weathering and erosion painted desert, arizona. weathering vs. erosion weathering –the general...
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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!)