Weathering and Erosion Painted Desert, Arizona

Weathering Vs. Erosion Weathering Erosion 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: Pressure-Release 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 Basic 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!)