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 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
- 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
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) -
- 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
- Karst topography forms on limestone terrain and is characterized by:
- caves/caverns,
- sinkholes,
- disappearing streams,
- 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.
- Oxygen combines with iron-bearing silicate minerals causing "rusting"
- olivine
- pyroxene
- amphibole
- biotite
- Iron oxides are produced
- limonite
- hematite
- goethite
- 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
Weathering Rind, Wilhite Formation, eastern Tennessee
- Silicate minerals weather by hydrolysis to form CLAY.
- Feldspar alters to clay (kaolinite) plus dissolved materials (ions)
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
iron oxides)
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
- 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
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?
- First, unweathered granite contains these minerals:
- Na Plagioclase feldspar
- K feldspar
- Quartz
- 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. (See section on 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 (bed load), 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 (suspended load); it may be deposited in
quiet water.
- Dissolved ions will be transported by rivers to the sea (dissolved load), and will
become part of the salts in the sea.
See sections on SEDIMENTARY ROCKS and RIVERS.
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