Weathering and Soil Earth - Chapter 6

Earth’s external processes Weathering – the physical breakdown (disintegration) and chemical alteration (decomposition) or rock at or near Earth’s surface Mass wasting – the transfer of rock and soil downslope under the influence of gravity

Weathering (Disintegration & Decomposition) Accentuates Differences in Rock Types

Earth’s external processes Erosion – the physical removal of material by mobile agents such as water, wind, ice, or gravity

Weathering Two types of weathering Mechanical weathering – breaking of rocks into smaller pieces Chemical weathering – decomposition into new compounds

Mechanical Weathering (Physical Breakdown of Rocks) Produces Smaller & Smaller Pieces that Increase the Surface Area Available for Chemical Attack https://mediaplayer.pearsoncmg.com/assets/secs-geology-SmartFigures-Physical_and_Chemical_Weathering

Weathering Four types of mechanical weathering Frost wedging – alternate freezing and thawing of water in fractures and cracks promotes the disintegration of rocks Unloading – exfoliation of igneous and metamorphic rocks at the Earth’s surface due to a reduction in confining pressure Thermal expansion – alternate expansion and contraction due to heating and cooling Biological activity – disintegration resulting from plants and animals

Frost wedging alternate freezing and thawing of water in fractures and cracks promotes the disintegration of rocks

Unloading – exfoliation of igneous & metamorphic rocks at the earth’s surface due to a reduction in confining pressure https://mediaplayer.pearsoncmg.com/assets/secs-geology-SmartFigures-Sheeting

Thermal expansion alternate expansion and contraction due to heating and cooling

Not all fractures are created by thermal processes Not all fractures are created by thermal processes. These “joints” near Moab, Utah, are cracks produced by tectonic forces generated during mountain building.

Biological activity disintegration resulting from plants and animals

Weathering Chemical Weathering Breaks down rock components and internal structures of minerals Most important agent involved in chemical weathering is water (responsible for transport of ions and molecules involved in chemical processes)

Weathering Major processes of chemical weathering Dissolution Aided by small amounts of acid in the water Soluble ions are retained in the underground water supply

Dissolution Halite (Rock Salt) Dissolving in Water. Sodium & Chloride Ions Are “Attacked” by Polar Water Molecules. Once Removed, the Na & Cl ions are Surrounded & Held by Numerous Water Molecules

The Effects of “Acid Rain” – Partially Dissolved Limestone Figure, Damaged Trees in Great Smoky Mtns.

Weathering Major processes of chemical weathering Oxidation Any chemical reaction in which a compound or radical loses electrons Important in decomposing ferromagnesian minerals

A very common example of oxidation is the reaction of iron with oxygen to create iron oxide (rust). Similar oxidation of iron in rock-forming minerals produces a red color

Similar oxidation of iron in rock-forming minerals produces a red color on outcrop

Acid mine drainage results when pyrite (FeS2) is oxidized & releases sulfur that combines with H2O to form sulfuric acid

Weathering Major processes of chemical weathering Hydrolysis The reaction of any substance with water Hydrogen ion attacks and replaces other positive ions

Weathering Alterations caused by chemical weathering Decomposition of unstable minerals Generation or retention of materials that are stable Physical changes such as the rounding of corners or edges

Generation of more stable minerals from weathering of igneous rocks

Rates of Weathering Factors affecting weathering Surface area The smaller the particles to be weathered, the more surface area is exposed to be weathered and the more rapidly the weathering proceeds

Mechanical Weathering (Physical Breakdown of Rocks) Produces Smaller & Smaller Pieces that Increase the Surface Area Available for Chemical Attack

Rates of Weathering Factors affecting weathering Rock characteristics Rocks containing calcite (marble and limestone) readily dissolve in weakly acidic solutions Silicate minerals weather in the same order as their order of crystallization

These 2 tombstones were erected at roughly the same time, but the marble stone on the right weathered much more rapidly

Silicate Minerals Weather in the Same Order in Which They Form: i. e Silicate Minerals Weather in the Same Order in Which They Form: i.e. the Highest Temperature Silicates Are the Least Resistant to Weathering

Weathering Others factors affecting weathering Climate Temperature and moisture are the most crucial factors Chemical weathering is most effective in areas of warm, moist climates

Weathering Differential weathering Masses of rock do not weather uniformly due to regional and local factors Results in many unusual and spectacular rock formations and landforms

Differential weathering controlled by jointing patterns

Joint-controlled weathering in igneous rocks https://mediaplayer.pearsoncmg.com/assets/secs-geology-SmartFigures-Spheroidal_Weathering

Successive shells are loosened as the weathering process continues to penetrate deeper into the rock

Soil Soil = combination of mineral and organic mater, water, and air Rock and mineral fragments produced by weathering (regolith) that supports the growth of plants Humus (decayed animal and plant remains) is a small, but essential, component

Soil is an ESSENTIAL resource that supports plant life, which in turn supports animal life

Composition of a “good” soil

Soil Factors controlling soil formation Parent material Residual soil – parent material is the underlying bedrock Transported soil – forms in place on parent material that has been carried from elsewhere and deposited

Soil Factors controlling soil formation Time Climate Amount of time for soil formation varies for different soils depending on geologic and climatic conditions Climate Most influential control of soil formation Key factors are temperature and precipitation

Soil Factors controlling soil formation Plants and animals Organisms influence the soil’s physical and chemical properties Also furnish organic matter to the soil Slope (Topography) Steep slopes often have poorly developed soils Optimum terrain is a flat-to-undulating upland surface

Variations in soil development

Soils in Alaska’s Denali Nat Soils in Alaska’s Denali Nat. Park are produced by intense acid leaching of conifer humus shed from the overlying trees

Soil The soil profile Soil forming processes operate from the surface downward Vertical differences are called horizons – zones or layers of soil

Soil The soil profile O horizon – organic matter A horizon – organic and mineral matter High biological activity Together the O and A horizons make up the topsoil E horizon – little organic matter Zone of eluviation and leaching

Soil The soil profile B horizon – zone of accumulation C horizon – partially altered parent material The O, A, E, and B horizons together are called the solum, or “true soil”

An idealized soil profile

Soil Profiles: A = well developed soil in South Dakota; B = indistinct soil horizons in soil from Puerto Rico A B

Soil Classifying soils Variations in soil formation over time and distances has led to a great variety of recognized soil types Groups have been established using common characteristics In the United States a system is used called the soil taxonomy

Soil Classifying soils Soil taxonomy Emphasis is placed on physical and chemical properties Six hierarchical categories exist, ranging from order (12 broad catagories) to series (>19,000) Descriptive names are derived from Latin and Greek 12 basic soil orders are recognized

Distribution of the 12 soil orders in the western hemisphere

Distribution of the 12 soil orders in the eastern hemisphere

An example of soil order Oxisol (rain forest iron & aluminum soils) An example of soil order Oxisol (rain forest iron & aluminum soils). Although these provide an excellent foundation for trees, they are NOT good for farming

Angkor Wat, Cambodia, is built of bricks cut from oxisols in the region. When exposed to the sun (trees removed) this soil hardens like stone, thus it is not very good for growing crops.

Soil Soil erosion Part of the natural recycling of earth materials Natural rates of soil erosion depend on Soil characteristics Climate Slope Type of vegetation

Soil Soil erosion In many regions the rate of soil erosion is significantly greater than the rate of soil formation Sedimentation and chemical pollution Related to excessive soil erosion Occasionally soil particles are contaminated with pesticides

Exposed soil particles respond to rain drops Soil dislodged by splash erosion is more easily moved by sheet erosion.

Evidence of soil erosion in Wisconsin 1 mm of soil eroded from a single acre of land amounts to about 5 tons of soil lost

Gully erosion is severe in this poorly protected oxisol soil in Columbia

Soil Erosion Due to Drought and Excessive Farming Was Common During the 1930’s