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Geological Oceanography
Plate Tectonics
Part 2
Plate Tectonics
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How do we know that there are plates and
that they move?
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Earth’s lithosphere is broken up into plates
Plates are in motion and continually changing
in shape and size
Earthquake Distribution
Lithospheric Plates
Plate Tectonics
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Plate boundaries
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All major interactions among individual
plates occur along their boundaries
Types of plate boundaries
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Divergent plate boundaries
Convergent plate boundaries
Transform fault boundaries
Processes of Plate Tectonics
Plate Tectonics
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Divergent Plate
boundaries
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Most are located
along the crests of
oceanic ridges
Also continental rifts
One presently is on
land
Divergent boundaries are located
mainly along oceanic ridges
Creation of New Ocean Basins
The East African rift – a
Divergent boundary on land
Ridges and Rises
Figure A is a cross-sectional view of the fast-spreading MidAtlantic Ridge. Figure B is a cross-sectional view of the
fast-spreading East Pacific Rise. On both cross sections,
vertical exaggeration is 50 times normal.
Plate Tectonics
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Convergent Plate Boundaries
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Where two plates collide
Types of convergent boundaries
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Oceanic-continental convergence
Oceanic-oceanic convergence
Continental-continental convergence
Ocean-Ocean Convergence
Cascade Mountains, Pacific NW
Ocean-Continent Convergence
Continent-Continent Convergence
The collision of India and Asia
produced the Himalayas
Plate Tectonics
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Transform fault boundaries
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Plates slide past one another
No new lithosphere is created or destroyed
Transform faults
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Most join two segments of a mid-ocean ridge as
parts of prominent linear breaks in the oceanic
crust known as fracture zones
Transform Faults
Plate Tectonics
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Hot Spots
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Caused by rising plumes of mantle material
Volcanoes can form over them (Hawaiian
Island chain)
Most mantle plumes are long-lived structures
and at least some originate at great depth,
perhaps at the mantle-core boundary
Examples of Plate Boundaries
Divergent Plate Boundaries
Divergent Plate Boundaries:
Juan de Fuca Ridge
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Juan de Fuca Ridge
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A spreading center
~500 km (300 miles)
long
medium spreading rate
(5-10 cm/yr) = 50-200
m deep rift
The rift is only 5 km
across.
Cross-section of East Pacific Rise at 21 degrees
N from Macdonald (1982).
Vertical exaggeration = 4x.
Divergent Plate Boundaries:
Iceland
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Iceland
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Straddles the Mid
Atlantic Ridge
Iceland is between the
North American and
Eurasian Plates
Two zones of spreading
and volcanism
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An eastern zone, the site of
most historical eruptions
A western zone.
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Surtsey
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Episodic eruptions began
on November 8, 1963
The volcano grew from
the sea floor, at a depth
of 130 m, to sea level by
November 15
Initial eruptions were
explosive, then changed
to gentle basalt flows.
The eruption stopped on
June 5, 1967
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Heimaey - 1973 Eruption
Divergent Plate Boundaries
East African Rift Zone
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East African Rift Zone
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Rifting between the
African Plate and the
Arabian Plate
“triple junction”
May eventually form the
next major ocean
Convergent Plate Boundaries
The Cascades
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Cascades Range
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Active for about 36
million years
Convergence continues
at about 4 centimeters
per year
Not a typical subduction
zone,
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Very little seismic
evidence of active
subduction
There is no trench
Only evidence for
ongoing subduction is the
existence of volcanic
activity
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Japan
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Most volcanoes result
from subduction of
westward-moving
oceanic crust under the
Asian Plate (continental
crust).
The volcanoes of this
region are unusually
explosive
In the Izu-Marianas
chain, the crust to the
west is also oceanic,
forming island arcs with
mostly basaltic
volcanoes
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The Himalayas: Two
continents collide
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225 million years ago, India
began to move north
towards Asia
Collision occurred 40 – 50
million years ago.
The Himalayas and the
Tibetan Plateau to the north
have risen very rapidly
After India collided,
Australia was released from
Antarctica and it began to
move northward towards S.
E. Asia.
Australia is currently in
collision with Asia.
Transform Plate Boundaries
The San Andreas Fault
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San Andreas fault zone
connects the East Pacific Rise
in the south, with the South
Gorda - Juan de Fuca - Explorer
Ridge to the north
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1,300 km long and in places
tens of kilometers wide
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Pacific Plate has been grinding
horizontally past the North
American Plate for 10 million
years, at an average rate of
about 5 cm/yr
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Movement can occur either in
sudden jolts or in a slow, steady
motion called creep.
Great San Francisco
Earthquake (8.3-magnitude) in
1906 ruptured along a
previously locked 430 km-long
segment of the San Andreas
The magnitude-7.1 Loma Prieta
earthquake of October 1989
occurred along a segment of the
San Andreas Fault which had
been locked since the great
1906 San Francisco earthquake
Hot Spots
Hawaii
Hawaiian Islands-Emperor
Seamount Chain
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The Hawaiian Ridge –
Emperor Seamounts
chain extends 6,000 km
Contains more than 80
volcanoes
Sharp bend in the chain
indicates a change in
plate about 43 million
years ago
Oldest islands are being
subducted in the
Aleutian Trench
Youngest is Lo’ihi
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Lo`ihi Seamount
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Located south of Kilauea
about 30 km from shore
969 m below sea level
3000 m high
Generates frequent
earthquake swarms, the
most intense of which
occurred in 1996.
High-temperature
hydrothermal venting at
its summit (200 ° C )
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Water normally 77 ° C
Tour of
Lo’ihi
Hot Spots
Yellowstone
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Facts:
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Location: Wyoming, Montana, Idaho
Height: 2,805 Meters
Latest Eruptions: 70,000 years ago
Yellowstone hot spot active for as long as 17
million years.
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Catastrophic
eruption = calderas
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Most recent:
650,000 y.o.
Produced a 53 x 28
mile caldera