Download Recording script Unit 5

Recording script Unit 5
Earthquakes and volcanoes
Part 1
The earth is made up of three main layers: the crust; the mantle; and the core. The crust is solid rock and varies in
thickness from 6 km under the oceans to 70 km under mountain chains like the Himalayas. The mantle is thick,
molten rock. The core is made up of an outer layer of very hot, liquid rock and an inner layer of iron.
The crust is much thinner than the other layers of the earth and it is broken up into a number of pieces called
plates. Huge currents of molten rock rise from the mantle and move the plates about, very slowly. The continents
are passengers on these plates and so they are moved around as well; for example, you can see on a world map that
South America and Africa used to fit together like the pieces of a jigsaw puzzle.
Most earthquakes happen near to the plate boundaries where the plates are moving into, pulling away from or rubbing
past each other. Stress builds up until the crust snaps apart, causing the ground to shake.
Most volcanoes also happen near to the plate boundaries. Where plates are moving together one piece of the crust is
forced down into the mantle and melts; this molten material pushes its way back to the surface to form a volcano.
Where plates are moving apart, molten rock rises up to fill in the gap.
However, some volcanoes, like those which make up the Hawaiian Islands in the Pacific Ocean, happen a long way
from the plate boundaries where there is a hot spot in the crust. The cause of these hot spots is still a mystery.
Part 2
The diagram shows what happens where plates move together. Here, on the west coast of South America, the Nazca
Plate is being forced towards and underneath the South American Plate. As the plates move together the friction
between them makes the rock melt. The liquid rock (magma) rises upwards and erupts on the surface as a volcano.
The movement of the plates scraping together also makes the ground shake and sets off earthquakes. South America
has over a hundred volcanoes caused in this way and in some places earthquakes happen every day.
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