Key Stage 4: Chemistry Unit 1 – C1.7 - Changes in the Earth and its Atmosphere
The Earth’s Crust.
Plate tectonics
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 natural
decay of radioactive elements in 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 happen and how bad it will be, even in places
known for having earthquakes.
The theory of plate tectonics and continental drift was proposed at the beginning of the last century by a
German scientist, Alfred 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’s Theory
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. It took more than
50 years for Wegener’s theory to be accepted. One of the reasons was that it was
difficult to work out how whole continents could move: it was not until the 1960s
that enough evidence was discovered to support the theory fully.
Alfred Wegener (1880 - 1930)
Volcanoes and Earthquakes
There are two main types of tectonic plate:
 Oceanic plates occur under the oceans.
 Continental plates form the land.
Oceanic plates are denser than continental plates. They are pushed down underneath continental plates if
they meet.
Volcanic Activity
Where tectonic plates meet, the Earth’s crust becomes unstable as the plates slide past each other, push
against each other, or ride under or over one another. Earthquakes and volcanic eruptions happen at the
boundaries between plates. Magma (molten rock) is less dense than the crust. It can rise to the surface
through weaknesses in the crust, forming a volcano.
Geologists study volcanoes to try to predict future eruptions. Volcanoes can be very destructive, but some
people choose to live near them because volcanic soil is very fertile.
The movement of tectonic plates can be sudden and disastrous, causing an earthquake. It is difficult to
predict exactly when and where an earthquake will happen, even when a lot of data is available.
Where has All the Carbon Gone?
The Earth’s early atmosphere was mainly carbon dioxide. Over millions of years, carbon from the
carbon dioxide has been taken out of the atmosphere. Some of the carbon became trapped in
the sedimentary rock, and some became trapped in fossil fuels.
1 Complete these sentences using words from the box, to show how carbon got locked into
sedimentary rock.
Carbon ......................... in the atmosphere dissolved in ......................... water. The ......................... carbon dioxide
made chemicals called ......................... . The carbonates were ......................... as sediment. The sediment was
......................... and formed ......................... rock. The ......................... is now trapped in the sedimentary
......................... as carbonates.
carbon
carbonates
dissolved rock
cemented
sea
deposited
sedimentary
dioxide
2 These diagrams and sentences show how carbon got trapped into limestone. The diagrams are in the
correct order. Draw lines from the diagrams to the correct sentences.
a) When the animals died
their shells were deposited
as sediment.
b) Carbon dioxide dissolved in
the sea water making
carbonates.
c) The shells were crushed
and cemented into
limestone. The carbon is
now trapped in the limestone
as calcium carbonate.
d) Sea animals used carbonates
to make calcium carbonate.
They needed the calcium
carbonate to make their shells.