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Transcript
Weathering and Erosion Painted Desert, Arizona Weathering Vs. Erosion • Weathering – The general process in which rocks are broken down at the Earth’s surface – Mechanical – rocks broken w/o changing composition – Chemical – minerals are altered or dissolved • Erosion – Move weathered materials under influence of gravity on grain-by-grain basis Intertwined Processes • Weathering breaks rocks down • Erosion carries away loose material exposing new, fresh surface to weathering Mechanical Weathering: PressureRelease Fracturing • Igneous and meta rx generally form deep in the crust • Tectonic forces may cause rocks to rise forming a mtn • As mtn erodes, less pressure on stuff beneath it • Rx expand due to lower pressure--fractures Mechanical Weathering: Wedging • Frost wedging • Can also occur when – Salt water seeps into cracks and evaporates—haloclasty • Or when – Tree roots=wedge— organic activity Rocks wedged apart by growing Tree roots Honeycomb texture Characteristic of a rock that Has undergone haloclasty Mechanical Weathering: Abrasion • Weathering of rocks by friction and impact • Actions make jagged edges round • Natural sandblasting in desert environments—weird shaped rocks Mechanical Weathering: Thermal Expansion • Occurs only in areas with dramatic diurnal temperature variation – i.e. deserts • Outside of rock cools/heats more quickly than interior— may cause fractures to form Chemical Weathering • Dissolution: dissolving stuff in water • Ex: Halite – Polar water molecule – Water can pull apart ionic bonds Acids and Bases • Acidic solution – High concentration of H+ • Basic solution – High concentration of OH- • Acids and bases dissolve minerals faster than water because they have more H+ and OH- ions to pull atoms away from minerals • Acid rain What Governs Weathering? Properties of the Parent Rock • Different minerals weather at different rates • Different solubilities – The extent to which their minerals will dissolve in water Properties of the Parent Rock • Type of rock and structure – Determines rate of P and C. weathering Ex. Niagara Falls Climate • Amount of rainfall and temperature • Water is the best agent for dissolving – Can hold lots of dissolved ions • Chemical weathering – Fastest where water abundant, hot (humid) – Slower in dry, hot climates – Slowest in cold climates because water frozen, inert Soil • Fragments of rock, clay formed by altering minerals, and organic matter – Can trap rainwater, host vegetation, bacteria, and organisms – Create an acidic environment which speeds weathering – Roots and organisms create fractures • A positive-feedback process Length of Exposure • Obviously, the longer the exposure time, the more the weathering – But dependent on other factors Colorado Rockies: Short Exposure Appalachians: Long Exposure Surface Area vs. Volume • The more surface area compared to volume, the faster that part will weather Works Much Like Coffee • Add hot water to coffee beans – Water can only affect the surface of the beans – Smaller beans means more reactions (Hooray!)