Rocks and Weathering

I. Weathering A. The process that breaks rocks

down on Earth’s surface. B. Erosion - moves rock particles

away from original location.

II. Mechanical Weathering A. Physically breaks rock into smaller

pieces. B. Abrasion

1. Rock particles grinding against other rock due to wind or water.

C. Heating and Cooling 1. Sun or forest fire can

heat the outside of a rock more quickly than the inside. This causes it to flake off in layers.

D. Ice Wedging (freezing and thawing) 1. When water freezes

inside a crack in the rock, it expands (ice is less dense than water, remember?) and pushes the rock apart.

E. Plant Growth 1. Roots of plants can enter a crack in a

rock. As the roots grow, they force the rock apart.

F. Animal Actions 1. Burrowing animals (moles, gophers,

prairie dogs, insects) loosen and break apart rocks in the soil.

III. Chemical Weathering A. The process that breaks down

rocks through chemical changes. B. Water

1. Water can dissolve many rocks or certain minerals in the rock.

C. Carbon Dioxide 1. CO2 becomes dissolved in rainwater as

it soaks into the ground. It becomes carbonic acid. This easily weathers limestone and marble. (marble is the slide background)

D. Oxygen 1. Oxygen reacts with iron when water is

present and rust (iron oxide) forms. Rocks that contain iron turn a reddish color. (This is why Mars is red, it has a lot of iron oxide in the soil.)

E. Living Organisms 1. Plant roots and lichens secrete acids

that chemically weather rock.

F. Acid Rain 1. Burning oil, coal, and gas produces

pollution. These nitrogen and sulfur compounds mix in clouds and causes rainfall to be more acidic. This can chemically breakdown rocks and damage statues and gravestones.

IV. Rate of Weathering A. Type of Rock

1. More permeable, softer rock weathers faster.

2. Limestone and marble weather fast.

3. Granite weathers slowly. B. Climate

1. Weathering occurs faster in warm, moist climates.

2. Cool, dry climates are where weathering is slowest.