The surface of the Earth is cracked into large ‘plates’ that are moved by convection currents in the upper mantle. Where they meet, we can identify three different types of boundary, depending on how the plates are moving relative to each other.
Before reading this article, you will find it useful if you know about the different layers of the Earth.
1. Constructive (or Divergent) Plate Boundary
This type of plate boundary arises where the plates are moving away from each other. As they move apart, molten magma from the upper mantle fills the space between them, creating new plate material as it solidifies. This is why it is called a constructive boundary. The magma forms volcanoes which produce a mobile and free-flowing lava, so they are not the explosive types, although they can produce large amounts of ash.
The lava solidifies to form a rock called basalt. Sometimes, the gap that opens between the plates can be several km long and this produces a ‘fissure eruption’ instead of a volcano. These are spectacular as there are fountains of red-hot molten magma spewing into the air in a line that can be several kilometres in length.
Constructive boundaries are usually hidden from view as they form the mid-oceanic ridges like the mid-Atlantic ridge. Where they occur on land, they form rift valleys, for example, the Great African Rift Valley and volcanoes like in Iceland.
2. Destructive (or Convergent) Plate Boundary
This type of plate boundary occurs where plates are moving towards each other. One plate is forced underneath the other where it is eventually melted by the heat from the mantle — this is why it is called a destructive boundary. This type of plate boundary is destructive in other ways too.
Rocks don’t slide past each other very easily, they tend to get stuck for many years and then when they suddenly move, a huge amount of energy is released as an earthquake. Some of the world’s most destructive earthquakes happen at these boundaries. As the descending plate melts, the magma which is formed rises, creating some of the world’s most explosive volcanoes.
But these boundaries are not all about destruction, they are also where the great mountain ranges are built, for example the Andes of South America, the Alps of Europe and the Himalayas of Asia.
3. Conservative (or Transverse) Plate Boundary
These occur when plates slide past each another. They are called so because no new material is added to the crust and no crust is taken back down into the mantle. You don’t get volcanoes at this type of plate boundary, but you do get major earthquakes. The usual example of this sort of boundary is the San Andreas fault in the USA. Here the Pacific plate slides sideways past the North American plate.
As with the destructive boundary, the rocks stick for many years and then, when the forces have built up sufficiently to overcome the friction between the two plates, they suddenly move, releasing vast amounts of energy. In 1906, San Francisco was hit by a major ‘quake, starting fires all over the city. The city’s water pipes were broken by the earthquake so it was impossible to put these fires out, destroying buildings that had survived the initial earth movements.