Weathering and Soils Lecture 5 Limestone dissolved by naturally acidic rainwater

Weathering and Soils Weathering and Soils Weathering – Physical breakdown and chemical alteration of rock at Earth’s surface Weathering – Physical breakdown and chemical alteration of rock at Earth’s surface Erosion – The transport of rock fragmentsErosion – The transport of rock fragments

Source: Tom Bean/DRK Photo Preview of 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 Salt wedging is similar

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

Rockfall caused by frost wedging ends up in a streamColluvium

Salt Weathering is similar, growing salt crystals in cracks split the rock apart. At the shore and desert playas.

2. Unloading = Mechanical Exfoliation Igneous rocks at Earth’s surface peeling like layers off an onion due to reduction in pressure Granite Batholith above subduction zone

Unloading = Exfoliation of a Pluton Batholith

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

3. Thermal expansion and contraction A lternate expansion and contraction due A lternate expansion and contraction due to heating and cooling to heating and cooling Minerals expand and contract at different rates Slide past one another, breaking any intergrowths Rock crumbles to individual mineral grains - Important in deserts - Important in deserts

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

Mechanical weathering causes increase in surface area Same volume of rock, but many pieces have much more surface area than one piece

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 mineralsBreaks down rock and minerals Important agent in chemical weathering is water (transports ions and molecules involved in chemical reactions)Important agent in chemical weathering is water (transports ions and molecules involved in chemical reactions) The ions form the cements in Sedimentary RocksThe ions form the cements in Sedimentary Rocks These ions make the ocean saltyThese ions make the ocean salty Important for metals concentration Important for metals concentration

Three types of Chemical Weathering

Dissolution Weathered and Unweathered Limestone Boulders Source: Ramesh Venkatakrishnan

Dissolution of Calcite DissolutionDissolution –By carbonic acid, CO 2 in water Soluble ions contained in underground water H 2 O + CO 2 H 2 CO 3 H + + HCO 3 - CaCO 3 +2H + H 2 O (l) + CO 2 (g) + Ca ++ (aq)

Oxidation Chemical reaction where compound loses electronsChemical reaction where compound loses electrons Usually while bonding with oxygenUsually while bonding with oxygen –Important in breaking down mafic minerals (contain Fe +2 or +3 ) –Rust- colored mineral (Fe 2 O 3 ) from weathering of Basalt Which contains Pyroxene (Fe, Mg) 2 Si 2 O 6 Which contains Pyroxene (Fe, Mg) 2 Si 2 O 6

Oxidation of Basaltic Lava Flows Rust-colored 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

22 Hydrolysis – Feldspar to Clay expands in water Mechanical fracture due to chemical weathering Feldspars become

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

Bowens Reaction Series and Weathering

Also very important in recognizing past climates Soils

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

Components in topsoil that support plant growth

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 (parent material) Climate also very important, see below

Variations in soil development due to topography: slope and time Variations in soil development due to topography: slope and time

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 Deep Regolith, shallow Soil Soil Bedrock Deeply weathered bedrock (~ meters deep) Temperature 30 degrees Latitude Shallow nutrients Rainforest Desert US & Europe Factors related to Climate

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

Soil Horizons Soil Horizons 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 – soluble minerals removed B horizon – zone of re-precipitation C horizon – partly altered parent material

An idealized soil profile ONLY ACTIVE EDUCATORS BECOME CHAMPIONS

Remember the different horizons OAEBCOAEBC Organic Animal Activity Soluble minerals Exited Soluble Minerals 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 and B: 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

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 Monsoonal Climate Intense chemical weathering Red Iron oxide - Topsoil not distinct from B horizon – bacterium responsible for dissolving soil iron is not present Deep soil but usable nutrients shallow

Laterite in Sarawak, Borneo Source: Fletcher & Baylis/Photo Researchers, Inc. The percolating rain water causes dissolution of primary rock minerals and decrease of easily soluble elements as sodium, potassium, calcium, magnesium and silicon. This gives rise to a residual concentration of more insoluble elements predominantly iron and aluminum.

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

Natural rates of soil erosion depend on: Source: Ramesh Venkatakrishnan Headed for the Sea Soil characteristics ClimateSlope Type of vegetation

Soil Soil Soil erosion In many regions the rate of soil erosion is significantly greater than the rate of soil formation Farmers level fields to slow loss of topsoil Farmers have been building terraces for thousands of years better-farms/