7. Periglacial Processes - Periglacial Processes & their landforms Flashcards
Explain Freeze-thaw action (frost shattering) and its landforms that arise as a result of it.
Freeze-thaw action (frost shattering):
In periglacial areas, **screes **develop at the foot of slopes as a result of frost shattering. On relatively flat areas, extensive spreads of angular boulders are left, which are known as **blockfield **or **felsenmeer **(sea of rocks).
{Freeze-thaw action/frost shattering occurs in areas where temperatures rise during the day but drop below freezing at night for a substantial part of the winter. Water which enters cracks in the rocks freezes overnight. Ice occupies more space than water (just under 10% more) and therefore exerts pressure on the crack. As the process continues, the crack widens, and eventually pieces of rock break off. On steep slopes this leads to the collection of material at the base, known as scree. In a glacial valley, much of this material falls from the valley side onto the edges of the glacier and some finds its way to the base of the ice via the numberous crevasses which cross the glacier’s surface.}
Explain Nivation and its landforms that arise as a result of it.
Nivation takes place beneath patches of snow in hollows, particularly on north- and east-facing slopes. Freeze-thaw action and possibly chemical weathering, operating under the snow, cause the underlying rock to disintegrate. As some of the snow melts in spring, the weathered particles are moved downslope by the meltwater and by solifluction. Over some period of time, this leads to the formation of nivation hollows which, when enlarged, can be the beginnings of a corrie (cirque).
Explain Solifluction and its landforms that arise as a result of it.
Solifluction:
When the active layer thaws in summer, excessive lubrication reduces the friction between soil particles. Even on slopes as shallow as 2°, saturation of this upper layer then encourages soil movement downslope. This leads to solifluction sheets or lobes - rounded, tongue-like features often forming terraces on the sides of valleys. Solifluction was widespread in southern Britain during the Quaternary ice age, and such deposits are often known here as head.
Explain Frost heave and its landforms that arise as a result of it.
As the active layer starts to refreeze, ice crystals begin to develop. They increase the volume of the soil and cause an upward expansion of the soil surface. Frost heave is most significant in fine-grained material and, as it is uneven, it forms small domes on the surface.
Within the fine-grained material there are stones which, because of their lower specific heat capacity, heat up and cool faster than the surrounding finer material.
Cold penetrating from the surface passes through the stones faster than through the surrounding material. This means that the soil immediately beneath a stone is likely to freeze and expand before the other material, pushing the stone upwards until it reaches the surface. On small domes, the larger stones move outwards, effectively sorting the material which, when viewed from above, takes on a pattern. This **patterned ground **on gentler slopes takes the form of stone polygons, but where the ground is steeper (slopes exceeding 6°), the stones move downhill to form stone stripes.
Explain what Pingos are and their formation.
Pingos are Ice-Filled Periglacial Hills.
A pingo is a **conical hill **with a **core **of ice. Pingos can be as large as 50m high and about 500m wide.
Pingos are formed through the periglacial process of Groundwater freezing.
There are **two types **of pingo - **open-system **and closed-system.
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 acore **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 the 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 **hollow **of meltwater surrounded by **ramparts **(walls of soil).
Explain Ground contraction and the landforms that arise as a result of it.
The refreezing of the active layer during winter causes the soil to contract. Cracks open up on the surface in a similar way to cracks on the beds of dried-up lakes. During melting the following summer, the cracks open again and fill with meltwater. As the meltwater contains fine sediment, this also begins to fill the crack.
The process (ground contraction) occurs repeatedly through the cycle of winter and summer, widening and deepening the crack to form an **ice wedge **which eventually, over a period of hundreds of years, can become at least 1m wide and 2-3m deep.
The cracking produces a pattern on the surface which, when viewed from above, is similar to the polygons produced by frost heaving. These are therefore known as ice-wedge polygons.
