Chapter 3

1. Chapter 3 Weather, Soil, and Mass Wasting

2. Earth’s External Processes • Weathering : disintegration and decomposition of rock at or near the surface. • Mass wasting : transfer of rock material downslope under the influence of gravity. • Erosion : incorporation and transportation of material by a mobile agent, usually water, wind, or ice.

3. Weathering is the Earth’s response to a changing environment. • Mechanical Weathering occurs when rock breaks into smaller pieces retaining characteristics of original rock. • Frost wedging : alternate freezing and thawing (talus slopes) • Unloading : slabs break loose due to reduction of pressure (sheeting and exfoliation domes • Thermal expansion • Biological activity • Chemical weathering involves processes that alter the internal structure of minerals by removing/adding elements. • Water is the most important agent of chemical weathering (oxidation, carbonic acid). • Contributes to physical changes (spheroidal weathering)

4. Rates of Weathering • Particle size : smaller pieces increases surface area and promotes chemical weathering. • Joints : provide path for water to depths that begin weathering even before rock is exposed. • Mineral make-up : different minerals respond to weathering differently (Bowen’s reaction series) • Climate : temperature and moisture influence vegetation which determines formation of carbonic acids (optimum environment for chemical weathering is warm temperatures and abundant moisture. • Differential weathering creates unusual and spectacular land forms.

5. Soil is a combination of mineral and organic matter, water, and air • Regolith: the layer of rock and mineral fragments produced by weathering. • Humus: the decayed remains of animal and plant matter; enhances the soil’s ability to retain water • Regolith and humus make up ½ volume of soil, the remaining ½ consists of pore spaces where air and water circulate.

6. Soil Texture and Structure • Soil texture refers to the proportions of different particle sizes, strongly influences the soil’s ability to retain and transmit water and air. Fig 3.12 pg 68 (Loams are best for plant life) • 4 basic structures (peds): platy; prismatic; blocky; spheroidal. Structure influences the ease of a soil’s cultivation as well as susceptibility to erosion. Structure also affects porosity and permeability. • Prismatic & Blocky peds allow for moderate water infiltration • Platy & Spheroidal have slower infiltration rates

7. Controls of Soil Formation • Parent material : source of weathered mineral matter • Residual soils form when parent material is bedrock. • Transported Soilsform when parent material is unconsolidated sediment. • Type of parent material affects the rate of weathering and thus soil formation. (soil formation progresses more rapidly in unconsolidated soils due to greater surface area) • Chemical make-up of parent material affects the soil’s fertility.

8. Controls of Soil Formation • Time: as processes continue, the influence of parent material on soil is overshadowed by other factors, esp. climate • Climate: most influential control of soil formation, determines whether chemical or mechanical weathering predominates, also determines the rate of weathering, and controls the plant and animal life

9. Controls of Soil Formation • Plant and Animals: furnish organic matter for soil which contribute to organic acids, microorganisms cause decay of matter, burrowing animal aid in passage of water and air • Slope: impacts the amount of erosion and water content. Slope orientation, the direction the slope is facing determines the degree of sunlight/energy and therefore the temperature

10. Soil Profile

11. Soil Types • Pedalfer : pedon means soil, al for aluminum and fe for iron (east US) • Pedocal : calcite or calcium carbonate (drier, western US) • Laterites : hot, wet tropics; chemical weathering is intense; contains little humus; poor for crops • Refer to table 3.2 pg 73

12. Soil Erosion • Human activities has increased the rate of soil erosion. • In some areas the soil is eroded at a faster rate than the soil forms and therefore has become a nonrenewable resource. • Increased amounts of eroded sediment is becoming a problem in the reservoirs, lakes, and streams in which they are deposited.

13. Mass Wasting: downslope movement of rock, regolith, and soil under direct influence of gravity. • Controls and Triggers • Water: saturation reduces the internal resistance of materials • Oversteepened slopes • Removal of vegetation • Earthquakes

14. Processes are defined by the type of material involved, the kind of motion, and the velocity of movement. • If soil and regolith are dominant: debris, mud or earth • If bedrock: rock • When movement involves the free-fall of individual pieces: fall • When movement involves fairly coherent material moving parallel to slope: slides • When material moves as a viscous fluid: flow

15. Slump : downward slipping of rock or unconsolidated material along a curved surface. (oversteepened slope) • Rockslides: when blocks of bedrock break loose. (where rock strata is inclined) Pg 81 • Mudflow: flow of debris containing large amount of water and largely confined to streams and canyons ( Lahars on slope of volcano) • Earthflow: occur on hillsides in humid areas during heavy precipitation or snowmelt (liquefaction type of earthflow in association with earthquakes) • Creep: gradual downhill movement of soil and regolith due to expansion and contraction of surface material.