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Chapter 4 – The theory of plate tectonics
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Geologists now think the earth’s crust is made up of about a dozen or more plates
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The plates are basically rigid slabs of lithosphere (lithosphere = the crust + the rigid
part of the mantle)
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These crustal plates move with respect to one another as they glide over the plastic,
easily deformed asthenosphere
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Remember that the athenosphere is that part of the mantle that is solid, but also flows
very slowly.
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There are three types of plate boundaries:
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Convergent boundaries (plates are moving towards one another)
• divergent boundaries(plates are moving away from one another, for example
at sea-floor ridges)
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transform boundaries (plates are sliding past one another).
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Nothing is fixed or static, every part of the earth is moving.
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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 (ex. 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, 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.
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Plates can also change in size.
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Example: new sea floor is being created at the Mid-Atlantic ridge on the
trailing edge of the NA plate.
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The leading edge (western side) of the NA plate is not being subducted
because it is made up of lightweight continental rock. Therefore, the NA
plate is growing larger.
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Continental lithosphere averages about 150 km (range 125 to 250 km) thick.
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Old oceanic lithosphere at continental edges may be 100 km thick
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Young oceanic lithosphere at mid-ocean ridges may be only 10 km thick.
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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.
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The image below shows what some geologists think are the boundaries of plates that make
up the lithosphere. Some plates are all ocean floor and some contain both continents and
oceans. 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
are plate boundaries and passive continental margins are not. Sediments accumulate along
passive margins and this portion is called the continental shelf.
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Example of a passive margin.
Earthquake belts define the position of most plate boundaries.
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Divergent plate boundaries and sea-floor spreading.
• A divergent boundary is a mid-ocean ridge 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 to fill a magma chamber located in the crust below the ridge axis.
• Magma that solidifies along the chamber sides makes gabbro, a coarse-grained
igneous, mafic rock.
• As the magma rises still higher, it may form dikes (wall-like sheets) of basalt.
• Magma that is ejected at the surface is called pillow basalt.
• As soon as it is created, the new oceanic crust moves away from the ridge on
both sides to allow still more crust to form. As the oceanic crust and upper
mantle move away from the ridges, it cools and becomes part of the lithosphere.
This causes the oceanic lithosphere to thicken and to sink (cool rock is denser
than warmer rock).
• Thus, the sea floor grows older with increasing distance from the ridge axis.
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The age of the sea floor
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The concept of subduction. A plate bends, and one pieces pushes or overrides the other. At
convergent plate boundaries, the downgoing plate grinds along the base of the overriding
plate, generating earthquakes. The downgoing plate is always oceanic lithosphere, never
continental because the continental slab is too buoyant compared to the denser, heavier
oceanic slab. Because the oceanic lithosphere is constantly being returned to the mantle via
subduction zones, it is less than 200 million years old. Continental lithosphere, however, can
be as old as 3.8 billion years.
Volcanoes form here
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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.
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Transform faults are the
actively slipping segment of a
fracture zone between two
ridge segments. At this kind of
boundary, one plate slides
sideways pas 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 dfines 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. The first figure shows a
ridge-ridge-ridge triple junction. The second figure shows a trench-transform-transform triple
junction.
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Hot spot volcanoes develop from rising mantle plumes
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A mantle plume is a narrow column of hot mantle rock that rises through the
mantle
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A mantle plumes are thought to have spherical or mushroom shaped heads rising
above a narrow tail
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Plumes form hot spots of active volcanism on the earth’s surface
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When the head of the plume nears the surface, it causes uplift and the eruption of
vast amounts of flood basalts
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As the head widens beneath the crust the flood basalt area widens and the crust is
stretched
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The narrow tail that follows produces a narrow spot of volcanic activity
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The outward flow of the expanding head may be strong enough to start plates
moving
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Some geologists feel plumes located at the mid-Atlantic ridge are forceful enough
to drive the NA plate westward
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Plumes are essentially stationary with respect to moving plates and to each other
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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 might now be rising
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Evidence includes volcanism in the area, high elevation (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, weathering will eventually
cause inactive volcanic islands to disappear beneath the sea (seamounts and
guyots)
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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 is the ultimate 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
• Basalt erupts from the faults, often in a flood (thus the term flood basalts)
• Some geologists feels rifting causes uplift
• Others feel uplift due to the mantle plume occurs first, then rifting
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Continental rifting is
when the continental
lithosphere pulls apart and
eventually separates to
form a new continent.
An example of
continental rifting
is the East African
Rift zone.
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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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A Wadati-Benioff zone (named after the scientists who discovered it) of
earthquakes is 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 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
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Continental plate – continental plate convergence
• Two continents may approach each other and collide.
• As the sea floor that lies between them is subducted, the ocean becomes
narrower and narrower until the continents 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.
• An example of a mountain range formed in this kind of collision would be
the Himalayan Mountains in central Asia. They are formed by the
collision of India and Asia.
An example of two continental plates colliding (example, India and Asia). Collision
causes mountain ranges to arise (example, the Himalayas). Crust beneath a mountain
range is about 60-70 km thick, twice the thickness of normal crust.
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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.
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