Tectonic Plates

The idea of tectonic plates was put forward by Alfred Wegener early last century. He noticed that the coastline of South America fits to that of Africa.

More than that he found that:

• the types of __________ match like a 'Jigsaw' on both continents and
• some of them contained the same __________.

Other scientist did not believe his theory as he could not explain how the two continents could have moved apart. We now use Wegener's theory to explain mountain formation, but that was not always the case.

Scientists believe that the Earth was formed, along with the Sun, from a very hot cloud of dust and gas. The Earth started life as a ball of magma (liquid rock)
As the magma cooled on the surface the __________ was formed
Scientists once thought that the mountains and valleys were the result of the __________ of the crust as the Earth cooled down further.

Since Wegner's death further evidence has been discovered such as:

• Satellite images
• Mid Alantic Ridge:

  
  From Google Maps

We now know that the Earth's crust and the upper part of the mantle are cracked into a number of large pieces called __________ plates.

The mantle is mostly __________, but that it is able to move slowly. The Earth's core produces vast amounts of heat by natural __________ processes. This causes __________ currents within the Earth's mantle which in turn cause the plates to move in a process known as crustal movement or continental drift. This is at average relative speeds of a few __________ per year.

The movements can be sudden and disastrous. They can cause __________ and/or __________ eruptions at the boundaries between tectonic plates. If plates are moving relative to each other they can become stuck when there will be a build up of strain. This may __________ in a number of smaller quakes or result in a large one.

Scientists find it difficult to __________ the exact time of earthquakes. The best that can be done at the moment is to look at the __________ time between quakes and apply that to the future.