Weathering
Sedimentary CoverSedimentary Cover+ Sedimentary rocks form a veneer over much olderigneous and metamorphic “basement” rocks.+ This veneer varies in thickness from 0 to 20 km
Types of WeatheringWeathering+ Physical – Mechanical breakage, disintegration.+ Chemical *- Decomposition by reaction with water. Producesdissolved matter and an insoluble residue.+ Biological – Action of organisms often combines physical andchemical weathering processes.
+ Coffee metaphor for weathering:! Physical – Grinding coffee beans.! Chemical – Leaching coffee with hot water8 Generates dissolved material (coffee), and8 Depleted residue (coffee grounds).
Rock Disintegration
• + Weathering – Processes that
• break-up solid rock.
• ! Weathered Granite
• ! Fresh Granite
Rock disintegrates to form sediments which can form
sedimentary rocks.
Physical Weatheringaka mechanical weathering
-Mechanical breakup with no change in mineral composition.
-Often occurs with chemical weathering, except at climate extremes.
-Physical weathering dominates in polar and desert regions.
-Everywhere else, chemical weathering dominates.
+ Types of Physical Weathering-Jointing (release of overburden
pressure)-Frost Wedging-Root Wedging-Thermal Weathering-Alternate wet / dry
Jointing
• Rocks expand when rock erodes off of the top of a formation from reduction in pressure.
• + Rocks develop cracks called joints.
• + Igneous rocks crack in parallel layers,
Frost Wedging
+ Water enters a rock fracture, freezes in fractures.
+ Ice expansion wedges the fracture open.
+ During thaw, water infiltrates deeper into crack.
+ Repetition results in rock disintegration.
+ Dominant process in mountainous regions.
Frost wedging
-Rocks wedged loose
by freeze-thaw
accumulate as talus
at the base of steep
slopes.
- Talus (broken rock)
accumulate at the
bottom of the slope
Chemical weathering
-Breaks minerals into chemical components.
-Requires water “the universal solvent”.
-Significant in humid (water-rich) landscapes.
-Chemical weathering virtually absent in deserts.
- 3 major processes1. Dissolution2. Oxidation3. Hydrolysis
Types of Chemical WeatheringDissolution--Some minerals (halite, gypsum, calcite) dissolve.-Acidity (i.e. acid rain) enhances this effect.
Oxidation-- A reaction whereby a metal loses electrons.-Important in decomposing minerals.- Rusting is a familiar example of oxidation
Hydrolysis--Breaking (lysis) by water (hydro).-Water breaks apart elements that hold silicates together.
Alteration minerals (residues) such as-Clay minerals-Iron oxides (rust)
Biological weathering
• Mosses and plants release acids during decomposition that cause rock to deteriorate
Effects of weatheringUnstable minerals
decompose.
Rock volume is reduced.
Corners and edges attacked rapidly = spheroidal weathering.
Chemical Weathering in GraniteChemical Weathering in Granite
+ Biotite and feldspar weather to clay minerals.
+ These minerals are removed by running water.
+ Mineral removal causes the rock to crumble.
+ Quartz, resistant to weathering, is left over.
Weathering Rates
-Temperature and wetness extremesHot and Wet – Chemical weathering at a maximumCold and Wet – Physical weathering at a maximumHot and Dry – Physical weathering dominatesCold and Dry – All weathering at a minimum
-Biological effects – Active biota accelerates weathering.
-Geologic past – Rates of weathering have varied with atmospheric and biotic changes.
Differential weathering
- Rock masses do not weather uniformly.
-This results in many unusual and spectacular rock formations,
landforms and landscapes.
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