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.