Higher Geography Physical Environments Lithosphere
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Higher Geography Physical Environments Lithosphere 1 Glaciated Landscapes Glacial History About every 200 million years the Earth experiences a major period of ice activity - an ice age. The most recent of these started about 2 million years ago and finished about 10,000 years ago. An ice age consists of glacials (cold periods ) separated by interglacials (warmer periods). About 30% of the world was covered by glacial ice when the last ice age was at its maximum. The UK was covered by ice between 1-3km thick as far south as a line from London to Bristol. Causes of Glaciation There are many theories as to the cause of glaciations: 1. Milankovitch cycle – changes in incoming solar radiation due to changes in orbit, tilt and position in space. 2. Variations in sunspot activity 3. Changes in the amount of carbon dioxide in the atmosphere 4. Changes in the movement of the ocean currents 5. Periods of extreme volcanic activity which put huge amounts of ash into the atmosphere Formation of Glaciers During the onset of a glaciation, more and more precipitation falls as snow In addition, less and less snow melts each summer so that successive layers of snow gradually build up until there is a year-round snow cover in more and more places. As snow becomes more compacted, the air is driven out and density increases. Eventually, this process forms neve or firn (compacted snow). After 20-40 years the firn will turn into glacial ice which contains little air Glacial ice can begin to flow downhill under the influence of gravity as a glacier 2 Cross Profile of a Glacier Glaciers, like rivers, behave as a system with inputs, outputs, stores and transfers. The main input for glaciers is falling snow but avalanches can add considerable volumes of ice and snow. The glacier itself is the store in the form of frozen water. The outputs include evaporation, calving (where ice breaks off into water) and melting. The upper part of a glacier where inputs exceed outputs is called the zone of accumulation. The lower part of a glacier where outputs exceed inputs is called the zone of ablation. In between the two is the line of equilibrium which is the same as the snow line. Processes Ice is capable of transporting huge quantities of rock. Some rocks fall on to the surface of the ice from the valley sides and are transported as supraglacial debris. Some material finds its way into the ice via crevasses to be transported as englacial debris. Where there is basal sliding, debris may also be picked up below the ice and be transported as subglacial debris. Glaciers that move relatively quickly and that transport large amounts of debris at the base, are capable of powerful physical erosion which can drastically alter the pre-glacial landscape. Types of Glacial Erosion Exam tip: these should be described and explained in detail when asked about the formation of erosional features. 1. Abrasion If debris is incorporated into the sides and base of the ice, abrasion becomes active, sandpapering the rock surfaces to produce smooth, gently sloping landforms. Striations, scratches or grooves are found on bare rock surfaces and are useful to indicate direction of glacier movement. 2. Plucking Plucking occurs when rocks and stones become frozen to the base or sides of the glacier and are plucked from the ground or rock face as the glacier moves. Plucking produces jagged slopes to landforms. 3 Due to cold temperatures, weathering also has an impact on glaciated landscapes. The main type of weathering found in these locations is freeze-thaw action. Freeze-thaw Action Water (e.g. from rainfall or melting snow and ice) becomes trapped in a crack or joint in the rock If the air temperature drops below freezing, the water will freeze and expand by 9-10% putting pressure on the rock. The ice will melt when the temperature rises above freezing. If this process happens repeatedly, the rock will weaken and eventually shatter into angular fragments. The fragments may then be deposited as scree at the foot of a slope. Rates of Erosion Rates of erosion will vary considerably but are greatest where: temperatures fluctuate around freezing point where rocks are more jointed and faulted providing weaknesses where slopes are slightly steeper leading to more rapid glacier movement (very steep slopes can lead to extended flow, a thinning of the ice and reduced erosive power two or more glaciers meet and combine to give an increased depth of ice ice moves by rotational flow in corrie glaciers leading to over-deepening of the hollow Features of Glacial Erosion Corries, Cirques or Cwms Cirques (France), corries (Scotland) or cwms (Wales) are glacial hollows with a very steep backwall and a basin that may contain a lochan or tarn when the glacier retreats and melts. 4 Formation of a Corrie Snow accumulates in mountain hollows when more snow falls in winter than melts in the summer. North/north-east facing slopes are more shaded so snow lies longer and builds up. The accumulated snow compresses into neve/firn and eventually glacial ice. Plucking (when ice freezes on to bedrock, pulling loose rocks away) makes the backwall steeper. Abrasion (when the angular rock embedded in the ice grinds the hollow) makes the hollow deeper. Freeze thaw action continues to steepen the sides of the hollow when water in cracks in the rock turns to ice when temperatures drop below freezing; expansion and contraction weakens the rock until fragments break off. Rotational sliding further deepens the central part of the hollow floor as gravity causes the ice to move. Friction causes the ice to slow down at the front edge of the corrie, allowing a rock lip to form, which traps water as ice melts, leaving a lochan or tarn. During spring/summer, thawing takes place, allowing water to penetrate cracks in the rocks at the base of the hollow. The broken fragments build up over time and are removed by meltwater, further enlarging the hollow. Frost shattering on the backwall supplies further abrasion material as loose scree falls down the bergschrund. This is a large crevasse separating moving ice from the ice still attached to the backwall. Landforms caused by Corries Where a series of corries form around a mountain peak, they create other unique landforms. Two corries eroding into the mountain eventually leave a narrow, knife-edged ridge or arête between them. Striding Edge in the Lake District (UK) provides a classic example. Where three or more corries erode backwards around a mountain, they create a characteristic triangular pyramidal peak or horn. One of the most spectacular examples is the Matterhorn. 5 Identifying Corries & Associated Landforms on an OS Map Contour lines form a horseshoe Peak found shape. Contours between 3 or close together on more corries = steep back wall pyramidal peak and sides. Water in rock basin = lochan / tarn Bare rock in- between 2 corries = arete Glacial Troughs & Associated Landforms In mountain environments, valley glaciers severely modify former river valleys to produce very deep, steep- sided, flat-floored U-shaped valleys or glacial troughs. 6 Glacial Trough During glaciation, glaciers follow the paths of existing V-shaped valleys. Plucking (when ice freezes on to bedrock, pulling loose rocks away) wears away the valley sides making them steeper and abrasion (when the angular rock embedded in the ice grinds the rock beneath) deepens and widens the valley. As glaciers ‘flow’ downhill, they erode the ends off the interlocking spurs found previous to glaciation forming truncated spurs. Tributary valleys flowing into the main valley will also be glaciated, but as the glaciers in these valleys are much smaller, the erosive power is less. This results in the formation of hanging valleys - valleys that are not eroded as deeply and so are post-glaciation are left ‘hanging’ above the main glacial trough. Sections of the valley floor can be over-deepened due to differential erosion, rotational sliding or a confluence of glaciers increasing erosive power. These sections can be filled with meltwater to form a ribbon lake. After glaciation, a steep sided glacial trough is left, rivers may return to the valley forming misfit streams. Freeze thaw action (water in cracks in the rock turns to ice when temperatures drop below freezing; expansion and contraction weakens the rock until fragments break off) continues on the valley sides loosening rock and forming scree slopes. 7 Identifying Glacial Troughs & Associated Landforms on an OS Map Small valley flowing into Lake in base main trough = of valley = hanging valley ribbon lake Bare rock on steep valley side = truncated spur River running through valley= misfit stream Flat valley floor and steep sides = U shaped valley Depositional Landforms Material eroded and subsequently transported by glacial ice may be deposited as unsorted till as the ice melts or it may be further transported by glacial meltwater and then deposited as sorted fluvioglacial material. Till deposits, sometimes referred to as boulder clay, are a mixture of unsorted sand, clay and rock particles. The rock fragments are sub-angular in shape. The majority of this material has been transported as supraglacial debris and is dropped in situ at the glacier snout or more generally at the ends of ice ages when glaciers disappear. Some of the till deposits form distinctive landforms but much of it is simply deposited as a layer which masks the former pre-glacial landscape. Sometimes glaciers pick up and transport rocks with distinctive geological characteristics. Once deposited, these erratics can be used to trace back the route followed by the glacier.