GEOG 101 Week 6 Notes
GEOG 101 Week 6 Notes Geog 101
Minnesota State University, Mankato
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This 3 page Class Notes was uploaded by Hallie Notetaker on Wednesday October 5, 2016. The Class Notes belongs to Geog 101 at Minnesota State University - Mankato taught by Phillip Larson in Fall 2016. Since its upload, it has received 13 views. For similar materials see Introductory Physical Geography in Geography at Minnesota State University - Mankato.
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Date Created: 10/05/16
Weathering Weathering vs. Erosion Weathering – breakdown in place Erosion – moving pieces (dissolved or as fragments) First – weathering = decay in place Second – detach = break off Third – erode = move Sediment stages o Weathering o Erosion o Transportation via water, glaciers and wind o Deposition o Burial and compaction o Diagenesis Joints – Setting the Stage Joints – fractures that allow water to penetrate and weather the rock Many ways to make joints: o Cooling and contraction o Tectonic stresses o Columnar jointing – from contraction after lava flow cools or faulting/folding stresses the rock Joints can even fracture sedimentary strata from regional tectonic pressures Physical (Mechanical) Weathering Frost weathering o Body of water – top freezes first then water under pressure exerts stress Breaks rock along small fractures and large fractures (produces jagged alpine topography) Pressure release weathering o Erosion removes overburden and shell pop off as pressure is released o Glacial erosion is a great way to generate pressure release Salt weathering o Common in deserts and along coasts o Mechanics Salt crystal growth – extreme pressures in cracks and rock pores are caused by salt crystal growth from solution; varying causes of and extents to which salt growth occurs Thermal expansion – during temperature fluctuations, salts trapped in pores may expand to a greater degree than the surrounding rock minerals Hydration – the hydration of various salts causes expansion and contraction, pushing apart the silicate host minerals o Caverns – tafoni: larger cavernous forms Alveoli: smaller cavernous forms Thermal expansion/contraction o Abit of moisture and sudden heat makes the rock pop Wedging (fissuresols and calcrete) o Fissuresol can pry apart bedrock, mountain sides and giant boulders Wetting/drying Root pressure Chemical Weathering Physical weathering sets up chemical weathering Egyptian Obelisk – chemically weathered when brought to wetter environment; water matters Dissolution – rocks dissolve; limestone is best example Chemical weathering of carbonates o Easily soluble in water (especially with some acid present) Other rocks dissolve too, but slower than limestone Acid rain accelerates decay Oxidation Hydrolysis Hydration – water alters structure Complexation – metals released from primary minerals such as iron and manganese build complexes with organic components, such as fulfill acids and humid acids, causing an imbalance between cations and anions that leads to mineral decay Weathering Products Quartz sand – Quartz is one of the last minerals to decay; it survives weathering and erosion to be deposited in Nutrients released from mineral weathering o Calcium, sodium, magnesium, potassium Clays form – represent Earth’s ultimate decay of rock Weathering Landscapes Transport-limited landscapes – where the rate of transport (detachment and erosion) is smaller than the rate of weathering o Weathering > transport Weathering landscapes where o Transport > weathering The balance between weathering and erosion defines the landscape We see bedrock because weathering particles eroded away Humans can upset the balance and accelerate erosion o When transport (detachment and erosion) becomes faster than weathering, landscapes are not sustainable Granitic weathering landscapes o Consider a common rock – granitic rocks (granite, grandiosity, tonality, diorite, etc.) made up of interlocking minerals o Decay of weak minerals (biotite, feldspar) separates grains and makes granite sand called GRUS Most rapidly produced where joints intersect Core stones are made when corners of granite blocks weathered into grus Grus washes away easily with rain, leaving piles of core stones called tors Tors are very common in the Sonoran Desert o Granite weathering took a long time in the subsurface (groundwater) Spheroidal forms were then exposed by erosion of grus Dome forms produced the same way Granite that is not heavily joined becomes domes after grus washed away Limestone cave formation o Groundwater dissolves limestone, most aggressively at the water table o Groundwater follows lines of weakness in the limestone enlarging caves o When the water table drops stalactites and stalagmites can form on the roof and floor, respectively Water table usually drops when the stream has cut down to a lower level o Stalagmite – require lots of time with water table much lower
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