Plate Tectonics Revisited Flashcards
How is heat generated from inside the earth transmitted?
Conduction and convection
Define conduction
Heat energy transmitted through solid by molecular forces
Define convection
Heat energy transmitted by density
What mode of heat transfer does the mantle use?
convection
Define post glacial rebound, and what it proves
Crust pushed by glaciers rebounds upwards when glacier melts. This proves that the mantle is viscous
How do we know that the mantle convects?
It has a very high Rayleigh number
Rayleigh number. Define it
A parameter that indicates whether or not convection occurs in a fluid being heated from below. Convection occurs when Ra > 2000
How is Ra calculated?
(Variables that promote convection)/(variables that hinder convection)
Some problems with thermal-convection-driven model of plate tectonics (4)
• Plate vary significantly in size, so regular-sized convection cells not possible.
• Convection cells in mantle should be stable long-term features, but spreading ridges
move rapidly over Earth’s surface (in places at rates approaching the spreading rates
themselves), indicating they are superficial features.
• Some plates (e.g., African plate) surrounded by ridges; no subduction zones that could
correspond to downwelling limb of convection cell.
• Spreading ridges can be subducted (nail in coffin)
What is responsible for convection of the mantle?
Plate tectonics, when plates get secreted and subducted, will cycle is the heat in convection
What is the Beinoff zone?
Planar zone os seismicity associated with down-going slab at subduction zone
Is there evidence of mantle convection independent of plate tectonics?
Hotspots
What three types of plate tectonics
Divergent, Transform, and Convergent
What characterizes divergent plate boundaries?
Normal faults); crustal thinning; rifting of continental crust;
eventually mafic volcanism
What characterizes transform plate boundaries
Plates slide horizontally past eachother.
no thinning or
extension of crust; lithosphere not created or detroyed; limited to no volcanism
How many plate boundary types?
3
Oceanic-oceanic plate boundaries characteristics (4)
- Older plate subducts under younger one;
- water brought with it acts like flux (flux melting) that lowers the melting point of
the slab, - mafic to intermediate magmas are
produced, - generates a volcanic island arc.
Define Volcanic island arc
A chain of volcanic islands in the middle of the ocean
What characterizes Oceanic-continental plate boundaries (4)
- Oceanic lithosphere subducts under continental lithosphere; 2. water is brought in with it, acting as a flux (flux melting)
- mafic to felsic magmas
produced - generates a
continental arc
Define continental arcs
A chain of volcanic mountains along the edge of the continent
What defines Continental-continental plate boundaries (4)
- Neither plate subducts completely (too buoyant) 2. no volatiles being delivered to
depth ,therefore no volcanism. - Significant crustal thickening occurs (intense folding and
thrust faulting). - Shallow earthquake activity only (no subduction, no Benioff zone).
Define Seamounts
older, extinct volcanoes generated at spreading ridges or hotspots
What are guyuts?
flat-topped seamounts
How are many of the smaller morphological elements on the seafloor are generated ?volcanism
volcanism
What do hotspots generally look like?
Big heads, narrow tails
Which of the following observations suggest the mantle is viscoelastic?
a) The mantle transmits P-waves and S-waves.
b) The mantle transmits P-waves, as does the outer core.
c) The mantle transmits S-waves. Post-glacial rebound occurs.
d) The mantle transmits S-waves but the outer core does not.
c
The mantle convects because
a) of its low viscosity.
b) of its high thermal conductivity.
c) of the minimal temperature gradient between its top and bottom.
d) None of the above.
d
Which of the following challenges the idea that mantle convection drives plate
tectonics?
a) Spreading ridges can migrate rapidly across the Earth’s surface
b) Spreading ridges can be subducted
c) Some plates (e.g., African plate) are bordered almost entirely by spreading
ridges
d) All of the above.
d
Many researchers hypothesize that plate tectonics is driven by the plates
themselves. Explain the forces that likely facilitate this.
Slab pull (dominant) and ridge push (subordinate).
Earthquakes and faults are confined to the lithosphere because
a) the mantle is incapable of brittle failure.
b) the lithosphere can deform by brittle failure whereas the mantle that underlies
it cannot.
c) the mantle deforms through viscous dissipation.
d) All of the above.
c
Define the term Benioff zone and explain why it occurs.
Benioff zones are dipping, planar zones of increased earthquake activity caused
by subduction of one tectonic plate beneath another. See slide 18.
Volcanoes are only common at some types of convergent plate boundaries.
Why?
Because subduction does not occur at continental-continental convergent plate
boundaries. See slide 36.
In contrast to the Pacific margin, the Atlantic margin of North America is
aseismic (i.e., characterized by few earthquakes) and lacks volcanoes, even
though both margins occur at boundaries between continental and oceanic
lithosphere. Why?
The Atlantic margin is a passive margin, whereas the Pacific margin is an active
one. See slides 47 to 49.
Define aseismic
characterized by few earthquakes
