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Plate tectonics
The Structure of the Earth
A thin crust 10-100km thick
mantle – has the
properties of a solid
but it can also flow.
1000 – 2500 o C
core – made of molten
nickel and iron. Outer
part is liquid and
inner part is solid.
4000 – 4700 o C
The average density of the Earth is much higher than
the crust, so the inner core must be very dense
Tectonic theory
People once thought that the oceans and the continents were formed by
shrinkage from when the Earth cooled down after being formed.
Alfred Wegener proposed something different. Consider Africa and South
America:
These continents look
like they “fit”
together. They also
have similar rock
patterns and fossil
records. These two
pieces of evidence led
me to believe that
there was once a single
land mass. This is my
TECTONIC THEORY.
The plates
Continental Drift
What causes the plates to
move?
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The lithosphere describes the
crust and the upper mantle. These
both move like plastic (even
though they are solid).
The move because of convection
currents.
As heat is produced from the core,
it heats rock in the mantle. This
rock rises as it is less dense
When it reaches near the top of
the mantle the rock starts to cool
and will eventually become denser
and start to sink.
The lithosphere is moved by these
convection currents
Movements of the crust
1) Plates move past each other, causing
earthquakes. These are called transform plate
boundary
2) Plates move away from each
other – a “constructive plate
boundary”. The gap is filled with
magma which cools to form basalt.
This is called sea floor spreading.
3) Plates move towards each other – a
“destructive plate boundary”. The
less dense one slides underneath
(“subduction”) and partially melts. This
causes volcanoes and earthquakes.
Transform plate boundaries
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The relative motion of the
plates is horizontal. They can
occur underwater or on land,
and crust is neither destroyed
nor created.
Because of friction, the plates
cannot simply glide past each
other. Rather, stress builds up
in both plates and when it
exceeds the threshold of the
rocks, the energy is released –
causing earthquakes.
Conservative Plate Boundary
Why do most volcanoes occur
around plate boundaries?
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Plates colliding (destructive)
New Zealand examples
Destructive Plate Boundary
Forming mountains
Where continental crust meets continental crust
Both continental crusts are too light to subduct so a continent-continent collision occurs,
creating especially large mountain ranges. The most spectacular example of this is the
Himalayas.
This is where
India is now
This is where
India was millions
of years ago
The intense heat and pressure from this process causes the
rocks to change structure into metamorphic rocks.
Mountain formation
Or…
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Plates moving apart (constructive)
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The space created can also fill
with new crustal material sourced
from molten magma that forms
below. Divergent boundaries can
form within continents but will
eventually open up and become
ocean basins.
i. On land
Divergent boundaries within
continents initially produce rifts,
which produce rift valleys.
ii. Under the sea
The most active divergent plate
boundaries are between oceanic
plates and are often called midoceanic ridges.
Constructive Plate Boundary
New Zealand examples