A2 : Plate tectonics
The Pacific Ocean is getting closer and closer whereas the Atlantic Ocean is getting wider and wider.
Evidence also shows that the surrounding areas are often associated with vulcanicity.
With reference to the theory of plate tectonics, describe and explain the dynamic nature of oceans. To
what extent is global vulcanicity associated with plate margins ? Illustrate your answer with examples.
I.
Introduction :
1. The earth is broken into 6-7 continental and oceanic plates.
2. Owing to the presence of convection currents in the upper mantle, plates are drifting.
3. They may converge, diverge and slide past each other.
II.
A.
Contents :
D + E why the Pacific Ocean is getting closer :
1. The Pacific Ocean is narrower because margin of Pacific plate is destructive in nature.
2. On the west of the Pacific Ocean, there is convergence of Eurasian and Pacific plate.
3. On the east of the Pacific Ocean, Juan de Fuca plate converge with North American plate.
4. As convection current sinks, compressional force is exerted.
5. The denser oceanic plate, i.e. Pacific plate, subducts beneath the continental plate.
6. The subducted part of the oceanic plate melts under great heat and pressure upon entering the
asthenosphere. So, the oceanic crust is being destroyed.
B.
D + E why the Atlantic Ocean is getting wider :
1. There is sea-floor spreading in Atlantic Ocean because it is a constructive plate margin.
2. North American plate and South American plate move apart from the Eurasian and African
plate.
3. As convection currents rise, tension is created.
4. Magma rises from the upper mantle and fills the gap. It cools down and solidifies to form
new oceanic crust.
5. With repeated divergence of plates, a mid-oceanic ridge ,e.g. Mid- Atlantic Ridge, is formed.
Transitional paragraph :
1. Define vulcanicity : intrusive – ejection of magma from the mantle to the crust.
Extrusive – ejection of magma from the mantle to the earth’ surface
2.
Global distribution of vulcanicity :
a. Destructive plate margin : Circum-Pacific Ring of Fire
b.
c.
C.
Volcanic features along destructive plate margin
1. Features include volcanoes, volcanic islands and volcanic island arc.
2. When the oceanic and continental plate converge, the former will be subducted under the latter,
due to its higher density. Ocean trench will be formed. Examples are Japan trench,
Aleutian trench, Peru-Chile trench and Philippine trench.
3. The subducted part of the oceanic plate melts under great heat and pressure upon entering the
4.
5.
6.
D.
asthenosphere. So, the oceanic crust is being destroyed.
Being lighter, molten magma rises along lines of weakness and eject out as volcanoes.
Examples include Mt. St. Helens, Mt. Fuji.
Sometimes, a chain of volcanic islands may be formed offshore, parallel to the plate margin.
This is known as the volcanic island arc, e.g. Japan Islands and Aleutian Islands.
Volcanic features along constructive plate margin
1. Features include mid-oceanic ridge, submarine volcanoes, volcanic island arc and lava plateau.
2. When the oceanic plate and continental plate diverge, magma rises from the upper mantle. It
cools down and solidifies to form the oceanic ridge, with an axial rift on the top.
3. If magma ejects from a vent, submarine volcanoes which in time may grow above sea level,
4.
5.
III.
Mediterranean- East African Belt
Constructive plate margin : Mid-Atlantic Ridge
Hot spot : Hawaiian Islands
e.g. Surtsey, south of Iceland on the mid-Atlantic Ridge, may be formed.
Volcanic island arc may be formed when there is a chain of volcanic islands offshore.
If the constructive plate margin is found on land, magma will rise along the fissure and
accumulate layer upon layer. Therefore, a lava plateau will be formed, e.g.Deccan Plateau in
India.
Conclusion
Vulcanicity is also found on interior of plates. Due to the decay of radioactive elements, there is
uneven distribution of heat in the mantle. So, hot spot, e.g. Hawaiian Islands, may be formed.