Ch. 2, Part-IIRocks, Rock Materials &

Geologic Structures

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.)

•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.

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

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

Basic Silicate Structure: The silica tetrahedron

Silicate Mineral Structures

Main Rock Forming MineralsFewer than 20 minerals account for the bulk of the earth’s crust: Most are silicates (See Hand Specimens)

Percentin 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 X11 % 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.

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)

 

Rock Strength: Stess-Strain Relationships

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

Relationship between Rock Types and Plate Tectonics

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

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

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

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

Relationship between Rock Types and Plate Tectonics

Classification of Igneous Rocks: By Physical Criteria

Exam ples:- L ava- A sh

Further Subdivided By Eruptive Style:-E xp los ive (w / g as , w a te r)-N on -exp los ive (H aw iian -typ e)

Characteristics/Features:- C rys ta ls : ve ry sm a ll o r ab sen t- R ock = F in e-g ra in ed o r g lassy

Setting:- E xtru s ive , i.e ., V o lcan ic-E ru p ted ; on th e su rface o r ve ry sh a llow

Rapid

Exam ples:- G ran ite- G ab b ro

Further Subdivided By Depth & Relative G rain-S ize:-V ery d eep = ve ry s low = very la rg e c rys ta ls-M ed iu m o r sh a llow d ep th = m ed iu m -s ize c rys ta ls

Characteristics/Features:- C rys ta ls : L a rg e- R ock = C oarse-g ra in ed

Setting:- In tru s ive (p lu ton ic )- D eep w ith in th e earth

Slow

Cooling Rate

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)

Chemical & Physical Properties of Igneous Rocks and Plate Tectonics

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

Types / Classification of Sedimentary Rocks1. 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

Clastic Sedimentary Rocks: Further classified by grain size

Chemical Sediments

e.g., evaporite salt deposits

Organic Sediment

Chalk

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)

Metamorphic RocksFormed 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)

Foliated Metamorphic Rock

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

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?

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)

Relationship between Rock Types and Plate Tectonics