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 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

Tree Roots Growing in Rock Fractures Animal Burrows 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

Important for metals concentration 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 By carbonic acid, CO2 in water CO2 + H2O => H2CO3 H2CO3 => H+ + HCO3- 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 (Fe2O3 ) from weathering of Basalt [which contains Olivine (Fe, Mg) SiO4 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

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

Climate also very important, see below Factors related to bedrock composition (parent material) Weathering-resistant sandstone (mostly quartz) yields little soil Soil Soil Soil Chemical weathering by hydrolysis Iron-rich basalt Chemical weathering by oxidation Feldspar-rich granite Chemical weathering by dissolution Limestone Climate also very important, see below

Variations in soil development due to topography: slope and time

Equator to Poles Factors Annual precipitation Temperature Equatorial and tropical rain forests Low-latitude deserts and semi-deserts Temperate regions and mixed boreal forests Arctic and tundra regions Grasslands (steppes) Savannahs 30 degrees Latitude 40ºC Precipitation 1800 mm Temperature 30ºC Evaporation 20ºC 600 mm 10ºC Rainforest US & Europe Bedrock at or very near surface Desert Shallow nutrients Deep Regolith, shallow Soil Soil Deeply weathered bedrock (~40 - 50 meters deep) Bedrock Increasing depth of weathering 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

An idealized soil profile ONLY ACTIVE EDUCATORS BECOME CHAMPIONS An idealized soil profile

Remember the different horizons A E B C ONLY ACTIVE EDUCATORS BECOME CHAMPIONS Organic Animal Activity Soluble minerals Exited Soluble Minerals Back Crushed Rock

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

E and B: Eluviation & Illuviation exited back

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

Pedalfer Pedocal Laterite 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 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 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-build-better-farms/