Weathering: the decay of rocks and the source of sediments inand the source of sediments in

sedimentary rocks

Geology 200Geology 200Geology for Environmental Scientists

Physical Weathering

• Ice Wedging (Geo-wedgies!) - the most effective form of physical weathering. Ice p y gcan exert 1500 lbs/in2. Most effective with multiple freeze-thaw cycles.p y

• Sheeting or exfoliation - expansion joints• Thermal expansion/contraction• Thermal expansion/contraction• Plant roots• Animal burrows

Fig. 10.2a. Ice wedging

Physical weathering of columnar basalt

Weathering by exfoliation, Half-Dome at Yosemite National Park

Fig 10 4 Sheeting in graniteFig. 10.4. Sheeting in granite

Exfoliation: horizontal joints formed by h i b d i l dsheeting as overburden pressure is released.

Chemical Weathering

• Two variables control most aspects of chemical weathering.g

• Water - the more precipitation, the greater the rate of weathering Dry climates have athe rate of weathering. Dry climates have a slow rate.

• Temperature warm wet climates have• Temperature - warm, wet climates have the greatest rate of chemical weathering. Very cold climates have a slow rateVery cold climates have a slow rate.

Fig. 10.20: Precipitation, temperature, and rates of weathering are related to latitude (precipitation andweathering are related to latitude (precipitation and

vegetation are reversed).

Fig 10.19. Climate and weathering

Chemical Weathering - processes

• Dissolution• Hydration

O id ti• Oxidation

DissolutionW t ill di l i l b it i• Water will dissolve many minerals because it is a bipolar molecule. It acts to loosen the bonds

f i t th f f i l S lt dof ions at the surface of minerals. Salt and gypsum are easily dissolved.

• Carbon dioxide dissolved in water forms carbonic acid, which dissolves calcite.H2O + CO2 H2CO3

CaCO3 + H2CO3 Ca+2 + 2HCO3- (bicarbonate)CaCO3 H2CO3 Ca 2HCO3 (bicarbonate)

Dissolution

• Some silicates, such as pyroxene, will also dissolve in carbonic acid:

MgSiO + H 0 + 2H COMgSiO3 + H20 + 2H2CO3 Mg+2 + 2HCO3

- + H4SiO4 (silicic acid)

Hydration

• A mineral reacts with either the H+ or the OH- (hydroxide) from water to produce a ( y ) pnew mineral. Aluminum silicates do not dissolve in water. Feldspars weather this pway to form clay; e.g. plagioclase to clay:

• (equation not balanced)( q )

NaAlSi3O8 + H20 + H2CO3 Na+ + HCO3

- + H4SiO4 + Al2Si2O5(OH)4Na HCO3 H4SiO4 Al2Si2O5(OH)4

Oxidation

• The chemical combination of oxygen with a mineral. Important in weathering iron-rich p gsilicates: olivine, pyroxene, amphibole, biotite. Final product is hematite or limonite.p

2Fe SiO + 4H O + O 2Fe O + 2H SiO2Fe2SiO4 + 4H2O + O2 2Fe2O3 + 2H4SiO4

Fe2O3 + H2O 2FeO(OH) (limonite)

Results of Chemical Weathering

• Tables 10.1 and 10.2: Review• Na K Ca and Mg are removed into solutionNa, K, Ca, and Mg are removed into solution.• Al and Si are concentrated in clays.

F i i d i id• Fe is incorporated into oxides.• Clays and oxides are in equilibrium with the

earth’s surface.

Figure 10 8 - Susceptibility toFigure 10.8 Susceptibility to weathering

• Least stable: halite, gypsum, pyrite, calcite, dolomite

• Olivine, Ca-plagioclase, pyroxene, amphibole biotite Na-plagioclase K-amphibole, biotite, Na plagioclase, Kfeldspar, muscovite

• Most stable: quartz clay aluminum oxides• Most stable: quartz, clay, aluminum oxides (bauxite), iron oxides.

Quartz

Fig. 10.8. Relative stability of minerals

Weathering of silicate rocks is related to Bowen’s Reaction Series or Temperature of Crystallization

Weathering susceptibility follows the same sequence asWeathering susceptibility follows the same sequence as Bowen’s Reaction Series or Temperature of

Crystallization

Weathering of Major Rock Types

• Granite - physically weathers by exfoliation to form domes; chemically weathers to ; yquartz grains and clay. This is the source of quartz sand grains.q g

• Basalt - weathers totally to clay and iron oxides; forms red or brown soilsoxides; forms red or brown soils.

River Sand fromRiver Sand from eroded Granite

Weathering of Major Rock Types

• Sandstone - quartz grains are highly resistant to weathering and are recycled into g ynew sand deposits; cement type, calcite, iron oxide, or silica, controls erosion of , ,sandstones.

• Limestone - weathers rapidly in moistLimestone weathers rapidly in moist climates often forming karst features such as caves and sinkholes; forms cliffs in aridas caves and sinkholes; forms cliffs in arid regions.

Seneca Rocks -- vertical beds of the Tuscarora Sandstone

Karst forms in humid climates

LimestonesLimestones form cliffs in

id li tarid climates, such as the Red Wall Ls in the Grand Canyon

Weathering of Major Rock Types

• Shale or mudstone - weathers rapidly because it is fine-grained and soft. The gclays are transported in water by suspension to form muds which can later become mudstones and shales.

Pennsylvanian age mudstones and shales at Goshen Road

Differential Weathering

• Various rocks weather at different rates. This creates topography where the hills, p g p y ,ridges, or mountains are capped by resistant rock types. Most of the ridges of the yp gAppalachians are capped by sandstone.

Products of Weathering

• Rocks weathered into spherical shapes• Regolith including soil produced• Regolith, including soil, produced• Ions go into solution

Shape of Weathered Rocks

• Fractures and joints provide opportunities for weatheringg

• Bedding planes - rocks break into slabs or sheetssheets

• Cleavage or foliation - metamorphic rocks break along these planesbreak along these planes

• Shattering - in dense rocks like quartzite

Shape of Weathered Rocks

• Granular disintegration - sandstone or granitesg

• Spheroidal weathering - angular fragments become rounded as spheres have the leastbecome rounded as spheres have the least surface area per unit volume

• Exfoliation breaking into concentric layers• Exfoliation - breaking into concentric layers

Boulders showing spheroidal weathering

Regolith

• A blanket of loose decayed rock debris derived from the bedrock beneath it. The thickness may range from a few cms. to 100s of meters depending on climate, type p g , ypof rock, and length of time for weathering.

Regolith is thickest where there is abundant precipitation and warm temperatures (precipitation p p p (p p

and vegetation are reversed).

SoilSoil• The uppermost layer of regolith. ComposedThe uppermost layer of regolith. Composed

of weathered rock and clays, plus decomposed organic matter.deco posed o ga c atte .

• Soil horizons:A A A i l h lA0, A1, A2: organic layer, humus layer,

zone of leachingB: subsoil, zone of accumulation of clays

and oxidesC: weathered bedrock

Fig. 10.14gCommon soil profile

S il H iSoil Horizons

Soil and ClimateSoil and Climate

S bt i l il ft hi hl th d• Subtropical soils are often highly weathered and are red because of iron oxides.

• Tropical soils are highly weathered and are called laterites. They can be up to 60 m thick! They often accumulate aluminum oxides forming the mineral bauxite.

• Deserts and permafrost - thin soil• Temperate - best agricultural soilsTemperate best agricultural soils

Soil and Climate: tropical on theSoil and Climate: tropical on the left, temperate on the right

Soils: Parent Rock

• Good soils form on limestone and mafic igneous rocks. Many plant nutrients are g y preleased by chemical weathering.

• Poor soils form on quartz-rich rocks likePoor soils form on quartz rich rocks like sandstone, quartzite, or quartz-rich granites. Relatively few nutrients released for plantsRelatively few nutrients released for plants.

Ions in Solution

• Ions dissolved in water are invisible products of chemical weathering. Each year p g ythe world’s rivers carry about 4 million metric tons of dissolved material to the ocean (Table 10.2).

Rates of Weathering

• Controlled by the rock type and climate.• Note the different rates of weathering ofNote the different rates of weathering of

gravestones. Choose granite over marble.• Even the pyramids in a desert climate show• Even the pyramids in a desert climate show

substantial effects of erosion.V l i k i i l i• Volcanic rocks in tropical environments weather at a very rapid rate, >50 cm per 1 0001,000 yrs.

Weathering of tombstones: Granite on the left, gMarble (calcite) on the right (both 1888)

Weathering of the Pyramids in Egypt Less than 5000in Egypt. Less than 5000

years.