Ch. 2, Part-II Rocks, Rock Materials

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Ch. 2, Part-IIRocks, Rock Materials Geologic
Structures
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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.)

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• 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.

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• Minerals
• Systematic groupings of atoms
• e.g., salt (NaCl)

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• Relationship between
• Atoms
• Molecules
• Minerals
• Rocks
• Landforms

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Basic Silicate Structure The silica tetrahedron
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Silicate Mineral Structures
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Main Rock Forming Minerals

• Fewer than 20 minerals account for the bulk of
  the earths crust Most are silicates (See
  Hand Specimens)

Percent in Crust Mineral (formula)
Igneous Sedimentary
Metamorphic   51 Feldspar (Na, K, Ca)
(Al,Si)4 O8 X X X (39) Plagioclase (Na, Ca)
(Al,Si)4 O8 (12) Alkali feldspar (Na, K)
(Al,Si)4 O8 12 Quartz- SiO2 X X X 11
Pyroxene- (Ca, Mg, Fe) Si2 O6 X X 5
Amphibole- X X 5 Mica- X X 5
Clay- X 3 Olivine (Mg, Fe)2 Si O4 X 8
Others (non-silicates)- X X X 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.
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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)
•  

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Rock Strength Stess-Strain Relationships
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Three (3) Major Rock Types

• Igneous
• Formed from molten material (e.g., lava, granite)
• Sedimentary (including sediment)
• Formed from the weathering of other rocks, as
  chemical precipitates, or biologic material
  (shells)
• Metamorphic (including hydrothermal rocks
  minerals)
• Rocks modified/changed by heat and/or pressure

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Relationship between Rock Types and Plate
Tectonics
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• Rock Cycle- Cycle of melting, crystallization,
  weathering/erosion, transportation, deposition,
  sedimentation, deformation metamorphism, repeat
  of crustal materials.

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Igneous Rocks

• Definition
• Rocks formed from high-temperature silicate
  liquid (molten) rock material (magma)
  high-temperature ? 800oC to
  1300oC
• 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

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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

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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

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Relationship between Rock Types and Plate
Tectonics
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Classification of Igneous Rocks By Physical
Criteria
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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. Helens-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)

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Chemical Physical Properties of Igneous Rocks
and Plate Tectonics
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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

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Types / Classification of Sedimentary Rocks

• Clastic Formed from the mechanical and/or
  chemical weathering of other rock materials
• Sandstone, shale
• conglomerate
• 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
• 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)
• Combinations
• e.g., Clastic or organic sediment with chemical
  cement

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Clastic Sedimentary Rocks Further classified by
grain size
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• Chemical Sediments
• e.g., evaporite salt deposits

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Organic Sediment Chalk
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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)

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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)

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Foliated Metamorphic Rock
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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

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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?

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Examples cont

• 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)

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Relationship between Rock Types and Plate
Tectonics
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