1. Ch. 2, Part-II Rocks, Rock Materials & Geologic Structures

2. Chapter (Section) Objectives Review of some of the important mineral and rock types and their environmental significance –Relationships between atoms, minerals, rocks, rock materials –Basic silicate building block(s) –Properties of rocks & minerals –Basic rock types, basis for classification, –Why this stuff is important & the types of information they provide Appreciation/significance of geologic structures –Layering –Folds –Faults –Other structures (joints, dikes/sills, etc.)

3. Rock: –A solid, cohesive aggregate of grains of one or more minerals Mineral: – Naturally occurring crystalline inorganic substance with a definite chemical composition; element or compound with a systematic arrangement of atoms / molecular structure (e.g., sulfur, salt, silicates such as feldspar) Crystallinity –Atomic arrangement imparts specific physical and chemical properties Physical properties of minerals: –color, hardness, cleavage, specific gravity, streak, etc.

4. Minerals: –Systematic groupings of atoms –e.g., salt (NaCl)

5. Relationship between: –Atoms –Molecules –Minerals –Rocks –Landforms

7. Basic Silicate Structure: The silica tetrahedron

8. Silicate Mineral Structures

9. Main Rock Forming Minerals Fewer than 20 minerals account for the bulk of the earth’s crust: Most are silicates (See Hand Specimens) Percent in Crust Mineral (formula) Igneous Sedimentary Metamorphic 51% Feldspar (Na, K, Ca) (Al,Si) 4 O 8 XXX (39)Plagioclase (Na, Ca) (Al,Si) 4 O 8 (12)Alkali feldspar (Na, K) (Al,Si) 4 O 8 12 %Quartz- SiO 2 XXX 11 %Pyroxene- (Ca, Mg, Fe) Si 2 O 6 XX 5 %Amphibole-XX 5 %Mica- XX 5 %Clay-X 3 %Olivine (Mg, Fe) 2 Si O 4 X 8 %Others (non-silicates)- XXX halides, sulfates, sulfides, carbonates, Fe-Ti oxides, phosphates, native elements, etc.) Minor and trace elements: Minor element minerals (sulfides, uranium mins., heavy metals, trace element substitutions.

10. Rock Materials & Properties Rock materials: Composed of one or more component minerals having discrete physical and chemical characteristics The physical (e.g., color, hardness) and chemical characteristics of rocks and rock material reflect the combined characteristics (properties) of the discrete component materials (i.e., minerals)

11. Rock Strength: Stess-Strain Relationships

12. Three (3) Major Rock Types 1.Igneous –Formed from molten material (e.g., lava, granite) 2.Sedimentary (including sediment) –Formed from the weathering of other rocks, as chemical precipitates, or biologic material (shells) 3.Metamorphic (including hydrothermal rocks & minerals) –Rocks modified/changed by heat and/or pressure

13. Relationship between Rock Types and Plate Tectonics

14. Rock Cycle- Cycle of melting, crystallization, weathering/erosion, transportation, deposition, sedimentation, deformation ± metamorphism, repeat of crustal materials.

15. Igneous Rocks Definition: –Rocks formed from high-temperature silicate liquid (molten) rock material (magma) [high-temperature  800 o C to 1300 o C] Igneous rock material –Formed by solidification of molten material –Usually with the formation of high-temperature minerals (as crystals) that form from the magma as it cools

16. Classification of Igneous Rocks By Physical Criteria, i.e., grain size –Cooling rate & where cooling occurs (determines grain size) Chemical Criteria, i.e., Composition –Mainly by relative amounts of iron (Fe), magnesium (Mg), silicon (Si), ± water Primary Materials –Material from which magma is formed (mantle, crust) –Material that is “melted” to form magma

17. Classification of Igneous Rocks: By Physical Criteria Slow cooling  produces large crystals (minerals)  Coarse-grained rocks –Example: Granite –Slow cooling due to intrusive, thermally insulated emplacement of magma Rapid cooling  produces small, or no crystals  Fine-grained rocks –Example: lava, ash –Rapid cooling due to “extrusion, i.e., eruption” of magma at surface

18. Relationship between Rock Types and Plate Tectonics

19. Classification of Igneous Rocks: By Physical Criteria

20. Correlations between composition and physical properties, such as eruptive style of volcanic rocks Silica Content –Si-poor magmas (Hawiian-type) are fluid (low viscosity) –Si-rich volcanic magmas (St. Helen’s-type) are viscous (sticky) Explosiveness –Explosive eruptions result from Si-rich magmas w/ water, gases –Explosiveness depends on how well gases and water are released from the magma Lower viscosity, less gas  non-explosive eruptions High viscosity + gas  violently explosive eruptions So where & why do these types occur??? (more later)

21. Chemical & Physical Properties of Igneous Rocks and Plate Tectonics

22. Sedimentary Rocks Rocks form from: –The mechanical and/or chemical weathering of other rocks –Material deposited/precipitated from water via chemical or biological (organic) processes

23. Types / Classification of Sedimentary Rocks 1.Clastic: Formed from the mechanical and/or chemical weathering of other rock materials –Sandstone, shale –conglomerate 2.Chemical: Formed as inorganic precipitates (i.e., water saturated with respect to chemical compounds) –Limestone (Ca-carbonates (caliche) –Other salts, e.g., sulfates, hydroxides, halogen salts (e.g., NaCl) –Silica 3.Organic: Formed from (and including) organic material such as: –Fossil materials (typically shells, diatoms, etc.); exoskeletons, or endoskeletons of aquatic (e.g., marine) organisms –Organic and/or chemical cements (carbonate, silica, phosphates) 4.Combinations –e.g., Clastic or organic sediment with chemical cement

24. Clastic Sedimentary Rocks: Further classified by grain size

25. Chemical Sediments e.g., evaporite salt deposits

26. Organic Sediment Chalk

27. Environmental Conditions Indicated From Sediment and Sedimentary Rocks Environment in which they formed, e.g., –Marine Deep (limestone, shale) Shallow (deltas, reefs) –Terrestrial Glacial River/stream Arid/desert Environmental conditions –Source(s) –Mode and distance of transport –Depositional processes, e.g., near-source vs. mature sediment (coarse sand vs. shale)

29. Metamorphic Rocks Formed from other rocks but modified (e.g.,recrystallized) by heat and/or pressure Types –Foliated (alignment or banding of planar minerals) Slate Schist gneiss –Non-foliated (no preferrential alignment of minerals) Quartzite Marble (sometimes foliated) Hydrothermal/baked rocks (skarn)

30. Foliated Metamorphic Rock

31. Significance of Rock Types to Environmental Geology Type and origin of rocks provide insight into present or past environmental conditions (e.g., flood deposits, volcanic mudflows) Differences in rock types can have important envirornmental implications (e.g., strata/layers) Physical Properties –Strength –Planes of weakness –Porosity, permeability Chemical Properties –Tendency to dissolve (solubility), leach, or react

32. Examples Limestone: –Typically formed in a reef or deep marine setting –Highly stable in arid climates, unstable in wet climates –Poor aquifer material –Highly conducive to formation of ore deposits when adjacent to igneous magmas or hydrothermal fluids Implications for finding them in high mountains?

33. Examples con’t Sandstone –Formed as near-shore marine and desert environments (w/ noteable differences) –Moderate strength –Generally porous and permeable Foliated Metamorphic Rocks –Implies formation under conditions of directed tectonic forces –Have potential planes of weakness Others (See charts/figures)

34. Relationship between Rock Types and Plate Tectonics