Where are we??? Earth’s matter & structure Rock forming processes Intro to geology Plate tectonics Minerals Rocks Igneous rocks Volcanism Weathering & erosion Sediments and Sedimentary rocks Metamorphic rocks Rock record and Geologic time Rock deformation 19. Earthquakes 20. Evolution of continents 21. Exploring Earth’s Interior 17. Earth beneath the ocean, shorelines 12. Mass wasting 13. Hydrologic cycle and Groundwater 14. Streams 15. Deserts & winds 16. Glaciers, Ice Ages 18. Landscapes 22. Energy and Mineral Resources 23. Earth’s environment, Global Change, Human Impacts Earth’s matter & structure Rock forming processes Where are we??? Dynamic motions External factors, resources

Chapter 7: Weathering and erosion Study questions…. 1) Overview 2) Physical weathering 3) Chemical weathering 4) Weathering rates 5) Soil basics Some MC questions….

Chapter 7: Weathering and erosion Study Questions: 1. What is the difference between chemical and physical weathering? 2. What is the difference between weathering and erosion? 3. What are the different types of physical weathering? 4. How does feldspar weather into clay? 5. What are the three main agents of chemical weathering? 6. How does silicate weathering influence climate? 7. What controls rates of weathering (e.g, rock characteristics,temperature, humidity). 8. What is soil? ?

weathering erosion mass wasting Chapter 7: 1) Overview Ways to move material to lower elevations: weathering erosion mass wasting rock disintegration (physical breakdown) rock decomposition (chemical alteration) removal of material by water, wind, ice Later chapter transfer of rock & soil downhill

Chapter 7: 2) Physical weathering Physical forces break rock into smaller & smaller pieces w/out changing mineral composition 4 ways this happens in nature: Frost wedging Exfoliation and spheroidal weathering Thermal expansion Biological activity

Frost wedging How? Where? Chapter 7: 2) Physical weathering Frost wedging Repeated cycles of freezing & thawing can break rock into smaller fragments How? Water expands ~ 9% upon freezing Where? Mountainous regions w/ daily freeze/thaw cycles

Chapter 7: 2) Physical weathering Frost wedging

Chapter 7: 2) Physical weathering Frost wedging Talus slopes

Why? Manifestation: Expansion of rock from removal/erosion Chapter 7: 2) Physical weathering Exfoliation and spheroidal weathering Expansion of rock from removal/erosion of overlying rock (“sheeting”) Why? Reduction in pressure (since less overlying weight) Manifestation: Slab-like layers/sheets break loose

Continued weathering causes rock slabs to separate & fall Exfoliation and spheroidal weathering Chapter 7: 2) Physical weathering Continued weathering causes rock slabs to separate & fall HALF DOME, YOSEMITE NATIONAL PARK (California) “exfoliation domes”

Chapter 7: 2) Physical weathering Exfoliation and spheroidal weathering Chapter 7: 2) Physical weathering Another “sheeting” example

“Devil’s Marbles” Australia Chapter 7: 2) Physical weathering Exfoliation and spheroidal weathering “Devil’s Marbles” Australia

Expansion/contraction of rock Chapter 7: 2) Physical weathering Alternating Heat and Cold Expansion/contraction of rock from heating/cooling

Expansion/contraction of rock Chapter 7: 2) Physical weathering Alternating Heat and Cold Expansion/contraction of rock from heating/cooling

Expansion/contraction of rock Chapter 7: 2) Physical weathering Alternating Heat and Cold Expansion/contraction of rock from heating/cooling

Weathering of rock from activities of Chapter 7: 2) Physical weathering Biological activity Weathering of rock from activities of organisms plants burrowing animals humans

Chapter 7: 2) Physical weathering Surface area increases due to physical weathering and more surface becomes available for chemical reactions.

Carbon dioxide (Hydrolysis) Oxygen (Oxidation) Chapter 7: 3) Chemical weathering Chemical weathering: Processes that break rock components and internal structures of minerals, making new minerals Most important agents in chemical weathering: Water (Dissolution/Hydrolysis) Carbon dioxide (Hydrolysis) Oxygen (Oxidation)

How? Dissolving minerals by a liquid agent (such as water) Chapter 7: 3) Chemical weathering Water (Dissolution) Dissolving minerals by a liquid agent (such as water) How? Many minerals are water-soluble example: Halite (salt), Calcite (calcium carbonate) Pure water is a weak acid Water molecules break down from H2O into H+ and (OH-)

Decomposition of granite Water (Hydrolysis) Chapter 7: 3) Chemical weathering Decomposition of granite Granite (mainly quartz + potassium feldspar) + water Liberates potassium and silica from feldspar, go into solution makes kaolinite - this left over mineral is clay - has water absorbed (=hydration) - main inorganic part of soil quartz - very stable, glassy appearance - carried to sea: beach sand, sand dunes - cements to form sandstone See Fig. 7.3

Carbon dioxide (Hydrolysis) Chapter 7: 3) Chemical weathering Carbon dioxide combines with water to carbonic acid CO2 + H2O H2CO3 A small portion dissociates into hydrogen ions and bicarbonate ions: H2CO3 H+ + H CO3 - Carbonic acid is a weak + most common natural acid Speeds weathering rates of feldspar and calcite compared with pure water See Fig. 7.6

Chapter 7: 3) Chemical weathering See Fig. 7.6 See Fig. 7.6

Chapter 7: 3) Chemical weathering See Fig. 7.6

Example Chemical reaction that causes loss of electrons Chapter 7: 3) Chemical weathering Oxygen (Oxidation) Chemical reaction that causes loss of electrons Example Iron (Fe) + oxygen (O2) (dissolved in water) oxygen (O) combines with Fe to form iron oxide = hematite (Fe2O3) Oxidation decomposes important minerals: mineral formula where originates from olivine (Mg,Fe)2SiO4 Earth’s mantle pyroxene (Mg,Fe)SiO3 Earth’s mantle hornblende Ca2(Fe,Mg)5Si8O22(OH)2 Continental rocks See Fig. 7.8

Bedrock characteristics Chapter 7: 4) Rates of weathering Several control factors: Bedrock characteristics Climate Topography Some minerals are more stable than others Temperature, moisture (vegetation) freeze-thaw cycles e.g., gentle or steep slopes Table 7.15 And 7.2

Chapter 7: 4) Rates of weathering Differential weathering

Chapter 7: 4) Rates of weathering

Chapter 7: 5) Soil basics Regolith Layer of rock and mineral fragments produced by weathering that cover most of Earth’s land surface Soil = decomposed rock + decayed animal/plant life (“humus”) water air 45% 5% 25% “good soil:”

Chapter 7: 5) Soil basics

Important Soil-forming factors Chapter 7: 5) Soil basics Important Soil-forming factors Parent material Climate Time Plants/Animals Slope affects fertility, weathering rate temperature & precipitation longer time thicker soil organic matter if too steep little/no soil

A few questions… 1. Which of the following processes is not an example of chemical weathering? A. dissolution of calcite B. breakdown of feldspar to form kaolinite C. splitting of a rock along a fracture D. rusting of a nail

A few questions…   2. __________ is a chemical reaction in which an atom or ion loses electrons. A. Carbonation B. Exfoliation C. Hydration D. Oxidation

3. Which of the following statements is true? A few questions…   3. Which of the following statements is true? A. High temperature promotes physical weathering. B. Frost wedging is a form of chemical weathering. C. Chemical weathering is promoted by gentle slopes. D. Clay minerals are produced primarily by physical weathering.

A few questions…   4. Which of the following will decrease the rate of chemical weathering of a rock at the Earth’s surface? A. increasing the amount of acid in the rainwater B. decreasing the temperature C. breaking the rock into smaller pieces D. increasing the amount of surrounding soil

5. Which of the following statements is true? A few questions…   5. Which of the following statements is true? A. Silicate weathering and volcanism both increase the amount of carbon dioxide in the atmosphere. B. Silicate weathering increases and volcanism decreases the amount of carbon dioxide in the atmosphere. C. Silicate weathering decreases and volcanism increases the amount of carbon dioxide in the atmosphere. D. Silicate weathering and volcanism both decrease the amount of carbon dioxide in the atmosphere.