Weathering and Soils Chapter 7

Weathering Vocabulary Exfoliation: outer layers of rock are stripped away like an onion Hydrolysis: reaction of water with substance Frost wedging: repeated thawing and freezing of water in the cracks of rocks Oxidation: reaction of oxygen with a substance

Earth’s surface processes Earth’s surface processes First a definition: Weathering – Physical breakdown and chemical alteration of rock at Earth’s surface

Types of weathering Chemical: breakdown of material using changes in chemical composition Physical: breakdown of material without chemical change.

Physical or Mechanical Weathering Physical or Mechanical Weathering Physical weathering – Physical weathering is the disintegration of earth material without undergoing a chemical change. Frost wedging – freezing and thawing of water in cracks disintegrates rocks Exfoliation– exfoliation of igneous rocks at Earth’s surface due to erosion of “overburden” and reduction in pressure Biological activity – disintegration resulting from plants and animals - root wedging, animal burrows

Frost Wedging

Evidence of Frost Wedging in Wheeler Park, Nevada Source: Tom Bean/DRK Photo

Rockfall caused by frost wedging

Salt Wedging

Mechanical Exfoliation in Yosemite National Park Source: Phil Degginger/Earth Scenes

Mechanical Exfoliation of granite

Tree roots growing in rock fractures, plus animal burrows, expose deep rocks to water Source: Runk/Schoenberger/Grant Heilman

Physical weathering increases the rate of chemical weathering by increasing surface area.

Joint-controlled weathering in igneous rocks Joint-controlled weathering in igneous rocks

Joints in sandstone - Canyonlands Natl Park, Utah

Chemical Weathering Breaks down rock and minerals Important agent in chemical weathering is water Water dissolves and transports ions and molecules The dissolved ions later bond & form the cements in sedimentary rocks

Chemical Weathering Major processes of chemical weathering Dissolution –Soluble ions contained in underground water Oxidation –Chemical reaction where compound loses electrons –Important in breaking down mafic minerals –Rust colored weathering of Basalt Hydrolysis H + or OH - ions –Reaction of any substance with water –Water’s ions replace different ions in mineral –Feldspars, most abundant crust minerals, become fine clay particles. Easily transported.

Dissolution Weathered and Unweathered Limestone Boulders Source: Ramesh Venkatakrishnan

Angular Boulder Decomposes and Rounds Source: Paul McKelvey/Tony Stone Images

Hydrolysis Cleopatra’s Needle, (Egypt) Source: New York Public Library, Locan History and Genealogy Division Granite in a Dry Climate

Hydrolysis Cleopatra’s Needle, (Central Park, NYC) Source: Runk/Schoenberger/Grant Heilman Granite in a Wet Climate

Oxidation of Basalt Rust (Iron Oxide) forms

Weathering Rates of weathering Mechanical weathering aids chemical weathering by increasing surface area Others factors affecting weathering Rock characteristics e.g. minerals –Marble and limestone easily dissolve in weak acidic solutions -Dissolution

Our food grows in soils. Also very important in recognizing past climates Soils

Soil Soil - combination of mineral and organic matter, water, and air It is that portion of the regolith (weathered rock and mineral) that supports the growth of plants

Components in soil that support plant growth

Soil Factors controlling soil formation Parent material –parent material is the underlying bedrock - composition affects soil types

Soil Factors controlling soil formation Time –Soils get better developed (Thicker, with greater differences between layers) with more time Climate –Biggest control on soil formation –Key factors are temperature and precipitation

Soil Factors controlling soil formation Plants and animals –Organisms influence soil properties –Also furnish organic matter to the soil (especially plants) Slope –Steep slopes have poorly developed soils (due to faster erosion and downslope transport –Flatter terrain accumulates soil faster

Variations in soil development due to topography Variations in soil development due to topography Note location of agriculture

Soil Profile The soil profile Soil forming processes operate from the surface downward Vertical differences are called horizons – zones or layers of soil

Soil Horizons Layers in Soil Not Deposited, but Zones of Chemical Action Soil Profile Suite of Horizons at a Given Locality Soil Horizons and Profiles

An idealized soil profile with horizons

Soil Soil The soil profile O horizon – organic matter A horizon – organic and mineral matter –High biological activity (animals live here) –Together the O and A horizons make up topsoil B horizon – zone of accumulation C horizon – partly altered parent material Mnemonic: Only Active Educators Become Champions

Soil Profiles

A mature soil profile with well developed A, B, and C horizons takes hundreds to thousands of years to develop. Horizon “A” Horizon “B” Horizon “C” Soils-3-6 Image: NRCS

Remember the different horizons OAEBCOAEBC Organic Animal Activity Exited Back Crushed Rock Soluble Minerals

Animal Activities in “A” horizon Source: Runk/Schoenberger/Grant Heilman

Soil types The characteristics of each soil type primarily depend on the prevailing climatic conditions Three very generic soil types Pedalfer Moist All Year Accumulation of iron oxides and Al-rich clays in the B horizon. Brown B horizon –Best developed under forest landscapes Pedocal - High Evaporation –White calcium carbonate (caliche) in B horizon –Associated with dry grasslands and dry brush vegetation Laterite e.g. Monsoonal Climate –Alternating Hot dry and Cool Wet tropical climates –Intense chemical weathering –Red Iron oxide – horizons not distinct –Very thick soils but thin useful topsoil-Trees are Buttressed

pedalfer pedocal laterite tropicsShortgrass Evergreen forests

Pedocal with Caliche in the B horizon OAEBCOAEBC Organic Activity Leached Accumulation Crushed Rock

Caliche Source: Teaching Collection/University of Washington, Department of Geological Science

Laterite in Sarawak, Borneo Source: Fletcher & Baylis/Photo Researchers, Inc.

Earth’s surface processes Earth’s surface processes Erosion – the physical removal of material by mobile agents like water, wind, ice, or gravity

Soil Erosion Soil Erosion Soil erosion Recycling of Earth materials Natural rates of soil erosion depend on –Soil characteristics –Climate –Slope –Type of vegetation

Erosion Source: Ramesh Venkatakrishnan Headed for the Sea

Soil Soil Soil erosion In many regions the rate of soil erosion is significantly greater than the rate of soil formation Farmers now level fields with lasers to slow loss of topsoil Terraces

Balance Between: Downward Lowering of Ground Surface Downward Migration of Soil Horizons If erosion rapid or soil evolution slow, soils may never mature beyond a certain point. Extremely ancient soils may have lost everything movable Limits of Soil Development

Soil Developed on a Lava Flow Source: Stanley Chernicoff/Patrick Spencer End of Chapter 7