Geology Exam 3

What are the two subdivisions of metamorphic rocks?

Foliated and Non-foliated

What are some common dangerous events caused by earthquakes?

Faulting/shaking, fires, tsunamis, landslides.

What are three characteristics of an isochrone diagram?

1. The slope of the isochron is used to calculate age of a rock. 2. The start of every isochrone diagram is flat as they all have the same initial starting ratio. 3. Isochrone never passes through the origin as the ratio is never 0. Always start with some ratio.

What is an index mineral map?

A map which identifies metamorphic zones.

What are metamorphic facies?

A set of metamorphic mineral assemblages indicative of metamorphism under a specific range of pressures and temperatures. Describes a metamorphic environment where certain mineral assemblages form.

Define what an earthquake is.

A sudden trembling of the earth caused by the release of accumulated elastic strain

What is an asperity? Describe its significance.

An area on an active fault where there is increased friction, such that the fault may become locked, rather than continuously slipping as in aseismic creep. Earthquake rupture generally begins with the failure of an asperity, allowing the fault to move.

Describe how earthquakes are located.

As S and P waves travel at different speeds, the difference in arrival times is proportional to the distance between a station.

How do fluids play a role in metamorphism?

As magma crystallizes, it releases H2O and CO2. These volatiles react with the minerals and create new minerals.

Why does metamorphism occur?

Because all minerals are not stable at all temperatures and pressures.

What are isograds?

Boundaries within an index mineral map which measure where index minerals first appear.

How are 3D models of the Earths mantle constructed?

By analyzing the velocity/refraction of p-waves as they travel through the mantle.

Why was tetraethyl lead in gasoline bad?

Caused lead contamination in the atmosphere which had lots of negative effects to public health, it was eventually banned.

Protoliths do not melt, but may undergo what changes?

Changes in grain shape, mineralogy, or orientation

Who measured the age of the Earth? What isotope did they analyze?

Clair Patterson. He analyzed lead (Pb) ratios in iron meteorite.

Polar wandering can be recorded in some rocks. Each continent reveal different paths of polar wandering. What is this evidence of?

Continental drift. The poles may have wandered a little, but this shows that it moves differently relative to each continent. The continents were moving.

What are the two types of lithosphere? What are some features of them?

Continental: 150km thick, lighter Oceanic: 100km thick, heavier

What is the elastic rebound theory?

Describes how rocks are strained elastically until they reach breaking strength. They then return to an undeformed stress-free state.

What is a metamorphic phase diagram?

Describes the pressure, temperature, and depth conditions utilized to produce a certain rock.

Define isostasy

Equilibrium is the state of gravitational equilibrium between Earth's crust and mantle such that the crust "floats" at an elevation that depends on its thickness and density.

What evidence did contour maps of the ocean floor provide?

Evidence that the continents have been drifting away from each other. (ocean crust near continents are old, near the Mid-Atlantic Ridge is young). Water depth relates to age.

What is a geobarometer and geothermomenter?

Geobarometer: Reaction insensitive to temperature. Geothermometer: Reaction insensitive to pressure.

What is the evidence that supports continental drift?

Geometric reconstruction, continuation of structural trends (Caledonian Belt), similar sequences of rock (similar bedding in southern continents), climatic reconstruction, fossil succession

Explain how non-foliated metamorphic rocks may change during metamorphism.

Grain sizes may increase as the initial minerals become unstable and change to new minerals.

Why might the Richter scale be not the best measurement of earthquakes?

Ground motion can be refracted and obstructed by rock layers and can only loosely correlate to a magnitude. Some areas have infrastructure to protect against earthquakes. Does not correlate with damage.

What were some initial problems with radioactive dating?

How much of the parent isotope did we start with? How much daughter isotope did we start with? How can we measure long half-lives? How do we know if a system was closed?

What is a daughter isotope?

Isotope that is produced as the result of radioactive decay.

What happens to the grain size of a rock during metamorphism?

It increases! Smaller grains are more reactive (Ex: corn dust burns up)

As a rock gets older, what happens to the slope of the isochron?

It increases.

Initially geometric reconstruction was denied as coastlines did not match. How was this refuted?

It was found that the continental edges were all below sea-level.

What took the measurement of the age of the Earth so long?

Large lead (Pb) contamination in the atmosphere largely caused by gasoline.

Describe the glacial evidence for continental drift.

Late Paleozoic glacial deposits can be found on each continent, only explained if they were all connected at that point.

How do you tell the difference between a left and right lateral fault?

Left: Road moved to the left. Right: Road moved to the right.

What does the Richter scale measure?

Magnitude of an earthquake, ground motion. (strength).

How can we measure long half-lives?

Measuring the change of parent isotopes over a small period of the half-life to find the 'slope'.

What are some characteristics about metamorphic grade?

Metamorphic grade increases as temperature and pressure rise. As grade rises, new and larger minerals are formed.

What is thermal/contact metamorphism?

Metamorphism which was formed due to a distinguishable heat source (intruding magma). High T and relatively low P. Higher grades form near the magma intrusion and decrease with distance from the intrusion

What is regional metamorphism?

Metamorphism which was formed from no specific heat source. Foliage developed, large area. Often seen in mountain ranges where the higher grade forms at the core of the mountain near the convergent boundary and lower grades further away.

Explain plastic deformation in foliated metamorphic rocks.

Mineral grains soften and deform allowing them to change shape without breaking.

What are index minerals?

Minerals that form under specific pressure and temperature conditions. Used to identify degree of metamorphism.

What is beta decay?

Neutron splits into an electron and a proton. The electron is emitted from the nucleus as a beta ray. The new proton increases the element's atomic number by one, forming a new element with the same atomic mass as the parent isotope.

Does pressure and temperature effect radioactive decay?

No!

What are the three types of dip-slip faults? Define them.

Normal: footwall has moved up relative to the hanging wall. Thrust: Reverse Fault dipping less than 45 degrees Reverse: Hanging wall has moved up relative to the footwall

What is liquefaction, when does it occur?

Occurs when an earthquake's violent shaking suddenly turns loose, soft soil into liquid mud. Can occur hours after an earthquake. (Buildings/cars can sink into ground).

Why might there be microearthquakes in areas not near tectonic plate boundaries?

Oil and gas wells contaminate a lot of water. This contaminated water is disposed through injections into ground. The increase in subsurface pressure caused by injecting this waste water will cause faults to slip and generate microearthquakes.

Why does oceanic crust only date back to the Jurassic period?

Older sea floor gets subducted back into the Earth's mantle.

What is a parent isotope?

Original, unstable isotope that undergoes decay.

Name and describe the three major types of seismic waves.

P Waves: Primary Waves (fast/travels through everything via compression) S Waves: Secondary waves (slower/only travels through solids via shear) Surface Waves: (Slowest/Ripples across surface and is responsible for most damage)

Describe the P-Wave Shadow Zone.

P-Waves refract when they enter and leave the core (liquid), this leave an area on the surface where p-waves will never reflect off of (like a blind spot on other side of earth corners)

I which direction does slaty cleavage develop?

Perpendicular to compression

What is an isochron diagram?

Plots the abundance of the daughter isotope divided by the abundance of a stable, non-radiogenic isotope of the daughter element against the ratio of the abundance of the parent isotope to the abundance of that same stable isotope of the daughter element.

How is Earth's magnetic field generated?

Produced by convection of liquid iron in the outer core

What is the difference between prograde and retrograde metamorphism?

Prograde: Occurs with increasing T and P. Increasing grain size. Loss of volatiles. Reactions slow as temperature decreases. Retrograde: Occurs with decreasing T and P

Define a seismogram.

The measured record of such motion.

What is radioactive decay?

Radioactive decay is the process of releasing particles/energy from unstable isotopes to create more stable isotopes.

Define a seismograph.

Records vertical or horizontal ground motion caused by passing seismic waves.

What is the difference between relative dating and absolute dating in geology?

Relative dating can explain which layer of rock was formed first. Absolute dating explains when in time it was formed.

Describe the S-Wave Shadow Zone.

S-waves cannot travel through liquids (core), instead they are reflected. S-waves will not reach/reflect off the opposite side of the earth. This shadow zone proves that the Earths outer core is a liquid.

Magnetic reversals and other anomalies can be found in oceanic crust. What is this evidence of?

Sea floor spreading. New crust preserves the negative/positive magnetic field. This forms layers of crust with different magnetic fields. These layers are symmetric across the plate boundaries.

Name the sequence of metamorphic grades from least to greatest.

Slate, phyllite, schist, gneiss

What are the three types of foliation?

Slaty Cleavage: Tendency to break along plane of weakness Can occur diagonally or vertically to bedding. Schistosity: Textural description of the parallel alignment of platy visible grains Gneissosity: Compositional banding. Can be coarse and folding.

Define stress and strain in terms of geology.

Stress: The collective force acting on a rock. Strain: The response of a rock to stress.

How was the Mid-Atlantic Ridge discovered?

Submarines during WW2 discovered it using sonar.

What is magnetic inclination?

Tendency/angle of a compass to point downwards towards the Earth. Higher inclination in polar regions.

What is a metamorphic grade?

The degree to which the parent rock changes during metamorphism. Determines the mineral assemblages.

What is the difference between the epicenter and focus of an earthquake?

The epicenter is the point on the earth's surface directly above the focus (not necessarily on fault). The focus is the origination of the earthquake (on fault).

What is metamorphism?

The mineralogical, chemical, and structural adjustment of rocks in response to changing temperatures, pressures, and fluid compositions.

What is another way to measure the scales of earthquakes? Explain what it measures.

The moment magnitude. This measures the size of the area effected during an earthquake. Correlates to the damage seen in an earthquake.

What is polar wandering?

The movement of the planet's poles relative to the continents overtime.

Why are isograds based on when index minerals first appear and not when they disappear?

The product of a chemical reaction may disappear if one of the reactants is missing, even if all other conditions for that reaction to proceed remain.

What is a half-life?

The time it takes for half of the parent isotope atoms to decay into the daughter isotope.

Where are tectonic plates located?

There are about 12 tectonic plates within the lithosphere which lays above the asthenosphere.

What is magnetic reversal?

This occurs when the Magnetic poles reverse. Magnetic North would become Magnetic South on a compass

Why was lead used in gasoline?

To prevent engine knock (makes gas more efficient).

How do we measure the amount of initial parent and daughter isotopes?

Utilizing equations developed with math.

What is alpha decay?

When an alpha particle (He-4), which consists of two protons and two neutrons, is emitted from the nucleus of an atom.

What is a protolith?

the original, unmetamorphosed rock from which a given metamorphic rock is formed