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Plate Movement
Plate tectonics theory
Convection currents and sea floor spreading
In 1912 a German, Alfred
Wegener, published his
theory that a single
continent existed about 300
million years ago. He
named this super continent
Pangaea and maintained
that it had later split into the
two continents of Laurasia
in the north and
Gondawanaland in the
south. Today’s continents
were formed from further
splitting of these two
masses.
Continental
drift
Evidence for Continental Drift: Puzzle
Tectonic Plates
Evidence of tectonic motion is based on similar fossils and
rock types on opposing sides of the ocean
Palaeomagnetism
Wegener’s theories were unable to explain
how continental movement could have
taken place. From the 1940s onwards
evidence began to accumulate to show
that Wegener could have been correct.
The mid-Atlantic ridge
was discovered and
studied.
Examination of the ocean crust either
side of the mid-Atlantic ridge suggested
that sea-floor spreading was occurring.
The evidence for this is the alternating polarity of the rocks that
form the ocean crust. Iron particles in lava erupted on the
ocean floor are aligned with the Earth’s magnetic field. As the
lavas solidify, these particles provide a permanent record of
the Earth’s polarity at the time of the eruption - called
paleomagnetism
Earth’s geomagnetic
field is
3-3
recorded as new crust cools.
New crust.
Parallel bands of
crust with the
same magnetism
form along the
ridge.
Driving Mechanisms for Plate Motions
Geomagnetic Polarity Reversals
However, the Earth’s polarity reverses
at regular intervals [approx every
400,000 years]. The result is a series
of magnetic ‘stripes’ with rocks aligned
alternately towards the north and
south poles.
The striped pattern, which is mirrored
exactly on either side of a mid-oceanic
ridge, suggests that the ocean crust is
slowly spreading away from this
boundary. Moreover, the oceanic crust
gets older with distance from the midAtlantic ridge.
Earth structure
Interior
Movement of plates is caused by thermal
3-3
convection of the “plastic” rocks of the
asthenosphere which drag along the
overlying lithospheric plates.
Driving Mechanisms for Plate Motions
Convection Cells
Plate boundaries
Earth’s surface is composed of
a series of lithospheric plates.
Plate edges extend through
the lithosphere and are
defined by seismicity.
• Plate edges are trenches, oceanic ridges
and transform faults.
• Seismicity and volcanism are
concentrated along plate boundaries.
There are four types of plate boundaries
•CONSTRUCTIVE /Divergent boundaries -- where new crust is
generated as the plates pull away from each other.
•DESTRUCTIVE /Convergent boundaries -- where crust is
destroyed as one plate dives under another.
•CONSERVATIVE /Transform boundaries -- where crust is neither
produced nor destroyed as the plates slide horizontally past each
other.
•Plate boundary zones -- broad belts in which boundaries are not
well defined and the effects of plate interaction are unclear.
Artist's cross section illustrating the main types of plate boundaries (see text); East
African Rift Zone is a good example of a continental rift zone. (Cross section by José F.
Vigil from This Dynamic Planet -- a wall map produced jointly by the U.S. Geological
Survey, the Smithsonian Institution, and the U.S. Naval Research Laboratory.)
Convection Currents
• Hot magma in the Earth
moves toward the surface,
cools, then sinks again.
• Creates convection currents
beneath the plates that cause
the plates to move.
CONSTRUCTIVE/Divergent
Boundaries
• Boundary between two plates
that are moving apart or rifting

• RIFTING causes SEAFLOOR
SPREADING
Features of
Constructive/Divergent
Boundaries
• Mid-ocean ridges
• rift valleys
• fissure volcanoes
Constructive
MidOcean
Ridge
Lava fountains (10 m high) spouting from eruptive fissures during the October 1980
eruption of Krafla Volcano. (Photograph by Gudmundur E. Sigvaldason, Nordic
Volcanological Institute, Reykjavik, Iceland.)
Aerial view of the area around Thingvellir, Iceland, showing a fissure zone (in shadow)
that is the on-land exposure of the Mid-Atlantic Ridge. Right of the fissure, the North
American Plate is pulling westward away from the Eurasian Plate (left of the fissure).
Large building (near top) marks the site of Lögberg, Iceland's first parliament, founded
in the year A.D. 930. (Photograph by Oddur Sigurdsson, National Energy Authority,
Iceland.)
In East Africa, spreading processes have already torn
Saudi Arabia away from the rest of the African
continent, forming the Red Sea. The actively splitting
African Plate and the Arabian Plate meet in what
geologists call a triple junction, where the Red Sea
meets the Gulf of Aden. A new spreading center may
be developing under Africa along the East African Rift
Zone.
Constructive
DESTRUCTIVE/Convergent
Boundaries
• Boundaries between two
plates that are colliding
 
• There are 3 types…
Type 1
• Ocean plate colliding with a less
dense continental plate
• Subduction Zone: where the
less dense plate slides under the
more dense plate
• VOLCANOES occur at
subduction zones
Andes Mountains,
South America
Ring of Fire
Challenger Deep got its name from the British survey ship Challenger II,
which pinpointed the deep water off the Marianas Islands in 1951.
Type 2
• Ocean plate colliding with
another ocean plate
• The less dense plate slides
under the more dense plate
creating a subduction zone
called a TRENCH
Type 3
• A continental plate colliding
with another continental plate
• Have Collision Zones:
–a place where folded and thrust
faulted mountains form.
CONSERVATIVE/ Converse
Fault Boundaries
• Boundary between two plates
that are sliding past each other
• EARTHQUAKES along faults
3-4 Conservative
The San Andreas fault in
southern California is a
transform fault that
connects the sea-floor
spreading ridge of the
Gulf of California with
the spreading ridge off
Oregon and Washington.
• If these plate motions
continue, Baja will
splinter off California.
Conservative
3-4
Because the San Andreas fault has an
irregular trace, strike-slip motion can
cause local compression or tension.
Hot Spots
• Mantle plumes originate deep within the
asthenosphere as molten rock which
rises and melts through the lithospheric
plate forming a large volcanic mass at a
“hot spot”.
Mantle Plume
Plate-boundary zones
Not all plate boundaries are as simple as the main types
discussed above. In some regions, the boundaries are not well
defined because the plate-movement deformation occurring
there extends over a broad belt (called a plate-boundary zone).
One of these zones marks the Mediterranean-Alpine region
between the Eurasian and African Plates, within which several
smaller fragments of plates (microplates) have been
recognized. Because plate-boundary zones involve at least two
large plates and one or more microplates caught up between
them, they tend to have complicated geological structures and
earthquake patterns.