Download 11.11 The Earth`s tectonic plates (2 pages)

11.11 THE EARTH’S TECTONIC PLATES
The Earth’s crust includes ocean floors, islands, and continents with plains, high mountains and
deep valleys. In some places, earthquakes shake the ground and volcanos spew out ash, smoke
and red hot larva. In the rest of this chapter, we will consider how these things come to be.
The Earth’s crust is not all one piece! It is made of a number of huge ‘plates’ which float on the
upper part of the restless mantle. These tectonic plates are jammed tightly together and jostle
one another as they move with the convection currents at the top of the mantle. The next page
shows where the plates are located on the world map. Look at the map and find out which plate
you live on. The plates jostle against each other in slow motion, moving only a few centimetres a
year, but huge forces are involved. If you live near the edge of a plate, you are probably familiar
with the earthquakes and volcanos that these movements can cause! If you live nearer to the
middle of your plate, life is probably more peaceful. There are three main kinds of boundaries
where plates meet: divergent boundaries, convergent boundaries and transform boundaries.
Divergent boundaries (shown on the map by
) occur when two plates are moving apart.
This happens where hot, semi-molten rock in the mantle rises by convection, and spreads out
under the plate. The plate is forced upwards, then cracks
rift valley
and divides, forming a ‘rift valley’ with parallel ridges on
each side. As the plates move apart, magma (molten
mountains
mountains
rock) fills the gap creating new crust. Some magma may
reach the surface and create volcanos. A divergent plate
boundary runs down the middle of the Atlantic ocean. This
PLATE
PLATE
gives rise to the mid-Atlantic ridge – a long range of
magma
underwater mountains with a rift valley between them.
There are volcanos at several places along the ridges
including Iceland and the islands of the Azores. Another
MANTLE
divergent plate boundary is between the African and
Arabian plates. This has created the Red Sea between
Africa and Asia. Running south from this, is the wellknown rift valley of Africa where the African plate is probably starting to break into two pieces.
Convergent boundaries (shown by
) occur when two plates are colliding. One plate often
gets pushed under another. The plate on top buckles forming mountains. If the plates meet under
the oceans, then there will be islands instead
of mountains, and there will be a deep ocean
mountains or islands
trench
trench where one plate slides under the other.
The lower plate is forced down into the mantle
where it is slowly absorbed. Materials in the
lower plate react with the mantle generating
heat and forming magma. This may force its
PLATE
PLATE
way to the surface creating volcanos. For
example, the Nazca plate is colliding with, and
magma
MANTLE
MANTLE
going under, the South American plate. This
has forced up the mountain range of the Andes
which runs along the west side of South
America and includes a number of volcanos. It
has also created the deep Peru-Chile trench in the ocean off the west coast. Other examples of
convergent boundaries include the mountains and volcanic islands of Indonesia where the
Australian plate collides with, and goes under, the Eurasian plate. The highest mountains in the
world are the Himalayas to the north of India. They have been created by the collision of the
Indian and Eurasian plates. Both plates are covered with thick, continental crusts so neither is
going under and both plates are buckling. The mantle is not disturbed so there are no volcanos.
Transverse boundaries (shown by
) occur when plates are sliding past one another. The
example shown on the map is where the Pacific plate is sliding past the North American plate.
This is the famous San Andreas Fault that causes earthquakes in California.

1. Explain how tectonic plates are created at divergent
boundaries and destroyed at convergent boundaries.

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2. Look at the map. Africa and South America were
once parts of the same continent. What happened?
mid-Atlantic
ridge
rift valley
mid-Atlantic
ridge
The Earth’s Main Tectonic Plates
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