Types of Weathering

1. Types of Weathering • Mechanical – physically breaks down rock into smaller pieces or grains • Abrasion • Frost/ice wedging • Thermal expansion/contraction • Vegetation – root growth • Exfoliation • Chemical – alters the mineralogy of rocks through ion exchange • Oxidation – oxygen reacts with iron to form iron-oxide minerals (eg. Hematite) • Dissolution : rain + carbon dioxide (from air) = carbonic acid (reacts with rocks) H2O + CO2 = H2CO3 • Hydrolysis – minerals react with water to form new minerals (eg. feldspar + water + H ions = clay + dissolved ions)

2. ExfoliationEnchanted Rock, Fredricksburg, TXGranite Pluton

3. Weathering Rates • Composition – minerals stable at the Earth’s surface don’t weather as quickly as those that crystallize at depth (e.g.. Qtz is generally unaffected by dissolution, oxidation and hydrolysis) • Structure – fractured or faulted rock provide an increased surface area for weathering processes to attack • Climate – direct correlation between availability of water/ temperature fluctuations and increased weathering processes

4. How would weathering processes be affected by these different climate regions?1) Tropical2) Arid desert3) Temperate mountain4) Sub-polar

5. Lithification – turning sediment into rock • Diagenesis – a set of processes that collectively alter the physical and chemical nature of the rock • Burial – overlying sediments, subsidence, subduction • Heat – radioactive minerals, Earth’s mantle/core • Pressure – overlying deposits • Percolating water and solutions

6. Diagenesis • Burial - Upper few kilometers of the Earth’s crust • Temperature < 400F • Compaction – pressure reduces volume of sediments • Cementation – materials dissolved in solution percolate and precipitate between grains and bind them (usually either silica or calcite)

7. Erosion Cycle • Erosion • Fluvial • Aeolian • Glacial • Gravity • Transportation • Water • Air • Ice • Gravity • Deposition • Downslope • Basins • Layering (Stratification)

8. Sedimentary Structures • Sediments are usually deposited on horizontal planes • Bedding (stratification) • Graded bedding • Normal • Reverse • Cross-bedding • Ripple marks • Mudcracks • Salt casts • Biogenic forms and casts

9. Classifying Sedimentary Rocks • Clastic (detrital) • Depends on particle size NOT composition • All detrital rocks are clastic – mineral grains bound by cementation • Chemical • Depends on composition and are formed of interlocking crystals • Inorganic • Biogenic

10. Clastic Texture • Grain size • Depends on parent rock • Depends on mineral hardness • Depends on nature and energy of transport medium • Inversely dependant on distance from source i.e. larger = nearer smaller = farther • Sorting • Well sorted (aeolian sands) • Poorly sorted glacial till • Depends on nature and energy • Wind most selective • Glacial least selective • Grain shape (rounding) • Depends on source rock • Energy of transport • Distance from source • Composition of grains

11. Detrital Sedimentary Rocks • Mudstones - very fine grained, low-energy environments • Shales – fissile mudstone, mostly clays and micas • Siltstones – gritty, quartz rich • Lakes, lagoons, deep ocean, low energy river environments • Red – Fe-rich with oxygen • Green – Fe-rich without oxygen • Black – carbon-rich

12. Detrital Sedimentary RocksMudstones/ShalesTilted shales and sandstones in Rainbow Basin, near Barstow, California

13. Sandstones • 1/16 – 2 millimeter grain size • Silica or carbonate cement • Quartz arenite • 90% quartz • Little matrix • White to buff color • Very pure, well rounded, well sorted • Arkoses • 25% feldspars • Pinkish to reddish • Typically from granites • Poorly sorted • Angular grains • Greywackes • Dark gray to greenish • Qtz, felds, rock fragments, typically volcanic • Matrix of clays and micas • Angular grains

14. Aeolian Cross-bedded SandstoneCoconino SS Oak Creek Canyon

15. Conglomerates and Breccias • Largest grain size > 2 millimeters • Poorly sorted with lots of matrix • Cong. = rounded clasts • Breccias = angular clasts • Easy to identify source rock for large clasts • High energy environments

16. Detrital Sedimentary Rocks Conglomerates and Breccias

17. Chemical Sedimentary Rocks • Inorganic • Precipitate directly from waters • Often in deep ocean basins • Either temp. change or evaporation • Inorganic limestones • Chert – occurs everywhere • Evaporites – arid, shallow, temporary waters • Biogenic • Formed thru biologic activity • Commonly form carbonate muds in marine environments • Shallow, continental shelves • Fossiliferous limestones • Coquina • Coal • Cherts

18. Chemical Inorganic Sedimentary RocksTravertine LimestoneJemez Mtns. New Mexico

19. Chemical Organic Sedimentary RocksCoal SeamBlack Mesa, Arizona

20. Chemical Organic Sedimentary RocksBlack Mesa, Arizona

21. Environments of deposition(Fig. 7.20 pp.210-211 – note for lab) • Continental • Mostly detrital • Grain size relates to energy of depositing medium • Size and rounding ~= proximity to source rock • Marine • Vary according to depth and distance from continents • Continental shelves – continental detritus • Deep ocean floor – chemical and biogenic deposits • Transitional • Continental and marine processes merge • Beaches • Deltas • Estuaries • Lagoon • Subduction zones

22. Sedimentary Facies • Horizontal change in physical properties • Due to changes in energy • Distance from source rock (generally > distance = smaller grain size and more rounding; also removal of less resistant minerals) • Change in environment (subaerial to subaqueous) • Transgression/regression • Deposited at same time under sometimes vastly different conditions and/or environments • Figure 7.21

23. Carbon (carbonate) CycleBox 7.1 pp. 200-201 • Earth cycles/stores carbon between spheres • Dominantly found bound in compounds • Carbon dioxide – atmos to bio to hydro to litho • Calcium carbonate (CaCO3) – long-term storage • Depletion of C in atmos = reduction in greenhouse effects • Hydrocarbons – bio to litho, released in fossil fuel consumption • Moderate-term storage of paleo-solar energy • Increases C in atmos = greenhouse effects • Important to planetary climatic conditions • Venus vs Earth