Ch 9. Weathering

A. Mechanical Weathering

B. Chemical Weathering

C. Soil

Weathering

Mechanical Weathering Physical disintegration of rock

(with no chemical alteration) Chemical Weathering

Chemical alteration of minerals within the rock

Usually softening or dissolving the minerals

Forming clays, oxides and solutes

Weathering

Weathering, a geologic process:1. Separates out mineral grains2. Converts silicates to clay3. Oxidizes iron minerals (i.e., ferromagnesians) 4. Dissolves some ions (e.g, calcium) Erosion Transports Sediments

Quartz

Na Plagioclase

Biotite

Quartz1

Feldspar1

Clay2

Hematite3

Calcite4

Sediments of:

Weathering and Plate tectonics

Tectonic forces lift the land up Compression and uplift at

convergent boundaries Isostatic uplift due to crustal

thickening and buoyancy Stresses fracture rocks

Weathering and Erosion break the land down Mechanical and chemical

breakdown Erosion by gravity, wind, water,

ice moves material downhill

A. Mechanical Weathering Rocks that have been

exhumed Pressure release forms

sheet joints and exfoliation Frost wedging propogates

fractures and breaks rocks into smaller fragments

Crushing and Abrasion break rocks down further in tallus slopes, streams and beneath glaciers

An exfoliation dome in Yosemite, CA (Half Dome)

Mechanical Weathering

Talus Slopes

The result of Mechanical

weathering Rock falls and slides Crushing and

abrasion (more mechanical weathering)

Rock Avalanches Slopes of rock

fragments may let go and careen downhill as a very fast flow

Sediments of: Parent rock Mineral particles Angular fragments(Near source)

Sediments from Mechanical Weathering

Weathering

Mechanical Weathering Frost Action Pressure Release Crushing and Abrasion

Chemical Weathering Dissolving Dissolved ions Oxidation Iron in Ferromag. Minerals Iron

Oxides (e.g., Hematite) Formation of Clays from silicates (e.g., Feldspar)

Chemical Weathering (Table 9.1)

Oxidation: 4FeSiO3 + O2 + H2O FeO(OH) + 4SiO2

Hydration: CaSO4 + 2H2O CaSO4·2H2O

Hydrolysis

Soil Formation and Weathering Related to Climate

Weathering of Granite

1. Mechanical Weathering forms rock and mineral

fragments of parent rock

Chemical Weathering2. Converts silicates* to clay3. Oxidizes iron in minerals

(i.e., ferromagnesian minerals) 4. Dissolves some ions

(e.g, Ca, Na, K, Mg)

*except quartz

Quartz

Na Feldspar

Quartz1

Feldspar1

Clay2

Hematite3

Ions4

Sediments of:

Biotite

Rates of Weathering Stable at High

Temperatures Quickly Weathered

Slowly Weathered Stable at Atmos.

Temperatures

Con

vert

ed t

o cl

ay,

oxid

es,

and

ions

by

che

mic

al w

eath

erin

g

Olivine

PyroxeneGroup

AmphiboleGroup

MicaGroup

ClayGroup

FeldsparGroup

Quartz

IsolatedSilicateStructure

Single ChainStructure

Double ChainStructure

SheetStructure

FrameworkStructure

SilicateStructure Mineral

Mech. Weath.

Cool mountainous regions accelerate mechanical weath.

Warm moist climates accelerate chem. weath.

Increased slopes accelerate weathering Gravity caries sediments

away from slopes Exposing fresh surfaces

Other factors increasing

Rates of Weathering

Joints or fractures accelerate both mech. and chem. weath.

Other factors increasing Rates of Weathering

John Ford Point MonumentAlso See Fig. 13.7 of the

Colorado Plateau

Figure 3.8: Ship Rock ArizonaVolcanic Neck

Fig 5.3

Ship Rock, New MexicoVolcanic Neck

John Ford Point Monument, Arizona.

Differential Weathering

Weathering forms Soils

Acidic water percolates downward and A. Chemically weathers minerals in soil to form

• Clays• Iron Oxides• Dissolved Ions

E. Leach downward and B. Accumulate. C. Mechanical weathering breaks down bedrock

Sediment: Products of Weathering

Immature Mature

Transport Distance

Near source Far from source

History of Weathering

Short history of mechanical weathering

Long history of mechanical & chemical

weathering

Products of Weatheringi.e., types of

sediment

Forms rock and mineral fragments

Angular at first Rounded with time

and transport

Converts silicates to clays, iron oxides and dissolved ions which are transported to the ocean by streams

leaving stable minerals, eg. qtz. (& fld.)

cm2cm2

m2