Ch 14Weathering

Two Types:

1.Physical or Mechanical

2.Chemical

Mechanical Weathering

• Rock is broken down into smaller pieces by physical means.

• Agents: wind, water, ice, gravity, plants & animals.

EXFOLIATION

• Rocks break into curved sheets that peal away

Ice Wedging

• Water gets into cracks and freezes

• Freeze/Thaw cycle splits rock

ABRASION WIND

WATER

ICE

Wearing away of rocks by other rock particles

ROOT WEDGING

Animals - Burrowing

Chemical Weathering

• Rock is broken down by chemical interaction with the environment.

OXIDATION (rusting)• Elements combine

with oxygen to form rust

• Iron bearing rocks

HYDROLYSIS: Minerals decompose in water as hydrogen replaces cations infeldspar (turns into clay).

HYDROLYSIS: Minerals dissolve in water.

LEACHING: Water carries dissolved minerals away,forms beds of aluminum(bauxite)

CARBONATION: Limestone caves

Carbon Dioxide and water react to form carbonic acid...Carbonic acid “eats” limestone.

Acid Precipitation

• Acids formed from the burning of fossil fuels combining with atmospheric water; bad for plants & structures.

Factors that affect Rates of Weathering

DIFFERENTIAL WEATHERING

Softer, less-resistant rock wears away, leaving harder resistant rock behind.

Bryce Canyon “Hoodoos”

Rock composition

• Sedimentary rocks containing calcites weather rapidly (limestone).

• Quartz containing rocks are strong and weather slowly

Surface area

The more surface area per volume, the faster the rock’s weathering rate.

Fractures and Joints

• Increase surface area of rock

• Allow weathering to happen faster

Climates

• Hot/Cold climates with freeze/thaw cycles have fastest weathering rates.

• Hot/Dry climates weather the least.

Topography

• Elevation or slope of the land.

• High Elevations – cold temp, ice wedging

• Steep Slopes – gravity pulls rock down

Human Activities

• Mining

• Construction

Plants and Animals

• Roots, Burrowing, Waste (Bat Guano)