Volcanoes Tutorial


Volcanoes and Earthquakes - it is Earth's fault!

Plate Tectonics

Perhaps it was not surprising that scientists are not always on the same wave length all the time; the ‘Drifting Continents’ theory is a case in point.


People believed that features like mountains, were formed when the Earth’s early crust shrank as it cooled down.  However, after Alfred Wagener's death, scientists started to notice things that supported his theory. Once again, the death of a great man aroused the enthusiasm of a subject which had been hotly debated.


The presence of same rock and fossil features  between other pairs of continents revitalized the debate of once-being the same. Then explorers of the oceans discovered evidence that the sea bed was spreading.  In other words, continents were actually moving apart.  At last, people had to take the ideas of Alfred Wegener seriously.  This says that the Earth’s crust is made up from huge slabs of rock, called plates, and it explain how they move?

The plates are still moving today, although only a few centimetres a year, significant enough to take matters seriously, when taking into account their sheer size. Volcanoes appear where molten rock from beneath the crust rises to the surface through joints between the plates.


Moving Plates

Nowadays, we can explain why the plates move.  It is pretty simple. Layer underneath the crust tell the tale. It is the upper part of the mantle described earlier as ‘very thick treacle’.

You can imagine the plates almost floating on the mantle.  The plates move because of huge forces caused by convection currents in this partly molten rock. 
We know that the deeper we go into the mantle, the hotter it gets; Natural  radioactive atoms are decaying inside the Earth all the time. These nuclear reactions give out lots of energy.  They give out enough energy to melt rock and to set up the convection currents that move the Earth’s plates.

Plates slipping past each other

Some plates are trying to slip past each other.  The most famous example of this is the San Andreas fault.  It runs down the west coast of the USA.  It marks the boundary between the Pacific plate and the North American plate.  When the plates slip, there is an earthquake.

As yet, we cannot predict exactly when the plates will slip.  So people in California have to live with the threat of an earthquake at any time. Some says it is thirteen months pregnant!

Plates moving towards each other

There are 2 types of material forming the plates.  In some places the plates are thin and dense.  These are found under the oceans (basaltic crust).  Other plates are thicker and less dense. These make up the continents' (granites crust).

In some places 2 plates are moving towards each other.  Where they meet, the denser oceanic crust slips under the continental crust.  This is called Subduction.  The friction between the plates can cause earthquakes.   The rock can even get so hot that it melts and rises to the surfaces to form volcanoes.


Finally, the whole oceanic part of the plate slips under the continental crust.  Then the two continental plates collide.  The plates fold upwards, forming mountains.  Metamorphic rocks can form under this great pressure.   The Himalayas and the Alps were formed like this. Formation of them is not a rocket science anymore.





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