Dr. Pamela Gore
Georgia Perimeter College
- 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):
- 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:
- 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.
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This section addresses, in whole or in part, the following National Science Education Standards:
- 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.
- 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.
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
Frost wedging - water expands when it freezes
Talus slope, Lost River, West Virginia
Shale chips, West Virginia
Exfoliation or unloading -
Thermal expansion -
- repeated daily heating and cooling of rock;
- heat causes expansion; cooling causes contraction.
- different minerals expand and contract at different rates causing stresses along
B. Chemical weathering
Rock reacts with water, gases and solutions (may be acidic); will add or remove elements
Dissolution (or solution)-
- Several common minerals dissolve in water
- 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
- Caves and caverns typically form in limestone
- speleothems are cave formations
- speleothems are made of calcite
- form a rock called travertine
- stalactites - hang from ceiling
- stalagmites - on the ground
- Areas underlain by limestone in humid climates typically have karst topography,
which is characterized by:
- disappearing streams,
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 - 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:
Iron oxides are produced by oxidation of iron-bearing silicate minerals.
These minerals are iron oxide minerals:
- Iron oxides are red, orange, or brown in color
- 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
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
- Silicate minerals weather by hydrolysis to form CLAY.
- 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:
Kaolin mine, central Georgia
- Feldspars are stable at high temperatures and pressures (but not at the
temperatures and pressures of the Earth's surface)
- Clays are stable under conditions at the Earth's surface
- Feldspars and clays are similar in composition.
- 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
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
- Roots of trees and other plants
- Lichens, fungi, and other micro-organisms
- Animals (including humans)
Lichen on boulder, Cartersville, GA
Closeup of lichen, Stone Mountain GA
What happens when granite is weathered?
- First, unweathered granite contains these minerals:
- Na Plagioclase feldspar
- K feldspar
- Lesser amounts of biotite, amphibole, or muscovite
- 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.
- What happens after this?
- Quartz grains may be eroded, becoming sediment. The quartz in granite is sand-sized; it becomes quartz sand.
The quartz sand will ultimately be transported to
the sea where it accumulates to form beaches.
- Clays will ultimately be eroded and washed out to sea. Clay is fine-grained 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:
- 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.
- 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.
- 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,
Page created February 25, 2005
Links updated October 13, 2008