Chapter 16: Weathering and Erosion Copyright © 2007 by W. H. Freeman & Company With adaptaions from lectures by Peter Copeland Bill Dupré

What do you see in the following picture? What are the differences in the amount of weathering of each tombstone?

Concepts Factors in weathering: composition, structure, climate, soil and vegetation, time Chemical weathering: C02,carbonic acid, chemical stability of silicates,oxidation Physical weathering: frost wedging, spheroidal weathering

Chemical Weathering in the Graveyard Fig. 7.1

Destruction of rock by either physical (e.g., pressure) or chemical means (e.g. dissolution) or biological means (e.g. tree roots, bacteria) Erosion includes weathering AND transport WEATHERING

Chemical Weathering The principle agent of chemical weathering is water. This process occurs because minerals formed deep in the earth’s interior are not stable under the conditions on the surface of the Earth. Stability is generally the reverse of the order in which minerals form out of a cooling magma (a.k.a. “Bowen’s reaction series.

Principles of chemical weathering, e.g. coffee making. Why do we use hot water to make coffee? Why do we grind coffee before percolation? Is soil a chemical, physical, or biological weathering product? Why is the ocean salty?

What are the major factors that control weathering? –Composition, –Structure, –Climate, –Soil and vegetation, –time

Chemical Weathering of Silicates Quartz: very stable Feldspars: form clay minerals Mafic minerals: decompose to oxides

What is the role of CO 2 (Carbon Dioxide) in weathering and climate? Carbon dioxide is removed from the atmosphere through the process of weathering and can lead to global cooling. Carbon dioxide dissolves in water to form carbonic acid which weathers feldspars to clay, and dissolves limestone.

Biosphere I failed because the builders failed to understand the relation between Carbon Dioxide and weathering

Columbia University’s Biosphere Project 2 Tucson, AZ

Why are deserts and Mars red? Oxidation of iron (rusting)

Water dissolves Fe2+(“blue”) from silicates and provides a medium for chemical reactions to further oxidize iron ions to Fe3+ (“red”) Fe3+ is not as soluble and tends to precipitate out of solution as hematite

Physical weathering Frost: water expands by ~10% when it freezes Thermal expansion: differential thermal expansion of minerals creates stress in rocks Organic activity: tree roots to micro- organisms

Spheroidal weathering and...

Spheroidal weathering and exfoliation (large scale!!)

Soil vs. “clay” in Louisiana A soil usually consists of material weathered from a rock AS WELL AS mixed organic material. A soil is often the product of organic chemical activity. Soil often means life activity. Clay alone only means chemical breakdown of feldspars.

Weathering Terminology 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

Chapter 16: Weathering and Erosion THE END