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 Establishment: Amman Valley School Date: 22:04:05