GEO100 05 plate tectonics

PLATE TECTONICS
The Basic Premise of Plate Tectonics
The lithosphere is divided into plates that move relative to one another, and
relative to the earth’s asthenosphere. Movement occurs at very slow (cm/yr)
rates. As a plate moves, its interior remains relatively intact (but it is not
totally ridged) while its boundaries become deformed.
Relative motion:
30 cm/yr with
respect to red star
60 cm/yr with respect to red star
30 cm/yr with respect to blue star
Plate: a distinct piece of lithosphere that has boundaries on all sides which are
called plate boundaries .
Continental margins: edges of continents where they meet the oceans/seas.
These lead to active margins (continental margins that are plate boundaries) and
passive margins (continental margins that are not plate boundaries).
pa
ss
ive
active marg
in
ma
rg
in
South American
plate
Major plates : very large tectonic plates
•
Pacific Plate
•
North American Plate
•
South American Plate
•
Eurasian Plate
•
African Plate
•
Australian-Indian plate
Minor plates:
plates: small to moderate size tectonic plates
•
Juan de fuca Plate
•
Cocos Plate
•
Caribbean Plate
•
Scotia Plate
•
Arabian Plate
•
Iran Plate
•
Philippine Plate
•
Nazca Plate
Micro plates:
plates: very small tectonic plates that occur as non-ridged segments along plate
boundaries (e.g. Tonga Plate)
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Plate boundaries are common place for earthquakes to occur. This figure shows
the coincidence of areas with frequently occurring earthquakes and plate
boundaries.
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Mid-ocean ridges:
ridges: A 2-km-high submarine mountain belt that forms along a divergent
oceanic plate boundary.
Seamount:: An isolated submarine mountain.
Seamount
Trench: A deep elongate trough bordering a volcanic arc; a trench defines the trace of a
Trench:
convergent plate boundary.
Ridge axis:
axis: The crest of a mid-ocean ridge; the ridge axis defines the position of a divergent
plate boundary.
Fracture zone:
zone: A narrow band of vertical fractures in the ocean floor; fracture zones lie
roughly at right angles to a mid-ocean ridge, and the actively slipping part of a fracture zone
is a transform fault.
Volcanic arc:
arc: A curving chain of active volcanoes formed adjacent to a convergent plate
boundary.
There are three types of plate boundaries:
•
Divergent
•
Convergent
•
Transform
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Divergent plate boundaries: where two plates are moving away from one
another. this is where mid-ocean ridges occur.
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Convergent plate boundaries: where two plates are moving towards one
another. This is where trenches form in oceanic crust, and are commonly
associated with volcanic activity (such as the Cascade volcanoes).
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Transform plate boundaries: where two plates are moving past one another.
The San Andreas fault is an example.
True divergent plate boundaries occur along mid-ocean ridges. New ocean crust
is formed along the ridge in order to ‘fill in the space’ missing as the two plates
move away from each other. As spreading continues, older crust moves farther
away from the boundary. Spreading can be equal on both sides of the divergent
boundary, or more new crust can be formed on one side then the other.
When new crust is created at mid-ocean ridges (divergent plate boundaries) by
what is termed seafloor spreading, new sea floor is created by volcanic activity.
This volcanic activity is the result of magma rising up from the mantle and
forming a magma chamber. This magma is rich in Fe and Mg, and forms several
distinct rock types called Gabbro, Diabase, and basalt
Gabbro from rocks that are believed to have formed at a mid-ocean ridge
Diabase dikes from rocks that are believed to have formed at a mid-ocean ridge
Basalt from rocks that are
believed to have formed at
a mid-ocean ridge
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Just like basaltic rocks on the
continents, basalts on the ocean
record the magnetic polarity of
the earth at the time they were
erupted. This leads to the
stripped pattern that is seen on
the modern sea floor. It also
allows us to date the age of the
sea floor.
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The further away from the plate boundaries you move, the older the ocean floor gets.
True divergent plate boundaries occur as mid-ocean ridges; however, rifting is a
lesser form of divergent plate boundary.
Rifting: The process by which a continent stretches and splits along a belt; if
successful, this process separates a larger continent into two smaller continents
separated by a divergent boundary.
Rifting can lead to true
divergent boundary oceanic
spreading.
Convergent plate boundaries are associated with subduction zones (the region
along a convergent boundary where one plate bends and sinks down into the
asthenosphere beneath another plate).
Convergent plate boundaries can be an oceanic plate being subducted under a
continental plate (i.e., the Pacific plate under the South American plate to form the
Andes Mountains), or
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Convergent plate boundaries can be an oceanic plate being subducted under a
second oceanic plate. This is where volcanic island arcs form. A back-arc basin
is a depression formed behind a volcanic arc.
There are a series of terms that go with subduction zones such as:
downgoing plate : plate the is being subducted under the overriding plate (plate
that is not sinking/ not being subducted).
Accretionary prism : A wedge-shaped mass of sediment and rock scraped off the
top of a downgoing plate and accreted onto the overriding plate at a convergent
plate margin.
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Accretionary prisms are not too different than a plow scraping off material as it
plows along.
Forearc basin: depression in front of a volcanic arc where sediment is commonly
deposited.
Volcanoes are
associated with
subduction zones
because the down going
plate brings water with
and within it. This water
is then heated up at
depth, and is released
into the mantle. This
commonly happens at
100-150 km in depth.
This water allows the
mantle to partially melt,
which allows magma to
form.
Partial melting: The melting in a rock of the minerals with
the lowest melting temperatures, while other minerals
remain solid.
Mt Rainier, and the other Cascade volcanoes are the result of mantle partial
melting due to subduction of the Juan de Fuca plate beneath the North American
plate.
Subduction is a process that continues deep down into the earth’s mantle.
Also, the top of a downgoing plate is marked by a series of earth quakes. This
zone is termed the Wadati-Benioff zone.
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Subduction ends when two continental plate collide (called a collision). The
density of continental plates is so low that they cannot be subducted, so the
former trench becomes a suture (a liner belt of highly deformed rocks), a
collisional mountain belt forms where the two plates collided and the downgoing
plate breaks off (becomes detached ).
Transform plate boundaries are where two
plates are sliding past each other. A great
example of this type of plate boundary is
the San Andreas fault, which id the
boundary between the North American
plate and the Pacific plate. the average rate
of movement on the San Andreas fault is
~3.5 cm/yr; however not all parts of the
fault are always moving.
Transform faults can also be seen offsetting mid-ocean ridges. Where there is
active movement between two plates is called a transform fault, and where there
is no longer movement it is termed a fracture zone.
Special locations called Triple junctions are points where three lithosphere plate
boundaries intersect.
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Hot spots are special locations within plates where deep plumes of magma rises
up from the mantle and breaks through the lithosphere.
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It is thought that
hot spots do not
move, and that
they can record
plate movement as
a plate moves over
the plume that
rises up from the
mantle.
As a plate moves
over a hot spot a
chain of volcanoes
is formed. As
these extinct
volcanoes sink
below sea level,
they become
seamounts.
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How do you explain the bend in the chain of islands formed by the Hawaiian hot spot?
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