Geology 229Engineering Geology

Lecture 23

Rock Weathering

(West, Ch. 8)

Outline

Introduction of weatheringMechanical weatheringChemical weathering

Comparison of surface and subsurface conditions

Subsurface

• High temperature but constant at which minerals reach equilibrium

• high confining pressure (stress)

• less water or no water • no oxygen

Surface

• low temperature, and highly variable

• little or no confining pressure (stress)

• abundant of water • abundant of oxygen

Conclusion can be drawn from the comparison:

Rock at the surface will undergo changes

---- This change is called Weathering

Weathering is the physical breakdown (disintegration) and chemical alteration (decomposition) of rocks to form soil or loose particles at or near Earth's surface. Weathering causes deterioration of building materials. It also weakens rocks, a great concern when weathered rocks are used for foundation.

Two types of weathering

Mechanical weathering:Physical disintegration or degradation of rock pieces without a change in composition

--size reduction

Chemical weathering:decomposition whereby one mineral species is changed into another through various chemical processes. Water plays a major role, through:

1, provide oxygen, 2 provide mobility for moving ions.

Mechanical weathering always involves fracturing--but that can occur by a whole host of causes.

Chemical weathering tends to weaken rock, thereby making it easier to break. Likewise, mechanical weathering creates additional surface area that is prone to chemical attack. In this way, the two processes work together.

Weathering is controlled largely by climate. The more water available, the more likely that chemical processes can proceed. Additionally, if temperatures are warm, then chemical weathering can proceed even faster. Then, mechanical weathering can move more quickly also. In arid climates, however, weathering processes move very slowly. Mechanical weathering will be the dominant process in arid climates; however, because of its reliance on chemical weathering, it will also be quite slow.

mechanical weathering:

With or without water makes distinguished difference in rock weathering process, in arid region, since no water or little water presence, even the carbonate rocks (e.g., limestone) are not subject to solution but persist as resistant rocks. The rock mass may be sharp angular, and topography developed.

Mechanical Weathering (cont.):

Mechanical weathering processes include:

1) freezing & thawing (frost wedge)Example: Pottery container with water in winter time, when water is frozen and become ice it can have 9% of relative volume change, i.e.,

dv/v = 9% = 0.09This is a very large strain! Similarly if there is water in rock fractures it will force the fracture to propagate into further depth.

Mechanical Weathering (cont.):

The stress generated by frozen water is about 550 psi, check with Table 6.1 you can find out that this is a value comparable to the tensile strength of most rock types. Or only one order of magnitude less. But remember rocks are experiencing many freezing-thawing cycles before they are finally breaking down.

σ

Mechanical Weathering (cont.):

2) differential expansion and contractionAs temperature changes (in deserts or from forest fires), not all parts of a rock or all its minerals expand or contract by the same amount. So when rocks are heated or cooled, the mineral grains are subjected to differential stresses, which may be sufficient to make the rock spall, or break off in sheet-like pieces.

#

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.

The thermal expansion coefficient αL and αV

characters how much a mineral change its dimension in response to a unit degree increase in temperature . Using a finite expression

∆L=LαL∆T

PL Tl

L)(1

∂∂

=α PV TV

V)(1

0 ∂∂

=α

Mechanical Weathering (cont.):

The linear thermal expansion coefficient αL is on the order of 10-5/ºC, and it could be anisotropic, i.e.,

αLx ≠ αLy ≠ αLz

For example, αmax is in the elongation axis (C-axis).

Differential weathering

Processes of mechanical weathering unloading: jointing, exfoliation, and sheeting

Upon removal of overburden, the elastic component of rock deformation is recovered and the rock expands. The unloading may occur when the overlying rocks are eroded or rocks are removed from a quarry. The expansion caused by unloading may be sufficient to fracture the rock. Such naturally formed cracks are known as joints.

Typically, large plutons (bodies of igneous rock) or metamorphic bodies split into sheets that are parallel to the mountain face, a process known as exfoliation. It is also known as sheeting if the expansion from unloading occurs in granite to form rock slabs.

3), Sheeting and exfoliation

When subsurface confining stress is released fast, and the rock exposed to the surface, if the rock has large residual stress, it will be broken into blocks. The fracture in vertical direction form exfoliation. If the fracture is in horizontal direction, it calls sheeting.

The famous examples of exfoliation are the North Dome and Half Dome in the Yosemite National Park, CA.

joints are parallel cracks in which rocks on either side are not offset; SheetingSheeting rock layers peel like rock layers peel like layers of an onionlayers of an onion

Another “sheeting” example

See also T&L Figure 5.8

Chapter 5: 2) Mechanical weatheringChapter 5: 2) Mechanical weathering Unloading

North Dome

North Dome

Half Dome

Half Dome

Vertical columns frommagma cooling &shrinking

“columnar jointing”

Glacially polished basalt columns (end view)

Columnar jointing in basalt

4), Spheroidal weathering

Spheroidal weathering is caused by the combination of any of the following causes:

pressure relief, frost wedging, expansion, wind, and chemical weathering.

Spheroidal Weathering. Granite illustrates weathering forms quite well. Chemical weathering attacks to granite along joints and makes rounded boulders (Alabama Hills near Lone Pine) .

“Devil’s Marbles” Australia

5), Other mechanical weathering

erosion by wind and water(only the mechanical part);

forest fires;plants’ rooting;

Fire and water quenching was used till 16th

century to do rock quarrying.

Chemical weatheringChemical weathering=chemical processes that dissolve and decay earth materials;

Chemical weathering needs the rock exposes to air and water. Mechanical weathering could enhance chemical weathering by disintegration, i.e., increase the surface area of rock blocks and debris and this will greatly accelerates chemical weathering.

Chemical weathering rate depends on1. Temperature2. Amount of surface area3. Availability of water or natural acid

Thus, rocks in tropical environment experience most severe chemical weathering.

Because of its dipolar nature water is able to dissolve many chemical compounds. In addition to the solution effect, water aids decomposition through acid action, oxidation, and hydrolysis.

Acidity of Natural WatersWater is a good solvent. Acidic water is better!pH of most natural

waters ranges from 4 to 9

pH > 9 or < 4 occurs in extreme environments

Chemical Weathering of silicate minerals by carbonic acid

feldspar + water + carbonic acid = clay minerals + dissolved ions

2KAlSi3O8 + H2O + 2H2CO3 = Al2Si2O5(OH)4 + 2K+ + 4SiO2(aq) + 2HCO3

-

Why is rainwater naturally acidic?

Rainwater contains dissolved CO2 from atmosphere.

Dissolved CO2 reacts with water to form carbonic acid (H2CO3)

CO2 + H2O ⇔ H2CO3

Carbonic acid dissociates to produce hydrogen ion (H+) and bicarbonate

H2CO3 ⇔ H+ + HCO3-

Chemical weathering (cont.)

Common chemical weathering processes are solution (dissolution), oxidation, and hydrolysis.

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

Solution (or dissolution)

* Several common minerals dissolve in water

i), halite; ii), 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 one New Orleans graveyard.

A 16th-century monastery in Mexico shows the ravages of weathering mostly from wind and wind-driven rain. The rock is volcanic tuff.

Karst landscape of Guilin, China, caused dissolved Carbonate rocks.

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:

* olivine

* pyroxene

* amphibole

* biotite

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

* 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

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 (below) .

Mafic rocks such as basalt (which may contain olivine, pyroxene, or amphibole) weather by oxidation to an orange color (above)

Hydrolysis: Hydration-reaction between mineral and water

Since many igneous rocks contain feldspar minerals (K-feldspar and plagioclase) as a major component in mafic, intermediate, and felsic rocks, clays are common products of chemical weathering of igneous rocks. Clays are a group of minerals that are all sheet silicates with various amounts of water in their crystal structures (added to feldspar structure, which is a framework silicate). An example is kaolinite (a chalky white mineral)

Feldspar + water -> kaolinite + silica + potassium ions

KAlSi3O8 + H2O -> Al2Si2O5(OH)4 + SiO2 + K+

The difference between Weathering and Erosion

Weathering involves two processes Chemical weathering and Mechanical weathering, that often work in concert to decompose rocks. Both processes occur in place. No movement is involved in the weathering.

If a particle is loosened, chemically or mechanically, but stays, call it weathering.

Once the particle starts moving, it is erosion.

God’s Garden, Colorado

Twin Tower, God’s Garden, Colorado

Summary

Mechanical weathering is accomplished by physical forces that break rock into smaller and smaller pieces without changing the rock's mineral composition.

Chemical weathering involves breaking down rock components and internal structure and forming new compounds.

Whereas weathering breaks rocks apart, erosion removes rock debris by mobile agents such as water, wind, or ice.