Classroom presentations to accompany Understanding Earth, 3rd edition prepared by Peter Copeland and William Dupré University of Houston Chapter 6 Weathering and Erosion

Tim Davis/Photo Researchers Weathering and Erosion

Weathering Physical and chemical changes that occur in sediments and rocks when they are exposed to the atmosphere and biosphere. Not the same as erosion.

Factors Controlling Rates of Weathering

Chemical weathering This process occurs because minerals formed deep in the earth’s interior are not stable under surface conditions. Stability is generally the reverse of Bowen’s reaction series. The principle agent of chemical weathering is water.

Chemical weathering of silicates Quartz: very stable Feldspars: form clay minerals Mafic minerals: decompose to oxides

Chemical weathering of carbonates Easily soluble in water (especially with some acid present) Ca and Mg taken into solution

Fig. 6.1 R. Siever Slate Marble Weathering rates of gravestones

Fig. 6.2 Berner & Holden, 1977 Etched and corroded feldspar in the soil zone

Fig. 6.3a

Fig. 6.3b

Fig. 6.3c

Analogy of weathering: making coffee fresh grounds + water = coffee + residue (a solution) K-feldspar + water = K + + kaolinite (a clay mineral)

Weathering and Making coffee Fig. 6.4

Weathering by solution The complete breakup of minerals into ions in solution NaCl (halite) is the best example, but is geologically unimportant calcite (limestone) = CaCO 3 CaCO 3 + H 2 CO 3 = Ca HCO 3 - Mafic silicates dissolve much more slowly

Mechanical Weathering Changes the Surface to Volume Ratio Fig. 6.5

Fig. 6.6

CO 2 and the Atmosphere, Weathering, and the Climate Fig. 6.7

Fig. 6.8

Fig. 6.9 Betty Crowell Weathering Oxides Provide Color to the Desert Landscape

Fig Ric Ergenbright Weathered Limestone

Joint-controlled Weathering Fig Jeff Foott/DRK

Mechanical weathering Frost — water expands by 9% when it freezes Thermal expansion — differential thermal expansion of minerals creates stress in rocks Organic activity — tree roots to micro- organisms Mechanical abrasion — things go bump

Fig Peter Kresam Role of Organisms in Weathering

Gneiss Boulder Fractured by Frost Action Fig Michael Hambrey

Fig Tony Waltham Exfoliation Dome in Yosemite

Fig Michael Follo Spheroidal Weathering

Weathering terms Bedrock: unaltered rock of any kind Regolith: a layer of broken pieces of rock and slightly altered rock that overlies the bedrock Soil: a layer of altered mineral material usually mixed with organic material

Fig Weathering, Soil Formation, and Erosion

Fig. 6.16

Change in U.S. soil erosion rates

Fig Soil Profile

Laterite Fig. 6.18a

Pedalfer Fig. 6.18b

Fig. 6.18c Pedocal

Sand Fig Rex Elliot