GEOL 101 Weathering and Soils
GEOL 101 Weathering and Soils GEOL 101
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This 4 page Class Notes was uploaded by Victoria Williams on Sunday March 13, 2016. The Class Notes belongs to GEOL 101 at George Mason University taught by Mark Uhen in Fall 2016. Since its upload, it has received 23 views. For similar materials see Introductory Geology in Geology at George Mason University.
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Date Created: 03/13/16
Weathering and Soils Where is weathering the most intense? The Rainforest! The rain, heat, animals, and vegetation all contribute. Weathering – The physical breakdown and chemical alternation of existing rocks at or near the earth’s surface. The physical or mechanical weathering changes appearance. The chemical weathering can change its chemical components Processes o Erosion – The physical removal of material by mobile agents (like floods, dust storms, wind storms, and glacial movement) Mechanical Weathering – The physical breakdown of rocks into smaller pieces, each retaining the characteristics of the original. See the diagram on mechanical weathering in your book. As a rock weathers, its surface area increases but its volume stays the same. Types of Mechanical Weathering – o Frost Wedging – When water gets in cracks between rocks and then it freezes, the water expands and takes up more space. The ice then will break the rock it’s wedged between. On mountains that have been effected by frost wedging, there are talus slopes composed of angular rock fragments that are very poorly sorted. o Unloading – The erosion or removal of material that is originally formed at depth. This mostly happens on plutons (solidified magma chambers) that emerge to the surface. The pluton expands because it is no longer under pressure, and since it is a solid it will crack and fracture. The pluton will break into sheets. o Fractures and Joints – Cracks in the rock along which there is no displacement. This is ONLY the physical breaking of rock, there is no major movement. o Thermal Expansion – Heating a rock at a high temperature but with only surface pressure, the rock will expand and crack. o Biological Activity – Trees send down their roots to stabilize themselves and reach the water table for nutrients. If there isn’t much soil and they are growing on bedrock, they will force their roots through the rock and find any weakness and crack the rock. And since the tree is removing water from the area, this is also a chemical change. Chemical Weathering – Set of processes that break down rock components and minerals Processes o Dissolution is the breakdown of a crystal lattice into individual ions, atoms or molecules and their transport into a solvent. An example is dissolving table salt in water. o Dissolution in rock weathering is found in limestone caves where water dissolves the limestone and deposits it to create the spikes. Types of Chemical Weathering o Oxidation – A chemical reaction where electrons are lost from one element. An example is rusting, exposing iron to oxygen and water. The oxygen is stealing an electron from the iron atoms, which will cause the iron to rust. Oxidation can also leave rusty stains on rocks. o Hydrolysis – A reaction of any substance with water in which the water is split + into its constituent ions (H and OH). An example of this is the weathering of feldspar into clay. Potassium feldspar exposed to water, the water will go through hydrolysis and will take out the potassium to leave behind an aluminum silicate, which is Calanite (spelling?). The aluminum silicate is bonded with the OH. o Spheroidal Weathering – There should be a diagram of this in your book. Water penetrates an extensively jointed rock, the chemical weathering decomposes minerals and enlarges the joints. The rocks are attacked more on corners and edges and take on a spherical shape. This weathering mainly attacks sharper edges on rocks. Rates of Weathering: Rock Composition o Take tombstones for example. A granite headstone that is four years older than a marble headstone will weather less than the marble. One can still read the granite, but the marble is worn away. o Relate back to the Bowen’s reaction series. The first to crystallize (ex. Olivine, Pyroxene, Calcium Feldspar, etc.) are the least resistant to chemical weathering while the last to crystallize is the most resistant (Potassium feldspar, Muscovite, Quartz). Rates of Weathering: Climate o Freezethaw cycles, frost wedging. o Temperature Cycles o Precipitation o Vegetation Cover Differential Weathering – Capstones out in the west, the cap is probably a sandstone and then the stone under it, which is probably shale, weathers away faster. Soil – a combination of mineral and organic matter that supports the growth of plants. Variables of Soil Formation – o Parent material o Time o Climate o Biological Components o Topography Soil Formation – o No soil development because of a very steep slope o transported soil developed on unconsolidated stream deposits o Residual Soil in developed on bedrock o Thicker soil develops on flat terrain Soil Terminology o Humus – Stable remains of decayed organic matter o Leaching – The dissolution of minerals from the upper layers of soil o Eluviation – Downward transport of particles in the soil o Solum – “True Soil”, includes O, A, E, and B horizons. These horizons indicate how mature the soil is. O is loose and partly decayed organic matter A is mineral water mixed with some humus E is the zone of eluviation and leaching B is the accumulation of clay transported from above C is the partially altered parent mineral (not soil, transition zone) And at the bottom is the nonweathered parent material (bedrock) In the textbook there is a table on the soil classification. We won’t be tested on this but it’s to show just how many soils there are and what each one is made of. Distribution of Soils. When looking at this map we can see the climate bands of the soil, similar climates will develop similar soils.
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