Earthquakes and Tectonic Plates

by Deb on February 28, 2010

Unfortunately it seems like an appropriate time for this.

Tectonic Plates

The earth’s crust is made up of several tectonic plates, floating on softer, moveable rocks.  They move because of convection currents.  When anything is heated it becomes less dense and lighter, because there is less matter in the same area, and the denser, heavier matter pushes it up out of the way.  When it gets to the top, in this case the surface of the earth, it can’t go any higher so it spreads out.  Eventually it gets cold and dense again so it sinks back down.

The plates floating on the surface are carried along with the material underneath.

This map shows the major plate boundaries, there are several things to notice –

  • Lots of plate boundaries are along the edges of continents.
  • There are plate boundaries near the world’s major mountain chains – Himalayas, Andes, Rockies.
  • Several plate boundaries go through island chains.
  • The major boundary running between the Americas and Africa and Europe is roughly parallel to the coastlines.  (Others are too, that one’s just really obvious on this map.)

These are all consequences of the three things plate boundaries can do.

Diverging Boundaries (Spreading ridges) –

Most of these are the ones in the middle of the ocean, although the Great Rift Valley in Africa is also spreading.  These are the places where the hot rock is coming up from the middle of the earth, it’s like an ongoing volcanic eruption.  The lava comes up and pushes the ridges apart, that one running North / South is what has formed the Atlantic ocean and is spreading the Red Sea.  It’s also the cause of the volcanos on Iceland, in fact all the islands in the mid-Atlantic are just very high parts of the mid-Atlantic ridge that are currently above sea level.

Converging Boundaries –

This is where two plates are moving towards each other, one has to go underneath the other.  This is what is happening along the west coast of the Americas.  When there is a continent near the edge, the sea floor always goes underneath because it is denser and the continent floats on the top.  This is called subduction.  Put your hands with one on top of the other facing together and they don’t slip past each other smoothly – they bend, get caught, and suddenly let go.  This is exactly what happens to the plates – the bending causes the large mountain ranges to form along continental boundaries, and the catching and letting go is an earthquake.  There are often volcanos associated with it as well, because the plate going under starts to melt and the magma bubbles up.

There is a major converging boundary that isn’t on the edge of a continent – the north of India.  This used to be a normal converging boundary covered by a shallow sea, until the subcontinent of India ran up against Asia.  Neither of them can go under because they are too light and thick so both plates are buckling, throwing up the Himalayas and all the mountain chains through central Asia.  This has had a major effect on plate movements around the world, changing the direction of movement of all the other plates.

Transform Boundaries –

This is where two plates are sliding sideways past each other, it’s similar to a converging boundary because it gets caught then releases suddenly, causing an earthquake.


When you try to slide your hands, you usually notice a few small slips, then a big slip, then more small slips as they settle into a new position.  The same happens with earthquakes.  I’ve recently heard of ‘pre-shocks’ that come before a big earthquake, and then there can be aftershocks that continue for decades.  Geological time is very different to our time.  And of course your hands are only dealing with pressure in one direction, but in the earth there are multiple convection currents and multiple plates, all moving against each other.

Haiti is on the northern border of the Caribbean plate, a transform boundary.  But the south of the Caribbean plate is subducting under the South American plate, as are the Cocos and Nazca plates.  This is what has caused the 2010 Chilean earthquake, in 1960 the same area experienced the strongest earthquake ever recorded.

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