WeatheringCHAPTER 3

HONORS EARTH SCIENCE

Def: the breaking apart of rock over long periods of time and without obvious motion.

◦ Mechanical (Physical)- break into smaller pieces

◦ Chemical- chemical change in composition

Weathering

Weathering occurs at a slower rate than erosion.

Erosion involves obvious motion when breaking down rock

WEATHERING vs. EROSION

Cause by water freezing Common in high elevations and mid

latitudes (Michigan)

Physical Weathering1. FROST (ice)WEDGING

Bryce Canyon, utah

Potholes

Removing overburden relieves pressure Outer layer peels off like onion skin

Occurs mainly in intrusive igneous rocks◦ batholiths

Exfoliation (sheeting)

Half-dome; Yosemite

3. Mudcracks

When water evaporates- mud shrinks

Wet/dry climates

Septarian nodule;

BIOLOGIC ACTVITY

Lichen secrete aciddissolve rock

Tree roots- growth breaks rocks

Humid climates Usually sed. rock

Change in composition (decomposition)

Chemical weathering

Water + carbon dioxide = carbonic acid H2O + CO2 H2CO3

Acid reacts (dissolves) with calcite (limestone, marble)

May create caves and sinkholes

Dissolution

sinkholes

Sinkhole formation

Areas prone to sinkholes

Sinkhole in Corvette Museum, 2014

http://av.rds.yahoo.com/_ylt=A9ibyKddoLVHFNEAH09vCqMX;_ylu=X3oDMTBvMmFkM29rBHBndANhdl9pbWdfcmVzdWx0BHNlYwNzcg--/SIG=12f4ctvf1/EXP=1203171805/**http:/detroit.metblogs.com/archives/2006/01/sinkhole.phtml

Guatemala, Sinkhole

MI sinkholes (near Alpena)

Marble statue

Dissolution

where iron minerals produce iron oxide ("rust").

Often in ss from iron in groundwater OR mafic rocks

Fe + O2 (atm) Iron Oxide

Fe2O3

CHEMICAL WEATHERING

2. Oxidation

Result- reddish stain

Pictured Rocks, MI

Hydrolysis Reaction with water

feldspar + carbonic acid +water clay + salt

common in granite

Result- holes in granite

CHEMICAL WEATHERING

Water enters cracks in rock and weathers the edges and creates a rounded shape

Any rock type Result- rounded spheres

Spheroidal weathering

t

Spherodial weathering

Time- more time; more weatheringRock Type/Resistance

◦ Sedimentary least resistant- esp. calciteClimate

◦ Wet- chemical dominates◦ Freeze/thaw- physical dominates

Surface area

Factors that affect Weathering

Smaller pieces (more surface area)= more weathering

Surface area

Climate

Cleopatra’s Needle

(Egypt)

3,000 years old

Cleopatra’s Needle

(New York)

100 years old

Not all rocks weather equally

Differential weathering

Different rock types weather differently

OR

Different rates of weathering due to cracks

Differential weathering

Arches National Park

                                                                                                                                                                                         

Differential erosion

limestone shale

Hoodoos-

Tall spires of rock Often “balancing” Softer rock (shale) erodes faster than cap

rock

hoodoos

Thoughts?

Honeycomb weathering