Plate Tectonics Flashcards
The most common model to explain occurrence of EQ is the elastic rebound theory. What famous EQ provided the first evidence for this theory?
1906 San Francisco Earthquake
Hypocenter
- Focus.
- Where the EQ originates below ground
Mohorovicic discontinuity
Boundary between crust and mantle
Low Velocity Zone
- An area within the Asthenosphere.
- Seismic waves slow down as they pass through.
= Partial melt lowers velocity of waves.
450 km and 650 km Discontinuities
- Pressure induced mineral phase transitions.
- Collapse of mineral structures - creates deep quakes
pP seismic wave
- P wave moves from focus and reflects at the Earth’s surface close to epicenter region.
- Used to find depth of EQ.
Travel-time residual
- Difference in expected vs actual arrival times on seismograph.
- Used to explore Earth’s interior.
Elastic Rebound Theory
The gradual accumulation and release of stress and strain. How energy is released during an EQ.
Aftershocks occur due to….
… further slippage along original fault and asperities
(irregular surfaces along fault plane that produce local resistance to sliding)
Seismic observatories operate a minimum of 6 seismograph
- Vertical, E-W and N-S horizontal short period and
- Vertical, E-W and N-S horizontal long period instruments.
Which seismograph/instrument best captures motion of P-waves, S-waves, Rayleigh & Love waves
- P-wave: vertical
- S wave: horizontal short period
- Rayleigh wave: long period
- Love wave: E-W long period
Body waves
- 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
Surface waves
- Love and Rayleigh waves:
- slowest, highest amplitude, longest duration of shaking
- reach depth of 1000 km
- produces simultaneous horizontal and vertical shaking
How did Charles Richter calculate his famous magnitude scale (now local magnitude ML)
- 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.
How does the calculation of the seismic moment of an EQ differ from any other magnitude scales that are in use?
- 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
What is a major advantage of the seismic moment scale?
- 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
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?
- 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.
What is the most significant source of heat within Earth’s interior?
Radioactive decay
Why is heat flow over MOR lower than expected?
Deep sea hydrothermal vents - Hydrothermal circulation through MOR fractures produces a significant cooling effect
What is the nature of the correlation between the age of a continental terrain and its average heat flow? Why is this correlation observed?
- 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.
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?
Thin oceanic crust allows mantle heat closer to the surface - responsible for higher heat flow
What are Euler pole jumps
- 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.
Magnetic Field Intensity
- Sum of vertical and horizontal components.
- At maximum near the poles, at minimum close to the equator
Paleomagnetic Time Scale
- 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”.
How do rocks acquire magnetic properties?
- 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.
Types of magnetic behavior in minerals
- 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)
Thermal Remanent Magnetization
- 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
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.)
- 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
Magnetic cleaning
-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.
Paleomagnetic data has been instrumental in the discovery of definition of exotic or “suspect” terranes
- 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.
Continental Drift Hypothesis
-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.
Arthur Holmes (1890-1965) - 1944 Model of mantle convection and continental mobility
- 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
Passive model of plate tectonics
- 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.
Four problems of convection traction force
- 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
Forces associated with plate margins
- 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.
What specific observations did Hutton make that brought significant change in how geologists regard our planet?
- 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)
At the end of the 19th century what was the dominant model to explain the formation of mountains?
- 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
What is a “land bridge” and how did this idea originate?
- 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.
What specific argument did Wegener use to argue that landbridges were impossible?
- 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.
Concept of isostasy
- 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.
What were the principal weaknesses of Wegener’s theory that causes most geologists of his day to recent continental drift?
- 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
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?
- Identified common paleozoic gondwana sequence of rock stratigraphy on southern hemisphere continents.
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)
- 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
Describe the initial controversy regarding the interpretation of the meaning of APW curves. How was the controversy resolved?
- 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)
Hospers proposed a useful application of the flipping of the Earth’s magnetic polarity
- Able to identify normal vs reversed polarity in rocks w/magnetite
- led to geo-paleomagnetic timescale.
- Used to date seafloor and stratigraphic correlations.
Runcorn is most often remembered for the first calculation of the Apparent Polar Wander Curves. What steps are necessary to determine APW curves?
- 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.
Corrected and uncorrected natural remnant magnetization in folded Rose Hill (silurian) formation in Pinto, Maryland….
….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.
Describe two of Harry Hess’ incorrect early theories of the ocean floor
- 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
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?
- 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.
Marie Tharp contribution to plate tectonics
- 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.
Seismic Waves
- 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
Abraham Werner (1749-1817)
Neptunism = Theory stating that the majority of rocks on Earth’s surface were precipitated out of a global ocean
George Cuvier (1769-1832)
Catastrophism = Episodes of extinction were caused by catastrophes that were also responsible for shaping Earth’s surface
James Hall and James Dana
Geosynclinal Theory = Mountain ranges result from vertical movement w/o significant horizontal displacement.
- Also Dana -> contracting Earth theory
Eduard Suess
- 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
Evidence of Continental Drift
- 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)
S. Warren Carey (1911-2002)
Mostly remembered for his “Expanding Earth” theory
Paleomagnetism
- 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
Bernhard Brunhes (1867-1910)
- Paleomagnetism of igneous rocks in France preserved a record of reverse magnetization.
- Proposed as evidence of whole Earth reversals of geomagnetic field
John Tuzo Wilson (1908-1993)
- 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)
Slip vector
- The pole to the auxiliary plane is always the slip vector
- and plots on the fault plane
P and T are always 45 degree from slip vector…
…and all 3 lie on a common plane/great circle
Mechanisms to drive seafloor spreading
- 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
Ophiolite sequence
- Composition of oceanic lithosphere:
- Fine sediments
- Pillow basalts
- Vertical dikes (sheeted basalt dikes)
- Gabbro
- Peridotite (uppermost mantle)
