Transcript Chapter 7: Weathering and Erosion With adaptaions from lectures by Peter Copeland • Bill Dupré Copyright © 2004 by W.

Chapter 7:
Weathering and Erosion
With adaptaions from lectures by
Peter Copeland • Bill Dupré
Copyright © 2004 by W. H. Freeman & Company
What do you see in the
following picture?
What are the differences in
the amount of weathering of
each tombstone?
Chemical Weathering in the Graveyard
Fig. 7.1
WEATHERING
•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
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?
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 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 microorganisms
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 7:
Weathering and Erosion
THE END
Copyright © 2004 by W. H. Freeman & Company