Download Intro to plate tectonics

Earth structure
Interior
Plate boundaries
The three plate boundaries involved in
tectonic Hazards:
•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.
•There are also….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.)
Plate Movement
True or False?
• The Earth is 4.6 billion years old.
• The earliest human remains are dated at
less than 2 million years old.
• Systematic study of the Earth only began
about 300 years ago.
• No major understanding of its structure or
evolution was achieved until the past
century.
• The country of Iceland did not exist 20
million years ago.
Supercontinents!
• The Earth did not always look as it does
today.
• It is believed that ~ 250 million years ago
all the continents were joined together to
form one “supercontinent” called Pangaea.
• Pangaea was a roughly “C” shaped
landmass that spread across the equator.
Alfred Wegener put forward this theory in 1912…
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.
Wegener’s model
of continental drift
What might happen 200
million years from now?
What evidence did Wegner use?
1. Continental fit
Why don’t all the
continents fit
together snugly
today?
2. Geological Evidence
• The Appalachian Mountains of eastern North America are
thought to link to the Caledonides of Ireland, Britain,
Greenland, and Scandinavia and the Anti-Atlas Range in
Morocco.
3. Climatological evidence
• Coal deposits that would have been formed in tropical
climate conditions are found in places that do not have a
tropical climate. Therefore, they must have drifted. E.g.
Antarctica, South Wales
4. Biological Evidence
• Mesosaurus remains were found in southern Africa and eastern South
America, two far away places. Mesosaurus was a freshwater animal,
and could not have crossed the Atlantic Ocean, this indicates that the
two continents used to be joined together.
• Marsupials are only found in Australia because it drifted away from
the main supercontinent before the predators that wiped them out
elsewhere had migrated there
So far:
Wegener had convincing evidence for
continental drift.
However, sceptics were quick to point out
that there was no explanation of the
mechanism by which continents could
move over a solid earth.
It was not until the second half of the 20th
century that major discoveries began to
suggest how this might be possible.
The Puzzle of Continental Drift
• New evidence in 1948
• A survey of the floor
of the Atlantic Ocean
revealed a continuous
ridge running north to
south.
• It was 1000km wide.
• Heights of 2.5km
• Composed of volcanic
rocks.
• Similar found in Pacific
Ocean.
1950’s Evidence =
Paleomagnetism
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 and
new rocks are being added equally on either side.
This process is known as sea floor spreading.
Geomagnetic Polarity Reversals
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.
This is known as ‘Polar Wandering’
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
Pull slabs of oceanic crust
back down into the mantle
at suduction zones.
Pull crust apart at
spreading ridges.
The Driving Mechanism for Plate Movement
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
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.
CONSTRUCTIVE/Divergent
Boundaries
• Boundary between two plates
that are moving apart or rifting

• RIFTING causes SEAFLOOR
SPREADING
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.
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/ Transform
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.