Earth & Space ScienceChapter 7

Weathering, Erosion, and Soil

Mechanical/Physical Weathering

Changes size and shape of rocks, not composition

• Temperature– Water freeze/thaw cycle (water expands when it

freezes): frost wedging• Pressure– Plant roots enlarge cracks in rocks– Exfoliation of layers

Examples:

Frost wedging, exfoliation, and root wedging

Chemical WeatheringChanges the chemical identity of the rock• Water– Dissolves minerals– Chemical reaction: hydrolysis (decomposes silicates)

• Oxygen– Chemical reaction: oxidation (especially iron)

• Carbon dioxide– CO2 + H2O Carbonic acid (dissolves calcite)

• Acid precipitation– Nitric and sulfuric acid from air pollution

Examples:

Effect of temperature and rainfall

What affects the rate of weathering?

• Climate– Warm/rainy = more chemical weathering– Cool/dry = more physical weathering

• Rock type/composition– Sedimentary = most weathering

• Exposed surface area– Greater surface area = greater weathering

• Topography: slope of the land– Steeper = greater weathering

Exposed surface area affects weathering

Erosion and Deposition

• Erosion transports weathered material until it is deposited in a new location.

• Wind, running water, glaciers, glaciers, ocean currents, and waves

• The importance of gravity:– Loose rock falls downhill– Water flows downhill– Glaciers flow downhill

Water Erosion

• Greatest when water is moving rapidly• Rivers/streams: billions of tons of sediment

carried downstream, deposited at deltas• Waves: erosion and deposition of sand forms

barrier islands

Glacial Erosion

• Move as dense rivers of ice

• Grind up, transport and deposit large amounts of rock and debris

• Much of the northern U.S. landscape has been shaped by glaciers

Wind Erosion

• Especially important in arid (dry) locations and coastal regions without much vegetation

Soil Formation

• Soil consists of weathered rock and organic material

• Soil texture is determined by the amount and type of different sized particles

Soil Horizons

• O: organic material and leaf litter

• A: weathered rock and organic material

• B: zone of accumulation/ clay and minerals

• C: broken down bedrock

Factors in Soil Formation

• Climate—rock weathers more rapidly in humid, temperate climates

• Topography—slope affects deposition of soil • Parent material—type of rock that forms the

soil• Biological organisms—help break down soil

and add to humus• Time—soil is renewable, but can take long

periods of time