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The Active Earth
Continental plates
Trench
The Earth’s shell or crust is split into a number of plates. These
plates fit together like a huge jigsaw puzzle. The plates float on the
semi-molten rocks of the Earth’s mantle.
Outer core
of molten
metal
Convection
current
Heating from the Earth’s core causes the semi-molten material in
the mantle to churn in currents. These currents carry the crustal
plates slowly along up to as much as 15 centimetres per year. This
movement is known as continental drift.
Ridge
Inner core of
solid metal
Sometimes the Earth’s plates collide, pushing up mountain
ranges. The contact between these converging plates can also
cause earthquakes and volcanoes. Plates beneath the ocean move
much more quickly than plates beneath the continents.
Mantle
Crust
Mountain building
Most of the world’s great mountain regions are formed when crustal rocks are buckled as the plates slide underneath
each other. This folding process formed the Himalayas in Asia and the Rocky Mountains in North America. The process
where rocks crack and sections move up or down is known as faulting and it forms both mountains and rift valleys such
as those in Africa.
Sediments folded and pushed
upwards e.g. Himalayas
Direction of plate
movement
Continental plate
e.g. Indian Plate
Continental plate
e.g. Asian Plate
Convection current
Convection current
Mountain building
Volcanoes
A volcano forms when magma, hot molten rock from beneath the crust, emerges on the Earth’s surface through a fissure
or opening. When it does, this molten rock is called lava.
A volcanic cone forms when there is an explosive eruption of ash and cinders. These fall back to Earth, solidifying to
form a steep volcanic cone like Mt Paricutin in Mexico.
A composite cone volcano, such as the one in the diagram on the next page, is made up of layers of ash and lava
because the eruptions alternate between explosive (ash) and quiet (lava). Mt Fuji in Japan and Mt Etna in Italy are
examples of a composite cone.
© John Wiley & Sons Australia, Ltd 2005
2005-02-earth.pdf
(Page 1 of 2)
Ash and gas explode
from the crater.
Main volcanic vent
Pressure from gas and
magma mixing underground
forces magma up the main
vent and branch pipes.
Ash settles in a layer
over the volcano.
Lava covers the ash
and solidifies.
Branch pipe
Magma gathers in a
magma chamber
before it is forced to
the surface.
Volcanic eruption
Earthquakes
Earthquakes are caused by movements in the Earth’s crust where the continental plates converge. Earthquakes always
have an epicentre, the point on the earth’s surface from which the shock waves radiate. It is directly above the focus, the
area underground where the stresses that result in the earthquake have built up. The shock waves decrease in intensity
the further they are from the epicentre. Aftershocks of lesser intensity may occur for weeks after the main earthquake.
Epicentre
Strongest shock
Focus
Weakest shock
The focus and epicentre of an earthquake
© John Wiley & Sons Australia, Ltd 2005
2005-02-earth.pdf
(Page 2 of 2)