Rock Weathering
Dr. Pamela Gore
Georgia Perimeter College
Objectives
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Briefly contrast weathering and erosion.
Contrast chemical and physical (or mechanical) weathering.
List and describe the types of physical (or mechanical) weathering.
List and describe the types of chemical weathering.
List the products resulting from the chemical weathering of Igneous rocks.
List and discuss the factors that influence the type and rate of rock weathering.
This section addresses, in whole or in part, the following Georgia GPS standard(s):
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Describe processes that change rocks and the surface of the Earth.
Describe soil as consisting of weathered rocks and decomposed organic material.
This section addresses, in whole or in part, the following Benchmarks for Scientific
Literacy:
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Some changes in the earth's surface are abrupt (such as earthquakes and volcanic
eruptions) while other changes happen very slowly (such as uplift and wearing
down of mountains). The earth's surface is shaped in part by the motion of water
and wind over very long times, which act to level mountain ranges.
Although weathered rock is the basic component of soil, the composition and
texture of soil and its fertility and resistance to erosion are greatly influenced by
plant roots and debris, bacteria, fungi, worms, insects, rodents, and other
organisms.
This section addresses, in whole or in part, the following National Science Education
Standards:
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Land forms are the result of a combination of constructive and destructive forces.
Constructive forces include crustal deformation, volcanic eruption, and deposition of
sediment, while destructive forces include weathering and erosion.
Some changes in the solid earth can be described as the "rock cycle." Old rocks at
the earth's surface weather, forming sediments that are buried, then compacted,
heated, and often recrystallized into new rock. Eventually, those new rocks may be
brought to the surface by the forces that drive plate motions, and the rock cycle
continues.
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Soil consists of weathered rocks and decomposed organic material from dead
plants, animals, and bacteria. Soils are often found in layers, with each having a
different chemical composition and texture.
Living organisms have played many roles in the earth system, including affecting
the composition of the atmosphere, producing some types of rocks, and
contributing to the weathering of rocks.
Rock Weathering
Weathering is the BREAKDOWN of rock to form sediment.
Erosion is the TRANSPORTATION of rock particles (or sediment) that have formed
by weathering processes. Types of weathering:
A. Physical or mechanical weathering
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Frost wedging water expands when it freezes
Talus slope, Lost River, West
Virginia
Shale chips, West Virginia
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Exfoliation or unloading o rock breaks off into leaves or sheets along joints which parallel the
ground surface;
o caused by expansion of rock due to uplift and erosion; removal of
pressure of deep burial;
example: Stone Mountain, Georgia.
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Thermal expansion o repeated daily heating and cooling of rock;
o heat causes expansion; cooling causes contraction.
o different minerals expand and contract at different rates causing stresses
along mineral boundaries.
B. Chemical weathering
Rock reacts with water, gases and solutions (may be acidic); will add or remove
elements from minerals.
Dissolution (or solution)
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Several common minerals dissolve in water
o halite
o calcite
Limestone and marble contain calcite and are soluble in acidic water
Marble tombstones and carvings are particularly susceptible to chemical
weathering by dissolution. Note that the urn and tops of ledges are heavily
weathered, but the inscriptions are somewhat sheltered and remain legible.
Photo taken in an above-ground cemetery in New Orleans
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Caves and caverns typically form in limestone
o speleothems are cave formations
o speleothems are made of calcite
o form a rock called travertine
 stalactites - hang from ceiling
 stalagmites - on the ground
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Areas underlain by limestone in humid climates typically have karst
topography, which is characterized by:
o caves/caverns,
o sinkholes,
o disappearing streams,
o springs
General view of karst topography, St. Paul Group, Chambersburg
Limestone. Pennsylvania, north of Clear Spring, MD. Note small closed
depressions.
Small sinkhole within a larger sinkhole, west of Albany, GA
Street detours around a large sinkhole in Albany, GA near Radium
Springs
Chinese Tower Karst. Photo from Microsoft Clip Gallery.
Oxidation Oxidation - Oxygen combines with iron-bearing silicate minerals causing "rusting".
Iron oxides are produced that are red, orange, or brown in color.
Iron-bearing silicate minerals that undergo oxidation include the following:
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olivine
pyroxene
amphibole
biotite
Iron oxides are produced by oxidation of iron-bearing silicate minerals.
These minerals are iron oxide minerals:
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limonite
hematite
goethite
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Iron oxides are red, orange, or brown in color
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Mafic rocks such as basalt (which may contain olivine, pyroxene, or
amphibole) weather by oxidation to an orange color
"Georgia Red Clay" derives its color from the oxidation of iron bearing
minerals
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Broken piece of fine-grained basalt from a dike near Stone Mountain, GA. Note the
black color of the unweathered rock, and the weathering rind colored by iron oxides.
The weathering rind has two distinct layers, an inner yellowish layer and an outer
orange layer. Sample is about 10 cm in width.
Weathering Rind, Wilhite Formation, eastern Tennessee
Hydrolysis 
Silicate minerals weather by hydrolysis to form CLAY.
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Feldspar alters to clay (kaolinite) plus dissolved materials (ions).
Kaolinite (or kaolin) is a pure, white clay mined in central Georgia along a line
from Augusta to Macon to Columbus.
Kaolinite is used for shiny coating on paper, and is used in rubber (tires),
paints, plastics, ceramics, and many other products.
References on kaolin mining in Georgia:
o http://www.kaolin.com/ccpkaoln.htm
o http://www.moles.org/ProjectUnderground/drillbits/4_17/vs.html
Kaolin mine, central Georgia
Kaolinite
Feldspars are stable at high temperatures and pressures (but not at the
temperatures and pressures of the Earth's surface)
o Clays are stable under conditions at the Earth's surface
o Feldspars and clays are similar in composition.
o Feldspar readily alters to clay when in contact with acid and water.
Iron-bearing silicate minerals weather to form clays by hydrolysis (in addition
to iron oxides)
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Spheroidal weathering in jointed basalt, Culpeper Basin, Virginia
Spheroidal weathering is caused by chemical weathering of jointed rocks. The jointed
rocks weather to form roughly spherical shapes.
C. Biological weathering
Organisms can assist in breaking down rock into sediment or soil.
Photo from Microsoft Clip Gallery
1. Roots of trees and other plants
2. Lichens, fungi, and other micro-organisms
3. Animals (including humans)
Lichen on boulder, Cartersville, GA
Closeup of lichen, Stone Mountain GA
What happens when granite is weathered?
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First, unweathered granite contains these minerals:
o Na Plagioclase feldspar
o K feldspar
o Quartz
o Lesser amounts of biotite, amphibole, or muscovite
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The feldspars will undergo hydrolysis to form kaolinite (clay) and Na and K
ions
The Na and K ions will be removed through leaching
The biotite and/or amphibole will undergo hydrolysis to form clay, and
oxidation to form iron oxides.
The quartz (and muscovite, if present) will remain as residual
minerals because they are very resistant to weathering.
Weathered rock is called saprolite
Weathered rock fragments are one of the constituents of soil.
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What happens after this?
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Quartz grains may be eroded, becoming sediment. The quartz in granite
is sand-sized; it becomesquartz sand. The quartz sand will ultimately be
transported to the sea where it accumulates to formbeaches.
Clays will ultimately be eroded and washed out to sea. Clay is finegrained and remains suspended in the water column; it may be deposited
in quiet water.
Dissolved ions will be transported by rivers to the sea, and will become
part of the salts in the sea.
Rates of weathering
Factors influencing the rate of weathering include:
1. Amount of surface area exposed to chemical weathering.
As the rock breaks down into smaller pieces, more surface area is exposed, and
the rock weathers faster. The presence of cracks or joints in the rock can allow
water to penetrate and increase the rate of weathering.
2. Different minerals weather at different rates.
Marble and limestone, which are composed of calcium carbonate, dissolve
readily in weakly acidic solutions.
Silicate minerals weather in the same sequence as they crystallize. (Bowen's
Reaction Series). Olivine crystallizes first from a magma, and so is the first to
weather. Quartz crystallizes last from magma, and so it is the most resistant to
weathering.
3. Climate influences weathering rates, particularly temperature and the
availability of water. Warm temperatures and abundant moisture lead to
RAPID WEATHERING.
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Page created by Pamela J.W. Gore
Georgia Perimeter College,
Clarkston, GA
Page created February 25, 2005
Links updated October 13, 2008