Ch 9 Weathering A Mechanical Weathering B Chemical

Ch 9. Weathering A. Mechanical Weathering B. Chemical Weathering C. Soil

Weathering n Mechanical Weathering n n Physical disintegration of rock (with no chemical alteration) Chemical Weathering n n n Chemical alteration of minerals within the rock Usually softening or dissolving the minerals Forming clays, oxides and solutes

Weathering Quartz Sediments of: Quartz 1 Feldspar 1 Clay 2 Hematite 3 Calcite 4 Biotite Na Plagioclase 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

Weathering and Plate tectonics n Tectonic forces lift the land up n n Compression and uplift at convergent boundaries Isostatic uplift due to crustal thickening and buoyancy Stresses fracture rocks Weathering and Erosion break the land down n n Mechanical and chemical breakdown Erosion by gravity, wind, water, ice moves material downhill

A. Mechanical Weathering n Rocks that have been exhumed n n n 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

Mechanical Weathering n An exfoliation dome in Yosemite, CA (Half Dome)

Talus Slopes n The result of n n Mechanical weathering Rock falls and slides Crushing and abrasion (more mechanical weathering) Rock Avalanches n Slopes of rock fragments may let go and careen downhill as a very fast flow

Sediments from Mechanical Weathering n Sediments of: n n n Parent rock Mineral particles Angular fragments (Near source)

Weathering n Mechanical Weathering n n Frost Action Pressure Release Crushing and Abrasion Chemical Weathering n n n 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) Hydrolysis Oxidation: 4 Fe. Si. O 3 + O 2 + H 2 O Fe. O(OH) + 4 Si. O 2 Hydration: Ca. SO 4 + 2 H 2 O Ca. SO 4· 2 H 2 O

Soil Formation and Weathering Related to Climate

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

n n Stable at High Temperatures Quickly Weathered Isolated Silicate Structure Olivine Single Chain Structure Pyroxene Group Double Chain Structure Amphibole Group Sheet Structure n n Slowly Weathered Stable at Atmos. Temperatures Mineral Framework Structure Mica Group Clay Group Feldspar Group Quartz Converted to clay, oxides, and ions by chemical weathering Rates of Weathering Silicate Structure Mech. Weath.

Other factors increasing Rates of Weathering n n n Cool mountainous regions accelerate mechanical weath. Warm moist climates accelerate chem. weath. Increased slopes accelerate weathering n n Gravity caries sediments away from slopes Exposing fresh surfaces

Other factors increasing Rates of Weathering n Joints or fractures accelerate both mech. and chem. weath.

Differential Weathering John Ford Point Monument Also See Fig. 13. 7 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.

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 cm 2 Immature m 2 Transport Distance History of Weathering Products of Weathering i. e. , types of sediment cm 2 Mature Near source Far from source Short history of mechanical weathering Long history of mechanical & chemical weathering n n n Forms rock and mineral fragments Angular at first Rounded with time and transport n n Converts silicates to clays, iron oxides and dissolved ions which are transported to the ocean by streams leaving stable minerals, eg. qtz. (& fld. )