Periglacial Processes And Landforms Flashcards
What is frost contraction?
Frost contraction occurs when temperatures drop very low in winter causing the ground to contract and form cracks in the permafrost.
What is permafrost?
Permanently frozen ground with a top layer than melt in the summer called the active layer. 20-25% of the earths land surface is permafrost. Permafrost can be either continuos or discontinuous.
How do ice wedges develop?
Frost contraction creates cracks cracks in the permafrost when the temperature drop low which form the basis of the wedge. When temperatures increase in spring, the active layer thaws and meltwater seeps into the cracks, however the permafrost layer is still frozen so the water freezes in the cracks. The ice filled cracks are what become the WEDGES. If frost contraction continues in following years the cracks can re-open, splitting the wedge. More water seeps in and freezes, widening the ice wedge. The ice wedge gets bigger each time this happens.
What’s frost heave?
Frost heave is the process of the soil and other surface deposits lifting upwards due to the expansion of groundwater unfreezing.
How are ice lenses formed?
Ice lenses are formed by frost heave as water freezing in the ground can make humps on the surface and when the active layer freezes in the winter the ice forms a ‘lens shape’
They can also form underneath stones as that is the coldest place. As the ice lenses expand the stones are pushed upwards and the lenses underneath stop the stones from slipping back down. If the ice thaws fine materials fill in the space where the ice was, so the stones don’t fall down and eventually the stones rise above the surface of the ground.
What is patterned ground?
Patterned ground is when the stones on the surface of the ground are arranged in either circles, polygons or stripes. It can be formed by either frost heave or frost contraction.
Explain how patterned ground is formed by frost heave.
By frost heave; stones get pushed to the surface. Once they have reached the surface they roll down the edges of the mounds that have formed, so they either form circles or polygons if the mounds are closer together. If the mounds are on a slope, the stones roll downhill and form lines.
Explain how patterned ground is formed by frost contraction.
Frost contraction causes the ground to crack into polygon shapes which then get filled in with stones, forming the polygon patterns on the surface.
What is solifluction?
Because the layer of permafrost is impermeable, if the temperature gets above 0c in summer, the active layer melts. The active layers becomes water logged, as the water can’t go anywhere, and will easily flow downhill wherever there’s a gradient.
What formations can frost heave make?
Frost heave can make ice lenses and patterned ground in all three patterns; circular, polygonal and striped.
What formations can frost contraction make?
Frost contraction can make ice wedges and patterned ground in a polygonal pattern.
What is a lobe formation and what is it formed by?
Because 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 e.g. Because it’s on steeper ground so it flows down further to create a tongue shape.
What are pingos?
Periglacial hills with a core of ice that can be as large as 800m high and 500m wide.
How are open- system pingos formed?
Open system pingos form where there’s discontinuous permafrost. Groundwater is forced up through the gaps between the 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.
How are closed system pingos formed?
Closed system pingos form in areas of continuous permafrost where there is 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 the permafrost advances around the area of unfrozen ground. This causes water to collect in the centre of the unfrozen ground which then eventually freezes and creates the core of ice which pushes the ground above it upwards.
