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Shaping Earth’s Surface
PowerPoint Lectures to accompany Physical Science, 8e Chapter 20 Shaping Earth’s Surface Start 12 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Core Concept The surface of Earth is involved in an ongoing process of destruction and tearing down of higher elevations.
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Weathering Slow changes resulting in the breakup, crumbling and other destruction of solid rock Includes physical, chemical and biological processes Contributes to The rock cycle Formation of soils Movement of rock materials over Earth’s surface Erosion The process of physically removing weathered materials Then transported by wind, water, gravity, glaciers
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Mechanical Weathering
The physical breakup of rocks without chemical change Disintegration processes Wedging By frost By trees Exfoliation Reduced pressure effect Fractures caused by expansion of underlying rock Like peeling an onion
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Stone Mountain Georgia
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Chemical Weathering Decomposition of minerals by chemical reactions
Oxidation Reactions with oxygen Produces red iron oxides (soils in Georgia) Carbonation Reactions with carbonic acid (carbon dioxide dissolved in water) Easily dissolves limestone Hydration Reactions with water Includes dissolving in water and combining with water Halite (salt) dissolves in water Water & feldspars react to form clay & SiO2
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Soils Mixtures of unconsolidated weathered Earth materials and humus (altered, decay-resistant plant material) Weathered materials include quartz sand, clay minerals, metal oxides and salts Loam Soil containing varying amounts of sand, silt and clay, mixed with an abundance of humus Also known as topsoil Subsoil Less fertile soil beneath the topsoil Contains more rocks and mineral accumulations Contains less humus
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Erosion Mass movement Erosion caused directly by gravity
Aided by loss of cohesiveness (too much water) Creep The slow movement of soil down a steep slope Landslide Any slow to rapid downhill movement of materials
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Running Water Most important of all gravitational erosion processes
Three stream transport mechanisms Dissolved materials Suspended materials Rolling, bouncing and sliding along stream bed (most effective) Streambed evolves over time
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Stream Development Youth Maturity Old age Landmass recently uplifted
Steep gradient, V-shaped valley w/o floodplain Boulders, rapids and waterfalls Generally found near source or headwaters Maturity Stream gradient smoothed and lowered Meanders over floodplain Old age Very low gradient Broad, gently sloping valleys Sluggish flow; more floods Generally found near mouth
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Deltas Deposits of sediment at the mouth of a river or stream
Stream flow dissipates into an ocean or lake Erosive and sediment-carrying abilities lost
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Glaciers Masses of ice on land that move under their own weight
Form from snow accumulated over a number of years (5-3500) Alpine glaciers Form at high elevations Flow through valleys Also “valley glaciers” Continental glaciers Cover large area of a continent Today in Greenland and Antarctica
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Glacier Erosion Three mechanisms Bulldozing Abrasion Plucking
Forms deposits called moraines Abrasion Produces powdery, silt-sized rock flour Plucking Glacier water freezes into surrounding rock and pulls it along
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Wind Considerably less efficient than water or ice Two major processes
Abrasion Natural sandblasting Produces ventifacts (rocks sculpted by wind) Shape can depend on prevailing winds Deflation Loose material picked up and carried away by the wind Wind-blown deposits Dunes: low mound or ridge of sand or other sediment Loess: fine dust deposited over a large area
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Loess Hills
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Sand Dunes
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Development of Landscapes
Landforms: Earth’s surface features Broad features Mountains, plains and plateaus Minor features Hills, valleys, canyons, … Generalizations based upon Rock structure Weathering and erosion Stages of erosion
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Rock Structure Determines the shape of minor landforms Refers to
Type of rock Igneous Metamorphic (resistant to chemical weathering) Sedimentary (susceptible to chemical weathering) Attitude of rocks Effect of disturbance by faulting or folding Faulting and folding can hasten erosion
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Weathering and Erosion Processes
Influenced and controlled by such factors as climate and elevation Warm, moist climates Chemical weathering dominant Landforms have softer, rounded outlines Dry climates Mechanical weathering dominates Sharper, angular outlines more common
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Stage of Development Describes cumulative effects of formative processes Youthful Weathering and erosion effects in beginning stages Mountains have steep peaks and narrow, steep valleys Maturity Original structure worn down into rounded peaks and slopes Old age Characterized by nearly flat rolling plains Peneplains The nearly flat surfaces Monadnocks Hills of resistant rock Rejuvenation Land uplifted and then erosion processes begin again
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