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.

WEATHERING AND 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 TYPES BY SIZE Clay particles - Silt particles Sand 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 Mature 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 Assignment 2/24 Section 12.3 Homework Assignment 2/24 Section 12.3 Worksheet and Question page 230 2/24 Section 12.3 Key Terms