The surface of the Earth is divided into seven large tectonic plates and many smaller ones.
We know where they are, how fast they move (between as fast as a fingernail growing and
a hair growing) and what direction they move in, but we are not completely sure why they move
Most plates are being created at constructive plate boundaries in a process known as sea-floor
spreading, which pushes plates outwards. And most plates are then being destroyed at
destructive plate boundaries, where one plate is pushed down underneath another into the mantle.
But some plates are not associated with any
constructive plate boundaries and their movement is not yet fully understood.
A constructive plate boundary is where new crust is being created. A good example is
the constructive boundaries between the Eurasian Plate and the North American Plate, or between
the South American plate and the African Plate.
These are pushing these continents apart -- you can see from the shape of their coastlines
how Africa and South America once fitted together.
A destructive plate boundary is where one plate is pushed against another. The new crust
created at constructive plate boundaries is called oceanic crust and it is denser than
the older continental crust. So at a destructive boundary the denser oceanic crust is pushed
under the lighter continental crust and destroyed in a process called subduction. This is a
violent process that produces a lot of energy!
A good example of a destructive boundary with a subduction zone is between the Nazca plate
and the South American plate. The Nazca plate is being subducted under the South American
plate. And what happens when two continental crust plates meet? This is called a continental
collision. One example of where this is happening is between India and Asia -- forming the Himalayas
-- the world's highest mountain range. And what about when two oceanic plates meet? One
gets subducted under the other -- this is happening on the boundary of the Philippine
Sea Plate and the Pacific Plate. The subduction zone is the Mariana Trench -- the world's
deepest ocean trench.
A conservative plate boundary is where plates are sliding past each other. The most famous
example of this is the San Andreas Fault -- the boundary between the Pacific Plate and the
North American Plate.
If you compare the distribution of the world's volcanoes with the map of plate boundaries,
you can see a very close match. This is because most volcanoes form either at constructive
boundaries, where the Earth's crust is being pulled apart, or at destructive boundaries,
where oceanic crust is being pulled down and melted into the mantle.
You can sometimes get volcanoes away from plate boundaries: this is thought to happen
where a plume of superhot magma rises up from the mantle or where the crust is particularly
thin or weak. These locations are often called 'hot spots'. A good example is the Hawaiian
Island chain, which has formed as the Pacific Plate has passed over a hot spot.
And if you compare the distribution of the world's earthquakes with the map of plate
boundaries, you also a really close match as well.
This is because earthquakes are the result of the vast amounts of energy produced at
plate boundaries. As you can imagine, plates don't just gently glide past each other. They
often stick. Then pressure builds up along the fault until the plates unstick. And when
they unstick, all that stored energy is released in an earthquake.
Constructive boundaries usually have the weakest sorts of earthquakes. When conservative boundaries
get stuck, as shown in this diagram, they can produce powerful earthquakes. But the
most powerful earthquakes of all usually happen at destructive boundaries because this is
where the most energy is generated.
If an earthquake at a plate boundary happens under the sea, a tsunami can result.
Tsunamis occur when the sea floor is suddenly pushed up; this pushes up the water above
it , generating an enormous movement of water. Ships in the deep water out at sea might not
notice any difference but when the tsunami reaches the shore it slows down and its amplitude
(its height) increases. This can result in a massive movement of water inland. This movement
might seem more like a rising tide than a wave, although the very biggest tsunamis do
sometimes break like a wave on the shore.
You can see from this map that the most serious tsunamis have been produced by earthquakes
on plate boundaries, mainly destructive plate boundaries. But not all tsunamis are produced
by earthquakes. Some can be triggered by landslides, some even by meteor strikes.