Plate Tectonics
Our Restless Planet
Chapters
Introduction
Patterns in our restless Earth
Theory of Plate Tectonics
The Mechanics of the Plates
The Mid-Atlantic-Ridge
At the Edges of Plates
Sliding Plates
Alfred Wegener and the Continental Drift
Pangaea, the Super Continent
Conclusions
1. Patterns in our restless Earth
Each year there are reports of the Earth being shaken by earthquakes and disturbed by
volcanoes. In fact there are over 1 million of these events per year, but three quarters are
detected only by sensitive instruments. By plotting these events on a world map we find that
95% occur across the surface in bands or linear zones. This distribution allows us to identify
the geologically or tectonically active regions: young active mountain ranges, island arcs off
the edges of continents, subduction zones near the edges of continents and also mid-ocean
ridges. They also define the edges of the crustal plates which are relatively stable segments of
material made of continental crust, oceanic crust or a combination of both.
2. Theory of Plate Tectonics
The Earth's surface is divided into 8 major plates and a number of minor ones. Each has a
general direction of movement, away from one zone and towards another. Where adjacent
plates move in different directions events occur to accommodate this movement and we see a
resulting set of tectonic features. Other evidence, from earthquake wave studies, crustal
temperature distribution, heat flow data, magnetic field intensity distribution, and gravity
anomalies, assist in building a model of plate tectonics.
3. The Mechanics of the Plates
In examining a cross-section of the layers within the Earth we note that the temperature
increases to 6000°C at the centre, but is not a gradual increase. Definite boundaries occur
where the physical and chemical properties of the materials change and make up four distinct
layers. Another feature is the relatively very thin outer layer of crust.
With thicknesses varying between 5 and 70 km, the crust is almost not seen on scaled
drawings compared to the total diameter of 12,756 km. The heat within the Earth is the
powerful source of energy and so as it moves or is transferred, it will have a major effect on
the crust and on the plates that make it up. Convection currents within the mantle rise and fall
at a rate of centimeters per year. Where the crust is very thin and above a convection cell
carrying molten or plastic rock it will cause the lithosphere to arch and finally rupture to form
a rift valley. Most rifts are on the ocean floor but some occur on land as in the Rhine Valley
and the Great African Rift valley. New ocean crust moves upward to fill the rift as it forms.
Although the Red Sea is rifting apart it is a region of extreme complexity as it also sits on the
end of a ridge extending from the Indian Ocean. It could expand to become an ocean, at a rate
of about 10 cm per year.
Plate Tectonics – Teachers’ Notes
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4. The Mid-Atlantic-Ridge
About 200 my ago the Atlantic Ocean began to form as a rift valley. The convection cells
within the mantle provide the mechanism with the hot rising magma pushing America and
Africa apart. As the magma surfaces in the rift it cools and welds onto the edges of the
continental plates so they move apart. In this way new plate material is added to the ocean
floor and through this spreading the Atlantic grows a few cm each year. The rift centered on
the Mid-Atlantic Ridge is a region of extreme activity which is seen where it surfaces at
Iceland.
5. At the Edges of Plates
With matter being constantly added, somewhere there has to be a reversing process where
crustal material is consumed. Along some edges of the more dense oceanic plates subduction
occurs, where they are pushed under less dense continental plates and are forced deeper into
the mantle. Oceanic trenches result. Recording, positioning and analysing the earthquakes
that occur in these regions show that the ocean plates must finally melt or become sufficiently
fluid so that this downward movement occurs with no associated earthquakes. Further study
of these regions, termed geosynclines, using the distribution of the depths of the earthquakes
and the strange gravity anomaly associated with trenches or subduction regions shows that
two kinds of active margins to continents occur:
Trench – island arc margins such as the New Hebrides Trench NE of Australia.
Trench – continental margins such as the Pacific margin of South America.
But there are several variations. Where two continental regions collide and subduction of crust
comes to a standstill, the edges of the plates wedge into each other and are pushed up by
enormous pressure to form mountains as seen in the Himalayas.
In other cases subduction can cease when the sediment load on the continental side of island
arcs increases past a critical point. Horizontal squeezing of the sediment basin raises and folds
them into mountain chains still edged by a volcanically active region.
6. Sliding Plates
Activity in some regions does not result in vertical movements of crustal material. In these the
forces result in a longitudinal or sideways movement where the two plates slide past each
other, and produce a transformed fault as the plate boundary, no crustal material is produced
or destroyed. The boundary is often marked by deformed structures and chasms such as the
San Andreas Fault.
7. Alfred Wegener and the Continental Drift
Several geologists and geomorphologists including Snider-Pellegrini in 1859, and Taylor in
1910, had drawn attention to the fit of the continents. Several models were proposed with
some based on a contracting Earth. Wegener in 1915 offered evidence for continental drift
other than the geometric matching of their coastlines. Firstly he noted that there were
geological features and fossil correlations that continued from one continent to the other,
Secondly, the present distribution of plants and animals required some form of land bridging
to explain their distribution. Thirdly, long lasting glaciation had affected the southern parts of
the continents and the pattern of this glaciation made sense when they were repositioned
together. What Wegener didn’t know was that continents do not move alone and he could not
propose a mechanism to move them.
Prior to the Second World War the ocean floor was thought to be vast flat, relatively featureless
regions covered with a consistently thin layer of sediment of pelagic ooze, its origins not from
land but from the death and accumulation of marine organisms. At this time sonar detection
developed sufficiently to be used to map the ocean floor. It was found to be anything but flat.
Massive mountain chains running almost continuously through the centers of the oceans for
thousands of kilometers were found and were far more impressive than mountains on land.
Plate Tectonics – Teachers’ Notes
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The pelagic ooze was also much thicker near the continental margins. A third equally amazing
observation was that the ocean crust has different ages at different points. Near the ocean
ridges the crust was found to be very young. The further away form the ridge the older the
crust appeared to be. The simple picture of 60 years ago was wrong.
The mechanism began to unfold in the late 1950’s, particularly through the work of the
Australian geologist S. Warren Carey. Also at this time studies of the magnetic properties of
rocks began and results were quickly used to determine the paths of the relative movements
between continents and the drift of the Earth’s magnetic poles. The next important step
involved solving the puzzle of the magnetic anomaly stripes found in ocean floor crust. In 1963
Vines and Matthews, from their work on the Carlsberg Ridge in the Indian Ocean, suggested
that the ocean floor crusts may show the same magnetic pole reversals seen in continental
volcanic lavas. The symmetrical pattern of the magnetic anomaly stripes across mid ocean
ridges was found in 1965 and in 1967. McKenzie and Parker published a paper linking sea
floor spreading, island arcs and transform faults to plate structures that interacted at their
boundaries.
The oceanic crust was actively moving away from the ridges. Continents were moving and
some were being changed in shape as well as position. The patterns in these movements
suggest that the present land masses were together as one large super-continent named
Pangaea, and began breaking up about 200 million years ago. Several methods are used to
correlate regions of continental plates that once fitted together, including sediment basins,
fossil correlation, structures, palaeological, paleogeographic and paleoclimatic data. The
evidence from age determinations of oceanic crust suggests the breaking up started 200
million years ago and began with the Atlantic Ocean forming as a rift.
Several questions in hot debate at the moment centre around the continental structures that
existed before Pangaea and whether they had changed before this time. The history of the
extension of continental shield regions by mountain building or orogenic cycles are features of
all continents.
A model for Wegener’s continental drift had been established base on the hypothesis of plate
tectonics.
8. Pangaea, the Super Continent
So, mainly with the use of palaeomagnetic data, we are able to follow the movements of plates
over the past 300 million years, beginning from the one super continent of Pangaea. 200
million years ago it began to break apart. The first major movement was of North America
breaking away and the Atlantic Ocean beginning to form in its northern region. The African
and South American plates formed and began moving from the rift that became the mid
Atlantic Ridge. India moved away from Pangaea and migrated northwards where its oceanic
parts submerged under the Eurasian plate. Eventually the continental part of the Indian plate
collided with the Asian continental crust which resulted in the uplifting of the Himalayan
mountain range. The African plate reached the south-west of the Eurasian plate where the
continents pushed into each other and mountains around the Mediterranean Sea began to
form.
9. Conclusions
In these movements the processes of Mid-Ocean Ridge formation, the growth of oceanic crust,
migration of continental plates, island arc formation, subduction and trench formation are
clearly seen. Mountain building is still going on with centimeters being added to the Earth’s
mountain chains each year.
The theory of Plate Tectonics serves as a master key for understanding the history, structure
and dynamics of the Earth’s crust. It explains how mountains form and oceans grow, why
volcanoes erupt and earthquakes shake the land.
Plate Tectonics make us aware that the face of our planet is in constant change.
Plate Tectonics – Teachers’ Notes
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Questions and Activities
1. Explain the following key words and phrases:
Restless Earth, Mid-Ocean Ridge, earthquakes, volcanoes, distribution, mountain ranges,
island arc, ocean trench, ocean floor, continent, subduction zone, crustal plates,
continental crust, oceanic crust, tectonic features, earthquake wave, crustal temperature
distribution, heat flow, magnetic field intensity, gravity anomalies, layers within the Earth,
boundaries, convection currents, mantle, lithosphere, rift valley, hot rising magma, new
plate material, more dense oceanic plates, less dense continental plates, Trench - island
arc margins, Trench - continental margins, plate boundary, deformed structures,
Wegener's theory , Pangaea, Plate Tectonics.
2. Assemble a short report of a volcano that is active at present. Find some information about
a major volcanic eruption that has occurred in the past. Where is the nearest extinct
volcano nearest to your school? Is it active? If not, when was it last active? How do people
know this?
3. Collect newspaper reports on the most recent major earthquake. When was the last
earthquake that was felt in your area? Where was its centre? How did geologists find this
out?
4. Find a world map that shows the distribution of active volcanoes and earthquakes. Can
you find any patterns? What does this tell us about these areas?
5. What are crustal plates? Plot the major plates on another world map? How is the
distribution of volcanoes and earthquakes related to the plates?
6. The Earth is said to be made up of four layers: inner core, outer core, mantle and the crust.
Make a model of the Earth or draw a cross-section to show these layers.
7. How large a circle do you need to draw that shows the crust with mountains and ocean
trenches drawn to scale? Compare your drawings to photos of the Earth from space. How
do they compare? What does this say about the surface of the Earth?
8. Find out how scientists have used earthquake waves to determine that the structure of the
Earth is as you have drawn.
9. Seismographs are the instruments used to record earthquake waves. What are the features
of these graphs? Is there only one type of earthquake wave? How can you tell?
10. Explain how convection currents occur. Find and complete an experiment to show
convection currents. How do they move heat energy?
11. What is the source of the heat within the Earth? How do we know that the temperature
increases closer to the centre?
12. Lava from a volcano is molten rock material. As it flows as a liquid how hot is it? Is there
only one kind of lava?
13. Find out about an area on the Earth that has hot springs and active geysers. What is
special about the liquids that come from beneath the surface in these areas?
14. What would happen if there was no mountain building on the Earth? There are several
interesting answers here; try discussing each.
15. List the evidence for "continental drift". Where are they drifting from? Where are they
drifting to? What would happen if the process of subduction did not occur at all?
16. Because mountains are built out of sedimentary rocks what should we be able to find in the
layers of rocks high in mountains? What other kinds of rocks should we be able to find?
Why should they be there?
17. Some text books make charts of the "Rock Cycle". Prepare one to display in your room.
What does this cycle have to do with Plate Tectonics?
18. Prepare a chart to display geological time. Place the names of the Epochs next to their
time in millions of years. Record some of the important geological events on the chart.
19. Mountain building has been going on throughout all of geological history. Find where there
are old mountain chains near you. What is it that you should look for to find them? Do
geological maps of your state help? When in geological history were they formed as young
mountain chains? How would they have been different then to what they are now?
Plate Tectonics – Teachers’ Notes
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20. The Earth’s magnetic poles move. Find out how they have moved over the past million
years. Do we always have a magnetic field around the Earth? What would change if there
was no field for a period of time?
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