WEATHERING

GEOLOGY NOTES

Chapter 5

WEATHERING

Breakdown of rock due to weather

no visible motion

MECHANICAL (PHYSICAL) WEATHERING

Disintegration

Breaks into small pieces

NO visible motion

MECHANICAL WEATHERING

Ice (frost wedging)–water expands

when freezes– angular pieces

MECHANICAL WEATHERING

Sheeting(exfoliation)–pressure is removed

from above–Fractures “peel” off

Half-dome

MECHANICAL WEATHERING

Other types– burrowing animals– Plant roots– Mud cracks

Mudcracks

When water evaporates- mud shrinks

CHEMICAL WEATHERING

Decomposition

chemical change takes place

CHEMICAL WEATHERING

Hydrolysis reacts with acid/water to form clays

feldspar + carbonic acid + water clay + soluble minerals

common in granite

CHEMICAL WEATHERING

DISSOLUTION

ANY CARBONATE + CARBONIC ACID CALCIUM CARBONATE (soluble)– common in limestone– creates caves and sinkholes

Parthenon, Greece

sinkholes

Sinkhole formation

Areas prone to sinkholes

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Marble statue

Dissolution

CHEMICAL WEATHERING

OXIDATION–reacts with O2

–Groundwater with Fe is powerful agent

–minerals with high Fe content• hematite, limonite, olivine• intense in tropical climates

Oxidation of basalt

Pictured Rocks, MI

PLANES OF WEAKNESS

WHERE ROCKS WILL BREAK– Joints- vertical fractures–bedding planes- horizontal fractures–Granular disintegration- cement in clastic

rocks breaks down

joints

BEDDING PLANES

Granular disintegration

ROCK TYPES

Granite - exfoliation, hydrolysis (k-spar to clay)– quartz doesn’t weather

Basalt- chemical (oxidation) Sandstone- granular disintegration limestone- dissolution shale- breaks down into clay

Sandstone tombstone

Granite

Which rocks weather most rapidly?

sedimentary

Which rocks are most resistant to weathering? Igneous / metamorphic (exception?)

Bowen’s reaction series– Olivine most

Differential weathering

Different rock types weather differently

OR

Different rates of weathering due to exposure

Stairway to heaven, CO

Bryce Canyon, UT

arches

                                                                                                                                                                                         

Differential erosion

limestoneshale

Hoo-doos

CLIMATE AND WEATHERING

Chemical- warm and moist

Physical- freeze/thaw cycle

CLIMATE

TROPICAL– Strong chemical

DESERT– Slight mechanical

POLAR– Slight mechanical

MID LATITUDE (MI)– Strong chemical and mechanical