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GEOG 203 Midterm 2 Review.doc

Course Code
GEOG 203
Bruce Murphy
Study Guide

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Midterm #2 Review
Lecture 1: Rock types, mineralogy, physical and chemical weathering
Biogeochemistry: The movement of elements and chemical species between components of the
Human Impacts – The effect of human activities: deforestation, acid precipitation, and accelerated
rates of soil erosion
Processes include:
Gas exchanges with soils: CO2, CH4, N2O
Nutrient cycling: Ca, Mg, K, N, P
Weathering and soil formation
Atmosphere<>Vegetation<>Soil organic matter<>soil mineral matter<>bedrock
Important questions to answer in this section
What is the composition of major rock types which form the parent material of soils?
What are the processes of physical and chemical weathering?
What are the important properties of soil and what controls them?
How do soil profiles develop? Controlled by climate, parent material, vegetation,
topography and time, how do they vary in properties?
How do soils erode? Predicting rates of soil erosion, causes, effects, and alternatives
Biogeochemical cycling. Human impact on biogeochemistry, effects of forest removal and
acid rain
Rock types:
Igneous: rocks composed of minerals from molten magma
Intrusive: Igneous rock that cools slowly in the crust. Formers a pluton a
large batholith of rock that makes up certain mountain ranges. Ex. Sierra Nevada, Idaho, or
coastal range B.C. Granite and Diorite are examples of intrusive.
Extrusive: Formed as a result of melting of the earth’s crust at the surface. Glossier
surface considering they cool much quicker than intrusive. Examples include obsidian and
volcanic lava.
Igneous minerals
Sedimentary: Rocks composed of minerals from other rocks and minerals at near-surface
temperature and pressures
Metamorphic: formed under high heat, and pressure. “secondary process”
***Good figure, 11.5-11.6 in book for rock cycle
Chemical composition of earth’s crust

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Mainly Oxygen (46.6%) and Silicon (27.7%). Aluminum (8.1), Iron (5.0), Calcium (3.6),
Sodium (2.8), Potassium (2.6), Magnesium (2.1).
Ca, K, Mg, N, P, S are all important plant nutrients
Refer to slide that shows mineralogy of various minerals and their compositions. Mainly
Silicon and Oxygen
The silicon tetrahedron
Silicon and Oxygen form a tetrahedral structure, 4 oxygen atoms and 1 silicon atom in the
middle. The overall charge of a silicon tetrahedron is -4
This negative charge is shared with adjacent Si atoms, or with positively charged
atoms cations such as Fe2+, Mg2+ etc. Within the mineral “lattice”
Olivines: All Fe/Mg to “absorb” negative charge from silicon tetrahedron
Pyroxenes: single chain…50% Si-O-Si
Amphiboles: double chain…62.5% Si-O-Si
Mica : sheet, 75% Si-O-Si
Quartz : 3-D crystalline structure all sharing Si-O-Si
Chart in notes is good reference
Relative size of major ions: K > Ca > Na > Fe > Mg > Al > Si
Isomorphous (no change in structure) substitution: Replacement of the central Si atom by
Al as minerals form from molten magma. Results in alumunosilicate minerals
This results in the loss of one positive charge for each atom replaced because
Si4+ >> Al3+
To balance the loss of positive charge, a cation must be incorporated into the
structure to provide the extra positive charge (K+, Na+, Ca2+). These are called, feldspars.
Albite (Na) NaAlSi2O8
Anorthite (Ca) Ca2Al2Si2O8
Orthoclase (K) KAlSi2O8
Mineral structure affects weathering ability, based on the strength of bonding between
element and oxygen in the mineral. Si-O-Si is resistant, Al-O-Al is weaker, Fe/Mg – O, and Na/K/Ca
– O is most easily weathered. Quartz is strongest to weathering (pure O-Si-O bonding)
Sedimentary Rocks
Clastic: composed of weathered and transported minerals: mixed mineralogy
Conglomerate (boulders), Sandstone (sand size), Shale (clay size)
Chemical: composed of new minerals formed in situ(underground) in sedimentary

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Ex. Limestone CaMg(CO3)2
Evaporites: Na and Ca chlorides and sulfates
Ironstone: Oxides and hydroxides of Fe and Al
Organic: Coal, and oil
Weathering: The breakdown of rocks and minerals to produce soil materials
Rate of weathering is dependent on the intensity of weathering process, and resistance to
rocks and minerals
Two major types of weathering
Physical: Disintegration of rocks and minerals, decreasing particle size and
increasing surface area
Chemical: Chemical transformations into new products
Neither chemical or physical weathering can occur without the other
Physical weathering processes
Free-thaw: Due to increase in volume of water when it freezes
Thermal changes: variations in temperature lead to expansion and contraction of
of rocks and minerals. Ex. Exposure to strong sunlight, or fires burning in soil
Wetting/Drying: Clays increase in volume when “wetted”
Biological: Plant root penetration, earthworm churning of soil
Salt weathering: formation and growth of salt crystals in arid climates
Pressure release: When igneous rocks are exposed at the surface
Chemical weathering processes
Direct solution: dissolution of soluble salts
Hydration: absorption of water, hematite to limonite. Anhydrite to gypsum
Oxidation: Change in valency (Fe3+ <> Fe2+)
Chelation: reaction of normally insoluble elements with complex organic
compounds produced by decomposition of organic matter, Eg. Fe, Al
Carbonation: reaction of carbonic acid produced by dissolving CO2
Hydrolysis: reaction of H+ ion with cation in mineral
H+ + KAlSi3O8 >>> H4SiO4
***Look at slides for helpful diagrams
Rates of weathering
Dependent on intensity of weathering processes and strength and resistance of
rocks and minerals
Strength of rocks generally:
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