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 grains 2.Converts silicates to clay 3.Oxidizes iron minerals (i.e., ferromagnesians) 4.Dissolves some ions (e.g, calcium) Erosion Transports Sediments Quartz Na Plagioclase Biotite Quartz 1 Feldspar 1 Clay 2 Hematite 3 Calcite 4 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: 4FeSiO 3 + O 2 + H 2 O  FeO(OH) + 4SiO 2 Hydration: CaSO 4 + 2H 2 O  CaSO 4 ·2H 2 O  Hydrolysis

Soil Formation and Weathering Related to Climate

Weathering of Granite 1. Mechanical Weathering forms rock and mineral fragments of parent rock Chemical Weathering 2. Converts silicates* to clay 3. Oxidizes iron in minerals (i.e., ferromagnesian minerals) 4. Dissolves some ions (e.g, Ca, Na, K, Mg) *except quartz Quartz Na Feldspar Quartz 1 Feldspar 1 Clay 2 Hematite 3 Ions 4 Sediments of: Biotite

Rates of Weathering Stable at High Temperatures Quickly Weathered Slowly Weathered Stable at Atmos. Temperatures Converted to clay, oxides, and ions by chemical weathering Olivine Pyroxene Group Amphibole Group Mica Group Clay Group Feldspar Group Quartz Isolated Silicate Structure Single Chain Structure Double Chain Structure Sheet Structure Framework Structure Silicate Structure 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 Monument Also See Fig of the Colorado Plateau Figure 3.8: Ship Rock Arizona Volcanic Neck Fig 5.3 Ship Rock, New Mexico Volcanic 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 sourceFar from source History of Weathering Short history of mechanical weathering Long history of mechanical & chemical weathering Products of Weathering i.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.) cm 2 m2m2