones rise above the surface of the ground.

sometimes Stone's on the surface of the ground are arranged in circles, polygons or stripes - this is called patterned ground. pattern ground can be formed in two ways - by frost heave and by frost contraction.

• stones can get pushed to the surface by frost heave. once they reach the surface, they roll down the edges of the mounds that have formed, so they form circles around them (polygons form when the mounds are close together). if the mounds are on a slope, the stones roll downhill and form lines

• frost contraction causes the ground to crack in polygon shaped. the cracs get filled in with stones, forming polygon patterns on the surface

• when snow gets into a hollow in the ground it can increase the size of the hollow.

• the temperature in periglacial environments often fluctuates around zero degrees Celsius so a lot of freezing and thawing happens - when the temperatures above zero degrees Celsius the snow melts and when its below zero degrees Celsius the water refreezes as ice.

• every time the ice freezes, it expands, so frost shattering eventually breaks bits off the rock at the base of the hollow. when the snow melts, the melt water carries the broken bits of rock (debris) away.

• slopes collapse because they're waterlogged and they've been eroded - this material is washed away by melt water

• eventually the hollow becomes deeper and wider. the process that causes this are collectively called nivation and the hollows formed by nivation are called nivationhollows. nivation hollows can be at the beginning of a corrie

• the waterlogged active layer of soil flows easily over the frozen impermeable layer beneath.
• solifluction produces lobe formations where one section of the soil is moving faster than the soil around it.

• a pingo is a conical hill with a core of ice. pingos can be as large as 80m high and about 500m wide.

• there are two types of pingo - open systems and closed systems.

• open-system pingos form where there's discontinuous permafrost. groundwater is forced up through the gaps between areas of permafrost (from unfrozen layers lower down). the water collects together and freezes, forming a core of ice that pushes the ground above it upwards

• closed-system pingos form in areas of continuous permafrost where there's a lake at the surface. the lake insulates the ground, so the area beneath it remains unfrozen. when the lake dries up, the ground is no longer insulated and permafrost advances around the area of unfrozen ground. this causes water to collect in the centre of the unfrozen ground. the water eventually freezes and creates a core of ice that pushes the ground above it upwards

• if the ice core thaws, the pingo collapses, leaving behind a pond of melt water surrounded by ramparts (walls of soil)