Chapter 16:Weathering and Erosion

Copyright © 2007 by W. H. Freeman & Company

With adaptaions from lectures byPeter 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 CO2 (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 weatheringFrost: 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 weatheringand ...

Spheroidal weatheringand 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 kindRegolith: 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