Plate Tectonics (the unifying theory in Earth Sciences) Summary Data and Questions
Definition: Theory that large segments or plates of the lithosphere move relative to one another
A. Continental Drift Hypothesis
Although Suess (1885) noted similarities of Glossopteris flora and glaciation and named the southern super-
continent Gondwana, Alfred Wegener is credited with developing the hypothesis of Continental Drift. He suggested
a super-continent of Pangea that formed in the Paleozoic that subsequently split apart.
Evidence:
1. Continental fit
2. Glacial evidence
3. Similarities of rock sequences (compositions and types including coal distribution)
4. Mountain ranges of the same age and rock type on different continents
5. Fossil evidence (Glossopteris flora, Mesosaurus, Cynognathus, Lystrosaurus)
This evidence, though persuasive, lacked a means to describe how the continents moved.
B. Plate Tectonics (move to a theory)
Evidence plus a mechanism:
1. Paleomagnetism
a. Polar wandering
b. Magnetic reversals (striping) on the sea floor; led to idea of seafloor spreading
2. Age of ocean floor materials
3. Distribution of earthquakes
4. Distribution of volcanoes
5. Hot spot “trails”
C. Types of Plate Boundaries (see Table 2.2): Know locations and examples, types of rocks, stress
regimes, and types of geohazards
1. Divergent (spreading ridges, split apart)
2. Convergent (colliding margins)
3. Transform (strike-slip)
D. Plate Names (the big seven and there are many smaller ones), Rates of Movement (mm’s to
cm’s per year), and Mechanisms of Movement (convection cells and slab push/pull models)
E. Resources
1. Where do they occur and why? At what plate boundaries?
2. Types of resources: How does the previous and current distribution of plates dictate the occurrence of oil and gas,
coal, copper, gold, metals, etc.
3. Where would you look for these resources?
F. Questions for Your Consideration
1. Where are active hazardous earthquakes and volcanoes today? Where would I go to find the most exciting
geology?
2. Where are most of our resources located? What are the controls on their distribution?
3. Are there different rock types on continents and ocean floors, and if so, Why?
4. How does a hot spot reveal information to support the Plate Tectonics Theory?
G. Interior of the Earth: Composition and How do we Know and what is the significance?
1. The Earth is composed of many “layers” but you should know the main 4 including their composition, thickness
etc.:
a. Lithosphere (continental and ocean) – the thin outer shell
b. Mantle - plastic
c. Outer Core - liquid
d. Inner Core – solid
2. How do we know the composition and liquid vs. solid?
Earthquake waves, seismic waves and models, nuclear explosions, meteorites
How do we know what is
inside the earth and why do
we care?
How do we know?
1. Seismic data
2. Earthquakes
3. Magma and rock comp.
Why do we care?
1. Heat of the earth
2. Geothermal energy
3. Geomagnetism
(magnetic field protects us)
4. Location of resources
We only know directly about the
“skin” of the Earth; but even that
upper 5 – 40 km is continuously
in motion leading to the formation
of ocean basins, mountains, and all
of the features we observe.
Granite: Light in color w/ less
dense minerals like quartz,
feldspar, mica
(Grain density = 2.8 g/cc)
Basalt and peridotite: Dark in
color w/ dense minerals like
olivine, augite
(Grain density = 3.1 g/cc)
Earth Materials: Minerals, Rocks, and Sediments
Mineral Definition:
1. Must occur naturally
2. Must be inorganic
3. Must be a solid
4. Must possess an orderly internal structure; atoms must be arranged in a definite pattern
5. Must have a Definite chemical composition that varies within small limits.
Ex. Graphite – lubricant, pencil lead, golf – C Halite – Salt – NaCl
Quartz – used for glass, computer chips - SiO2 Talc – baby powder – Mg3Si4O10(OH)2
(Are coal, pumice, and/or cubic zirconia minerals?)
ELEMENTS (Basic building block of minerals:)
a. 112 elements, 92 are naturally occurring.
b. Only eight make up the bulk of minerals and represent 98% (by weight) of the continental crust.
What are the most abundant elements on or in the earth? Or is it the same thing? The interior has more Mg, Ni, Fe and less Al, Na,
K because of early differentiation (density)
CRUSTAL (ELEMENTAL) COMPOSITION
ELEMENT SYMBOL % BY WEIGHT Mineral Percent in Crust
Oxygen O 46 Plagioclase 39
Silicon Si 28 Quartz 12
Aluminum Al 8 Orthoclase 12
Iron Fe 5 Pyroxene 11
Calcium Ca 4 Micas 5
Sodium Na 3 Amphiboles 5
Potassium K 3 Clay minerals 5
Magnesium Mg 2 Other silicates 3
All Others >1 Nonsilicates (calcite/halite, etc) 8
Properties of Minerals
1. Crystal Form – external expression of the orderly internal arrangement of atoms.
Ex. Quartz (hexagonal) with pyramidal shaped ends
2. Hardness – measure of the resistance of a mineral to scratching
Use MOHS Scale of Hardness
Relative Scale Mineral Hardness of Common Objects
Hardest 10 Diamond
9 Corundum
8 Topaz
7 Quartz 6.5 Ceramic Streak Plate
6 Feldspar 5.5 Glass Plate
5 Apatite
4 Fluorite 3.5 Copper Penny
3 Calcite 2.5 Fingernail
2 Gypsum
1 Talc
3. Color – obvious feature of minerals but not truly diagnostic (inclusions)
Ex. Olivine – green; Quartz – clear, smoky, amethyst (purple), rose; sulfur – yellow
4. Luster – appearance or quality of light reflected from surface of mineral
a. Metallic – pyrite, galena, magnetite
b. Non-metallic – vitreous (glassy, quartz), silky (talc), earthy (soft hematite)
5. Streak – color of mineral in powdered form. Use streak plate.
Ex. Hematite – red to reddish brown/gray
6. Cleavage – tendency of mineral to break along planes of weak bonding. All minerals do not have cleavage; some
fracture such as quartz.
Ex. Calcite – 3 directions not at 90°, muscovite – 1 perfect direction
7. Tenacity – resistance a mineral offers to breaking (brittle, ductile, elastic, malleable, etc.)
8. Specific Gravity – compares weight of a mineral to the weight of an equal volume of water
9. OTHER: Magnetism – magnetite; Double Refraction – calcite; Malleability – gold; Taste – halite; Feel – talc, graphite;
Smell – sulfur; Reaction to HCl – calcite, dolomite
The BIG FIFTEEN: Rock-forming Minerals you MUST know
Ferromagnesium (also called Mafic) Silicates (Fe, Mg, Si, Al, O) Primarily Occurs In:
Olivine (Mg, Fe)2SiO4 Ig., meta rks
Pyroxene Ca, Mg, Fe, Al silicate Ig., meta rks.
Amphibole Hydrous Na, Ca, Mg, Fe, Al silicate Ig., meta rks.
Biotite Hydrous K, Mg, Fe silicate All rk. types
Non-Ferromagnesium (also called Felsic) Silicates (Si, Al, Na, K, O)
Quartz SiO2 All rk. types
K-feldspar KalSi3O8 All rk. types
Plagioclase Ca(Na)Al2Si2O3 All rk. types
Muscovite Hydrous K, Al silicate All rk. types
Clay minerals Various Soils, sed rks
Carbonates
Calcite CaCO3 Sed. rks
Dolomite CaMg(CO3)2 Sed. rks
Sulfates
Anhydrite CaSO4 Sed. rks
Gypsum CaSO4.H2O Sed. rks
Halides
Halite NaCl Sed. rks
Sylvite KCl Sed. Rks
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Rock Cycle and Rock Types
Rock – solid aggregate of mineral particles (mostly). Some rocks are composed of volcanic glass (obsidian) and of organic
particles (peat, coal, skeletal limestone).
Rock Cycle – conceptual model that explains how all rocks are formed (cooled from magma, cemented grains), transformed (heat
and pressure), destroyed (broken down to grains), and/or re-formed as a result of environmental factors and natural processes.
All rock-forming materials come from:
a. Earth’s Mantle – molten rock: magma or lava
b. Organisms – plants and animals
c. Fragmentation/Chemical Decay of Other Rocks – sediments
d. Space
Three Rock Groups (Identification of rock types rely on their Composition and Texture; form and color)
Igneous – formed when magma or lava cool to a solid state, either glass or masses of intergrown mineral crystals
Sedimentary – formed when fragments of plants, animals, or other rock are compressed or cemented together; or when masses of
mineral crystals precipitate from water
Metamorphic – rocks are changed from one form to another (transformed) by intense heat, pressure, or by the action of fluids.
North Carolina Mineral and Rock Facts: (Go to website: www.geology.enr.state.nc.us/ for additional information)
N.C. has 310 minerals (3700-4000 in the world)
First gold rush in 1799 near Charlotte (17lb. nugget found)
N.C. official state rock is granite; state precious stone is emerald
Mineral industry is $1billion industry in N.C.
N.C. leads nation in production of feldspar (most common mineral in earth’s crust and used to make ceramics and abrasives),
mica (filler in paints, plastics, cosmetics, drilling mud), and high purity quartz (glass, industrial materials, computer
components; chips in silicon valley and in the Palomar telescope)
Dimension stone (like granite for building faces and monuments); ?largest in Mt. Airy in world
Other important minerals/rocks in N.C.
- Phosphate (Aurora quarry: fertilizer, feed, ceramics)
- Common clay (bricks and tiles)
- Olivine (refractory liner; only in N.C. and Washington)
- Aggregate (sand and gravel for roads and construction)
- Peat (organic matter for soil additive, filtering, and energy)
Name 10 rocks/minerals/soils/sediments that are resources and their uses. Five should include quartz, feldspar, coal,
clay, lime/limestone.
Igneous Rocks
Igneous (“Fire”) Rocks – formed by the cooling of molten rock.
Magma – molten rock below surface Lava - molten rock at the surface
Why doesn’t molten rock stay at depth? Liquid is less dense than rock confining it so confining pressure and low density
causes the molten material to squeeze up through weak points in earth’s lithosphere.
1. Intrusive – magma cools below the surface. Large crystals can form because of slow-cooling.
Types of Intrusive Bodies A. Batholiths – very large (~100 km2) B. Sills – sheet-like, parallel to
bedding C. Dikes – cut across bedding
2. Extrusive – magma cools at the surface. Rapid cooling either leads to small or no crystal formation from
lava. Explosive forms are the geohazards.
Types of Extrusive Bodies A. Lava flows B. Pyroclastics (ash, flows, etc.)
The location, type of material, and hazard potential of intrusives, lava flows, and volcanic activity is primarily dictated by plate
tectonics.
Formation of Igneous Rocks: There are 3 locations where igneous rocks form that bring them near or to the surface
of the earth.
1) Mid-oceanic ridges (Divergent) 2) Subduction zones (Convergent) 3) Hot Spots
Ex. Mid-Atlantic Ridge (Iceland) Ex. Japan, Western S. America Ex. Hawaii, Yellowstone
Geologic hazards associated with volcanoes are concentrated along these areas though the type of magma (amount of
gas it contains and whether it is felsic or mafic) controls the explosivity of the volcano. This too is controlled by plate
tectonics. Spreading centers and hot spots have lower hazard potential than volcanoes at converging boundaries.
Why do we have different kinds of magma?
1) Source material - spreading centers and hot spots are more mafic, subducting zones are more felsic
2) Bowen’s Reaction Series – cooling of magma leads to a predictable igneous rock composition
as different minerals crystallize at different temperatures. As minerals form they are removed
from the melt changing composition. At what temperature would you melt a rock?
Classification of Igneous Rocks
Three Properties
1) Mineralogic composition, 2)Color Index (mostly controlled by minerals), 3)Texture
Minerals: Eight minerals (rock-forming minerals) occur most commonly in igneous rocks.
A. Felsic (light colored) – quartz, potassium feldspar, muscovite, plagioclase (Na mostly)
B. Mafic (dark colored) – biotite, amphibole, pyroxene, olivine
C. Intermediate – contains both felsic and mafic minerals
2. Color Index (CI) – percentage (by volume) of mafic minerals in the rock. Used as a rough measure of the amount of mafic and
felsic minerals. Know minerals!
3. Texture – description of components of rocks including their sizes, shapes, and arrangement. Intrusives usually cool slowly and
have large crystals; Extrusives rapidly cool and fine-grained
Textural Terms
1) Phaneritic – coarse-grained crystals (1-10 mm)
2) Aphanitic – fine-grained crystals (requires hand lens or scope to see: 1 cm)
5) Porphyritic – two distinct crystal sizes
Phenocrysts are large crystals (larger than matrix or groundmass). May have phenocrysts in aphanitic or
phaneritic textures.
Porphyritic indicates two different cooling rates. Larger crystals formed more slowly and then cooling rate
increased to have smaller crystals.
6) Vesicular – trapped gas bubbles leave “holes” in rock. May have many or few.
7) Pyroclastic – rocky material broken and ejected during eruption; may contain ash (64 mm).
Classification of Igneous Rocks
STEPS: Identify the rock’s texture, dominant material (Aphanitic, phaneritic, glassy), Identify the mineral composition and color index
(determine felsic, intermediate, mafic so light minerals are felsic and dark are mafic), Add qualifying terms such as porphyritic (name
phenocrysts if possible) or vesicular.
Igneous Rock Questions you Should be able to Answer for Lab and Lecture
Everyone should of course be able to completely diagram and explain in detail the rock cycle – I mean
detail. How about drawing it on the back of this sheet?
1. Where do igneous rocks form and occur?
2. a. What are the types of volcanic geohazards? Name at least two areas that have been impacted by a
volcanic geohazard in the last 20 years.
b. Does the U.S. have any potential volcanism geohazards?
3. What are the best properties to use to classify igneous rocks?
4. Why are some rocks light and some rocks dark? Be specific.
5. I have three rocks (I think they are igneous). Let me describe them to you and you tell me where they
were formed, how, and give me a possible mineral composition and rock type.
a. Very small crystals, black
b. Some large crystals (plagioclase) in a finely crystalline matrix (dark)
c. Vesicles throughout rock (dark, except on the surface which is brown – why would it be brown?)
6. There are upwards of 4000 minerals in the world, or some such huge number. However, if you know
the rock forming minerals you will be able to make most of our rocks. What are those rock-forming
minerals?
7. THOUGHT Questions:
A mineral resource is a concentration of a naturally occurring material in or on the crust of the earth in a
form that may be currently or potentially extracted economically. U.S. mineral raw materials are worth $32
billion annually. Igneous processes (e.g. plate tectonics) concentrate many minerals such as gold,
diamonds, copper, magnetite, etc. Why do you think there are these “exotic” minerals with igneous rocks?
If many of these are related to plate boundaries, why was N.C. the site of the most gold production in the
U.S. prior to the California Gold Rush in 1848-49?
Rock Cycle and Rock Types
Rock – solid aggregate of mineral particles (mostly). Some rocks are composed of volcanic glass
(obsidian) and of organic particles (peat, coal, skeletal limestone).
Rock Cyle – conceptual model that explains how all rocks are formed (cooled from magma, cemented
grains), transformed (heat and pressure), destroyed (broken down to grains), and/or re-formed as a result
of environmental factors and natural processes.
All rock-forming materials come from:
1) Earth’s Mantle – molten rock: magma or lava
2) Space – meteorites
3) Organisms – plants and animals
4) Fragmentation/Chemical Decay of Other Rocks – sediments
Three Rock Groups (Identification of rock types rely on their Composition and Texture; form and color)
Igneous – formed when magma or lava cool to a solid state, either glass or masses of intergrown mineral
crystals
Sedimentary – formed when fragments of plants, animals, or other rock are compressed or cemented
together; or when masses of mineral crystals precipitate from water
Metamorphic – rocks are changed from one form to another (transformed) by intense heat, pressure, or
by the action of fluids.
Sediments and Sedimentary Rocks
Sediment – loose grains or chemical residues derived from inorganic (rock and mineral) and organic (plants and
animals) materials, and/or from chemical precipitates (halite, gypsum).
The formation of sediments, other than the precipitates, is from the weathering of pre-existing materials (rocks, trees,
shells)
Weathering – breakdown of pre-existing rocks and sediments
1. Physical (mechanical) – break larger particles into smaller ones (cracking, abrading)
Ex. Break granite into rock fragments and minerals, shells into smaller pieces, logs into plant fragments
2. Chemical – dissolution or decomposition
Ex. Halite dissolves to salty water, calcite decomposes to calcium and bicarbonate, feldspar from granite
decomposes to clay minerals
Sedimentary Rocks are consolidated sediments composed of organic or inorganic materials, or chemical
precipitates. Hardening of sediments to rock occurs by:
1. Compaction – grains pack closer together: Sand ---- Sandstone (Compaction and cementation)
2. Cementation – precipitates such as calcite, halite, silica, etc. “glue” the sediments or grains
together
3. Precipitation of crystals from aqueous solutions, often form crystalline masses
Ex. Salt in Solution ---- Evaporation of Water ---- Halite (solution to crystal)
Composition and Texture of Sediments and Sedimentary Rocks
Sediments have different COMPOSITIONS based on their origin and different TEXTURES because of how
they are transported and then deposited. These are the two keys to the Description, Identification, and
Interpretation of Sedimentary Rocks.
COMPOSITION
Types and abundance of grains that make up sediment or sedimentary rock.
1. Detrital (Clastic or Terrigenous) – decomposed pieces of pre-existing rock. Includes rock
fragments, mineral grains (quartz, feldspar, micas), and clay minerals (decomposed
feldspars and other minerals).
Ex. Breccia, conglomerate, sandstone, siltstone, shale
2. Biochemical – remains of organisms such as shells (corals, forams, clams, etc.) and plants
EX. Skeletal limestone, peat, coal
3. Chemical – composed mostly of intergrown mineral crystals precipitated from aqueous
solutions (halite, gypsum, calcite, dolomite)
Ex. Chemical limestones (travertine, micrite, oolites), Dolostones, Gypsum, Chert, and Halite
Texture – description of a sediment’s or sedimentary rock’s constituent parts and their sizes, shapes, and
arrangement.
1. Grain size – particle size. Usually relate size to energy of transport and deposition (larger equals higher energy)
Gravel – grains larger than 2 mm (pebbles, boulders)
Sand – grains 62 um to 2 mm. Feel gritty like sandpaper
Silt – grains 4 – 62 um. Grains are mostly too small to see
Clay - 1 to 0.5 b.y.) and abundant (43) peaks over 6000’. The highest is
Mt. Mitchell at 6,684’.
Information for Geologic Maps/Structure/Stratigraphy
Stratigraphy – study of layers of sediment and rock and how they were deposited.
1) Principle of Original Horizontality – sediments are originally deposited as near horizontal sheets. If they are at a
high angle that means they have been deformed.
2) Law of Superposition – The oldest rock is at the bottom and the youngest is at the top (true unless severe
structural deformation)
3) Law of Faunal Succession – fossils change with time.
Index fossil – key fossils, used to represent named subdivisions of the geologic time scale
4) Principle of Cross-Cutting Relationships – any feature that cuts across a rock or sediment must be younger than
what it cuts across.
5) Lateral continuity, 6) Principle of Inclusions, 7) Baked zones
Geologic Cross Section – a cutaway view of the earth. It’s like digging a trench and looking at the sides of the trench.
The cross section shows how the rocks are arranged, what they are, and even the surface topography.
Structural Geology – how rocks deform after they were originally deposited. Stresses such as compression, tension,
and shear cause these deformations.
Ex. Outcrop shows deformation, what type of stress caused it?
Types of Structures: Unconformities, Faults, Folds
Unconformity – rock surface that represents gap in the geologic record either by removal of sediment by erosion or
non-deposition
a. Disconformity – unconformity between parallel strata
b. Angular – unconformity between non-parallel strata
c. Nonconformity – stratified sedimentary rock deposited on igneous or metamorphic rock.
Faults – a break in the rock, shown by offset of layers or rock types. There is either vertical and/or lateral offset
a. Normal – footwall moves up relative to hanging wall
(Form in a tensional regime such as divergent zones/rifts)
b. Reverse – footwall moves down relative to hanging wall
(Form in a compressional regime such as convergent zones) Thrust – low angle reverse fault
c. Lateral or strike-slip – lateral movement of rock
(Form in Shear stress regime as at transform boundaries)
Folds – upward or downward arching of layers of rock. Folds have limbs on each side of axis (axial plane), elongate
in plan view, may plunge (angle between fold axis and horizontal)
a. Anticline – convex fold, oldest rocks in middle
b. Syncline – concave fold, youngest rocks in middle
More circular forms
c. Dome – upward arch, strata oldest at center
d. Basin – downward arch, strata youngest at center
Strike – compass direction (bearing) of a line formed by the intersection of horizontal plane and an inclined surface
Dip – angle between horizontal plane and inclined surface. Dip is measured perpendicular to strike
Geologic Map – shows the distribution (describes rock types and age) of rocks at or near the surface (sometimes the maps ignore
the surficial recent material). The colors represent ages/rock types/formations
Formations - mappable rock units
Ex. Castle Hayne Limestone in North Carolina; Coconino Sandstone in Grand Canyon
Boundaries between these formations are called contacts.
Block diagram – combination of the geologic map and the geologic cross section.