Earth image Rock Weathering

Dr. Pamela Gore
Georgia Perimeter College

Objectives

  1. Briefly contrast weathering and erosion.
  2. Contrast chemical and physical (or mechanical) weathering.
  3. List and describe the types of physical (or mechanical) weathering.
  4. List and describe the types of chemical weathering.
  5. List the products resulting from the chemical weathering of Igneous rocks.
  6. 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.

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.

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

B. Chemical weathering

Rock reacts with water, gases and solutions (may be acidic); will add or remove elements from minerals.

Dissolution (or solution)-


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: Iron oxides are produced by oxidation of iron-bearing silicate minerals.
These minerals are iron oxide 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


Hydrolysis -


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?


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