WEATHERING and EROSION Changes at the Earthıs Surface Physical - a reduction in grain size without an accompanying change in chemical composition Chemical - chemical changes as Earth materials react at the surface in contact with a reactive atmosphere Chemical Weathering CO2 and Oxygen are two highly reactive components in the atmosphere H2O + CO2 = H2CO3 [carbonic acid] H2CO3= HCO3 + H+ pH is a measure of the concentration of the hydrogen ion - as pH goes up, the concentration of H+ goes down....pH=7 is neutral and pH = 6 is weak acid. The worldıs oceans have a pH of about 8 - a weak base Rainwater has a pH of about 6 - a weak acid Some elements, like Fe can exist in more than one oxidation state....Fe metal, Fe2+ (has lost two electrons) and Fe3+ (has lost three electrons). Some components (like Oxygen) are good oxidizing agents and remove electrons. Other components produce a reducing environment and supply electrons Clay Minerals Kaolinite - stable geologic environment ... all of the easily oxidized materials have been leached away. Illite - like mica but some potassium has been leached away Expandable clays - will accept water molecules into their structure Organic matter (tissue, vegetation, etc.) breakdown rapidly if the environment is oxidizing. To preserve organic matter (to eventually produce coal and hyrdocarbons) the environment must be reducing....an environment which does not have a strong oxidizing agent present Some material, like calcium carbonate is soluble in a weak acid; others, like quartz are soluble in a base. Stability of Earth Materials Bowenıs Reaction Series is a good guide High temperature minerals break down more rapidly than low temperature minerals Thus, quartz is chemically stable at the Earthıs surface whereas olivine alters rapidly With the exception of quartz, most common minerals react to form clay minerals Physical Weathering Changes in size but not in composition Decrease in size is accompanied by an increase in surface area Chemical reactions begin at surfaces - therefore, an increase in surface area will speed up chemical changes Abrasion history - usually use quartz Particle Sizes > 2 mm : pebbles, boulders 1/16 to 2 mm : sand 1/256 to 1/512 mm : silt < 1/512 mm : clay