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Plate
Tectonics
• Plate tectonics is the theory (paradigm) that the earth’s crust and outer mantle (lithosphere) is divided into a
small number of plates that behave rigidly and move relative to each other about the earth. Deformation
occurs primarily at plate boundaries, which are of three types: divergent, convergent, and transform.
• The plates are floating on the asthenosphere, which is plastic or fluid (on a long time-scale) and maintain
isostatic equilibrium by sinking or rising.
• Oceanic crust is created at divergent boundaries and subducted back into the mantle at convergent
boundaries. Continents are more-or-less passive passengers (like pond scum) on the plates, in that they don’t
subduct. Transform boundaries are secondary features caused by convergent and divergent boundaries.
• The other planets and moons seem to not have horizontal plate tectonics like Earth, but the plate
loading/unloading aspect is seen elsewhere, like Mars' Olympus Mons. Europa looks a lot like Archean earth.
v 0023 of 'Plate Tectonics' by Greg Pouch
at 2011-03-28 10:28:57
LastSavedBeforeThis 2010-04-02
Plate Tectonics
3 Plates Map
4 Plate boundaries
5 Plate boundaries>Divergent
6 Breakup of Continent
7 Plate boundaries > Convergent
8 Plate boundaries > Convergent (Slides)
9 Plate boundaries > Transform
10 Plate interiors
11 Evidence in favor of plate tectonics
12 Evidence>Earthquakes, Volcanoes, and Geologic Features
13 Evidence>Magnetic Stripes and Polar Wandering
14 Evaluation of the Theory
15 Possible driving mechanisms for plate tectonics
Plates Map
Plate boundaries
• Divergent
–plates moving apart
–extensional features
–results in ocean basin
• Convergent
–plates moving towards each other
–compressive features
–features depend on the combination of continental and oceanic crust. Oceanic crust can be subducted,
continental crust can't
• If both plates at convergence line are continental, Collision (folding, thrust faults, granites)
• If one is oceanic, Subduction (volcanic chain, maybe thrusts and folds)
• Transform
–plates moving past each other
–strike-slip features
–small areas of divergence or convergence due to irregularities in boundary
Plate boundaries>Divergent
• Divergent (plates moving apart) (extensional features)
• Once a divergent boundary has started, new oceanic crust is extruded at the mid-ocean ridge as the two plates
move apart. As the crust cools, it subsides. (Gives age relationships, symmetric magnetic anomalies, depth)
• A continent (or ocean, I suppose) can split, forming a new divergent plate boundary. Initially, doming occurs,
followed by rifting (extensional faulting or block/normal faulting, like that found in the Basin and Range).
Continued extension leads to a narrow sea and eventually an ocean basin.
• Examples: East African Rift Zone, Dead Sea, Red Sea, Atlantic).
• Rifting does not start with a single graben that eventually becomes the ocean. There are a series of parallel
grabens, only one of which eventually becomes the sea. Rifting can also stall, resulting in failed rifts.
Because there is uplift (and consequent high erosion rates) and a topographic basin, rifts can accumulate huge,
thick sequences of sediments. Old rifts are common sites of oil deposits (anoxic conditions, heating by
basalts, thick sedimentary sequences often with well sorted sandstones to act as traps).
• Oceanic crust is initially hot and buoyant, and forms a rise. As it ages and cools, it gets denser and eventually
becomes denser than the underlying mantle, a very unstable situation. (Fe-rich material melts first, but has a
higher density at a given PT).
Breakup of Continent
Plate
boundaries
>
Convergent
• Convergent (plates moving towards each other) (compressive features)
• Oceanic lithosphere can be subducted. The oceanic crust is richer in Fe than the
mantle and, when cold, is denser than mantle material, so it can sink. Presently, it
does so by subducting as a big, coherent sheet that bends down and sinks into the
mantle. Oceanic plate bends down at the trench. Sediments and some crust can be
scraped off to form an accretionary wedge.
• Partial melting of oceanic lithosphere and maybe some mantle gives rise to an
arcuate chain of volcanoes on the overlying plate where the oceanic plate gets to
100km, which depends on age/density of downgoing slab. With older oceanic crust,
the plate descends at a steeper angle and the volcanic chain is closer to the trench.
• Continental crust does not get subducted, because it is less dense than mantle
material and oceanic crust, although small bits of continental material (mainly deepsea sediments) can be carried down.
• There are often minor extensional features in convergent zones. A diffuse spreading
center sometimes starts between an island arc and the continent, and you get a backarc basin. (Maybe magma/heat plasticizes mantle lithosphere and platyness vanishes)
• Depending on whether there is subductable oceanic crust at the convergence
–Subduction
• Ocean-continent (Andes) Volcanic chain, some compressional deformation of
sediments, and optional accretionary wedge. Example of old, eroded remnants
is the Sierra Nevada.
• Ocean-ocean (Tonga) fairly rare, common in Pacific, seems to usually happen
when oceanic crust gets very cold. Results in an island chain built on oceanic
crust.
–Continent-continent (Himalayas) a.k.a Collision
• Leads to compressional mountain chains, like the Himalayas-Alps and the
Appalachians. Extensive folding and thrust faulting in sediments, and deeper
metamorphism and intrusion (granites). The compression propagates outward
from the line of collision (suture zone), sort of like two cars crumpling in a
collision. (Look at Asia north of the Himalayas on GoogleEarth)
• Continental crust can be doubled or tripled in thickness, resulting in a mountain
range with a root, sort of like an iceberg embedded in pack ice.
Plate boundaries > Convergent
Plate boundaries > Transform
• Transform (plates moving past each other) (strike-slip features)
–These are secondary features that mainly take up the slack from convergent and divergent boundaries.
–Unless the boundary is a perfect great circle (straight line-equivalent), there will be tiny areas of
convergence and of divergence, resulting in small compressive and tensional regions.
Plate Interiors
• In plate tectonics theory, most deformation occurs at the boundaries. Plate interiors are more passive and
only respond to loading and unloading, as thin, elastic plates (a civil engineering theory of how to handle
sheets and the origin of the name plate tectonics)
• Hot spots are trails of volcanic activity that appear unrelated to plate boundaries, but do trace plate motion.
These seem to be fixed with the mantle, and the tracks record plate motion. Typically, there is a chain of
volcanic features of progressive age. The youngest feature marks the current position of the hotspot.
Evidence in favor of plate tectonics
• Distribution of earthquakes and volcanoes
• Fit of continents, geologic features (including ice ages) across ocean basins
• Distribution of organisms and fossils.
• Age distribution of sea floor
• Magnetic stripes on ocean floor
• Polar wandering from sediments
• Odd lack of old sediments in the oceans
• Over-abundance of basalts at sea and granites on land.
• Velocity distribution of crustal material
• ****Surveying****
• Why was it rejected initially? (Mainly lack of believable driving mechanism, and the claim that continents
moved over or through oceanic crust, which they clearly don’t) Was widely accepted in Gondwanaland.
• We still don’t have a good driving mechanism.
Evidence>Earthquakes, Volcanoes, and Geologic Features
Evidence>Magnetic Stripes and Polar Wandering
Evaluation
of
the
Theory
• What plate tectonics explains
–Ocean basins, mid-ocean ridges, trenches, distribution of sediments in oceans, magnetic striping, ages
–Many recent compressive mountain chains along continental margins or former continental margins, such
as the Appalachians and the Himalayas and continental margin volcano chains like the Andes and Japan.
–Polar wandering (magnetic and climatic)
–Most volcanism and igneous activity
• What plate tectonics doesn’t explain
–Deformation within continents, like
• Michigan basin and Illinois Basin
• uplifts like Black Hills
–Flood basalts
–Intra-plate earthquakes
–Certain mountain chains (especially in the Western US)
–Archean and earlier geology
Possible
driving
mechanisms
for
plate
tectonics
•Facts to explain:
–Plates are massive and slow moving.
–Lateral plate motion accounts for a lot of momentum but very little kinetic energy.
–Mid-ocean ridges are primarily tensional and trenches are primarily compressive.
–Lots of deformation at edges, but little in interior.
so some sort of dispersed force is needed, like dragging by the mantle or gravitational forces.
•Mantle convection
–Early thought centered on mantle convection injecting magma at ridges and pushing oceans apart at the
mid-ocean ridge and pulling them down in the trenches, but there should be compressive features at ridges,
tensional features in trenches. Requires linear convection patterns that persist for 10 Ma-1Ga [unlikely]
–Alternatively, the plates could be moved by being dragged by mantle convection currents, which disperses
the force over the entire plate interior, but you still need very stable convection.
•Density of oceanic crust
–Current thought leans towards oceanic crust density driving almost everything. If the asthenosphere acts
as a good lubricant (ice skating analogy goes here), observable density contrasts can explain most motion,
once it starts. Maybe it gets started by mantle plumes, and then runs by sliding.
–Cold oceanic lithosphere is denser than mantle and subducts, pulling oceanic crust and any embedded
continents with it.
–The mid-ocean ridge is a landslide, with partial melting of mantle producing basalt due to pressurerelease. Eruption and intrusion of magma keeps the ridge hot and the landslide going, so it’s stable once it
starts.
Plate Tectonics
• Plate tectonics is the prevailing paradigm in geology. It holds that the
earth's lithosphere (crust + upper_mantle) is broken into a small number of
plates that behave rigidly, and that almost all interesting deformation
happens at the edges between plates.
• Plate boundaries come in three or four varieties
–Divergent (going apart)
–Convergent (coming together)
• Subduction zones involving oceanic crust
• Collision zones, not involving oceanic crust,
–Transform (going past)
• Evidence in favor of plate tectonics comes from polar wandering,
distribution of ages of seafloor, matching geologic features and fossils
across oceans that make sense with drifting continents, most earthquakes
and volcanoes…
• Once plate boundaries are started, density contrasts and gravity will keep
them going.
• Plate tectonics explains oceans very well, much continental deformation in
the last 1Ga pretty well (with some exceptions)