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: 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 growingTree roots

Honeycomb textureCharacteristic of a rock thatHas 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 Colorado RockiesRockies: Short : Short

ExposureExposure

AppalachiansAppalachians: : Long ExposureLong 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!)