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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 5 • 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 20 • • • • • 15% of the Earth’s surface Limestone Pitted and bumpy surface poor drainage Vast underground caverns 21 Formation of Karst • • • • 80% calcium carbonate Pattern of joints zone of air between ground and water table Vegetation to supply organic acids 22 Features of Karst Landscapes • Sinkholes • • • • Solution sinkhole collapse sinkhole Watertable lowering Construction 23 • Caves and Caverns • • • • Limestone easily dissolves Forms just below water table Flowing water Unique plant and animal species 24 25 26