Tectonic plates

The Earth consists of four concentric layers: inner core, outer core, mantle and crust. The crust is made up of tectonic plates, which are in constant motion. Earthquakes and volcanoes are most likely to occur at plate boundaries.

The structure of the Earth

The Earth is made up of four distinct layers:

1. The inner core is in the centre and is the hottest part of the Earth. It is solid and made up of iron and nickel with temperatures of up to 5,500°C. With its immense heat energy, the inner core is like the engine room of the Earth.
2. The outer core is the layer surrounding the inner core. It is a liquid layer, also made up of iron and nickel. It is still extremely hot, with temperatures similar to the inner core.
3. The mantle is the widest section of the Earth. It has a thickness of approximately 2,900 km. The mantle is made up of semi-molten rock called magma. In the upper parts of the mantle the rock is hard, but lower down the rock is soft and beginning to melt.
4. The crust is the outer layer of the earth. It is a thin layer between 0-60 km thick. The crust is the solid rock layer upon which we live.

There are two different types of crust: continental crust, which carries land, and oceanic crust, which carries water.
The diagram below shows the structure of the earth. In geography, taking a slice through a structure to see inside is called a cross section.


structure of the Earth

Tensional margins

At a tensional or constructive boundary the plates are moving apart. The plates move apart due to convection currents inside the Earth.

The Helgafjell volcano on Westman Island, Iceland

As the plates move apart (very slowly), magma rises from the mantle. The magma erupts to the surface of the Earth. This is also accompanied by earthquakes.
When the magma reaches the surface, it cools and solidifies to form a new crust of igneous rock. This process is repeated many times, over a long period of time.
Eventually the new rock builds up to form a volcano. Constructive boundaries tend to be found under the sea, eg the Mid Atlantic Ridge. Here, chains of underwater volcanoes have formed along the plate boundary. One of these volcanoes may become so large that it erupts out of the sea to form a volcanic island, eg Surtsey and the Westman Islands near Iceland.
The diagram below to see how magma pushes up between the two plates, causing a chain of volcanoes along the constructive plate boundary.