WEATHERING AND EROSION Chapter 12

WEATHERING

• The process which breaks rocks into smaller fragments, eventually producing soil.

Two types• 1.Mechanical – (Physical) A

process that breaks rocks apart without changing their chemical composition.

• 2. Chemical – occurs when water, air, and other substances come into contact with a rock. This will change its chemical composition!

• Mechanical

• Frost action – occurs when water freezes in cracks, expands, and breaks rocks apart.

• Abrasion – occurs when rocks come into contact with one another and smooth off jagged edges.

• Root pry – occurs when plant roots grow in a crack and cause rock to break.

• Exfoliation – rocks breaking off in sheets due to expansion of underlying rocks.

• Chemical Weathering• Hydrolysis – water

combines with minerals in rock which break it down into new materials.

–Example: Orthoclase weathers to form clay.

Acids• Co2 and So2 combine with

rain to form weak acids.

• Organic acids produced by decaying matter of Lichens.

• Acids completely dissolve some rocks, change others to clay.

Acid Rain

Lichen - a symbiosis between algae and fungus. Secretes acid. It is the

most important producer of soil from rock.

• Oxidation (rusting) - occurs when metals in the rock come into contact with oxygen in the air.

• * Iron turns red

• * Aluminum turns black

• * Copper turns green

Rates of Weathering• Surface exposure – more exposed

surface area means faster weathering.

• Composition of rock affects its rate of weathering

• Shale – made of mud – Fast

• Sandstones, Quartzites – quartz – Slow

Effects of Climate• Wet climates tend to speed up weathering

– Good for chemical and mechanical weathering

– Dry Climates weather rocks mores slowly

– Mostly Mechanical weathering

– Limestone is very durable in dry climates but weathers rapidly in wet areas.

Differential weathering

• difference in weathering of rock layers in the same rock formation. Easily weathered rock will form a gentle slope, while resistant rock will form vertical cliffs.

Grand Canyon

Badlands

Bryce Canyon

Soil • loose weathered rock and

organic material where plants with roots can grow.

• (80% rock, 20% organic material)

• Three basic sediment sizes are silt, sand, and gravel.

• Silt = < .063mm

• Sand = .063 – 2mm

• Gravel = 2 – 16mm

• Pebble = 16 - 64mm

• Cobble= 64 – 256mm

• Boulder= > 256mm

• Parent material - material from which a soil is formed.

• Residual soil - soil that has original bedrock as its parent material.

• Transported soil - bedrock that has been covered by deposits left by wind, water, and ice.

• Leaching -the downward movement of minerals and chemicals through the soil by water.

• Soil profile - cross section of earth exposed by digging through soil layers until parent material is reached.

A soil profile has three horizons

• Three Soil Horizons A, B, + C.

• 1. A Horizon - Topsoil

• a. Gray to black.

• b. Some sand - little clay.

• c. Contains humus.

• Humus - organic material formed by decaying plant and animal material.

• 2. B Horizon - Subsoil• a. Red to brown from iron oxides that

formed in A horizon and leached down.

• b. May contain water soluble minerals CaCO3 and MgCO3

3. C Horizon - parent material

• slightly weathered parent material and rock fragments.

• a. Under C is unweathered bedrock

Soil as a resource• Soil Fertility – ability of soil to grow plants

• Soil depletion – soil loses nutrients and fertility

• Soil Conservation

– Windbreaks, terraces, etc.

• Irrigation - Adding water to soil

• Salinization – deposits of salt left behind after irrigated water evaporates

Windbreak

Terrace

Contour Farming

• Tilling and planting across the natural slope creates a series of dams which redirect and slow runoff. This allows water to soak into the ground or to flow gently between the rows to grassed headlands at the edge of the field or to grassed waterways within the field. On short slopes, cross-slope farming can reduce erosion by up to fifty percent.

Strip Cropping

Dust Bowl

Irrigation

Salinization

MASS MOVEMENTS

–– the downhill movement of soil and rocks (sediments) due to the force of gravity.

–Dry soils or rock moves quickly downhill.

Landslide

• TALUS – fallen material that collects at base of hill.

Creep-Soil or rock soaked with water move slowly downhill.

Slump• Soil moves downhill leaving a curved

surface behind

Earthflow – soil or rock soaked with water move at a noticeable rate. Ex. Mudflow

–Subsidence – soil or rock material sinks to a lower level without sliding down a hillside

• Ex. Sinkhole

Landforms created by the weathering process.

• Mesa – large areas with flat tops and nearly vertical sides.

• Buttes – flat-topped hill that rises abruptly above the surrounding landscape.

Volcanic Neck

• The Legend of Devils Tower• One day, an Indian tribe was camped beside the river and

seven small girls were playing at a distance. The region had a large bear population and a bear began to chase the girls. They ran back toward their village, but the bear was about to catch them. The girls jumped upon a rock about three feet high and began to pray to the rock, "Rock, take pity on us; Rock, save us." The rock heard the pleas of the young girls and began to elongate itself upwards, pushing them higher and higher out of reach of the bear. The bear clawed and jumped at the sides of the rock, and broke its claws and fell to the ground. The bear continued to jump at the rock until the girls were pushed up into the sky, where they are to this day a group of seven little stars (the Pleiades). The marks of the bear claws are there yet.