7006EDN Learning and Teaching in the Science Curriculum
Susan Simonini 2171045
Assessment 2 Part B
CDSU - PLATE TECTONICS
Millions of years ago, soon after the earth formed, the top layer of the Earth’s surface, called the
crust, solidified. However, instead of being a solid shell, it is broken up into huge, thick, segments
that drift or ‘float’ on top of the soft, underlying layer called the mantle. These enormous blocks or
plates are solid rock and can be 80 to 400km thick. They vary in shape and size and have borders
that cut through continents and oceans. There are nine large plates and several smaller plates. Of
the major plates, six are mostly comprised of land (continental plates) and the other three are under
the ocean (oceanic plates).
These plates move extremely slowly on their journey around the surface of the earth, moving at
about the same speed as a human fingernail grows (between 1 and 10cm a year). They move both
horizontally (sideways) and vertically (up and down) and over long periods of time they can also
change in size as they are added to, crushed together, or pushed back into the Earth’s mantle.
The theory of plate tectonics (meaning ‘plate structure’) was developed in the 1960’s. It explained
the movement of the Earth’s plates and also explains the cause of earthquakes, volcanoes, oceanic
trenches and mountain range formation. Most of the Earth’s seismic activity (volcanoes and
earthquakes) occurs where plate boundaries meet. A plate boundary is where two tectonic plates
interact. There are three types of boundaries – divergent, convergent and transform.
Divergent boundaries – the word diverge is used by scientists to explain what happens when two
objects move away from each other. Therefore a divergent boundary is a boundary where two
tectonic plates are moving away from each other. This mostly occurs at the bottom of the ocean
and forms a zone of intense volcanic activity. Divergent boundaries can also form on continents, and
in this case, a line of volcanoes emerges.
Convergent boundaries – the word converge is used by scientists to describe two objects that come
together. Therefore a convergent boundary is a boundary where two tectonic plates slide towards
each other. When two continental plates collide, the plates crumble and compress, forming
extensive mountain ranges. When oceanic plates collide, an oceanic trench is formed. When an
oceanic plate collides with a continental plate, the oceanic plate (which is thinner and denser) is
forced under the lighter, thicker continental plate and eventually re-melts into the mantle. This
phenomenon is called subduction. Subduction zones can also generate a tsunami, when the stress
on the upper plate causes it to move and displace the sea water above, resulting in a series of
powerful waves.
Transform boundaries – these occur where tectonic plates slide or grind past each other along a line
called a fault. Because of the friction caused by this movement, stress builds up in both plates and
this energy is released as motion along the fault. Occasionally massive amounts of energy are
suddenly released in the form of earthquakes.
REFERENCES
Orme, David. (2010) Moving Earth Australian Geographic, Sydney.
Ferrett, Russell. (2008) When the Earth Moves : Earthquakes and Volcanoes Young Reed, Sydney.
Earth’s Continental Plates
http://enchantedlearning.com/subjects/astronomy/planets/earth/Continents.shtml access date
05.08.2011
Plate Tectonics: How Plates Move http://www.platetectonics.com/book/page_4.asp access date
05.08.2011
Unit at a Glance
Continental drift
Volcanoes – pompeii (English/history) build volcano
Tsunamis – demonstration (English – personal account)
Mountain ranges