Geology: Weathering and Soil Formation - Earth's External Processes and Soil Composition, Study notes of School management&administration

Download Geology: Weathering and Soil Formation - Earth's External Processes and Soil Composition and more Study notes School management&administration in PDF only on Docsity! 1 Earth’s external processes • Weathering – the physical breakdown (disintegration) and chemical alteration (decomposition) of rock at or near Earth’s surface • Mass wasting – the transfer of rock and soil downslope under the influence of gravity Earth’s external processes • Erosion – the physical removal of material by mobile agents such as water, wind, ice, or gravity Weathering • Two types of weathering • Mechanical weathering – breaking of rocks into smaller pieces • Four types of mechanical weathering – Frost wedging – alternate freezing and thawing of water in fractures and cracks promotes the disintegration of rocks Frost wedging Weathering • Mechanical Weathering continued – Unloading – exfoliation of igneous and metamorphic rocks at the Earth’s surface due to a reduction in confining pressure – Thermal expansion – alternate expansion and contraction due to heating and cooling – Biological activity – disintegration resulting from plants and animals Exfoliation of igneous rocks 2 Weathering • Chemical Weathering • Breaks down rock components and the internal structures of minerals • Most important agent involved in chemical weathering is water (responsible for transport of ions and molecules involved in chemical processes) Weathering • Major processes of chemical weathering • Dissolution – Aided by small amounts of acid in the water – Soluble ions are retained in the underground water supply • Oxidation – Any chemical reaction in which a compound or radical loses electrons Weathering • Major processes of chemical weathering • Oxidation continued – Important in decomposing ferromagnesian minerals • Hydrolysis – The reaction of any substance with water – Hydrogen ion attacks and replaces other positive ions Weathering • Alterations caused by chemical weathering • Decomposition of unstable minerals • Generation or retention of materials that are stable • Physical changes such as the rounding of corners or edges Weathering • Rates of weathering • Advanced mechanical weathering aids chemical weathering by increasing the surface area • Others factors affecting weathering • Rock characteristics – Rocks containing calcite (marble and limestone) readily dissolve in weakly acidic solutions 5 Soil • Soil erosion • Recycling of Earth materials • Natural rates of soil erosion depend on – Soil characteristics – Climate – Slope – Type of vegetation Soil • Soil erosion • In many regions the rate of soil erosion is significantly greater than the rate of soil formation • Sedimentation and chemical pollution – Related to excessive soil erosion – Occasionally soil particles are contaminated with pesticides What is a sedimentary rock? • Sedimentary rocks are products of mechanical and chemical weathering • They account for about 5 percent (by volume) of Earth’s outer 10 miles • They contain evidence of past environ- ments • Provide information about sediment transport • Often contain fossils Turning sediment into rock • Many changes occur to sediment after it is deposited • Diagenesis – all of the chemical, physical, and biological changes that take place after sediments are deposited • Occurs within the upper few kilometers of Earth’s crust Turning sediment into rock • Diagenesis • Includes – Recrystallization – development of more stable minerals from less stable ones – Lithification – unconsolidated sediments are transformed into solid sedimentary rock by » Compaction » Cementation by calcite, silica, and iron oxide Types of sedimentary rocks • Sediment originates from mechanical and/or chemical weathering • Rock types are based on the source of the material • Detrital rocks – transported sediment as solid particles • Chemical rocks – sediment that was once in solution 6 Detrital sedimentary rocks • The chief constituents of detrital rocks include • Clay minerals • Quartz • Feldspars • Micas • Particle size is used to distinguish among the various types of detrital rocks Detrital sedimentary rocks • Common detrital sedimentary rocks (in order of increasing particle size) • Shale – Mud-sized particles in thin layers that are commonly referred to as laminea – Most common sedimentary rock Shale containing plant remains Detrital sedimentary rocks • Sandstone – Composed of sand-sized particles – Forms in a variety of environments – Sorting, shape, and composition of the grains can be used to interpret the rock’s history – Quartz is the predominant mineral Quartz sandstone 7 Detrital sedimentary rocks • Conglomerate and breccia – Both are composed of particles greater than 2mm in diameter – Conglomerate consists largely of rounded gravels – Breccia is composed mainly of large angular particles Conglomerate Breccia Chemical sedimentary rocks • Consist of precipitated material that was once in solution • Precipitation of material occurs in two ways • Inorganic processes • Organic processes (biochemical origin) Chemical sedimentary rocks • Common chemical sedimentary rocks • Limestone – Most abundant chemical rock – Composed chiefly of the mineral calcite – Marine biochemical limestones form as coral reefs, coquina (broken shells), and chalk (microscopic organisms) – Inorganic limestones include travertine and oolitic limestone Coquina