Chapter 4 – The theory of plate tectonics
Geologists surmise that the earth’s crust is made up of ten or more rigid
pieces of lithosphere called plates
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These crustal plates or slabs move with respect to one another as they
glide on the hot, plastic asthenosphere
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Remember that the athenosphere is that part of the mantle that is solid,
but also somewhat plastic so it flows very slowly.
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Plates move around the earth’s surface and sometimes collide with or
slide past each other. Plates can also break or tear apart.
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There are three types of plate boundaries:
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convergent boundaries (plates move towards one another)
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divergent boundaries (plates move away from each other)
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transform boundaries (plates slide past one another).
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Nothing on earth is fixed or static; every part of the earth is moving although
normally at a rate so slow as to go unnoticed by humans.
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Not only are the plates moving, but plate boundaries also move or migrate
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Ridge crests (divergent boundaries) can migrate across the earth’s surface
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Convergent boundaries can also migrate (subduction stops in one place and starts
in another)
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Transform boundaries (for example: San Andreas fault where two plates are
sliding past one another) can also change positions over geologic time. The fault
has been in its present position for only about 5 M yrs. Before that, the fault was
located further west. The 1992 Landers earthquake in California took place on a
new fault located further west in the Mojave Desert. This suggests the fault is
trying to migrate further inland. If this is so, more of California may be attaching
itself to the Pacific plate instead of the NA plate and will continue to slide
northwestward relative to the rest of NA.
The image below shows what geologists believe are the boundaries of the major
lithospheric plates. Some plates consist of ocean floor and some contain both oceans
and continents. For example, the South American plate contains both ocean and
continental lithosphere and it’s eastern boundary lies far out in the ocean. Active
continental margins define a plate boundary. Passive continental margins are not
plate boundaries.
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Plates can change in size, growing larger or smaller, such as when new
sea floor is being created at a ridge zone. Plates can grow smaller when
crust is subducted or when a plate splits apart to become two at a
divergent zone.
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The leading edge of the NA plate (western edge of NA) is not being
subducted because it is made up of lightweight continental rock. More
NA plate material is being created at the mid-Atlantic ridge; therefore,
the NA plate is growing larger.
• Continental lithosphere averages about 150 km (range 125 to 250 km)
thick.
• Old oceanic lithosphere at continental edges may be 100 km thick
• Young oceanic lithosphere at mid-ocean ridges may be only 10 km
thick.
•Ocean basins exist because continental lithosphere is made of lighter
density rock and therefore “floats” higher than oceanic lithosphere, which
is made of denser rock.
Passive margins accumulate sediments washed down from the adjacent continent
and these sediments can, over time, form a continental shelf.
Earthquake belts (red markings in map below) help define the
position of plate boundaries.
Divergent plate boundaries and sea-floor spreading.
• A divergent boundary is usually a mid-ocean ridge and is a place where new sea floor
is created by rising of the hot asthenosphere. As the asthenosphere rises it melts to
form magma, which, because it is more buoyant that the surrounding rock, continues to
rise and push apart existing rocks as it fills the magma chamber located in the crust
below the ridge axis.
•Magma that solidifies along the
chamber sides makes gabbro, a
coarse-grained igneous, intrusive
mafic rock that is the chemical twin to
basalt, a fine-grained extrusive rock
that solidifies at or above the surface.
•Basalt is an extrusive igneous rock
that cools very quickly upon reaching
the surface and hitting the ice cold sea
water. There are a peculiar kind of
basalt formation that is found on the
sea floor called “pillow basalt”.
As it is created, the new oceanic crust moves away from the ridge on both sides to
allow room for still more crust to form. As the oceanic crust and upper mantle move
away from the ridges, it cools, becomes denser, sinks and becomes part of the
lithosphere (cool rock is denser than warm rock).
Thus, the sea floor grows older (green) with increasing distance from the ridge axis with the
oldest sea floor found near the continental margins and the youngest sea floor (dark red)
found directly over the ridge axes.
The concept of subduction.
Two plates converge and one plate is forced to bend and subduct beneath the
overriding plate. At convergent plate boundaries, the downgoing plate grinds along
the base of the overriding plate, generating earthquakes.
In a continental-oceanic convergence, the downgoing plate is always the denser,
heavier, oceanic slab, never the continental lithosphere because the continental slab is
too buoyant to subduct, being made of lighter, silica-rich rocks. Because oceanic
lithosphere is constantly being returned to the mantle via subduction zones, no oceanic
lithosphere is greater than 200 million years old. Continental lithosphere, however, can
be several billion years old.
A chain of volcanoes can develop behind the accretionary prism (wedge). If an
oceanic plate is being subducted beneath a continental plate, this arc is called a
continental volcanic arc (the Andes in S.A. and the Cascade Mtns in N.A.). If the
subduction occurs between two oceanic plates, it is called a volcanic island arc.
Example of a
volcanic island
arc. The Aleutian
Islands, where two
oceanic plates are
involved in a
subduction
process.
Transform faults are the
actively slipping segment of a
fracture zone between two
ridge segments. At this kind of
boundary, one plate slides
sideways past another but no
new plate is formed or
consumed. A famous example
of a continental transform fault
is the San Andreas fault in
California which defines part
of the plate boundary between
the North American Plate and
the Pacific Plate. LA. is part of
the Pacific Plate while east of
the fault is part of the N.A.
Plate. In 100 million years,
LA. will be a suburb of
Anchorage, Alaska. Transform
faults can generate huge
earthquakes as in the San
Francisco Bay area in 1906
and 1989.
A triple junction is where three plate boundaries intersect a point. Figure A shows a ridgeridge-ridge triple junction. Figure B shows a trench-transform-transform triple junction.
• Hot spot volcanoes develop from rising mantle plumes. A mantle plume is a narrow
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column of hot mantle rock that rises through the mantle. A mantle plumes are thought to
have spherical or mushroom shaped heads rising above a narrow tail
Plumes form hot spots of active volcanism on the earth’s surface
When the head of the plume nears the surface, it can cause uplift and the eruption of vast
amounts of flood basalts
As the head widens beneath the crust the flood basalt area widens and the crust is stretched
The narrow tail that follows produces a narrow spot of volcanic activity
Some scientists theorize that
the outward flow of the
expanding head of the plume
may be strong enough to start
plates moving
Some geologists feel plumes
located at the mid-Atlantic
ridge are forceful enough to
drive the NA plate westward
Plumes are essentially
stationary with respect to
moving plates and to each
other
If plumes are located beneath
a continent, they may
eventually cause that
continent to split apart
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Yellowstone National Park in northwestern Wyoming is a place where a
mantle plume may exist.
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Evidence for this includes past volcanism in the area, high elevation of the
region (uplift occurs when a plume is beneath), high heat flow, and hot spring
and geyser activity
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Radial flow of mantle rock beneath Yellowstone may also be causing
earthquakes in the area
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Eventually, the NA continent may split apart at this site, and a new ocean will
form
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Plumes may also rise beneath oceanic plates
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The plume under Hawaii rises in the center of the Pacific plate
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As the plate moves along the plume, a line of volcanoes form, creating an
aseismic ridge
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Eventually, the volcano is carried off the hot spot and becomes extinct (no
new material is added to it).
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Without the continuous addition of new rock by volcanic activity, weathering
and/or subsidence will eventually cause inactive volcanic islands to disappear
beneath the sea (seamounts and guyots)
Hot spot volcanoes develop from
rising mantle plumes underneath
that portion of the plate. As the
plate moves, the volcano is carried
with it and off the hot spot to
eventually becomes extinct. Over
eons of geologic time, the process
causes a chain of extinct
volcanoes to develop. The oldest
volcano is furthest from the hot
spot while the youngest, still
active volcanic island is directly
over the hot spot. The extinct
volcanoes gradually sink below
sea level to become seamounts.
• Divergent plate boundaries can occur in the middle of the ocean or in
the middle of a continent
• The result of divergence, ultimately, is the creation of new ocean basins
• The breakup of the ancient continent of Pangaea was caused by divergent
boundary activity
• The divergent boundary is marked by rifting, basaltic volcanism, and uplift
• During rifting, the continental crust is stretched and thinned, a process than
can produce earthquakes and normal faulting
• Some geologists feels rifting causes uplift
• Others feel uplift is caused by the mantle plume first, then rifting happens
Continental
rifting. This
occurs when the
continental
lithosphere thins
and pulls apart to
eventually separate
and form a new
continent.
Continental
rifting is the East
African Rift
zone.
Oceanic plate – continental plate convergence
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When an oceanic plate is subducted under the continental lithosphere, an
accretionary prism (wedge) and forearc basin form an active continental
margin between the trench and the continent
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The Wadati-Benioff zone (named after the scientists who discovered it) is an
earthquake zone caused by the subducting plate dipping under the edge of the
continental plate
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A belt of igneous activity called a magmatic arc (similar to an island volcanic
arc if in the ocean) is created
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An example of this on the NA continent would be the Cascade mountain range
in the Pacific NW
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The hot rising magma causes uplift and a mountain range is created by the
thickened crust which rises isostatically
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A second contributor towards mountain building is stacking up of thrust sheets
on the continental side of the magmatic arc
Continent – continent convergence
In this case, two continental plates approach each other and eventually
collide. A good example of this is the collision between the Indian plate
and the Asian plate.
• This happens when the sea floor that lies between them is subducted, and
the ocean becomes narrower and narrower until it is all gone and the
continents, which as we’ve learned cannot subduct, collide with each
other.
• One continent may slide a short distance beneath another, but it will not go
down into a subduction zone because they are both too buoyant to be
subducted into the mantle.
• The two continents become welded together along a suture zone that
marks the old site of subduction and quite large mountain ranges can form
• An example of a mountain range formed in this kind of collision would be
the Himalayan Mountains in central Asia.
Continent-continent convergence. Collision causes mountain ranges to arise Crust
beneath a mountain range is about 60-70 km thick, twice the thickness of normal crust.
Plates move in response to ridge-push force and to slab-pull force. Ridge-push force
develops because mid-ocean ridges are at a higher elevation than the adjacent abyssal
plain. Gravity causes the elevated lithosphere at the axis to push on the abyssal plain
lithosphere. Slab-pull force results from the more dense, older and cooler lithosphere
being subducted into a less dense asthenosphere. Once a plate starts to sink, it pulls the
rest of the plate along behind it. The velocity of plate motion varies from 1 to 15 cm per
year. Geologists using GPS can now track plate motion in rates as small as millimeters
per year.
End of Chapter 4.