1Weathering, Erosion, and SoilsWeathering vs. erosionTypes of weathering• Physical• Chemical1Rates of weathering and erosion• Climate• Rock composition• Other factorsSoils – a product of weatheringThe Grand Canyon, a landscape shaped by weathering and erosion2S. KuehnWeathering and ErosionWeathering- includes all processes which break down rocks at the earth’s surface- there are two general types of weathering: •physical(mechanical) weathering3py()g• chemical weatheringErosion- includes processes which remove weathered material from its source- water, wind, and ice, for example, can erode weathered materialWeathering and ErosionPhysical (mechanical) weathering- breaks rocks into smaller pieces4Examples: • abrasion by particles carried by wind, water, or ice• fractures created or widened by changes in pressureor temperature or by root action25Abrasion by glaciersGlaciers typically have many fragments of rock embedded in the ice. When the ice slides along, these rock fragments grind against the bedrock beneath the glacier. This wears down underlying rock and creates smoothed and grooved surfaces. S. Kuehn6Abrasion by flowing waterWater typically carries sand and gravel. These particles bump into each other and larger rocks wearing them down over time. 7Mechanical weathering caused by growing rootsPlant roots can grow into tiny fractures in rocks. As the roots grow over time, the fractures become wider.Fig 6.12 Understanding EarthMechanical weathering by frost actionFreezing and thawing widens fractures over time. Pieces can then fall off the cliff and move down the slope.Mount Rainier andesite 8lava flow at Burroughs Mountain, Mt. Rainier National ParkS. KuehnAnimation:Frost Wedging39Mechanical weathering by exfoliation (sheeting)Exfoliation occurs as rocks expand when brought to the surface from deep in the crust where pressures are much higher than at the surface.Photo: exfoliating granite in Yosemite National Park - Fig 6.14 Understanding EarthPart of the Sierra-Nevada batholith:Intrusive igneous rocks in Yosemite National Park10Animation:Uplift and erosion of a volcanic arcWeathering and ErosionChemical weathering- breakdown of minerals by chemical reactions toform dissolved material and/or other minerals11such as clayExamples:Dissolution - dissolves in waterOxidation - oxygen is added Hydration - water is added12The brown stain on the surface of this sandstone is called desert varnish. It is produced by chemical weathering and evaporation.S. Kuehn4Rates of Weathering and Erosiontype of mineral or rocksurface area13climateacidstimeUnderstanding Earth1415Understanding EarthTime16Different rates of weathering of slate and marble. Marble is composed of calcite which dissolves easily in acidic water. The faster weathering of the marble headstone is apparent in the blurring of the text.Fig 6.1 Understanding Earth5Weathering along natural fractures in rocksPhotos of weathered granite at Yosemite National Park17 18Spheroidal weathering of a basaltic lava flow in Oregon – Chemical weathering of these rocks began along the fracturesPhotos by S. KuehnWeathering to produce soilsIn the example to the left, the soil was produced by weathering of bedrock with little erosion. Soils also may be formed in di d i h il19sediment deposits such as silt on a river floodplain or dune sand.The type of soil formed depends on the source material, the climate, and how much time has passed since the soil began to form.Fig 6.12 Understanding EarthImportance of SoilSoil directly or indirectly plays a critical role in supporting life on land.Soils serve as an interface between the biosphere, hydrosphere, atmosphere, and solid Earth. They affect and are effected by life, air, and water.Soils help regulate the flow of water and dissolved material on land 20pgand help filter groundwater.Soils store and cycle nutrients.Soils support trees, buildings, bridges, etc.Most weathering occurs in or near the soil zone.Soils are strongly affected by human activities over very large areas (e.g. farming, urbanization). People depend on soils for growing food, waste disposal, etc.6Factors in soil formation1. Starting (parent) material2. Climate/weathering213. Biological activityFactors in soil formation4. Topography5Time225. TimeSoil Forming Factors: The Story of Rocks and Soil http://ltpwww.gsfc.nasa.gov/globe/soilform/parmat.htmDescribing and Classifying SoilsSoil scientists classify soils into 12 soil orders, 64 sub-orders, and other sub-categories on the basis of physical, chemical, and biological properties including:• Color• Texture (e.g. sand, silt)•Structure (e g blocky platy)23•Structure (e.g. blocky, platy)• Roots•Pores• Composition (organic material, clay, carbonates, salts, volcanic glass, etc.) • Climate (temperature, moisture)Soils are composed primarily of solids (mostly minerals and organic matter) plus some liquid (mostly water) and gasesSome Soil Profiles24Source http://soils.usda.gov/gallery/photos/profiles/ and http://soils.usda.gov/technical/classification/orders/Oxisola soil formed in siltstonean an EntisolEntisol formed in formed in sandy sedimentssandy sediments7Generalized (very!) Global Soil Map25Online ResourcesSoil Formation and Classification (USDA) http://soils.usda.gov/education/facts/formation.htmlDescriptions and U.S. distribution maps for major soil orders http://soils.usda.gov/technical/classification/orders/Understanding Soil Risks and Hazards http://soils.usda.gov/use/risks.html26Soil education section of NASA’s GLOBE programhttp://archive.globe.gov/tctg/tgchapter.jsp?sectionId=86Smithsonian Soils Exhibit