Download Plate INteractions - Fellows

Topic 3-Lesson 1
Ocean-Continental Convergence
Plate Interactions
We have previously discussed the evidence that supports the
concept of drifting continents and plate tectonics. Because of
the different directions the plates move, different processes
occur at plate boundaries. We have touched on these briefly
and now we will look at them in more detail.
Convergent Boundaries
When two plates collide, scientists call this a convergent plate
boundary. These slow collisions usually result in one plate
being forced beneath the
other. The overriding
plate normally grows
as folding and volcanism
produce mountain chains.
Convergent Boundaries
However this all depends on the types of crusts that are
colliding. There are three types of collisions:
 oceanic-oceanic,
 continental-oceanic
 continental-continental.
Oceanic-Continental Convergence
Because oceanic crust is more dense than continental crust,
when these two types of crust collide the oceanic crust is
always forced into the mantle (subducted).
Oceanic-Continental Convergence
Where the oceanic crust is
bent downward, trenches
form. These are the
deepest parts of the ocean
and are used to help
indicate the edge of a plate.
Oceanic-Continental Convergence
As the subducting oceanic plate is forced beneath the overriding
continental plate, the continental plate is lifted and folded
upwards producing mountains.
Oceanic-Continental Convergence
An example of a oceaniccontinental convergent
plate boundary is along the
west coast of South
America. Here we find the
Andes Mountains.
Oceanic-Continental Convergence
The Oceanic Nazac plate is slamming into the Continental
South American plate
Oceanic-Continental Convergence
If we visit this plate boundary we can see the evidence which
supports this theory.
Oceanic-Continental Convergence
Something else happens at this boundary which produces
mountains. As the oceanic crust is forced deeper into the
Earth, intense heat and pressure cause it to melt. And what
happens to materials when we heat them?
Oceanic-Continental Convergence
They rise! The oceanic plate turns into magma and rises into
the overlying continental crust. This is called a magma
plume. If this plume reaches the surface we get volcanoes!
Oceanic-Continental Convergence
A key feature to this magma is that it is rich in silica which
comes from the sediments in the oceanic crust or the melted
part of the continental crust.
Oceanic-Continental Convergence
This high silica magma makes
it more viscous (thicker)
which allows it to trap
gasses within it. Volcanic
eruptions at such
boundaries tend to be very
explosive because of the
intense heat and pressure
build up.
Oceanic-Continental Convergence
This type of silica rich and explosive eruption is called andesitic
volcanism (after the Andes Mountains).
Oceanic-Continental Convergence
But what happens if the magma never makes it to the surface?
That’s a great question! The magma that has collected within
the continental crust can slowly cool to form granite or
similar intrusive igneous rocks.
Oceanic-Continental Convergence
Eventually the surrounding material is eroded away and we can
see these frozen granite plumes today on the surface of the
earth.
Oceanic-Continental Convergence
Stone Mountain in Georgia (USA) is an example of just how big
these frozen granite magma plumes can get.
Oceanic-Continental Convergence
So what happens to the rocks on the continental crust that
come into contact with these magma plumes?
Oceanic-Continental Convergence
They are changed ‘metamorphosed’ into a different rock. This
is how we get metamorphic rocks.
Oceanic-Continental Convergence
When these rocks are uncovered due to weathering and
erosion, we can see where this contact metamorphism has
happened.
Oceanic-Continental Convergence
Oceanic-continental convergence boundaries also experience a
lot of seismic activity. Earthquakes are quite common as the
two plates are crushing into each other.
Oceanic-Continental Convergence
An earthquake’s focus is the exact point where the rocks in the
crust break or move. At a subduction zone, scientist can
measure both deep and shallow
earthquakes. This
provides further
evidence that the
oceanic plate is being
forced into the mantle.
Homework
 Read pages 135-136 Prelim Spotlight Text
 Complete Activity 4.9 Prelim Spotlight Text pg 137
 Complete DOT Point 4.6 pg 102
Topic 3-Lesson 2
Ocean-Ocean and Continental-Continental
Convergence
Plate Boundaries
Oceanic-Oceanic Convergence
This type of boundary occurs between two pieces of oceanic
crust. Because no buoyant continental crust is involved we do
not get large mountain ranges.
Oceanic-Oceanic Convergence
As one plate is forced beneath the other, it is melted and a line
of volcanoes form in the same way as described in a oceaniccontinental convergent boundary.
Oceanic-Oceanic Convergence
The volcanoes that are created at this boundary form a curved
line out of the sea and scientists call these island arcs.
Oceanic-Oceanic Convergence
Examples of island arcs
include the Philippine
Islands, Japan and
Indonesian Islands.
Oceanic-Oceanic Convergence
Metamorphism occurs at
these boundaries and the
trenches formed are the
deepest on the planet. The
Mariana Trench is 11
kilometres deep. These
boundaries also have deep
and shallow earthquakes.
Continental-Continental Convergence
This type of boundary is where two continental pieces of crust
slam into each other. This boundary is different than the last
because there is not complete subduction.
Continental-Continental Convergence
This is because the continental crust is so buoyant. Instead the
plates become intensely folded and uplifted. An example of
this is the Himalayas.
Continental-Continental Convergence
Earthquake foci are shallow at this boundary and there is no
contact metamorphism. Instead there is regional
metamorphism. Low heat, high pressure.
Activity
 Begin working on DOT Points in homework together if time
Homework
 Read pages 138-140 Prelim Spotlight Text
 Complete DOT Points 4.1-4.5
Topic 3-Lesson 3
Divergent and Transform Plate Boundaries
Divergent Plate Boundary
This is where two plates separate. These spreading zones are
where new crust is being generated. Mid-ocean ridges mark
divergent plate boundaries.
Divergent Plate Boundary
These boundaries are some of the most active on Earth.
Volcanoes at these boundaries produce basalts low in silica.
This means the lava has a low viscosity and releases gasses
easy. Eruptions are therefore less explosive.
Divergent Plate Boundary
Lava usually erupts in long cracks in the Earth’s surface called
fissures rather than mountains. This is because the crust is
being pulled apart rather than forced together.
Divergent Plate Boundary
Rocks at divergent boundaries do not usually undergo
metamorphism. Earthquakes are shallow as the forces cause
rocks to crack and sink along fault lines.
Transform Boundaries
This occurs when two plates slide past one another. Movement
is parallel to the direction of the boundary so neither
convergence or divergence occurs.
Transform Boundaries
Volcanoes rarely occur along these boundaries. Shallow
earthquakes are common because parallel movement of the
plates are not smooth. Elastic energy is stored within the
rocks and when the rocks give way the energy is released.
Transform Boundaries
The San Andres Fault in
California is an example of
a transform boundary. The
displacement of landforms
gives a clear indication of
plate movement.
Activity
 Begin working on ‘To Think About’ pg 140 Prelim Spotlight
Text together
Homework
 Complete To Think About Set 1 and 2 pg 140 Prelim Spotlight
Text
 Finish Electronic Vocab List