Transcript CHAPTER 2 Plate Tectonics and the Sea Floor

Weathering and Soils
GEOLOGY TODAY - Chapter 7
Barbara W. Murck
Brian J. Skinner
Soil Profile
N. Lindsley-Griffin, 1999
Acid Rain
Damage
Rock Cycle
and
Weathering
The physical
and chemical
breakdown of
rocks at or near
Earth’s surface
J.R. Griffin, N. Lindsley-Griffin, 1999
Mechanical Weathering
Joints and bedding
planes cause rocks to
break into blocks.
Joints and fractures
aid weathering.
Red sandstone, Wales
(Fig. 7.3, p. 194)
N. Lindsley-Griffin, 1999
Mechanical Weathering
Frost Wedging - rocks split
by repeated freezing and
thawing.
Temperature must cycle back
and forth across freezing point.
Important in temperate and
cold climates
Granite boulder, Sierra Nevada, CA
(Fig. 7.4, p. 194)
N. Lindsley-Griffin, 1999
Mechanical Weathering
Heat spalling - flakes of
rock loosened by expansion
from extreme heat of forest
fires.
Yellowstone National Park, WY
(Fig. 7.5, p. 194)
N. Lindsley-Griffin, 1999
Mechanical Weathering
Root Wedging - tree
roots pry rock apart by
growing into cracks.
As roots enlarge, flakes
of rock break off.
Ponderosa pine on granite
(Fig. 7.6, p. 195)
N. Lindsley-Griffin, 1999
Mechanical Weathering
Exfoliation - thin sheets
flake off.
When rocks formed at
depth are uplifted to the
surface, confining pressure
decreases.
They flake into concentric
layers.
Common in sandstone,
granite, and other evengrained rocks.
Granite dome, Yosemite
National Park, CA
© Lutgens & Tarbuck, 1999; N. Lindsley-Griffin, 1999
Mechanical Weathering
Exfoliation - Corners weather faster
because they are attacked on all sides,
causing boulders to become rounder.
The Devil’s Marbles, Australia (Fig. 7.7, p. 195)
N. Lindsley-Griffin, 1999
Chemical Weathering
Normal chemical weathering:
Slightly acidic rainwater in
which atmospheric carbon
dioxide is dissolved,
or more acidic ground water
from decaying organic matter.
Acid Rain - human-generated
sulfur and nitrogen compounds
in the atmosphere mix with
rainwater to form strong acids.
Marble statue, Italy
N. Lindsley-Griffin, 1999
Acid Rain
Damage
Acid Rain
10 to 1000 times
more acidic than
natural rain water
Cause: burning
sulfur-rich coal,
industrial emissions,
automobile exhausts
Damages lakes,
forests, buildings
© Houghton Mifflin 1998; N. Lindsley-Griffin, 1999
Great Smoky Mountains, Tennessee
Chemical Weathering
Hydrolysis - any
chemical reaction
that involves water
Ion Exchange hydrogen ions in
natural acid
solutions displace
cations to form new
minerals
(Fig. 7.8 A, B; p. 195)
N. Lindsley-Griffin, 1999
Chemical Weathering
Solution can dissolve some
minerals completely.
Calcite and dolomite are
especially vulnerable
(limestone and marble).
Sometimes solution of silicates
leaves a residue of clay behind.
1810 marble tombstone,
New England cemetery
(Fig. 7.9, p. 197)
N. Lindsley-Griffin, 1999
Chemical Weathering
Oxidation adds oxygen to iron in minerals to produce
hematite and stain rocks and soil red.
Oxidized soil, Hawaii
(Fig. 7.10, p. 197)
N. Lindsley-Griffin, 1999
Climate and Weathering
Climate - combination
of temperature and
rainfall.
Controls type and rate
of weathering.
Strength and type of
mineral bonds also
important.
(Fig. 7.12, p. 199)
N. Lindsley-Griffin, 1999
Differential Weathering
Limestone weathers quickly
in humid climates by
hydrolysis and solution.
In arid climates sandstone
weathers more quickly than
limestone.
Limestone and marble in New
England weather faster than
granite and basalt dikes
Folded marble cut by granite and basalt
dikes, central Maine
N. Lindsley-Griffin, 1999
Differential Weathering
In arid climate of the Grand Canyon:
Limestone and
sandstone
form steep cliffs,
resistant ledges.
Shale is easily
eroded, forms
gentle slopes.
N. Lindsley-Griffin, 1999
Soils
Soil-forming processes need water, solar heat
- both are climate dependent
Weathering of
sedimentary rock,
South Africa
Fig. 7.2, p. 192
N. Lindsley-Griffin, 1999
Soil Profile
Typical sequence of soil
horizons developed in a cool
moist climate.
(Fig. 7.13, p. 201)
N. Lindsley-Griffin, 1999
Soils and Climate
Different combinations of climate and vegetation
produce different soil horizons.
(Fig. 7.14, p. 201)
N. Lindsley-Griffin, 1999
Soils and Climate
Pedalfer soils are rich in clays, with aluminum and iron
oxides. Very fertile - moderate rainfall, temperate climates.
Fig. 7.15, p. 202
Houghton-Mifflin, 1998
N. Lindsley-Griffin, 1999
Soils and Climate
Pedocal soils are rich in
calcium carbonate,
gypsum, and other soluble
minerals.
Fertile with enough water.
Warm, dry climates.
White caliche horizon, central
New Mexico
(Fig. 7.16, p. 203)
N. Lindsley-Griffin, 1999
Soils and Climate
Laterite soils are made of
insoluble iron and
aluminum oxide residues
left by leaching - most
original materials removed
by solution.
Low fertility.
Warm, wet, tropical forests.
(See Fig. 7.17, p. 203)
N. Lindsley-Griffin, 1999