39 terms

Physical Geology exam 3

What are the agents of metamorphism?
Heat, pressure, and chemical activity.
What is the difference between differential stress and confining pressure?
Pressure is the external force acting over a unit surface area of a material.

Stress is an internal force acting over a cross sectional area within the material.
Which of these two stresses is most likely to produce foliation in a metamorphic rock
Describe hydrothermal metamorphism.
Where does hydrothermal metamorphism occur?
What types of reactions occur?
Occurs along mid-ocean ridges where heated seawater percolates through hot, fractured basalt
Chemical reactions between heated seawater and basalt
Crustal rocks on continents can react with invading, hot fluids associated with igneous intrusions
What metamorphic rock is a typical product of hydrothermal metamorphism?
Talc and other clay minerals
Describe contact metamorphism.
Where does it occur?
What type of geothermal gradient is associated with contact metamorphism?
Occurs at high temperatures
Restricted to a small area around the margins of a magmatic intrusion
Geothermal gradients are high
What is a contact metamorphic aureole?
The area surrounding the intrusion where the contact metamorphism effects are present
What are the differences between burial and regional metamorphism? Which is more extensive? Where does each type occur?
Describe the stresses associated with burial versus regional metamorphism.
Burial Metamorphism
• Sedimentary rocks that had undergone diagenesis are buried even deeper
• A relatively mild type of metamorphism
• Diagenesis grades into burial metamorphism
• Results in partial alteration of mineralogy and texture
• Original bedding and sedimentary structures usually preserved

Regional Metamorphism (Figure 8.22)
• When temperatures & pressures increase beyond range of burial metamorphism
• Takes place under high temperatures & pressures over large areas
• Intense alteration of rock mineralogy and texture
• Original sedimentary structures destroyed
• Caused by major tectonic forces
What is cataclastic metamorphism? Where does it occur?
• Results from the crushing and shearing of rock during tectonic movement
• Generally localized along fault planes
• Might produce sheared, highly deformed rocks called mylonites
What is the difference between a fault breccia and mylonite?
Mylonite is a fine-grained, compact rock produced by dynamic recrystallization of the constituent minerals resulting in a reduction of the grain size of the rock

Fault breccia, or tectonic breccia, is a breccia (a rock type consisting of angular clasts,) that was formed by tectonic forces
Describe how foliation is produced in a metamorphic rock (3 mechanisms).
Rotation of platy and/or elongated minerals
Recrystallization of minerals in the direction of preferred orientation
Changing the shape of equidimensional grains into elongated shapes that are aligned
Name two common platy minerals that produce foliation in metamorphic rocks
mica and chlorite
Can non-platy minerals exhibit foliation (think metaconglomerate)?
In what tectonic environment do blueschists and eclogites form?
Blueschist Facies (very high pressures, relatively low temperatures)
Occurs within subduction zones
Metamorphosed basalt and shale contain blue amphiboles
Eclogite facies (extremely high pressures,moderate-high temperatures)
Identify the parent rock for each of the following metamorphic rocks: slate, quartzite, marble, greenstone, and anthracite.
anthracite-bituminous coal
What are migmatites? How do migmatites form? Do they represent low or high temperatures
Migmatite is a rock at the frontier between igneous and metamorphic rocks.
Migmatites form under extreme temperature conditions during prograde metamorphism, where partial melting occurs in pre-existing rocks.
What are index minerals?
degree of metamorphism a rock has experienced
Know the stages of the Elastic Rebound Theory in explaining earthquakes.
As rock is deformed, it bends and stores elastic energy. Once strained beyond it's breaking point, the rock ruptures and releases energy in the form of seismic waves.
2. How does the focus (hypocenter) of an earthquake differ from the epicenter?
Epicenter: the point on the Earth's surface directly above the focus
Hypocenter- the site of the earthquake
3. Describe the particle motions for P-waves, S-waves and surface waves.
P waves move through solid rock, and move things in the direction of its path
s-waves push things at a 90 degree angle
surface waves clear everything in the path
5. Explain how seismologists locate the epicenter of an earthquake?
the time of interval between the p waves and the s waves
7. What are some of the problems associated with using the Mercalli scale to measure the strength of earthquakes?
sometimes the damage can be deceptive
8. How is the Richter magnitude determined?
measure the amount of ground movement, and measuring the maximum amplitude of the p and s waves
9. What does the moment magnitude of an earthquake measure?
the actual amount of energy
10. Describe the global distribution of earthquakes. Where do most earthquakes occur? Intermediate and deep earthquakes primarily occur along which of the three types of plate boundaries?
Normal faulting (divergent plate boundary)
• Reverse and thrust faulting (convergent plate boundary)
• Strike-slip motion (transform plate boundary)
11. What causes tsunamis? Describe the changes to tsunami waves as they approach the shoreline in terms of wave velocity, wave height, the spacing between waves, and amount of destruction.
• Destructive waves that are often inappropriately called "tidal waves"
• Result from vertical displacement along a fault located on the ocean floor or a large undersea landslide triggered by an earthquake
• In the open ocean height is usually < 1 meter
• In shallower coastal waters the water piles up to heights that occasionally exceed 30 meters
• Can be very destructive
12. What are the various ways by which future earthquakes may be predicted?
time scales
13. What are seismic gaps and what do they say about future major earthquakes?
Seismic gaps along a fault (Figure 11.26):
- Segments where fault is locked

- Strain not readily released along these segments and thus builds up

- Fault eventually slips within a segment (seismic gap) and enormous strain build-up is released as a major earthquake
Distinguish between relative and absolute (numerical) dating.
Relative dating
- Determine sequence or order that geologic events and processes occurred
- No actual dates attached, just the order
• Absolute dating
- Determine actual ages of rocks and events
- Utilizes isotopes and radiometric dating
2. What is meant by the correlation of rock units? Identify different criteria (e.g. lithology, sedimentary structures, color, fossils, etc.) that can be used to correlate rocks from one exposure to another.
matching a particular rock layer (formation) in one exposure with its counterpart at a different locality
3. What are fossils? Describe at least five different types of fossils.
traces or remains of prehistoric life now preserved in rock
The remains of relatively recent organisms - teeth, bones, etc.
- Entire animals, flesh included
- Given enough time, remains may be petrified (literally "turned into stone")
- Molds and casts
- Carbonization
4. What is the Principle of Fossil Succession? How is this principle useful for correlating rocks from one exposure to another? What are the criteria for a good index fossil?
fossil organisms succeed one another in a definite and determinable order
Geographically widespread fossil that is limited to a short span of geologic time.
5. What is a formation? What are the different ways (criteria) by which rocks can be grouped into formations?
The most basic local unit of stratigraphy is the formation
• A particular rock unit is grouped into a formation based on a distinctive appearance and/or other characteristics:
- Lithology (rock type)
- Color
- Sedimentary structures
- Certain depositional patterns
6. Define an unconformity? What do unconformities represent in terms of rock units and geologic time? How do unconformities form?
erosional surfaces or intervals of missing strata within a rock sequence.

Types of Unconformities
• Angular unconformity
• Nonconformity
• Disconformity
7. Define each of the following: Principle of Superposition, Principle of Original Horizontality, and Principle of Cross-cutting Relationships. How are each of these principles applied to relative dating?
Law of Superposition: In an undeformed sequence of sedimentary rocks, each bed is older than the one above and younger than the one below (Figure 9.2).

Principle of Original Horizontality:
• Layers of sediment are generally deposited in a horizontal position
• If resulting sedimentary rock is folded or faulted, then the deformation occurred after formation

Principle of Cross-cutting Relationships (Figure 9.4):
• If a fault cuts rocks or a magma intrudes rocks...
- The fault/magma are younger than the rocks
- Faulting and intrusion occurred after the sediments were deposited and lithified

Inclusions (Figure 9.5)
• Inclusions are fragments of a rock unit enclosed in another rock unit
• The rock unit that supplied the inclusions must be older than the rock containing the inclusions
8. What are inclusions? If a sedimentary rock layer contains inclusions of an adjacent igneous rock, what can we say about the relative ages of these rock units?
fragments of a rock unit enclosed in another rock unit
• The rock unit that supplied the inclusions must be older than the rock containing the inclusions
10. What is radioactivity? What types of atoms undergo radioactive decay?
Types of Radioactive Decay (Figure 9.13):
• Alpha emission
• Beta emission
• Electron capture
11. What are isotopes? Distinguish between a parent and daughter isotope. Which one is more stable? Can a parent isotope decay into a daughter isotope of the same element?
Parent is the original unstable radioactive isotope
• Daughter is the new, more stable isotope formed as a result of radioactive decay of parent
12. What is meant by half-life? Do different parent-daughter isotopes (e.g. uranium 238 - lead 206, potassium 40 - argon 40, etc.) have different half-lives?
Time required for half the number of radioactive nuclei in a sample to decay to a new element