Weathering and Soils

1. Weathering and Soils Dr. R. B. Schultz

2. Weathering and Soils Weatheringrefers to numerous processes that convert bedrock exposed at earth's surface to smaller pieces that can be eroded by wind, rivers, waves or glaciers, as well as processes that chemically attack minerals in rock to produce a variety of dissolved substances. Weathering is an important part of the rock cycle, because weathering products are the source for sedimentary deposits and sedimentary rocks. Rock weathering also plays a role in: • 1.Soil development • 2.Landslide potential • 3.Changes in rock strength • 4.Surface water chemistry. This section examines the common processes that weather rock and how soils can form from weathered rock. There are two (2) types of weathering: Physical-- a physical process occurs to make the change; processes that break rock apart Chemical -- chemical reactions between minerals and water/air Physical & chemical weathering work together in concert to break down rock. The Tetons in Jackson Hole, Wyoming are an example of how physical and chemical weathering create the landscape that we see around us.

4. Chemical Weathering Processes: Hydrolysis Reactions between minerals and water, e.g. feldspar weathers to clay. Products: material in solution and new mineral. Rain is naturally acidic because carbon dioxide in the atmosphere dissolves in water to form carbonic acid. Chemical Weathering Processes: Dissolution Dissolution -- mineral dissolves completely in acidic water e.g. calcite dissolves easily in weakly acidic water Chemical Weathering Processes: Oxidation Mineral (usually iron- bearing) reacts with oxygen to form oxide (iron oxide) or "rust" Mafic silicates like olivine, pyroxene, and amphibole weather readily due to oxidation Physical Weathering Processes: Freeze-thaw or frost wedging Ice grows in cracks Ice volume is 9% greater than water Volume increase exerts pressure that can widen cracks, break off pieces of rock Produces large, angular blocks of rock called talus Physical Weathering Processes: Pressure release due to unloading Rock splits due to expansion when material above or alongside it is removed Can result from natural erosion or human excavation. Fractures called joints develop

5. Fractures called joints develop in weathered rock

6. Factors that influence weathering 1. Climate -- more precipitation --> more weathering 2. Organisms -- can aid in breakdown of rock, e.g. burrowing organisms, tree roots 3. Rock type -- some minerals (and therefore rocks) weather more easily than others 4. Time -- the longer rocks are exposed to surface weathering, the more weathered they become Influence of rock type on weathering 1.Olivine, pyroxene, and amphibole form at high temperature and pressure. They are very unstable at surface T & P, and weather easily. 2.Igneous rocks that contain lots of mafic silicates weather more easily than ones that contain felsic silicates. 3. Limestone is very easily weathered. 4. Shales weather more easily than sandstone, which is composed mostly of quartz. 5.More water or fluid means quicker and more weathering 6.Warmer temperatures speed up reactions and act as a catalyst to weathering processes 7.Smaller fragments weather more quickly than large ones because more surface area id exposed

8. SOILS * Forms on weathered material but are distinctly different from weathered material What is soil? *Loose mixture of weathered rock, organic matter, water, and air that is chemically and physically different from parent material *Soils have distinct zones that roughly parallel land surface. Call these zones horizons. Typical soil profile O horizon -- abundant organic matter; dark A horizon -- contains some organic matter, minerals, but many minerals are removed from this layer; Lighter-colored than O (Zone of Leaching) B horizon -- contains material removed from A horizon: clays, iron oxides, calcite; may be red, yellow, or tan (Zone of Accumulation) C horizon -- slightly altered parent material 3 Soil Groups -- Forming in different climates (depending on precipitation) Pedalfer -- contain Al and Fe oxides in B-horizon, no calcite. Form in humid climate where calcite dissolves away. Pedocals -- contain calcite in B-horizon; form in dry climates. Laterites -- contain Al or Fe oxides, everything else is dissolved away; forms in tropical climates

10. Typical Soil Profile O Horizon A Horizon B Horizon C Horizon

11. Typical Soil Profile development in an arid region “E” Horizon: Eluviation; calcite-rich, only in arid regions.

12. Key Terminology Physical weathering Chemical weathering Hydrolysis Dissolution Oxidation Freeze-thaw/Frost wedging Pressure Release Joints Mineral stability Soil horizon Soil profile O Horizon A Horizon B Horizon C Horizon Zone of leaching Zone of accumulation Pedalfer Pedocal Laterite E Horizon Zone of eluviation

13. Pertinent Web Sites National Cooperative Soil Survey National Cooperative Soil Survey provides a dynamic resource of soils information for a wide range of needs. National Landslide Information Center (USGS) The National Landslide Information Center (NLIC) is responsible for distributing information about landslides to the lay public, researchers, planners, and local, state, and federal agencies. Planet Earth (Discovery.com) The Discovery Planet Earth site features a variety of interesting information, reports, and links on Earth science and related subjects. Recent Landslide Events (USGS) A listing with descriptions and explanations of recent landslide events from the United States Geological Survey (USGS). Soil Erosion Research Laboratory The United States Department of Agriculture's (USDA) National Soil Erosion Laboratory home page. Soil Profile Photo Gallery Soil profile photographs from the United States Department of Agriculture. Soil Science Journal—Soils Online This soil science online journal page offers many links to related Web sites. Soils and Substrates: World-Wide Web Virtual Library An informative site that includes a comprehensive listing of soils related WWW sites. Soils and Soil Science Links Soils and soil science links from National Soil Survey Center, Iowa State University. Soils Links This Central Michigan University site maintained by Dr. Mark Francek lists several good links to Web sites containing information relative to topics discussed in the chapter.

14. Soil Science Society of America The Soil Science Society of America (SSSA) is the professional home for over 6,000 professionals throughout the world dedicated to the advancement of soil science. The primary purpose of the Society is to advance the discipline and practice of soil science by acquiring and disseminating information about soils in relation to crop production, environmental quality, ecosystem sustainability, bioremediation, waste management and recycling, and wise land use. Soil Web Sites Links to soil related sites from the National Soil Survey Center (NSSC-USDA). U.S. Department of Agriculture Weathering and Soils Links (Houghton Mifflin) Links to several weathering and soils sites, including class lecture notes, arranged by topic. Weathering Images Several good photographs of various weathering phenomena hosted by Duke University.