Last time Viscosity determines the ability of a melt to releases gasses coming out of solution Viscosity depends on silica content and TEMPERATURE Mafic (Basaltic) melts are extremely hot and have low silica, do not explode much Felsic (Granitic) melts are only moderately hot and have very high silica. Explode violently
Limestone dissolved by naturally acidic rainwater Weathering and Soils Lecture 5
Earth s surface processes Weathering Physical breakdown and chemical alteration of rock at Earth s surface Erosion The transport of rock fragments
Preview of Mechanical Weathering Source: Tom Bean/DRK Photo
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 stream Colluvium
Salt Weathering is similar, growing salt crystals in cracks split the rock apart. At the beach and desert playas.
Unloading = Mechanical Exfoliation Granite Batholith above subduction zone Igneous rocks at Earth s surface peeling like layers off an onion due to reduction in pressure
Unloading - Exfoliation of a Pluton
Mechanical Exfoliation in Yosemite National Park Source: Phil Degginger/Earth Scenes
3. Thermal expansion and contraction Alternate expansion and contraction due to heating and cooling - Important in deserts
Biological Activity Tree Roots Growing in Rock Fractures Animal Burrows Source: Runk/Schoenberger/Grant Heilman
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 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 These ions make the ocean salty Important for metals concentration
Three types of Chemical Weathering
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 + + HCO 3 - Soluble ions contained in underground water CaCO3 +2H+ => H2CO3 + Ca++ H2CO3 => H2O + CO2
Dissolution Weathered and Unweathered Limestone Boulders Source: Ramesh Venkatakrishnan
Oxidation Chemical reaction where compound loses electrons Usually 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 Olivine (Fe, Mg) SiO 4 http://www.geol.umd.edu/~jmerck/gal04/events/htmls/dauttl.html
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
Hydrolysis Feldspar to Clay Feldspars become Mechanical fracture due to chemical weathering 23
Hydrolysis Cleopatra s Needle, (Egypt) Granite in a Dry Climate Source: New York Public Library, Locan History and Genealogy Division
Hydrolysis Cleopatra s Needle, (Central Park, NYC) Granite in a Wet Climate Source: Runk/Schoenberger/Grant Heilman
Vegetation and Soil Development Plants use Hydrolysis to get nutrient metals out of minerals
Bowens Reaction Series and Weathering
Soils Also very important in recognizing past climates
Soil - 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 plants
Components in topsoil that support plant growth
Factors related to bedrock composition (parent material) Weathering-resistant sandstone (mostly quartz) yields little soil Soil Soil Soil Iron-rich basalt Chemical weathering by oxidation Chemical weathering by hydrolysis Chemical weathering by dissolution Limestone Feldspar-rich granite Climate also very important, see below
Variations in soil development due to topography: slope and time
Increasing depth of weathering Annual precipitation Temperature Equator Equator to Poles Factors 1800 mm 40ºC 30ºC Equatorial and tropical rain forests Savannahs Precipitation Low-latitude deserts and semi-deserts 30 degrees Latitude Temperature Evaporation Grasslands (steppes) Temperate regions and mixed boreal forests Arctic and tundra regions 20ºC 600 mm 10ºC Rainforest Bedrock at or very near surface Desert US & Europe Shallow nutrients Deep Regolith, Deeply weathered shallow Soil bedrock (~40-50 meters deep) Soil Bedrock Factors related to Climate
Soil Profile The soil profile Soil forming processes operate from the surface downward as water descends Vertical differences are called horizons zones or layers of 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 soluble minerals removed B horizon zone of re-precipitation C horizon partly altered parent material
ONLY ACTIVE EDUCATORS BECOME CHAMPIONS An idealized soil profile
Remember the different horizons ONLY ACTIVE O A Organic Animal Activity EDUCATORS BECOME E B Soluble minerals Exited Soluble Minerals Back CHAMPIONS C Crushed Rock
E and B: Eluviation & Illuviation exited back
Illuviation and Eluviation Leaching and Precipitation of Iron E B Source: Jens/Gutzmer/Rand Afrikaans University/Geology
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 Evergreen forests Shortgrass tropics
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 O A Organic Activity E B Leached Accumulation C 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 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. Source: Fletcher & Baylis/Photo Researchers, Inc.
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: Soil characteristics Climate Slope Type of vegetation Headed for the Sea Source: Ramesh Venkatakrishnan
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 http://engineering.missouri.edu/news/2009/02/03/mizzou-engineering-helping-buildbetter-farms/