Weathering, Mass Wasting and Karst

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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