4.
DO
THE CONTINENTS MOVE? CONTINENTAL DRIFT AND TECTONIC PLATES
Do you think that continents can move?
The German
scientist Alfred Wegener proposed, in 1912, that millions of years ago all the
continents were joined together in a supercontinent called Pangaea. Then it
broke up into pieces which drifted along the ocean floor forming the actual
continents. This idea is known as Continental drift theory.
Although Wegener gave evidence to prove his theory, he wasn’t be able to explain what force was capable of moving the Earth crust. So, the other scientists didn’t accept his theory.
Evidence for continental
drift
·
Geographical evidence:
the coastline of some continents, such as South America and Africa, fit together like puzzle pieces.
·
Climatic evidence: there are remains of glaciers in Brazil and soft coal
deposits in Greenland, which indicate that these continents were in different
places in the past.
·
Biological evidence: there are land animals and plants
of the same species that live on both sides of the Atlantic Ocean, which show
that the continents were joined together.
·
Paleontological evidence: the same animal and plant fossils have been found on South America, Africa, Australia and India, demonstrating that these continents were joined together in that time.
Paleontological evidence: the same animal and plant fossils have been found on South America, Africa, Australia and India, demonstrating that these continents were joined together in that time.
1. Cut South America and Africa and try
to fit them like a puzzle in your notebook:
Plate tectonics theory
In the
1960s new discoveries completed the theory of continental drift and led to the
theory of plate tectonics. According to this theory:
·
The
lithosphere, the outermost solid part
of the Earth constituted by the crust and the upper part of the mantle, is
divided into blocks or plates, called tectonic plates, which fit together like
the pieces of a giant puzzle.
·
The
plates move over the asthenosphere.
The asthenosphere is denser than the lithosphere and is partially molten.
·
The
tectonic plates move because the materials of the inner mantle are so hot that
ascent to the upper mantle until they cool down enough to descend again. In
this way convection currents displace the tectonic plates and so the
continents.
There are
three types of plates:
- · Oceanic, formed by oceanic lithosphere.
- · Continental, formed by continental lithosphere.
- · Mixed, formed by oceanic and continental lithosphere.
Where plates meet, their relative motion determines the type of boundary:
·
Divergent,
when plates move away from each other. Materials from inside the Earth rise up
between the plates. Then volcanic eruptions occur forming oceanic ridges.
·
Convergent, when plates move toward each other and collide. One of the plate moves underneath the other one, what is called subduction. It causes intense earthquakes, volcanoes and mountain ranges.
· Animation
about subdution:
http://ieslamadraza.com/webpablo/webctma/1geosfera/subduct.gifTransform, when plates slide past each other, but don’t converge or diverge. The movement of these tectonic plates in opposite directions causes big earthquakes.
1. Watch this video about tectonic
plates
2. Have a dialogue with your partner
about the following questions:
a. Is there any relationship between
the areas where earthquakes and volcanoes occur?
b. Why do the plates move?
c. What is the Spanish tectonic
situation?
3. Working with vocabulary:
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