Chapter 14 Weathering and Erosion

Weathering ProcessesWeathering - the natural process by which atmospheric and environmental agents, such as wind, rain, and temperature changes, disintegrate and decompose rock at or near the surface.

There are two main types of weathering processes—mechanical weathering and chemical weathering.

Each type of weathering has different effects on rock.

Mechanical Weathering

Mechanical – produces smaller pieces, adds to effectiveness of chemical weathering, leads to development of talus slopes, does not change a mineral’s composition.

Mechanical Weathering

Mechanical weathering is strictly a physical process and does not change the composition of the rock.Common agents of mechanical weathering are ice, plants and animals, gravity, running water, and wind.Physical changes within the rock itself affect mechanical weathering.

Types of Mechanical Weathering

Ice wedging – water penetrates into cracks in rock and freezes.

When the water freezes, its volume increases by about 10% and creates pressure on the surrounding rock.

This process eventually splits the rock apart.Must have:

• Adequate moisture• Cracks in rocks• Freeze/thaw cycles

Ice Wedging

Thermal expansion

Repeated daily heating and cooling of rock;

Heat causes expansion; cooling causes contraction.

Expanding and contracting causes cracks.

Abrasion

Is the grinding and wearing away of rock surfaces through the mechanical action of other rock or sand particles

Abrasion is caused by gravity, running water, and wind.

Organic Activity

Plants and animals are important agents of mechanical weathering.Biological wedging – plant roots penetrate into cracks causing cracks to widen.

Must have:• Climate hospitable for plants• Adequate moisture and

temperature

Biological Wedging

Root wedging widens fractures in rock.

Organic Activity

Digging and burrowing activities.Earthworms and other animals that move soil expose new rock surfaces to both mechanical and chemical weathering.

Chemical Weathering

Chemical alteration of minerals.Chemical reactions commonly occur between rock, water, carbon dioxide, oxygen, and acids.Most effective in a warm, humid climate.Chemical weathering changes both the composition and physical appearance of the rock.

Types of Chemical Weathering

Ion Exchange – H+ replaces other cations.

Oxidation - reaction in which elements gain or lose electrons (example: rust).

Oxidation commonly occurs in rock that has iron-bearing minerals, such as hematite and magnetite.

OXIDATION

ROCKS / MINERALS CHEMICALLY REACT WITH THE OXYGEN IN

THE ATMOSPHERE, CAUSING A

DECOMPOSITION ON THE MATERIALS

Types of Chemical Weathering

Hydrolysis - any reaction in which water participates.Water plays a crucial role in chemical weathering.Minerals that are affected by hydrolysis often dissolve in water.Water can then carry the dissolved minerals to lower layers of rock in a process called leaching.

Dissolution - mineral completely dissolves, leaving only ions in solution.

Chemical WeatheringThe image below shows how water plays a crucial role in chemical weathering.

Carbonation

Is the conversion of a compound into a carbonate

When carbon dioxide, CO2, from the air dissolves in water, H2O, a weak acid called carbonic acid, H2CO3, forms.

H2O + CO2 H2CO3

Acid Precipitation

Such as rain, sleet, or snow, that contains a high concentration of acids, often because of the pollution of the atmosphere

Acid precipitation weathers rock faster than ordinary precipitation does.

Rainwater is slightly acidic because it combines with small amounts of carbon dioxide.

Factors influencing Weathering Rates

The processes of mechanical and chemical weathering generally work very slowly.Rock Structures – chemical/mineral composition, Physical featuresTopographyClimate

Spheroidal Weathering

Spheroidal Weathering

Differential WeatheringRocks weather at different rates.

Softer rocks are less resistant to weathering.

Amount of ExposureSurface Area

Both chemical and mechanical weathering may split rock into a number of smaller rocks.

The part of a rock that is exposed to air, water, and other agents of weathering is called the rock’s surface area.

As a rock breaks into smaller pieces, the surface area that is exposed increases.

Greater exposure on smaller pieces

Climate

Climates with alternating periods of hot and cold weather allow the fastest rates of weathering.In warm, humid climates, chemical weathering is also fairly rapid. The slowest rates of weathering occur in hot, dry climates. Weathering is also slow in very cold, dry climates.

Topography

Ice wedging is more common at high elevations than at low elevations.

On steep slopes weathered rock fragments are pulled downhill by gravity and washed out by heavy rains.

New surfaces of the mountain are continually exposed to weathering.

Soil Horizons

Bedrock

Horizon O

Erosionerosion a process in which the materials of Earth’s surface are loosened, dissolved, or worn away and transported from one place to another by a natural agent, such as wind, water, ice, or gravity

When rock weathers, the resulting rock particles do not always stay near the parent rock.

Various forces may move weathered fragments of rock away from where the weathering occurred.

Soil Erosion

Ordinarily, new soil forms about as fast as existing soil erodes.

Some farming and ranching practices increase soil erosion.

Soil erosion is considered by some scientists to be the greatest environmental problem that faces the world today.

Gullying and Sheet Erosion

Gullying is caused by furrow plowing.

Eventually land that is plowed in this way can become covered with deep gullies.

Sheet erosion is the process by which water flows over a layer of soil and removes the topsoil

Wind also can cause sheet erosion during unusually dry periods.

Results of Soil Erosion

Constant erosion reduces the fertility of the soil by removing the A horizon, which contains the fertile humus.

The B horizon, which does not contain much organic matter, is difficult to farm because it is much less fertile than the A horizon.

Without plants, the B horizon has nothing to protect it from further erosion.

All the soil layers could be removed within a few years by continuous erosion.

Soil Conservation

Certain farming techniques can increase the rate of erosion.

Land clearing accelerates topsoil erosions.

Soil erosion can be prevented by soil conservation methods

Soil Conservation

Contour Plowing: soil is plowed in curved bands that follow the contour, or shape of the land.

Strip-Cropping: crops are planted in alternating bands slowing the runoff of rainwater.

Terracing: construction of steplike ridges that follow the contours of a slope

Crop Rotation: farmers plant one type of crop one year and a different type of crop the next.

An earthflow on a newly formed slope

Some visible effects of creep

End of Chapter 14