WEATHERING

CHAPTER 12

Sections 12.1 and 12.2

Weathering

Weathering is the change in the physical form or chemical composition of rock materials exposed at the earth’s surface.

Weathering Outline

I. Mechanical(Physical) weathering A. Ice Wedging B. Abrasion C. Organic Activity D. Exfoliation

II. Chemical weathering A. Hydrolysis B. Carbonation C. Oxidation D. Plant acids E. Acid precipitation

Weathering Outline

III. Rate of weathering A. Rock composition B. Amount of exposure C. Climate D. Topography

I. Mechanical weathering

Mechanical weathering is the process of breaking down a rock into smaller pieces without changing the chemical composition of the rock

A. Ice Wedging

Ice wedging occurs when water seeps into cracks in the rocks and freezes.

When water freezes it increases in volume about 10%.

The freezing process widens the cracks in the rocks.

ICE WEDGING

B. Abrasion

Abrasion is the collision of rocks with one another resulting in the breaking or wearing away of rocks.

Agents of abrasion are gravity, running water, and wind.

C. Organic Activity

Organic activity is weathering caused by plants or animals.

D. Exfoliation

As a rock is pushed to the surface curved cracks, called joints, occur parallel to the surface.

Exfoliation is the peeling away of the sheets of rock

Weathering and exfoliation

II. Chemical weathering

Chemical weathering, or decomposition, occurs when chemical reactions take place between the minerals in the rock and water, carbon dioxide, oxygen and acids.

A. Hydrolysis

Hydrolysis is the change of composition of a mineral when it reacts chemically with water.

Leaching occurs when water carries the dissolved minerals to lower layers of rock or earth.

B. Carbonation

Carbonation is when carbonic acid reacts chemically with some minerals.

Carbonic acid is formed when carbon dioxide reacts with water.

CO2 + H2O --> H2CO3

C. Oxidation

Oxidation occurs when oxygen combines chemically with an element.

Rust results from the oxidation of iron.

D. Plants acids

Acids produced by plants can dissolve the surface of a rock.

E. Acid Precipitation

Rainwater is slightly acidic and can react with rocks.

Acid rain is formed when the oxides of sulfur and nitrogen react with the water. Acid rain greatly accelerates the weathering process.

III. Rates of weathering A. Rock composition The composition of the rock will determine

the rate at which it weathers.

Quartz is the least affected by weathering.

Sedimentary rock weather more easily than igneous or metamorphic rock.

B. Amount of exposure

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

C. Climate

Rainfall and the freezing and thawing produced by alternating hot and cold weather usually have the greatest effect on the rate of weathering.

Hot and humid climates produce good conditions for chemical weathering.

Weathering takes place slowly in hot and dry climates as well as cold climates.

D. Topography

The topography, or height and slope, of the land influences the rate of weathering.

Steeper slopes and higher elevations usually have a greater rate of weathering.

WEATHERINGAND SOIL

CHAPTER 12

Sections 12.3

SOIL

Bedrock is solid, unweathered rock beneath the earth’s surface.

Humus is the remains of plants and animals.

Regolith is a layer of weathered rock fragments on the surface.

Soil is a complex mixture of minerals, water, gases, and organic material.

SOIL TYPESBY SIZE

Clay particles - – less than 0.002 mm– Parent rock is feldspar

Silt particles – - between 0.002 to 0.05 mm

Sand particles – - between 0.05 and 2 mm– Parent rock is granite

Factors controlling soil development Climate

– (most important)

Time Type of bedrock Local land surface

– (flat vs. mountain side)

Type of organisms– (worms)

Soil Formation

It takes 100-600 years for 1 inch of soil to form

Soil profile

The arrangement of the soil horizons

Horizons -Layers in the soil A horizon - top soil, contains organic

material and small rock fragments B horizon - subsoil, contains materials

leeched from top soil, clay and maybe humus

C horizon - Infertile weathered rock D horizon - bed rock

Soil Ages

Immature soil– soil that is poorly developed with few or little

horizons

Mature soil– soil that has developed over a longer period of

time/many horizons

Soil Types

Tropical Soils – oxisols Forest Soils- spodosols Organic Soils – histosols Desert Soils – gridisols Tundra Soils- gelisols

Tropical Soils (Oxisols)

Tropical soils are highly weathered, reddish or yellowish soils of humid, tropical or subtropical regions. A road cut in Hawaii exposes an example of this homogeneous nutrient-poor soil. In tropical settings, extensive weathering leaches nutrients from the soil and leaves behind iron oxides. Such soils are extremely low in fertility and are commonly found in Hawaii & Puerto Rico.

Forest Soils (Spodosoils)

Forest Soils (Spodosoils) Portland, Oregon– Form in warm to cool, humid

regions with coniferous cover. Such soils predominate in the northeastern United States, New England, the upper Great Lakes region, and the Pacific North West. The light-gray upper horizon of forest soils overlies a reddish horizon rich in aluminum and/or iron. Subsurface accumulation of humus and Al & Fe oxides characterize these acidic soils.

Organic Soils (Histolsols)

Organic Soils are wetland soils, dark in color and rich in decomposed organic materials. They form in poorly drained and lowland environments in the Great Lakes region and coastal eastern United States. These soils play an important role in environmental protection by filtering contaminants from surface water.

Desert Soils (Aridisols) Aridisoils form in Arid

settings. These soils occur in the desert southwest of the United States like Utah and Arizona. These areas, if not irrigated, are used mainly for range, wildlife or recreation. Desert soils are commonly rich in calcium carbonate, which may for impermeable layers.

Tundra Soils (Gelisols) Tundra soils occur

throughout the tundra of Alaska and in the Artic areas. A tundra soil profile consists of a dark layer rich in organic material and a mineral rich layer overlying permafrost. Any soil, subsoil, or surficial deposit where temperature remains below freezing is considered to be permafrost.

Assignment

Reading Assignment2/24 Section 12.3

Homework Assignment2/24 Section 12.3 Worksheet

and Question page 230 2/24 Section 12.3 Key Terms