1.1.1 Alfred Wegener proposed the theory of continental drift at the beginning of the 20th
century. His idea was that the Earth's continents were once joined together, but gradually
moved apart over millions of years. It offered an explanation of the existence of similar
fossils and rocks on continents that are far apart from each other. But it took a long time for
the idea to become accepted by other scientists. Before Wegener Before Wegener
developed his theory, it was thought that mountains formed because the Earth was
cooling down, and in doing so contracted. This was believed to form wrinkles, or
mountains, in the Earth's crust. If the idea was correct, however, mountains would be
spread evenly over the Earth's surface. We know this is not the case. Wegener suggested
that mountains were formed when the edge of a drifting continent collided with another,
causing it to crumple and fold. For example, the Himalayas were formed when India came
into contact with Asia.
126.96.36.199 Wegener’s evidence for continental drift was that:
188.8.131.52.1 the same types of fossilised animals and
plants are found in South America and Africa
184.108.40.206.1.1 the shape of the east coast of South America fits the west coast of
Africa, like pieces in a jigsaw puzzle
220.127.116.11.1.1.1 matching rock formations and mountain chains
are found in South America and Africa
1.1.2 Alfred Wegener's ideas were
not accepted for the following
18.104.22.168 Alfred was not a qualified Geologist
22.214.171.124.1 People did not want to believe him; it would embarrass them
and also make decades of research obsolete.
126.96.36.199.1.1 Also, he had no way of showing how the plates
could have plowed through solid rock
188.8.131.52.1.1.1 Another thing was that the
theory that Alfred had could
be explaied by a theory that
includes a large chunk of
rock bridging from america
to africa which has sunk
into the sea.
1.2 An diagram to show plate tectonics
1.3 Tectonic plates
1.3.1 The Earth’s crust, together
with the upper region of the
mantle, consists of huge slabs
of rock called tectonic plates.
These fit together rather like
the segments on the shell of a
184.108.40.206 Although the mantle below the tectonic
plates is solid, it does move. This
movement is very, very slow – a few
centimetres every year. This means that the
continents have changed their positions
over millions of years.
1.4 The Earth's crust and upper part of the mantle are broken into large pieces called tectonic plates. These
are constantly moving at a few centimetres each year. Although this doesn't sound like very much, over
millions of years the movement allows whole continents to shift thousands of kilometres apart. This
process is called continental drift. The plates move because of convection currents in the Earth's mantle.
These are driven by the heat produced by the decay of radioactive elements and heat left over from the
formation of the Earth. Where tectonic plates meet, the Earth's crust becomes unstable as the plates
push against each other, or ride under or over each other. Earthquakes and volcanic eruptions happen at
the boundaries between plates, and the crust may ‘crumple’ to form mountain ranges. It is difficult to
predict exactly when an earthquake might happ
2.2 The early atmosphere was probably mostly carbon dioxide, with little or no oxygen. There were
smaller proportions of water vapour, ammonia and methane.
2.3 Why did it change?
2.3.1 The proportion of oxygen went up because of photosynthesis by plants. The proportion of carbon
dioxide went down because: It was locked up in sedimentary rocks, such as limestone, and in fossil
fuels. It was absorbed by plants for photosynthesis. It dissolved in the oceans. The burning of fossil
fuels is adding carbon dioxide to the atmosphere faster than it can be removed. This means that the
level of carbon dioxide in the atmosphere is increasing.