WEATHERING, EROSION,

AND SOILThe Walker School

Geology

Weathering vs. Erosion

Weathering the decomposition of earth rocks, soils

and their minerals through direct contact with the

planet's atmosphere. Weathering occurs in situ, or

"with no movement", and thus should not to be

confused with erosion, which involves the movement

and disintegration of rocks and minerals by agents

such as water, ice, wind, and gravity.

Formation of the Grand Canyonhttp://www.youtube.com/watch?v=ktf73HNZZGY

Debris flows

shown in this clip

erode rock along

the walls of the

canyon.

Arches National Parkhttp://www.nps.gov/arch/National

Fig. 6-CO, pp. 168-169

Erosion takes

place at different

rates – called

differential erosion

Produces:

hoodoos, spires,

arches, and

pedestals

Hoodoos, Bryson Canyon National Parkhttp://www.nps.gov/brca/

Types of Weathering

Mechanical

Chemical

Biological

Fig. 6-1a, p. 170

Weathering of Granite

PHYSICAL WEATHERING

Physical Weathering

Mechanical or physical

weathering involves the

breakdown of rocks and

soils through direct

contact with atmospheric

conditions such as heat,

water, ice and pressure.

Badlands, SD

Abrasion

Talus at the base of Rocky Mountains in Canada

The primary process in

mechanical weathering

is abrasion (the

process by which clasts

and other particles are

reduced in size).

Physical Processes of Weathering

Frost Action

Pressure Release

Thermal Expansion and

Contraction

Salt Crystal Growth

Activities of Organisms

Frost Action

Fig. 6-3b, p. 172

Frost Heaving in New York City

Pressure Release (Unloading)

Exfoliation at Stone Mountain, Georgia

Granite (igneous

rock) crystallizes far

below the surface, so

when it is uplifted

and the overlying

material is eroded, its

contained energy is

released by outward

expansion.

Thermal Expansion (Exfoliation)

Fig. 6-4a, p. 173

Slabs of granitic

rock bounded by

sheet joints in the

Sierra Nevada, CA

Rock is a poor

conductor of heat, so

its outside heats up

more than its inside;

the surface expands

more than the

interior.

Often occurs in areas, like

deserts, where there is a large

diurnal temperature range.

Salt Weathering (haloclasty)

Mechanical

Derives from an external source

(capillary rising ground water,

eolian origin, sea water along

rocky coasts, atmospheric pollution).

Favored by dry conditions in arid

climates.

The expanding salt crystals exert a

pressure on the walls of the rock

pores that exceeds the tensile

strength of the rock.

Marine Abrasion of Granite.

BIOLOGICAL WEATHERING

Biological Weathering from Plants

Fig. 6-6b, p. 174

Trees and other plants in

Lassen Volcanic National

Park, CA help break down

parent material into smaller

pieces and contribute to

mechanical weathering.

CHEMICAL WEATHERING

Chemical Weathering

Chemical weathering, involves the direct effect of

atmospheric chemicals, or biologically produced

chemicals (also known as biological weathering), in

the breakdown of rocks, soils and minerals.

Organisms

Fig. 6-6a, p. 174

Lichens are part

fungi and part

algae. They derive

their nutrients from

the rock and

contribute to

chemical

weathering.

Decomposition of Earth’s Materials

Dissolution (minerals dissolve in water,

limestone dissolves to form caves)

Hydrolysis (hydrogen ions in water

dissociate and attack minerals in rocks, ex.

forms hard water)

Oxidation (minerals react with oxygen, ex.

formation of rust)

Factors Affecting Chemical Weathering

Presence of Fractures

Particle Size

Climate

Parent Material

Granite rocks in Joshua Tree

National Park, CA. Chemical

weathering is more intense along

fractures.

Mechanical and Chemical

Fig. 6-10, p. 180

Mechanical weathering speeds chemical weathering by increasing

the surface area of the rock.

Temperature and Chemical Weathering

Fig. 6-11, p. 180

Chemical

processes proceed

more rapidly at

high temperatures

and in the

presence of

liquids.

Chemical

weathering can

extend to

depths of 10s of

meters in the

tropics, but only

a few inches in

arid or cold

climates

Table 6-1, p. 180

Stability is the

opposite of

Bowen’s reaction

series.

SOIL AND ITS ORIGINS

Regolith

A collective

term for

sediments.

Soil Production through Weathering

Fig. 6-1b, p. 170

Soil Production through Infiltration

Soil Composition

Fig. 6-14a, p. 183

Soil Profile

Fig. 6-14b, p. 183

Parent material, in soil

science, means the

underlying geological

material (generally bedrock

or a superficial or drift

deposit) in which soil

horizons form.

Variables in Soil Production

parent material

time

climate

atmospheric composition

topography

organisms

Climate and Soil Formation

Fig. 6-15, p. 184

Pedocal

Fig. 6-16a, p. 185

Pedalfer

Fig. 6-16b, p. 185

Laterite

Fig. 6-16c, p. 185

Soils in the Field

Fig. 6-18a, p. 186

Shows Laterite in

Madagascar, a

deep red soil that

forms in

response to

intense chemical

weathering.

12 Soil Typeshttp://soils.ag.uidaho.edu/soilorders/orders.htm

World Soils

U.S. Soil Orders

Most of the World’s Soils are Under Threat

38% of the

world’s

croplands have

serious topsoil

erosion.

Activities of Organisms

Churn soil; Break

down nutrients;

Provide

pathways for

gases to escape

SOIL PROBLEMS

Soil Expansion

Fig. 6-21b, p. 188

$6 billion in damage a

year to foundations,

roadways, sidewalks

and other structure.

Erosion

Fig. 6-22a, p. 189

Rill

Gully

Stalinization

Stunts Crop Growth;

Lowers Crop Yields;

Eventually Kills Plants;

Ruins the Land

Slash and Burn Agriculture

Fig. 6-19a, p. 187

Practices deplete tropical

soils of nutrients for

agriculture.

Desertification

70% of the world’s dry

lands are suffering.

Causes and Consequences of Desertification

Soil Conservation

Table 6-2, p. 190