The heat rising from the Earth's core creates convection currents in the plastic layer of the mantle (asthenosphere). The convection currents slowly move the tectonic plates above them in different directions. Convection currents occur because hot fluids are less dense than cold fluids. The hot rock rises and the cooler rock sinks due to gravity.
TASK 14: Negotiate a convection current activity with your teacher - you could try boiling rice (as they did in the video), or sprinkling cocoa on a saucepan of hot milk. Search the library and internet for other ideas. Write up your activity, including an explanation of how it explains the movement of the asthenosphere, and paste it in your notebook.
The lithosphere is broken up into tectonic plates (not unlike a cracked egg-shell). The direction the plates moves depends upon where they sit above convection currents.
What happens when tectonic plates collide?
Rising convention currents push the plates apart at divergent boundaries. New rocks are formed as magma rises to the surface and solidifies deep in the ocean. This is known as sea-floor spreading.
Plates are pushed together at convergent boundaries.
When continental crust collides with continental crust the land is pushed upwards to form huge mountains (eg. Himalayas).
Where oceanic crust collides with continental crust, the oceanic crust is forced downwards under the continental crust. This is called a subduction zone.
Some plates slide past each other at transformation boundaries.
Why are they so important? Earthquakes, volcanic activity, mountain building and oceanic trenches occur along these boundaries. The Earth's rocks are also recycled (more about that later).
Tectonic Plates and Volcanoes
The movement of tectonic plates is responsible for most volcanoes, but hotspot volcanoes are believed to form above a hot plume rising up from deep within the mantle. As the tectonic plate moves, new volcanoes form over the hotspot, creating a chain of volcanoes.
TASK 15: Create an annotated diagram that explains the structure and movement of divergent and convergent boundaries on Earth. Extra points if you include where mountain building, volcanoes and earthquakes are likely to occur.
Does Mars have tectonic plates?
While there may be some evidence of tectonic plate movement on Mars, scientists calculate that the massive size of Olympus Mons can be explained by the lack of tectonic plate movement. Over at least 115 million years Olympic Mons just kept getting bigger and bigger. By comparison Mt Etna is the oldest volcano on Earth at about 350,000 years, and most volcanoes are between 10,000 and 100,000 years old.
Next we'll look at how plates tectonics contributed to the tragic story of the Continental Drift Theory.