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EESC > Plate Tectonics > Flashcards

Plate Tectonics Flashcards

1
Q

The most common model to explain occurrence of EQ is the elastic rebound theory. What famous EQ provided the first evidence for this theory?

A

1906 San Francisco Earthquake

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2
Q

Hypocenter

A
  • Focus.
  • Where the EQ originates below ground
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3
Q

Mohorovicic discontinuity

A

Boundary between crust and mantle

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4
Q

Low Velocity Zone

A
  • An area within the Asthenosphere.
  • Seismic waves slow down as they pass through.
    = Partial melt lowers velocity of waves.
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5
Q

450 km and 650 km Discontinuities

A
  • Pressure induced mineral phase transitions.
  • Collapse of mineral structures - creates deep quakes
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6
Q

pP seismic wave

A
  • P wave moves from focus and reflects at the Earth’s surface close to epicenter region.
  • Used to find depth of EQ.
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7
Q

Travel-time residual

A
  • Difference in expected vs actual arrival times on seismograph.
  • Used to explore Earth’s interior.
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8
Q

Elastic Rebound Theory

A

The gradual accumulation and release of stress and strain. How energy is released during an EQ.

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9
Q

Aftershocks occur due to….

A

… further slippage along original fault and asperities
(irregular surfaces along fault plane that produce local resistance to sliding)

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10
Q

Seismic observatories operate a minimum of 6 seismograph

A
  • Vertical, E-W and N-S horizontal short period and
  • Vertical, E-W and N-S horizontal long period instruments.
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11
Q

Which seismograph/instrument best captures motion of P-waves, S-waves, Rayleigh & Love waves

A
  • P-wave: vertical
  • S wave: horizontal short period
  • Rayleigh wave: long period
  • Love wave: E-W long period
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12
Q

Body waves

A
  • Primary: fastest, travel through all material, produces high frequency, low amplitude vertical ground motion
  • Secondary: 2nd fastest, travels through solids only, produces horizontal ground motion
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13
Q

Surface waves

A
  • Love and Rayleigh waves:
  • slowest, highest amplitude, longest duration of shaking
  • reach depth of 1000 km
  • produces simultaneous horizontal and vertical shaking
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14
Q

How did Charles Richter calculate his famous magnitude scale (now local magnitude ML)

A
  • Measure amplitude of largest seismic wave in micrometer.
  • (convert to base 10 log).
  • Correct for distance to epicenter from seismic observatory.
  • Assign magnitude on log scale.
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15
Q

How does the calculation of the seismic moment of an EQ differ from any other magnitude scales that are in use?

A
  • Mo does not rely on measurement of a seismic wave amplitude.
  • Instead it is based on the seismic moment at the EQ focus which is defined as:
  • The product of the displacement, the area of the slip surface, and the shear resistance of the rock.
  • Mo= μDS
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16
Q

What is a major advantage of the seismic moment scale?

A
  • Moment Magnitude (Mw) is calculated by
  • the the seismic moment (Mo) converted to Mw scale (log 10)
  • More accurate for high magnitude EQ, avoids wave saturation effect
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17
Q

Name and define the variables that control the rate of heat flow (q-value) that occurs due to pure conduction. In what parts of the Earth does conductive heat flow predominate?

A
  • Heat flow q =
    thermal conductivity (k) x
    delta Temp/delta z
    (z-axis/depth).
  • Delta T/Delta z -> change of temp w/depth = geothermal gradient.
  • Lithosphere and inner core.
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18
Q

What is the most significant source of heat within Earth’s interior?

A

Radioactive decay

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19
Q

Why is heat flow over MOR lower than expected?

A

Deep sea hydrothermal vents - Hydrothermal circulation through MOR fractures produces a significant cooling effect

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20
Q

What is the nature of the correlation between the age of a continental terrain and its average heat flow? Why is this correlation observed?

A
  • Older continental cratons have lost a greater proportion of their radiogenic isotopes and therefore tend to have lower heat flow values than younger crustal regions.
  • Radioactive decay over time led to a sinking concentration of radioactive elements.
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21
Q

Why was it a surprise to find that heat flow over continental regions is lower than values measured over seafloor crust? What is the reason for the difference?

A

Thin oceanic crust allows mantle heat closer to the surface - responsible for higher heat flow

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22
Q

What are Euler pole jumps

A
  • Since plates interact with their neighbors it is common for a plate to change direction from time to time.
  • Each time period that is dominated by a particular motion of rotation has a defined Euler Pole to represent that behavior (called a State Pole).
  • When plates change direction a new Euler pole must be established.
  • These changes are called Euler Pole jumps.
  • Ex. Hawaiian Islands.
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23
Q

Magnetic Field Intensity

A
  • Sum of vertical and horizontal components.
  • At maximum near the poles, at minimum close to the equator
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24
Q

Paleomagnetic Time Scale

A
  • Recognition of magnetic poles led to creation of PTS.
  • A time interval in which a particular polarity dominates is called “chron”.
  • Short duration polarity events are called “subchrons”.
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25
Q

How do rocks acquire magnetic properties?

A
  • Many rocks contain iron-bearing minerals that act as tiny magnets.
  • As magma or lava cool, these minerals begin to form.
  • At this point the molten rock has not completely solidified, so the magnetic minerals floating in the molten mass, become aligned to the magnetic field.
  • Magnetic properties are lost above it’s Curie Point temp -> high curie point desirable!
  • Fe2+ and Fe3+ have unpaired electrons - the more unpaired electrons, the stronger the magnetism.
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26
Q

Types of magnetic behavior in minerals

A
  • Paramagnetic- Net moment 0 (disorder)
  • Ferromagnetic - Net moment ^
  • Antiferromagnetic - Net moment 0 (up/down)
  • canted antiferromagnetic - Net moment ->
  • Ferrimagnetism - Net moment ^ (strong up, weak/small down) (Magnetite, Hematite)
27
Q

Thermal Remanent Magnetization

A
  • Lava flow temp ~ 1000 C: paramagnetic behavior
  • Cooling below magn. crystall. temp (still above c.pt)
  • Cooling below c. pt. (585 C)-magn.domain begins to form
  • Cooling below bloching temp (grain size dep.)
  • Preserves magn.orient. for geological LT-periods
28
Q

Types of Remanence
(Remanence = A rock carries a natural remanent magnetization that is the vector sum of all the different possible components of magnetization acquired over its history.
After initially acquiring a primary magnetization, grains with low relaxation times may be susceptible to remagnetization by time, temperature or chemical changes and produce secondary components of magnetization.)

A
  • Thermoremanent Magnetization (TRM): Magnetization acquired during cooling from a temperature above the Curie Temperature in an external field
  • Chemical Remanent Magnetization (CRM): Magnetization acquired during chemical changes in an external field
  • Viscous Remanent Magnetization (VRM) Magnetization acquired over time in an external field
  • Depositional Remanent Magnetization (DRM) Magnetization acquired by sediments when grains settle out of water in an external field
29
Q

Magnetic cleaning

A

-Magnetic instruments are installed in shielded rooms so they are unaffected by the Earth’s current magnetic field.
- They measure:
magnetic intensity, inclination and declination of rock specimens
- They also magnetically clean by gradually increasing the temperature or increasing alternating currents to remove secondary magnetism.

30
Q

Paleomagnetic data has been instrumental in the discovery of definition of exotic or “suspect” terranes

A
  • Extensive regions in NA were bounded by faults and were geologically distinct from their neighbors.
  • Paleomagnetic data indicated they originated elsewhere and then collided and sutured onto the growing western margin of the continent.
  • If magnetic declination is consistent - rocks of same age, different lithology -> pole position the same
  • If magnetic declination is not consistent - rocks of same age, different lithology- terrane
    –>originated elsewhere.
31
Q

Continental Drift Hypothesis

A

-Alfred Wegener:
-Pangea began to break up 180 mya.
- He believed continental masses “plowed through” surrounding seafloor basalt rock due to westward tidal and centrifugal forces.
- The front end would be compressed creating mountains, back ends would be extended creating island arc.

32
Q

Arthur Holmes (1890-1965) - 1944 Model of mantle convection and continental mobility

A
  • Subcrustal currents are early part of convection cycle
  • then become vigorous enough to drag the two halves of original continent apart with
  • consequent mountain building in the front where currents descend and
  • ocean floor development on the site where currents ascend
33
Q

Passive model of plate tectonics

A
  • Motion of convective currents imparted a traction force to the base of the lithosphere.
  • Movement direction of a shallow portion of a convective cell would directly control the direction and speed of the overlying lithosphere.
34
Q

Four problems of convection traction force

A
  • Scale issues
  • What happens in vicinity of a trench
  • Explanation of large offset at MOR
  • Is convection consistent?
  • Ex: Pacific plate: one would expect it to have multiple convection cells - no reason for them to all move in the same direction
35
Q

Forces associated with plate margins

A
  • Slab pull at subduction zones due to negative buoyancy of the sinking slab and
  • Ridge push at ridge axis due to gravity sliding effect (gravity forces).
  • Frictional resistance balances driving forces.
36
Q

What specific observations did Hutton make that brought significant change in how geologists regard our planet?

A
  • The Earth is continually being formed.
  • Sedimentation takes place very slowly.
  • Even the oldest rocks are made up of ‘materials furnished from the ruins of former continents’.
  • Heat from within the Earth is involved in mineralization, which proves the existence of unconformities in rock layers.
  • Hutton proposed that the Earth constantly cycled through disrepair and renewal.
  • Exposed rocks and soil were eroded, and formed new sediments that were buried and turned into rock by heat and pressure.
  • That rock eventually uplifted and eroded again, a cycle that continued uninterrupted.
    ->Uniformitarianism:
    The present is the key to the past.
    Erosion and Transport, Sediment Deposition
    (Siccar Point - Scotland, angular unconformity)
37
Q

At the end of the 19th century what was the dominant model to explain the formation of mountains?

A
  • Geosynclinal Theory of Mountain building.
  • Deep depression in crust, long time accumulation of sediments, heating and uplift of mountain belts.
  • Coupled with the idea of cooling contracting Earth that squeezes sediments of geosynclines.
    -> Contracting Earth Theory: James D. Dana
38
Q

What is a “land bridge” and how did this idea originate?

A
  • Connection between continents.
  • The concept of land-bridge connections between the world’s continents used to be the basic explanation of palaeontological observations of the similarity of their past flora and fauna.
39
Q

What specific argument did Wegener use to argue that landbridges were impossible?

A
  • Wegener argued that rising and falling land bridges in the oceans were not likely considering the observation that the ocean crust was made of denser (basaltic) rock than the continents.
  • He argued that this denser oceanic crust could not rise up above sea level.
  • Likewise, if the land bridge was less dense (granitic) continental rock it would be too light to sink into the denser rock below.
40
Q

Concept of isostasy

A
  • Isostasy is
  • the rising or settling of a portion of the Earth’s lithosphere - that occurs when weight is removed or added in order to
  • maintain equilibrium between buoyancy forces
  • that push the lithosphere upward,
  • and gravity forces that pull the lithosphere downward.
41
Q

What were the principal weaknesses of Wegener’s theory that causes most geologists of his day to recent continental drift?

A
  • Proposed that continents plowed through surrounding ocean crust - critics knew that ocean crust is stronger than continental crust.
  • Wegener was unable to explain how the continents moved and proposed tidal and centrifugal forces
42
Q

Alexander du Toit was an early supporter of the idea of continental drift. What did he add in the way of additional evidence to prove that continents had moved?

A
  • Identified common paleozoic gondwana sequence of rock stratigraphy on southern hemisphere continents.
43
Q

What specific observations did Hospers make to support his view (he was able to demonstrate that the entire magnetic polarity of the the Earth’s magnetic field has flipped)

A
  • Hospers was the first to demonstrate that reversals of the field and not self-reversals were responsible for reversed magnetization.
  • Particular polar directions remained consistent within specific age ranges - reversals not found randomly within a given time period
44
Q

Describe the initial controversy regarding the interpretation of the meaning of APW curves. How was the controversy resolved?

A
  • Polar wandering model (w/fixed continent) vs
  • Fixed pole (w/drifting continents)
  • Runcorn believed in the polar wandering model
  • A student of him, Creer, believed that the evidence from calculated magnetic north pole positions of older rocks, supported continental drift.
  • Data from a separate continents showed that pole positions were different in NA than in Europe (Runcorn failed to recognize this)
44
Q

Hospers proposed a useful application of the flipping of the Earth’s magnetic polarity

A
  • Able to identify normal vs reversed polarity in rocks w/magnetite
  • led to geo-paleomagnetic timescale.
  • Used to date seafloor and stratigraphic correlations.
44
Q

Runcorn is most often remembered for the first calculation of the Apparent Polar Wander Curves. What steps are necessary to determine APW curves?

A
  • Obtain rocks of different ages.
  • Measure magnetic inclination -> tells us original latitude and distance to pole.
  • Measure magnetic declination -> direction to pole.
  • Plot results on map to find pole.
45
Q

Corrected and uncorrected natural remnant magnetization in folded Rose Hill (silurian) formation in Pinto, Maryland….

A

….This data was significant to establish validity of the data that was later used to calculate APW curves.
- All data plotted from one area in Maryland, after unfolding, magnetic data shows single area of magnetic pol.
- Fold Test demonstrates that paleomagnetic data is durable and survives tectonic deformation.

46
Q

Describe two of Harry Hess’ incorrect early theories of the ocean floor

A
  • Tectogene (long narrow downward fold of Earth’s crust into mantle below deep sea trenches filled w/sed. ) tectonic feature is responsible for sea floor trences and their gravity anomalies.
  • Serpentinization and Deserpentinization of seafloor crust would change density to rise/fall - seafloor topography.
  • Origin of guyots
47
Q

What research problem did Fred Vine set out to investigate? What resolution did he propose? How was his proposal tested and found to be true?

A
  • He wanted to understand how the Pacific ocean magnetic stripes pattern originated.
  • He proposed that Hess’ idea of seafloor spreading combined with episodes of geomagnetic pole reversals was responsible for the stripes.
  • Symmetry of magnetic data proves seafloor spreading.
48
Q

Marie Tharp contribution to plate tectonics

A
  • Tharp revealed global scale of MOR and was first to realize that a central rift axis goes down its axis.
  • She proposed that this was consistent with seafloor spreading
  • Marie Tharp was a pioneering American geologist and oceanographic cartographer who helped create the first scientific map of the Atlantic Ocean floor
  • Her revolutionary work revealed the detailed topography and multi-dimensional geographic landscape of the ocean bottom
  • and revealed the presence of a continuous rift valley along the axis of the Mid-Atlantic Ridge,
  • a shift in thinking that led to the acceptance of the theories of plate tectonics and continental drift.
49
Q

Seismic Waves

A
  • Primary (P) waves:
  • Alternating compressions (“pushes”) and dilations (“pulls”) which are directed in the same direction as the wave is propagating (along the ray path)
  • P motion travels fastest in materials, so the P-wave is the first-arriving energy on a seismogram.
  • Generally smaller and higher frequency than the S and Surface-waves.
  • P waves in a liquid or gas are pressure waves, including sound waves.
  • Secondary (S) waves:
  • Alternating transverse motions
  • S-waves do not travel through fluids, so do not exist in Earth’s outer core, air, water or molten rock (magma).
  • S waves travel slower than P waves in a solid and, therefore, arrive after the P wave.
  • Surface wave - Love wave:
  • Transverse horizontal motion
  • In general, Love waves travel slightly faster than the Rayleigh waves.
  • Love waves exist because of the Earth’s surface.
  • They are largest at the surface and decrease in amplitude with depth.
  • Surface wave: Rayleigh wave
  • Long waves
  • Ground roll motion is both in the direction of propagation and perpendicular
50
Q

Abraham Werner (1749-1817)

A

Neptunism = Theory stating that the majority of rocks on Earth’s surface were precipitated out of a global ocean

51
Q

George Cuvier (1769-1832)

A

Catastrophism = Episodes of extinction were caused by catastrophes that were also responsible for shaping Earth’s surface

52
Q

James Hall and James Dana

A

Geosynclinal Theory = Mountain ranges result from vertical movement w/o significant horizontal displacement.
- Also Dana -> contracting Earth theory

53
Q

Eduard Suess

A
  • Proposed existance of Gondwananland - a southern supercontinent
    -> SA, Af, Aus, India, Antarctica
  • Earth composition: SiAl Mg NiFe
  • Origin of volcanoes and EQ due to contraction theory
  • Landbridge concept
  • 1980’s book: The Face of the Earth
54
Q

Evidence of Continental Drift

A
  • Fit of Continents (best fit at ~ 1500 ft water depth).
  • Paleoclimate data Glacial: extensive glacial deposits, till, striations on southern “supercontinent”
  • Paleoclimate Data: Coal & Reefs: tropical swamps, shallow seas.
  • Fossil Data: Glossopteris, Mesosaurus
  • Rock Data: type, sequence, ages, styles of deformation match (Appalachian related to Caledonian)
55
Q

S. Warren Carey (1911-2002)

A

Mostly remembered for his “Expanding Earth” theory

56
Q

Paleomagnetism

A
  • Liquid lava (~1000 C): magnetic grains rotate freely
  • Part. crystallization (~800 C): magnetic crystals begin to align w/Earth’ magnetic field
  • Fully crystallized (<400 C): magnetic crystals fully aligened w/Earth’ magnetic field and locked in
  • Grains settling in water, rotate freely, settle and start to align in calmer waters.
  • Buried sediment magn. grains fully aligned and locked
57
Q

Bernhard Brunhes (1867-1910)

A
  • Paleomagnetism of igneous rocks in France preserved a record of reverse magnetization.
  • Proposed as evidence of whole Earth reversals of geomagnetic field
58
Q

John Tuzo Wilson (1908-1993)

A
  • Canadian geophysicist and doctoral student of H. Hess.
  • Demonstrates that many islands show increasing age w/distance to MOR
  • First suggestion of hotspot origin of Hawaiian Islands
  • Recognizes type of motion that characterizes transform boundaries that link MOR segments
  • 1965 paper defines 3 types of boundary motion areas of compression (convergent), horizontal shear (transform), MOR tension (divergent)
59
Q

Slip vector

A
  • The pole to the auxiliary plane is always the slip vector
  • and plots on the fault plane
60
Q

P and T are always 45 degree from slip vector…

A

…and all 3 lie on a common plane/great circle

61
Q

Mechanisms to drive seafloor spreading

A
  • Conveyor belt motion of the asthenosphere
  • Driven by radioactive decay adding heat to the interior of the Earth
  • Gravity sliding (ridge push) - aided by contraction of the slab as it leaves the ridges
  • Slab pull as the subducting ocean lithosphere gets progressively denser w/depth
62
Q

Ophiolite sequence

A
  • Composition of oceanic lithosphere:
  • Fine sediments
  • Pillow basalts
  • Vertical dikes (sheeted basalt dikes)
  • Gabbro
  • Peridotite (uppermost mantle)

EESC (3 decks)

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