Plate Tectonics

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Plate Tectonics
The surface of the Earth
To understand the theory of plate tectonics, you need to know some terminology for
features of the Earth's surface.
Exercise 1
Sketch a cross-section through the Earth's surface showing: continental crust, the
continental shelf, continental slope, continental rise, abyssal plain, ridge, rift, trench.
The lithosphere
The outer part of the Earth is called the crust; it is composed of solid rock and varies
in thickness from about 7 km (beneath the oceans) to 70 km (in mountain ranges).
Directly beneath the crust, separated from it by the Moho, there is the mantle. At the
top of the mantle, the rocks are rigid and this rigid material is stuck to the underside of
the crust. Together, the crust and rigid upper mantle are called the lithosphere.
Between 100 km and 350 km depth, there is a region of the mantle that has low
seismic velocities (the low velocity zone, LVZ), it is thought to be a region of mantle
that is partially molten and behaves more like a fluid. This region is called the
asthenosphere.
Exercise 2
(a) What are the differences between the crust beneath the oceans and the continental
crust?
(b) What is the composition of the mantle?
The lithosphere is broken into rigid pieces which "float" on the ductile asthenosphere
beneath, these pieces are called lithospheric plates and the theory that describes and
explains the way they move is called plate tectonics.
Plate tectonics, continental drift and sea floor spreading
Plate tectonics is a theory that has been used to explain other observations and
theories. You already know that Alfred Wegener suggested that the continents could
move across the surface of the Earth - a theory called continental drift - but until plate
tectonics was suggested, there was no way to explain how this could happen. When
magnetic stripes were discovered in the basaltic ocean crust, this led scientists to
suggest that the oceanic crust was created at the ridge and moved away from the
ridge, "spreading" sideways.
Exercise 3
(a) Sketch a diagram showing the magnetic stripes in oceanic crust (e.g. See page
176 McLeish).
(b) How is magnetism recorded in rocks of the oceanic crust? Explain "normal" and
"reversed".
Constructive plate boundaries (divergent margins)
The place where two lithospheric plates meet is called a boundary (or a margin). If
two plates are moving apart (diverging) a constructive plate boundary is formed. It is
called constructive because new crust is made.
Exercise 4
Sketch a constructive plate boundary (e.g. See page 178 McLeish)
As the plates are pulled apart, the asthenosphere below the rift melts.
Exercise 5
Sketch a geotherm for a constructive plate boundary. What does this geotherm show?
You have used the geotherm to show that at a constructive plate boundary, there is hot
material at shallow depth, so the geotherm crosses the melting line for peridotite.
The peridotite partially melts producing a magma which is has a higher percentage of
silica.
Exercise 6
(a) What is the composition of the magma produced?
(b) What does partial melting mean?
The result of this melting is magma which rises to the surface in underwater
volcanoes. On contact with seawater it cools quickly forming a solid skin that is
broken by more lava. This has the effect of producing a series of "blobs". As the new
oceanic crust slowly moves away from the ridge, it becomes colder and denser.
Exercise 7
What is the proper name for these igneous features?
All oceans must start somewhere. They begin when the continental lithosphere
becomes pulled apart, this causes it to stretch and get thinner. The surface of the
Earth drops and a rift valley is formed containing volcanoes. This is happening today
in East Africa. Eventually the valley becomes so deep that the sea floods in, covering
the floor of the valley, forming a shallow sea. This can be seen in the Red Sea. As
the lithosphere is pulled further and further apart, the sea gets deeper and new basalt is
made - oceanic crust. A new ocean has formed.
Exercise 8
Draw some simple diagrams to show the sequence described in the paragraph above.
Destructive plate boundaries (convergent margins)
A convergent margin is where two plates are moving towards each other; the result is
a destructive plate boundary. In this type of plate boundary oceanic lithosphere is
destroyed because it is forced to sink down into the mantle in a process called
subduction.
Exercise 9
Sketch the two types of destructive plate boundaries (e.g. See McLeish page 178).
Exercise 10
What is an accretionary prism? Add this to your diagram in exercise 9.
The oceanic crust with its covering of sediment, melts to form magma. This rises
through the lithosphere to the surface where volcanoes are formed.
Exercise 11
Sketch the geotherm to show why melting occurs at this type of plate boundary.
If they form on oceanic crust then a line of volcanic islands is formed called an island
arc. If the volcanoes form on continental crust then a chain of Andes-type mountains
are produced.
Exercise 12
Describe and explain the type of volcanic eruptions typical of destructive plate
boundaries. What are the products of these eruptions?
Destructive plate boundaries are also characterised by deep focus, powerful
earthquakes which follow an inclined plane down into the mantle, following the
subducting slab. This region of earthquake activity is called the Benioff Zone.
If two continental plates are in collision, there is no subduction. Instead the two
plates stick together a process called continental accretion. The rocks of the crust
buckle and fold to form a range of fold mountains where the two plates collide.
Exercise 13
Write a paragraph to explain how the Himalayas formed.
Conservative boundaries (transform faults)
Transform faults are places where two plates are sliding past each other. No new rock
is made and no rock is destroyed so they are called conservative boundaries.
Transform faults are huge fractures which run down through the lithospheres, at times
the fault "locks", the two plates become stuck and energy builds up. Eventually the
plates move again, releasing the built up energy as a massive earthquake.
Causes of plate movement
One possible cause of plate movement is convection in the mantle. We know that part
of the mantle is ductile so it can flow slowly. Convection currents may carry the rigid
plates along. There are other possible causes however. Ridge push is a mechanism in
which the weight of the oceanic crust at the ridge pushes it downwards so it moves
sideways. The other suggested mechanism is slab pull. The edge of the subducting
plate is heavy and pulls the rest of the plate along behind it.
Hot spots, sea mounts and guyots
Exercise 14
Write definitions for each of these terms:
Hot spot
Sea mount
Guyots
ESTA GEOTREX The Geology Teachers Resource Exchange Contributor: Owain Thomas
School Date: 22:04:05
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