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Weathering, Mass Wasting and Karst
• Capable of wearing down anything that the internal processes can build. • Gravity, water, wind
and ice • Denudation - the
overall effect of disintegration, wearing away and removal of rock material • Weathering • Mass wasting • Erosion
Weathering• The first step in shaping the Earth’s surface • Mechanical disintegration or chemical
decomposition • Rock fragments into smaller and smaller pieces • Occurs where atmosphere and lithosphere meet • Whenever bedrock is exposed, it weathers • Cracks and crevices allow weathering to penetrate
bedrock • Once started spreads
• Joints are the most common • Develops as a result of stress • Most important in facilitating
weathering • Microscopic openings are
tiny and numerous • Responsible for extensive
weathering
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• Faults are breaks in bedrock with displacement • Major landscape feature • Easy penetration of weathering agents
• Lava Vesicles • holes in cooling lava that allow gas to escape, later
allows weathering to occur • Solution Cavities • holes formed as soluble minerals are dissolved
Weathering Agents• Most atmospheric • Easy to penetrate into cracks and crevices • Oxygen, carbon dioxide and water vapor • Temperature
• Weathering influenced by a variety of factors • Structure of bedrock • Size of openings • Climatic conditions • Vegetative cover • Variety of digging animals
Mechanical Weathering• The physical disintegration of rock material
without any change to its chemical composition • Big rocks fractured into smaller rocks
• Frost Wedging • Most important mechanical weathering agent • Water freezes from the top down • Expanding wedge exerts pressure on rock • Repeated cycle of freeze and thaw
Exfoliation• Curved layers of peeled off bedrock • Curved joints break away in succession • Granite and other intrusive rocks • Half Dome, Yosemite Valley, CA • Not fully understood • Rock cracks after overlying weight is removed • Removal of weight allows expansion in rock
Chemical Weathering
• Decomposition of rock by the chemical alteration of its minerals • Some rocks are extremely resistant • Many others not resistant
• The greater the surface area exposed, the greater amount of weathering
• Requires moisture • Higher temperature is better
• Oxidation • Oxygen atoms combine with metallic
elements • Can form new materials (Iron +
Oxygen = Iron Oxide) • Rusting • Red stains on rocks • Oxides are softer and easier to
remove than original material
Biological Weathering
• Plants and animals contribute to weathering • Tree roots into cracks and crevices • Lichen • Live on bare rock and draw nutrients from rock and
weakens rock • Burrowing Animals
Mass Wasting• The process whereby weathered material is
moved a relatively short distance down slope under the direct influence of gravity.
• Occurs even on gentle slopes • Can fall abruptly or creep gradually • Can be gigantic boulders or tiny particle of dust
Fall• The falling off pieces of
rock downslope • Loosened by weathering • Characteristic in
mountainous areas • Pieces of unsorted rock is
known as talus • Forms a talus cone at base of
slope • Some sorting by size • Large talus cones can move
down slope under own weight
Slide• An instantaneous collapse
of a slope • Rainwater adds weight to
overloaded slope • Earthquakes • Detached along joints • Topographic changes
• Extensive scar exposing bedrock and debris
• In the valley bottom, massive pile of irregular debris
• Up-valley of debris a lake may form
Flow
• A sector of a slope becomes unstable and flows gently downhill • Normally caused by excess water
• Relatively small and shallow • Earthflow • Most common • During or after heavy rainfall • Low vegetation
Creep• Slowest and least noticeable • Very gradual downhill movement of soil and
regolith • Happens everywhere there is a slope • Water helps accelerate creep • Any activity that disturbs soil can contribute • Usually recognized only through displacement of
human built structures
Karst Topography
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• 15% of the Earth’s surface • Limestone • Pitted and bumpy surface • poor drainage • Vast underground caverns
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Formation of Karst• 80% calcium carbonate • Pattern of joints • zone of air between ground and water table • Vegetation to supply organic acids
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Features of Karst Landscapes• Sinkholes • Solution sinkhole • collapse sinkhole • Watertable lowering • Construction
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• Caves and Caverns • Limestone easily dissolves • Forms just below water table • Flowing water • Unique plant and animal
species
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