Weathering and Soils

Earth’s surface processes Earth’s surface processes First some definitions: Weathering – Physical breakdown and chemical alteration of rock at Earth’s surface Erosion – The transport of rock fragments

Source: Tom Bean/DRK Photo Preview of Mechanical Weathering

Mechanical Weathering Mechanical Weathering Mechanical weathering – breaking of rocks into smaller pieces Four types of mechanical weathering 1. Frost wedging – freezing and thawing of water in cracks disintegrates rocks

Frost Wedging

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

Rockfall caused by frost wedging

Thermal Expansion and Contraction and Salt Weathering

Weathering Weathering Mechanical Weathering (continued) 2. Unloading or Mechanical Exfoliation Igneous rocks at Earth’s surface peeling like layers off an onion due to reduction in pressure 3. Thermal expansion and contraction alternate expansion and contraction due to heating and cooling - important in deserts 4. Biological activity – disintegration resulting from plants and animals - root wedging, animal burrows

Unloading - Exfoliation of a Pluton

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

Unloading- Exfoliation of granite

Thermal Expansion and Contraction Source: Tom Bean

Tree Roots Growing in Rock Fractures Animal Burrows Source: Runk/Schoenberger/Grant Heilman

Increase in surface area by mechanical weathering

Joint-controlled weathering in igneous rocks Joint-controlled weathering in igneous rocks Definition: Joints vs Faults Increases surface area for chemical weathering

Chemical Weathering Breaks down rock and minerals Important agent in chemical weathering is water (transports ions and molecules involved in chemical reactions) The ions form the cements in Sedimentary Rocks The salt in the ocean

Dissolution of Calcite Dissolution –By carbonic acid, CO 2 in water CO 2 + H 2 O => H 2 CO 3 H 2 CO 3 = > H + + H CO 3 - –Soluble ions contained in underground water

Dissolution Weathered and Unweathered Limestone Boulders Source: Ramesh Venkatakrishnan

Oxidation Chemical reaction where compound loses electrons –Important in breaking down mafic minerals (contain Fe) –Rust- colored mineral (Fe 2 O 3 ) from weathering of Basalt [which contains Olivine (Fe, Mg) SiO 4 ]

Oxidation of Basalt Rust (Iron Oxide) forms

Hydrolysis Water makes H + and OH - ions –Hydrolysis is the reaction of any substance with water –Water’s ions replace different ions in a mineral –Feldspars, most abundant crust minerals, become fine clay particles. – clays are light weight, flat plates, easily transported by streams

24 Hydrolysis – Feldspar to Clay Mechanical fracture due to chemical weathering Feldspars become

Hydrolysis: 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

Vegetation and Soil Development Plants use Hydrolysis to get nutrient metals out of minerals

Calcite (Limestone) No solids Ca ++ CO 3 --

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

Chemical weathering by dissolution Limestone Weathering-resistant sandstone (mostly quartz) yields little soil Soil Feldspar-rich granite Iron-rich basalt Chemical weathering by  oxidation Chemical weathering by hydrolysis Factors related to bedrock composition

Weathering Other factors affecting weathering Rock characteristics continued –Silicate minerals weather in the reverse order as their order of crystallization (Bowens Reaction Series) –Olivine least stable, conditions of formation least like surface. –Quartz (sandstone) most stable Climate –Temperature and moisture most crucial factors –Chemical weathering most effective in warm, moist climates

Bowens Reaction Series and Weathering

Also very important in recognizing past climates Soils

Soil Soil - combination of mineral and organic mater, 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 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 E horizon – little organic matter –Zone of leaching B horizon – zone of accumulation C horizon – partly altered parent material

An idealized soil profile ONLY ACTIVE EDUCATORS BECOME CHAMPIONS

Remember the different horizons OAEBCOAEBC Organic Activity Exited Back Crushed Rock ONLY ACTIVE EDUCATORS BECOME CHAMPIONS

Animal Activities in “A” horizon Source: Runk/Schoenberger/Grant Heilman Worms ingest mineral grains because they are covered with living organisms – their food. Their burrows, not their feeding, increase chemical weathering by exposing the minerals to water and air

E luviation & Illuviation exited back

Illuviation and Eluviation Leaching and Precipitation of Iron Source: Jens/Gutzmer/Rand Afrikaans University/Geology E B

Soil Types Soil types The characteristics of each soil type primarily depend on the prevailing climatic conditions Three very generic soil types Pedalfer Pedocal Laterite

Equator to Poles Factors Equatorial and tropical rain forests Savannahs Low-latitude deserts and semi-deserts Grasslands (steppes) Temperate regions and mixed boreal forests Arctic and tundra regions Equator Annual precipitation Increasing depth of weathering 1800 mm 600 mm 40ºC 30ºC 20ºC 10ºC Precipitation Temperature Evaporation Bedrock at or very near surface Soil Bedrock Deeply weathered bedrock (~ meters deep) Temperature 30 degrees Latitude Shallow nutrients Rainforest Desert US & Europe

pedalfer pedocal laterite tropicsShortgrass Evergreen forests

Pedalfer Accumulation of iron oxides and Al-rich clays in the B horizon. Brown B horizon Best developed under temperate forest landscapes

Pedocal White calcium carbonate (caliche) in B horizon Associated with dry grasslands and brush vegetation

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

Laterite Hot and wet tropical climates Intense chemical weathering Red Iron oxide - Topsoil not distinct from B horizon Deep soil but usable nutrients shallow

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

Soil Developed on a Lava Flow Source: Stanley Chernicoff/Patrick Spencer End of Lecture 5