1.1 The outer layer of the Earth is called the
1.1.1 This layer is relatively cold and rigid and comprises the crust and top part of the
1.2 The lithosphere is made of tectonic plates which are less dense than the mantle below.
1.3 The crust is too thick to drill through, so most of our
information about the Earth is collected from seismic waves
produced by earthquakes and man-made explosions.
2 The mantle is the zone between the core and
the crust. It is cold and rigid just below the
crust, but at greater depths it is hot and
non-rigid and able to move.
3 The Earth's core transfers energy, so the
temperature of the mantle increases with depth.
4 Convection currents slowly move plates.
5 Oceanic crust is denser than continental crust. When these
plates collide, the oceanic plate which is cooler at the margins
sinks, pulling more of the plate down, and partly melting as it
reaches the hotter part of the mantle. This is called subduction.
6 Plate tectonic theory
6.1 Most scientists now accept the theory of plate
tectonics, which suggests that Africa and South
America could once have been one land mass, because:
6.1.1 - it explains a wide range of evidence
6.1.2 - it has been discussed and tested by many scientists.
6.2 Wegener's continental drift theory 1914 was
not accepted by scientists at the time.
6.3 In the 1960's, new sea floor spreading evidence was
found. Subsequent research led to Weigener's theory
slowly becoming accepted.
7 Magma and Rocks
7.1 Magma rises up through the Earth's
crust because it is less dense than
the crust. This can cause volcanoes.
7.2 Magma can have different types
of composition which cause
different types of eruption.
7.3 Geologists study volcanoes to try to
forecast future eruptions and reveal
more about the structure of the Earth.
7.4 Geologists are better able
to predict eruptions than
they used to be, but still
not with 100% certainty.
7.5 Different types of igneous rock are formed from lava.
7.6 Iron-rich basalt rock comes from runny
lava in slower volcanic eruptions.
Silica-rich rhyolite rock comes from
thick lava in explosive eruptions.