Weathering

Pamela J. W. Gore
Georgia Perimeter College

Objectives

1. Briefly contrast weathering and erosion.
2. Contrast chemical and mechanical weathering.
3. List and describe the types of 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.


The Rock Cycle

The three major types of rocks, igneous, sedimentary, and metamorphic rocks are interrelated by a series of natural processes. Igneous rocks form from the cooling and crystallization of hot molten lava and magma. Igneous rocks undergo weathering and erosion to form sediments. Sediments are deposited and lithified by compaction and cementation to form sedimentary rocks. Sedimentary rock become buried by additional sedimentary deposition, and when they are deep within the Earth, they are subjected to heat and pressure which causes them to become metamorphic rocks. With further burial and heating, the metamorphic rocks begin to melt. Partially molten metamorphic rocks are known as migmatite. As melting proceeds with increasing temperatures and depths of burial, eventually the rock becomes molten and becomes magma, which cools and crystallizes to form plutonic igneous rock, or which is erupted onto the Earth's surface as lava, and cools and crystallizes to form volcanic igneous rock.

Further complications within the rock cycle include (1) weathering of sedimentary and metamorphic rocks (in addition to igneous rocks), and (2) metamorphism of igneous rocks and repeated metamorphism of metamorphic rocks.


Definition of weathering:

Breakdown of rock to form sediment

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.


Weathering Rind, Wilhite Formation, eastern Tennessee



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

Tree roots in rock, Anna Ruby Falls, GA

  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


The Goldich Stability Series

This series describes the order in which silicate minerals weather.

Minerals which form at high temperatures and pressures are least stable, and weather most quickly because they are farther from their "zone of stability", or the conditions under which they formed.

Minerals which form at lower temperatures and pressures are most stable.

The order of mineral stability in the weathering environment is the same order as Bowen's Reaction Series.


Least stable (high temperature minerals)





	Olivine		Ca plagioclase feldspar

 	Pyroxene		

	Amphibole

	Biotite		Na plagioclase feldspar

		    Potassium feldspar

		          Muscovite

			   Quartz

		

Most stable (low temperature minerals)

What happens when granite is weathered?

See sections on SEDIMENTARY ROCKS and RIVERS.


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This page created by Pamela J. W. Gore
Georgia Perimeter College, Clarkston, GA

August 29, 1995
Modified January 24, 1997
Modified March 19, 1998
Modified July 17, 1999
Modified February 5, 2002
Links and email updated October 1, 2008