Weathering

Chapter 7.1

Earth-EnvironmentalWZPP

WZPP ees 7.1 2

Introduction

Read page 153 on Cleopatra’s needle and see figure 7-1

WZPP ees 7.1 3

Weathering of Cleopatra’s needle. The photograph taken to the left in Egypt before 1880, on the right was taken in New York in 1918 (courtesy of the Metropolitan Museum of Art)

WZPP ees 7.1 4

Introduction

Weathering Is the process by which rocks near the earth’s

surface break down and change Erosion

The removal and transport of weathered material

These processes have been going on for a long time

WZPP ees 7.1 5

Mechanical Weathering

Materials change size or shape Breakdown of material into smaller pieces

Holly LakeTetons NP

J B Photos

Backcountry lakeTetons NPJ B Photos

WZPP ees 7.1 6

Mechanical Weathering

TemperatureWater freezes and expands by 9%Exerts pressure on the rockFrost wedging

Causes potholes

WZPP ees 7.1 7

Mechanical Weathering

PressureWhen rock is deep underground and is

exposed at the surface, the pressure of the rock is reduced

Rocks break apart like layers in an onion (exfoliation)

Example: Half Dome in Yosemite NP

7.1a ws

WZPP ees 7.1 8

Mechanical Weathering

PressureRoots wedge themselves into cracks and

exert pressure as they grow

WZPP ees 7.1 9

Chemical Weathering

Rocks and minerals undergo changes due to chemical reactions

Chemical reactions form new minerals and release dissolved substances

WZPP ees 7.1 10

Chemical Weathering

Composition of the rock determines the effects of chemical weathering

WZPP ees 7.1 11

Chemical Weathering

Temperature determines the rate of chemical weathering Increase temperature means increased

chemical weathering

WZPP ees 7.1 12

Chemical Weathering

Agents of erosion:Water OxygenCarbon dioxideAcids

WZPP ees 7.1 13

Chemical Weathering

WaterDissolves many kinds of minerals and rockServes as a medium through which other

reactions occurs

WZPP ees 7.1 14

Chemical Weathering

Oxygen Combines with other substances Iron (& other minerals) readily combine with

atmospheric O2

WZPP ees 7.1 15

Chemical Weathering

Carbon Dioxide CO2 + H2O weak carbonic acid

Reacts with minerals (like calcite) in limestone, marble to dissolve rock

7.1b ws

WZPP ees 7.1 16

Chemical Weathering

Acid precipitation Caused mainly by oxidation of sulfur dioxide

and nitrogen oxide in atmosphere due to human activities

Is precipitation below 5.5 pH Is harmful to living organisms

WZPP ees 7.1 17

Rate of Weathering

Climate has a major influenceVariables include

Precipitation Temperature Evaporation

Interaction between temperature and precipitation is the greatest effect

WZPP ees 7.1 18

Rate of Weathering

Chemical weathering occurs readily in areas that have warm temperatures, abundant rainfall and lush vegetation

WZPP ees 7.1 19

Rate of Weathering

Physical weathering occurs readily in cool, dry climates Is highest where water undergoes repeated

freezing and thawing Remember, cool temperatures inhibit (slows)

chemical reactions

WZPP ees 7.1 20

Rate of Weathering

Harder rock is going to be more resistant to weathering than soft rockThis depends type of rock and their

composition (what they are made of) Sedimentary rock is generally going to be

more easily weathered than igneous or metamorphic

WZPP ees 7.1 21

Rate of Weathering Surface area

Increased surface area equals increased weathering

Both physical and chemical

WZPP ees 7.1 22

Rate of Weathering

Topography The steeper the slope, the greater tendency

for a material to move downhill

Indian Paint brush canyonTetons NPJ B Photos

WZPP ees 7.1 23

Rate of Weathering

OrganismsDecaying organic matter and plant roots

release carbon dioxide for chemical weathering

7.1c ws