The Earth as a Planet
▲The Earth’s crust is
divided into six large
tectonic plates and a
number of minor ones.
They are separated by
mid-ocean ridges, deep-sea
trenches, active mountain
belts and fault zones.
Volcanic eruptions and
earthquakes are largely
confined to the areas where
plates meet. During the
geological history of the
Earth, these plates have
moved around, creating
and re-creating continents.ATLAS OF THE UNIVERSE
T
he Earth’s crust, on which we live, does not extend
down very far – some 10 kilometres (6 miles) below
the oceans and 50 kilometres (30 miles) below the con-
tinents. Temperature increases with depth, and at the bot-
tom of the world’s deepest mines, those in South Africa,
the temperature rises to 55 degrees C. Below the crust we
come to the mantle, where the solid rocks behave as
though plastic. The mantle extends down to 2900 kilo-
metres (1800 miles), and then we come to the iron-rich
liquid core. Inside this is the solid core, which accounts
for only 1.7 per cent of the Earth’s mass and has been said
to ‘float’ in the liquid. The central temperature is thought
to be 4000–5000 degrees C.
A glance at a world map shows that if the continents
could be cut out in the manner of a jigsaw puzzle, they
would fit neatly together. For example, the bulge on the
east coast of South America fits into the hollow of west
Africa. This led the Austrian scientist Alfred Wegener
to suggest that the continents were once joined together,
and have now drifted apart. His ideas were ridiculed for
many years, but the concept of ‘continental drift’ is now
well established, and has led on to the relatively young
science of plate tectonics.
The Earth’s crust and the upper part of the mantle
(which we call the lithosphere) is divided into well-
marked plates. When plates are moving apart, hot mantle
material rises up between them to form new oceanic crust.
When plates collide, one plate may be forced beneath
another – a process known as subduction – or they may
buckle and force up mountain ranges. Regions where
the tectonic plates meet are subject to earthquakes and
volcanic activity, and it is from earthquake waves that
we have drawn much of our knowledge of the Earth’s
internal constitution.
The point on the Earth’s surface vertically above the
origin or ‘focus’ of an earthquake is termed the epicentre.
Several types of waves are set up in the globe. First there
are the P or primary waves, which are waves of compres-
sion and are often termed ‘push’ waves; there are also S
or secondary waves, which are also called ‘shake-waves’
because they may be likened to the waves set up in a matwhen it is shaken by one end. Finally there are the L or
long waves, which travel round the Earth’s surface and
cause most of the damage. The P waves can travel
through liquid, but the S waves cannot, and by studying
how they are transmitted through the Earth it has been
possible to measure the size of the Earth’s liquid core.
If earthquakes can be destructive, then so can vol-
canoes, which have been called ‘the Earth’s safety valves’.
The mantle, below the crust, contains pockets of magma
(hot, fluid rock), and above a weak point in the crust the
magma may force its way through, building up a volcano.
When the magma reaches the surface it solidifies and
cools, to become lava. Hawaii provides perhaps the best
example of long-continued vulcanism. On the main island
there are two massive shield volcanoes, Mauna Kea and
Mauna Loa, which are actually loftier than Everest,
though they do not rise so high above the surface because,
instead of rising above the land, they have their roots deep
in the ocean-bed. Because the crust is shifting over the
mantle, Mauna Kea has moved away from the ‘hot spot’
and has become extinct – at least, one hopes so, because
one of the world’s major observatories has been built
upon its summit. Mauna Loa now stands over the ‘hot
spot’, and is very active indeed, though in time it too will
be carried away and will cease to erupt.
Other volcanoes, such as Vesuvius in Italy, are cone-
shaped. The magma forces its way up through a vent, and
if this vent is blocked the pressure may build up until
there is a violent explosion – as happened in AD79, when
the Roman cities of Pompeii and Herculaneum were
destroyed. There have been many devastating volcanic
eruptions, one of the latest being that of Mount Pinotubo
in the Philippines, which sent vast quantities of dust and
ash into the upper atmosphere.
The Earth is not the only volcanic world in the Solar
System. There are constant eruptions upon Io, one of the
satellites of Jupiter; there are probably active volcanoes
on Venus, and we cannot be certain that all the Martian
volcanoes are extinct. However, it does not seem that plate
tectonics can operate upon any other planet or satellite, so
that in this respect the Earth is unique in our experience.Divergent plate
boundaries
Convergent plate
boundaries
Uncertain plate
boundariesDirection of plate
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