WEATHERING WEATHERING Nature of weathering and erosion Weathering

WEATHERING

WEATHERING • Nature of weathering and erosion • Weathering • chemical and/or physical breakdown of a rock or mineral material • weathering involves specific processes acting on materials at or near the Earth’s surface • Erosion • removal or transportation of material by agents as running water, glacial ice, wind, etc. • different degrees of weathering and erosion working together can form interesting geologic features as Bryce Canyon

Bryce Canyon National Park, Utah

WEATHERING • Kinds of weathering • Physical (Mechanical) • breakdown of a substance by disintegration • frost (ice) wedging • alternating freezing and thawing of moisture in rock openings causing rock disintegration • single most abundant form of physical weathering--potholes form from wedging • a slope of talus can form at base of a cliff or mountain as a mass wasting process

Frost wedging

Talus slope at the base of mountain

WEATHERING • unloading (exfoliation) • erosion of upper rock causes underlying rock to expand resulting in cracking and peeling of rock in slabs Half Dome in Yosemite National Park

WEATHERING • organic activity • burrowing worms or animals and roots of trees can cause rock disintegration

WEATHERING • Chemical weathering • breakdown of substance by decomposition resulting in formation of new minerals • water and acids especially carbonic acid are important agents • hydrolysis • chemical reaction between water and substance in which (OH) in water (HOH) becomes part of the new substance formed • often silicate minerals will weather by this process to form clay

WEATHERING • solution (leaching) • dissolving of substances by acids in water often leaving nonsolubilized remains • carbonate minerals are highly affected by leaching • oxidation • reaction of a substance with oxygen causing a rust material to form • manganese and especially iron minerals are affected by oxidation • often weathering of materials includes a combination of the 2 kinds of weathering

WEATHERING • Factors which affect the rate and/or extent of weathering • Total surface area of mineral or rock • chemical and physical weathering increases proportionately with amount of open space (cracks, holes, etc. ) at surface and extending through the material • finer grained rocks weather greater than a coarser grained equivalent in composition and size • basalt and gabbro; rhyolite and granite

WEATHERING • Climate • temperature and amount of moisture can influence the kind and magnitude of weathering • in a humid hot climate there would be a lot of hydrolysis, leaching, oxidation and certain types of physical weathering but little or no frost wedging • in a dry cold climate there would be a sparce variety and lower magnitude of chemical weathering

WEATHERING • Cleopatra’s Needle was in Egypt for nearly 3500 years and shows minimal weathering compared to the same in New York for about 80 years in Egypt in New York

WEATHERING • Composition of rock or mineral substance • as mentioned earlier certain composition materials are affected by specific kinds of chemical weathering • Silicate mineral weathering series • based on composition alone silicate minerals higher in Bowen’s mineral series will decompose more than those lower in the series • a gabbro should weather at a greater rate and more extensively than a granite if all other weathering factors are the same

WEATHERING • which would weather faster and a greater rate, a basalt or a rhyolite if all other weathering factors are the same?

WEATHERING • Time • obviously time is a factor in all geologic processes including weathering rate and magnitude or extent of weathering • Soil • Definition and basic nature of soil • unconsolidated material formed from chemical and/or physical weathering of substances as bedrock or outcrops • contains at least some organic matter • can be in situ (in place) or transported

WEATHERING • Soil profile and soil horizons • typical soil profiles include 3 main horizons • 2 upper horizons, A and B are known as solum • A horizon is concentrated in organic matter and acids and represents an area of high leaching or solution activity • B horizon is concentrated in clay and rich in iron • eluviation is the removal of clay and solubilized materials from A horizon

WEATHERING • illuviation is the precipitation of eluviated materials in the B horizon • B horizon forms and thickens as the degree of eluviation and illuviation increase • C horizon • comprised of fragmented bedrock

A

WEATHERING Mass Wasting Examples Related to Soils • Soil creep • a form of mass wasting of earth materials • creep is often initiated if ground becomes saturated with water allowing gravity to pull the water logged soil downslope • freezing and thawing in soil can also cause heaving thus loosening the soil allowing creep

Soil Creep

Debris Slide

Rock and Debris Flow

Landslides in La Conchita, California

WEATHERING • Solifluction • a form of mass wasting and soil creep that occurs in regions underlain by perafrost in arctic climates • creep occurs in active layer above permafrost-active layer becomes saturated in water during summer months but not the impermeable permafrost layer--this results in a creep downslope of the active layer

WEATHERING Solifluction—diagram and picture