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THE PLATE TECTONIC THEORY
PLATE BOUNDARIES
1. Divergent plate boundary (= constructive boundary) oceanic ridge (OR)
a. Volcanism
i. Magma (molten rock) wells up from the upper mantle along deep fissures formed by
extensional forces pulling the plates apart. As the magma cools and solidifies, it is
added onto the two plates. There are two plates formed at every spreading center. As
the plates continue to be pulled apart, more magma wells up from the mantle, solidifies
into solid rock on the plate boundaries, and new lithosphere/sea floor is formed. New
sea floor forms at full rates (sum of growth of both plates) as high as 14 cm/y at superfast spreading centers but intermediate rates are closer to 2 to 9 cm/y, slow rates <2
cm/y. Spreading rates are more often expressed as half rates (growth of one plate), so,
at a superfast spreading center, one plate moves at 7 cm/y whereas at a slow spreading
center one plate moves at <1 cm/y. Easy!
ii. At the crest of an OR, there is a spreading center, a narrow zone of plate accretion,
usually within a rift valley in which there are hundreds of small "volcanoes" (Nature,
348:152) or long fissures along which lava is extruded during growth of the plate. This
is the surface expression of plate growth. At ridges that spread faster than 3 cm/y (half
rate), the volcanism at the surface is fed from a magma chamber that lies 2 to 4
kilometers beneath the spreading center. The depth of the magma chamber depends on
spreading rate. The prism-shaped chamber is filled with magma that rises from the
upper asthenosphere (Sobelev and Shimizu, 1993, Nature, 363:151). The chamber is
about 1 km wide and 250 meters deep and lies within the oceanic crust between the
basaltic sheet dikes and layered gabbros (Phipps and Chen, 1993, Nature, 364:706).
The magma is no cooler than 1200oC. The aftermath of several recent eruptions has
been witnessed from submersibles at oceanic ridges (EOS, 1993, 74:619; Haymon et
al., 1993, Earth and Planetary Science Letters, 119:85; Reynolds, 1994, Nature,
367:115).
b. hydrothermal circulation and hot springs (hydrothermal vents) occur at spreading centers.
Cold seawater enters faults at the margin of the rift valley. The water descends several
hundred to thousands of meters along the faults, and is heated by the hot rock surrounding
the magma chamber in the crust below the rift valley. The water becomes buoyant due to
the heat and rises through vents and fissures at the axis of the rift valley. The water exits
the system in hot springs on the floor of the rift valley. The hot seawater leaches elements
from the basaltic rock as it circulates through the faulted crust. The water also picks up
some volcanic fluids rising from the magma chamber. The water that is vented at the
surface is often black with precipitates of material extracted from basalt during circulation
through the crust. The dissolved material (the fluids quickly become reducing when
circulated) precipitates when it hits the normal cold oxygen-rich seawater forming
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polymetallic sulfide and sulfate minerals enriched in various metals such as iron, copper,
zinc, manganese, nickel, silver, gold, and cadmium. In addition to sulfides, Ca sulfates
(anhydrite and gypsum) commonly form at vents. The basalt becomes altered during
hydrothermal circulation. For example, the basalt gives up Fe and Ca and takes up Mg
forming the new mineral, serpentine. The metamorphosed basalt is called serpentinite, a
metamorphic rock rich in the mineral, serpentine.
c. lithosphere (=crust above Moho + uppermost ~90 km of uppermost mantle), The
lithosphere is solid. It is very brittle at the top (crust) and less so at depth. This is a
conceptual model subject to change when more direct evidence becomes available.
d. asthenosphere (base is about 200 km below the surface of the earth, ~120 km thick). The
hypothesis is that the asthenosphere is plastic and flows, perhaps made a bit melted by
addition of water.
2. Convergent plate boundary (destructive boundary)
a. subduction zone (Benioff Zone, the subducting plate is the hypocenter of earthquakes
from shallow ones near the trench to deep ones (650 km) beneath the island arc. The plane
of earthquakes is called the Benioff Zone. The lithospheric plate melts as it subducts into
the asthenosphere and mantle. At about 100 km depth, the water in the fractured basalt,
pore spaces of ocean sediments, and bound in hydrated minerals is baked out and interacts
with the surrounding mantle, causing it to melt and form a magma. The magma is buoyant
and rises until it erupts in the volcanic arc found behind every deep sea trench. Some of
the basalt associated with this process is metamorphosed to a rock called eclogite with
garnets and olivine, and occasionally a tectonic accident will bring this rock type to the
surface. The ultimate fate of the descending plate is to be completely resorbed into the
mantle.
b. the trench is formed by the downward bending of the subducting plate as it dives into the
mantle.
c. the island arc is formed by eruption of lava formed when volatiles (mostly water) released
from the subduction plate mix with overlying mantle material. The buoyant magma rises
and erupts at the surface. The eruptions at such volcanic arcs are very explosive due to the
high viscosity of the volatile-rich magma. Mount Saint Helens is an example of an
explosive volcano located behind a trench and above a subducting lithospheric plate. In
contrast, magma at spreading centers and hot spots is volatile-poor and comparatively low
in viscosity.
d. The volcanism is explosive in part from the water that is added to the mantle from the
sediment and highly fractured top of the crust. But the biggest source of water is from
dehydration of minerals that contain water in their structure.
e. The ocean sediment that accumulated on the plate can be “offscraped” onto the adjacent
volcanic island arc or continental margins, or it is carried down the trench to melt with the
ocean crust (or some of both!).
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f. Backarc basin often forms behind the island arc. The crust is formed at a spreading center
that is poorly developed but sufficient to introduce new crust to the floor of the backarc.
The convection driving the crust formation might be a little cell formed above the
subducting plate.
3. Shear plate boundary (transform faults)
a. transform faults lie between segments of ORs that are offset by distances of tens to
hundreds of kilometers and in age by thousands to tens of millions of years.
b. At these boundaries, crust is neither created nor destroyed. The plates simply slide past
one another.
c. Some transform faults are extra-long and some pass through continental crust. A good
example of this is the San Andreas Fault of California.
d. Beyond and continuous with the transform faults are fracture zones. The seafloor remains
scarred by the faulting processes that formed it at the transform faults and spreading
centers. There is generally one side of the fracture zone that stands higher than the other,
and there are often seamounts and linear deeps that trace the position of the fracture
zone. There is no net movement of one side relative to the other, and fracture zones are
virtually aseismic.
e. Some plate boundaries are DIFFUSE, and spread over hundreds to thousands of miles.
Diffuse boundaries occur at transform plate boundaries through both continental and
oceanic crust. Some convergent plate boundaries are diffuse, like the Himalayan
boundary, but this is a somewhat special boundary where two continents have collided.
4. Midplate region
a. The rigid midplate region moves along like a conveyer belt to its site of destruction in the
subduction zone. Midplate regions are involved in several process. 1) Cooling; the plate
cools as it ages. 2) Some plates have lines of volcanism on them that might be caused by
mantle plumes beneath them or another process. Both subsidence and this volcanism are
discussed below.
b. The mid plate region is tectonically quiet for the most part. The oceanic crust cools after
it’s emplacement as hot young volcanic basalt at the spreading center of the oceanic ridge.
c. Mantle plumes (Morgan, 1971, Nature, 230:42; Morgan, 1972, AAPG, 56:203) are
streams of molten material that ascend from the mantle (Nature, 1993, 364:115) to Earth’s
surface where they erupt as volcanic hot-spots (Wilson, 1963, Can. J. Geophy.,41:863)
[http://www.geolsoc.org.uk/template.cfm?name=fbasalts]. A few mantle plumes originate
in the lower mantle below the asthenosphere perhaps some as deep as just above the coremantle boundary, rise through the asthenosphere and lithosphere, and erupt as gigantic
flows (floods) onto the Earth's surface. However, most hotspots probably result from
upper mantle processes with sources in the upper mantle (i.e., Yellowstone and Newberry
hotspots in the U.S.; Humphreys et al., 2000, GSA Today 10(12): 1-7). There are around
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100 plumes and associated volcanic flows on the surface of the Earth at present. Examples
of active plumes include Iceland, the island of Hawaii, and Yellowstone National Park. As
you can see from this listing, mantle plumes can pop up anywhere on Earth's surface, in
the middle of a plate in oceanic crust like Hawaii, the middle of a continent like India
(Deccan Traps) and Yellowstone National Park of North America, or at plate boundaries
like Iceland, which lies on the crest of the Mid-Atlantic Ridge. However, there is a
general correlation between the location of hotspots and the position deep mantle anaolies
in the geoid (implies a connection to a deep mantle process). Mantle plumes are slowmoving (e.g., Antretter et al., 2002, Earth and Planetary Science Letters. 203(2):
635-650), though were considered stationary until evidence from the last decade corrected
this misconception. Deep mantle plumes, like those below Hawaii and Reunion Island,
start with a massive eruption of flood basalts, then leave a trail of volcanic mounts on the
plates that move across them. Flood basalts are massive eruptions of over a million cubic
km emplaced in a very short amount of time, < million years (i.e., see Renne et al., 1996).
Examples of flood basalts that mark the onset of mantle plume eruption include the
famous Indian Deccan Traps (66 Ma, ~Cretaceous-Tertiary boundary in age), the
Rajmahal (116 Ma; latest Aptian Epoch of the Cretaceous Period), the Paraná Flood
basalts of southern Brazil and adjacent nations (early Cretaceous in age), Karoo in South
Africa (183 Ma, ~E-M Jurassic boundary), and Siberian Traps (249 Ma, ~Permo-Triassic
boundary). Examples of apparent plume trails include the Hawaiian Islands and the
Emperor Seamounts of the North Pacific Ocean,, and the Ninetyeast Ridge in the Indian
Ocean. However, magnetic anomalies do not correspond to the age of the seamounts in
the way a plume trail should, so the concept is under close scrutiny at the moment.
d. Motion of the plates is described by Euler’s Theorem from spherical geometry. Each plate
pair moves about a pole of rotation that passes through the Earth’s center.
i. The poles of rotation have no connection with the geographic north pole of Earth or
its axis of rotation, although at present many plate poles of rotation are near the
geographic north pole.
ii. The plates move slowest near the pole and fastest farthest from the pole. Every plate
pair has its own pole of rotation. Transform faults and their fracture zones sweep out
great circles that parallel the motion of the plates. See
http://math.rice.edu/~pcmi/sphere/ for more on Euler’s Theorem.
iii. GPS studies have measured this movement directly.
iv. Typically, the position of a pole of rotation shifts over time because
(1) a plate gets jammed, and its change in motion jostles all the other plates. This idea
is not holding up very well with new facts that are coming forward.
(2) some changes of pole location could reflect reorganization of the driving forces
that move the plates
(3) something not yet conceived?????
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MECHANISMS OF PLATE MOTION AND STRUCTURE OF THE CRUST AND
MANTLE
1. Convection cell hypothesis
a. convection in the asthenosphere
i. the asthenosphere is equivalent to the mantle excluding the uppermost mantle
ii. the asthenosphere is plastic and flows in convection cells driven by heat from
radioactive decay in Earth's interior. The movement in the mantle is on the order of a
few centimeters per year.
b. lithosphere
i. lithosphere includes the uppermost mantle, the Moho, and the crust above the Moho
(~100 km thick).
ii. the lithosphere is rigid and is dragged along by the movement of the convection cells in
the asthenosphere.
c. Movement
i. The rigid lithosphere is dragged along by the upper limb of the convection cell.
(1) Spreading centers form above rising limbs of the convection cells, and trenches and
Benioff zones are dragged down by descending limbs of convection cells. That’s
the hypothesis. This suggests that the cells are very big laterally with one cell per
plate.
ii. Gravity hypothesis
(1) plates cool and sink into the asthenosphere under their own weight. It used to said
that theey sink when they become denser than the underlying asthenosphere.
However, modeling does not support this. It is unclear how subduction is
initiated.
(2) Sinking margin of plates drags the rest of the plate behind it. (Lachenburch, 1976,
J. Geophys. Res., 81:1883).
(3) Others suggest that the plates roll back during subduction, stirring the
asthenoshere and creating its currents (see Hamilton, GSA today, Nov. 2003).
iii. A more sophisticated version combines the two hypothesis mentioned above.
(1) The mantle has many small convection cells.
(2) It is the sum of the motion of the many cells beneath a plate that determines its
direction of motion.
(3) The crust is jostled around by the convection cells, but location of ORs and
trenches aren't directly related to position of rising and falling limbs of individual
convection cells, but rather groups of cells.
(4) ORs form where crust is torn apart along zones of weakness in the crust, and
Benioff zones form where crust becomes too dense to float any longer on the
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mantle. This is in contrast to formation of convergence and divergence zones in
direct connection with rising and falling limbs of convection cells.
2. We need more technology to answer this, but here is an interesting view into the interior of
the Earth using seismic tomography (Dziewonski and Woodhouse, 1987, Sci. 236:37 and
subsequent publications.) The tomography is still too coarse to differentiate between the small
and large convection cell hypotheses.
OTHER OBSERVATIONS EXPLAINED BY PLATE TECTONIC THEORY
1. heat flow through the lithosphere
a. regions of high heat flow in ocean basins
i. oceanic ridges
ii. mantle plumes
iii. Backarc basins and volcanic arcs of subduction zones.
b. Concept: the depth of sea floor varies with heat flow.
i. hot rock is less dense than cold rock
ii. hot oceanic rocks float higher on the mantle than cool oceanic rock. Remember
isostasy where continental rock floats higher on the mantle than does oceanic crust
because it is made of less dense material. The same underlying physical law is at work
here. The hot part of the plate floats higher because it is more buoyant.
iii. So areas with high heat flow (for example, ORs and areas above mantle plumes) float
higher on the mantle than areas with low heat flow.
iv. This is what happens: (cooling of the plate and depth of the sea floor)
(1) The plate is hottest at spreading centers and coolest near trenches where plates are
subducted. The plate is hot at the OR because it has just cooled from molten lava
erupted at the spreading center. The plate is coolest at the trench because this spot
has had the most time to cool during transport from its birthplace at the spreading
center of the oceanic ridge (OR).
(2) Depth of seafloor increases with distance from spreading centers because the crust
becomes cool and dense as it loses heat over time.
(3) q = k/%t, where q is heat flow in 10-6 cal / cm2/s, k is a constant, and t is time. 10-6
cal is a micro-calorie.
c. density differences due to heat explain the following:
i. ORs are shallower than abyssal plains in midplate regions because they are hotter and
less dense.
ii. fracture zones keep their stepped profiles beyond transform faults, the zone of
movement between OR segments
iii. guyot formation and subsidence to depths greater than 1500 m
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(1) Guyots are volcanic islands with flat tops found at great ocean depths. Guyots are
formed as follows.
(a) They start as volcanic islands in temperate regions. Wave action erodes the
tops until they are completely flat. Subsidence on the sinking and cooling crust
eventually drowns the flat-top sea mount and carries it to great depth with the
subsiding plate.
(b) Guyots can also have started as atolls. Atolls are formed as follows. They start
as tropical volcanic islands where coral rapidly grows around its coast. The
island sinks with the plate it sits on, and over time the coral grows upward to
form fist a fringing reef, then a barrier reef around the island, then an atoll
when only a ring of coral and a shallow central lagoon are visible at the ocean
surface because the island has sunk out of sight. The extinct volcanic island is
now completely capped by the coral. Overall, the top of the island has been
made relatively flat by the ring of coral and its shallow coral lagoon. At some
point the coral is drowned for reasons that remain unclear and the coral-capped
seamount continues to subside with the sinking plate. These observations are
described by ODP Legs 143 and 144, i.e. Haggerty et al., 1993, JOIDES
Journal, 19(1):13-20).
2. Age of the crust increases with increasing distance from ORs.
a. Sediment thickness increases with increasing distance from ORs. Why?
b. The oldest ocean crust on the seafloor today is about 180 million years old, much younger
than the oldest continental rocks. Why?
3. Most other features discussed in the "feature" lecture can be explained by the theory.
a. island chains like the Hawaiian Islands and the Emperor Sea Mounts form on moving
plates above mantle plumes. (Or maybe not!) The idea was first developed by Tuzo
Wilson in 1963, who called the surface volcanoes "hot-spots". The molten material rising
from the lower mantle is called a "mantle plume." The volcanic trends do sink as they
cool.
b. Aseismic ridges like 90EE Ridge (just a really dense island chain)
PLATES INTERACT WITH CONTINENTS
1. Continents sit on the plates and are carried along passively by the conveyer-belt movement of
plates. At present, some continents lie in the middle of plates well away from active plate
boundaries (what continent fits this description?) and some lie against a plate margin and
undergo various types of tectonic activity.
a. active and passive continental margins
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i.
active margins lie at plate boundaries, like the Pacific coast of the U.S. The continental
margin is altered at the boundary.
ii. passive margins lie in midplate regions, like the Atlantic coast of the U.S. A spreading
center formed beneath a supercontinent and tore it apart and began forming oceanic
crust (the young Atlantic Ocean). The margin contains a complete rock history of its
formation and has suffered only subsidence as the plate cooled and sank
2. What happens when continents collide with plate boundaries?
a. Continents at ORs
i.
East African rift valleys; sit above a spreading center that failed. Convection began
beneath the continent but seems to be stalled.
ii.
OR overridden by a continent: history of San Andreas fault and basin and range.
(1) beginning about 36 million years ago, the continent overroad the spreading
center of the East Pacific Rise.
(2) Extensional forces of the spreading center stretched (basin and range region
including Nevada and the Mojave Desert of southern California) and tore the
continent (the San Andreas fault).
b. Continents at deep sea trenches
i.
Continental crust is too light to go down the subduction zone, so some pretty
spectacular events can result. India was too light to go down the trench, so it was
pushed across the subduction zone and pushed beneath Asia.
ii.
Collision of India and Asia; Himalayan Mountains thrust up as India is shoved
beneath Asia. (Collision occurred in a prolonged event from 65 to 45 Ma. The
northwest corner hit Asia first at about the end of the Cretaceous Period and the east
part of northern India hit Asia last during the Eocene Epoch. Uplift of the
Himalayas, a consequence of the collision, has been episodic with major pulses at
21-17, 11-7 and 2-0 Ma [Sorkhabi and Stump, 1993, GSA Today, 3(4):88]. The
mountains rise as India continues to be shoved under Asia at about 4 to 5 cm/yr
(Searle, 1995, Nature, 374:17). The mountains rise at about 10 mm/y at the fastest
spots - very different rates at different places.
c. Continents at transform faults
i.
San Andreas Fault
(1) shear plate boundary where lithospheres of two plates slide past one another
without creation or destruction of lithosphere.
(2) Baja California and Point Reyes are two pieces of continent severed from the
whole. Their fate is to be carried to the Aleutian Trench
(3) Observations from the Global Positioning System satellites show that the total
rate of movement between North American Plate and Pacific Plate is 47
mm/yr, and that 35 mm/yr of the movement is along the San Andreas Fault
(Hudnut, K.W., 1992, Nature, 355:681).
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ii.
accretionary terrains
(1) Slivers of continent sliced away by transform faults are too light to go down
trenches. Instead, they float across and get plastered onto island arcs or
continents on the other side of the trench.
(2) Examples are
(a) the terrains of Alaska
(b) the Shatski Rise in the future will be plastered onto the island arc of Japan
(c) many of the plateaus found in the oceans may have been severed from the
continents by transform faults and may eventually be plastered onto other
continents or island arcs.
d. age of oceanic crust compared to continental crust
i.
formation of oceanic crust which is no older than 200 million years
(1) created at ORs and “destroyed” at subduction zones beneath trenches
(2) 20% of Earth's surface has been subducted in the past 65 million years
ii.
formation of continents which are made of rock that ranges in age from recent to 3.8
billion years
(1) Precambrian shields may be made of rock formed by palte tectonics after the
original differentiation of mantle and crust during earth formation. More than
half of continental crust was formed before about 2500 million years ago
(McLennan, S.M., 1988, Pure appl. Geophys. 128: 683-724).
(2) sediments scraped off subducting oceanic plates onto continent
(3) collision and suturing of two plates like India and Asia
(4) accretion of terrains
(5) continued differentiation and fractionation of mantle elements; the light
elements float up in volcanism to form new continental rock.
(6) underplating of the continents by mantle magma accounts for about 17% of the
total thickness of 35 km thick continental crust (Voshage et al., 1990)
3. History of continental fragmentation and suturing
a. Plate tectonics has been active since 2 billion years after the planet's formation based on
observations of rocks of that age that are similar to modern rocks that have undergone
plate processes. It was thought that plates didn't sink during the first 2 billion years
because they were too hot and not dense enough to subside into the mantle. Recent
evidence from Greenland suggests that rock 3.8 billion years old bears the clear marks of
plate tectonic activity (see Richard Kerr, Science, 1991, v. 254, p. 802). Hamilton contests
this (GSA Today, Nov. 2003)
b. Sequence of events
i.
600 million years ago there were 6 major subcontinents distributed throughout low
latitudes
ii.
240 million years ago, continents had converged into one supercontinent, Pangaea.
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iii.
180 million years ago, Tethys Sea had opened in the middle of Pangaea along the
Equator, separating the supercontinent into two continents:
(1) cool, high-latitude Gondwanaland and
(2) warm, low-latitude Laurasia. Most land was in the southern hemisphere in
contrast to today when most land is in the northern hemisphere.
iv. 135 million years ago, opening of the south Atlantic Ocean began. Steps 1-4 suggest
repeated opening and closing of the ocean basin a concept developed by Tuzo
Wilson.
v.
75 million years ago, India rifted from Antarctica and moved toward Asia
vi. 65 million years ago, the Atlantic Ocean was well on its way to its present width
vii. 50 million years ago, Australia separated from Antarctic and began moving
northward
viii. 35-40 million years ago,
(1) India collided with Asia and the Himalayan Mountains were thrust up
(2) The impact may have caused plates around the globe to shift somewhat their
direction of motion. The date corresponds to the time when hot-spot islands in
the Pacific changed their lineation.
ix. 5 million years in the future, where will the continents be?
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