WEATHERINGWeathering, Soil, and Erosion

2011-2012

WEATHERING DEFINITIONS

Weathering – the breakup of rock due to exposure to processes that occur at Earth’s surface.

Mechanical weathering Also called disintegration. Takes place when rock is split or broken into smaller

pieces of the same material without changing its composition.

Chemical weathering Also called decomposition. Takes place when the rock’s minerals are changed

into different substances.

MECHANICAL WEATHERING – ICE AND WATER

Water occupies 10% more space when it freezes puts a lot of pressure on the container it’s in.

Frost wedging – water freezes in the cracks in rocks and pushes the rock apart.

Occurs in areas where the temperature varies from below freezing (0°C) to above freezing.

MECHANICAL WEATHERING – ICE AND WATER

Repeated wetting and drying can break up rocks that contain clay (i.e. shale). Clay absorbs water easily and swells when wet. Clay shrinks as it dries. Repeated swelling and shrinking causes the

rocks to break apart.

MECHANICAL WEATHERING – ABRASION

Rocks are moved by water, wind, and ice.

Abrasion occurs when moving rocks grind and scrape against each other, wearing the rock materials away.

The sand on the beach is a result of abrasion.

MECHANICAL WEATHERING – PLANTS AND ANIMALS

Plant roots can wedge into pores and crevices in rocks.

As the roots grow larger, they push out on the rocks and cause them to split.

Burrowing animals and insects dig holes in the soil, allowing air and water to reach bedrock and weather it.

MECHANICAL WEATHERING – UPWARD EXPANSION

Exfoliation – the peeling of surface layers from exposed bedrock.

Example: Granite is lifted up and the rocks above it are worn away. The rocks form sheets and break off as seen in the photo.

http://0.tqn.com/d/geology/1/0/H/L/exfoliation.jpg

CHEMICAL WEATHERING – WATER

Hydrolysis The chemical weathering by reaction of water

with other substances. When some minerals are exposed to water, they

dissolve into ions. These ions slowly react with the water and form

clay minerals.

CHEMICAL WEATHERING – WATER

Carbonic acid Weak acid that is formed when carbon dioxide

dissolves in rainwater. Seeps into the ground and reacts chemically with

many common minerals. Example: Carbonic acid reacts with calcite,

causing it to dissolve completely. This dissolving has formed underground caverns.

CHEMICAL WEATHERING – WATER

Acid rain Sulfur dioxide, nitrogen compounds, and carbon

dioxide react with water in the air. Causes structures made of concrete, stone, and

metal to wear out more quickly.

CHEMICAL WEATHERING – OXYGEN

Oxidation The chemical reaction of oxygen with other

substances. Very effective at weathering minerals that have

iron in their chemical formulas. The formation of different types of rust (iron

oxides).

RATES OF WEATHERING – SURFACE EXPOSURE

The rate at which a rock weathers is affected by the amount of surface that is exposed to chemical weathering processes.

The more of a rock that is exposed, the faster it will weather.

RATES OF WEATHERING – COMPOSITION OF ROCK

How fast or how slow a rock will weather depends on what it is made of.

Rocks with layers of different compositions will wear at different rates.

RATES OF WEATHERING – CLIMATE

Wet climates are conducive to both chemical and mechanical weathering processes.

Regions with cold or dry climates are more conducive to mechanical weathering.