Geography

Tectonic plates

The Earth consists of four concentric [concentric: Describe shapes (usually circles) which share the same centre point. ] layers: inner core, outer core, mantle [mantle: A zone of hot rock in the interior of the Earth, located between the outer crust and the molten core. ] and crust [crust: The outer layer of the Earth, on top of the mantle. It is between 6 and 48 kilometres thick and includes the continents and the ocean floor. ]. The crust is made up of tectonic plates [tectonic plates: Large areas of the Earth's crust that move slowly on the upper part of the mantle, often colliding and moving away from each other. ], 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.

Cross section showing structure of the Earth

Cross section showing structure of the Earth

Distribution

The Earth's crust is broken up into pieces called plates. Heat rising and falling inside the mantle creates convection currents generated by radioactive decay in the core. The convection currents move the plates. Where convection currents diverge near the Earth's crust, plates move apart. Where convection currents converge, plates move towards each other. The movement of the plates, and the activity inside the Earth, is called plate tectonics.

Plate tectonics cause earthquakes and volcanoes. The point where two plates meet is called a plate boundary. Earthquakes and volcanoes are most likely to occur either on or near plate boundaries.

The map shows the world's tectonic plates and the distribution of earthquakes and volcanoes.

The Earth's plates move in different directions.

Different plate boundaries

  • At a tensional, constructive or divergent boundary the plates move apart.
  • At a compressional, destructive or convergent boundary the plates move towards each other.
  • At a conservative or transform boundary the plates slide past each other.

Different exam boards and textbooks may use different names for each of the boundary types. For example, a destructive boundary may also be called a collision boundary. Use any term so long as you use it correctly, but it is best to stick to the terms you have been taught.

Tensional margins

At a tensional or constructive boundary the plates [Plates: Large areas of the Earth's crust that move slowly on the upper part of the mantle, often colliding and moving away from each other. ] are moving apart. The plates move apart due to convection currents [convection currents: Radioactive decay in the core of the Earth causes hot circular convection currents to rise and cold currents to fall. These currents cause the motion of tectonic plates. ] inside the Earth.

The Helgafjell volcano on Westman Island, Iceland

The Helgafjell volcano on Westman Island, Iceland

As the plates move apart (very slowly), magma [magma: Magma is molten rock. ] 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 [plate boundary: The region where two or more tectonic plates meet. It is a zone of intense seismic activity. ]. 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.

Compressional boundary

At a compressional or destructive boundary the plates are moving towards each other. This usually involves a continental plate and an oceanic plate.

The oceanic plate is denser than the continental plate so, as they move together, the oceanic plate is forced underneath the continental plate. The point at which this happens is called the subduction zone. As the oceanic plate is forced below the continental plate it melts to form magma and earthquakes are triggered. The magma collects to form a magma chamber [magma chamber: A region under the surface of the Earth where hot molten magma collects. ]. This magma then rises up through cracks in the continental crust. As pressure builds up, a volcanic eruption may occur.

The diagram below shows how the oceanic plate is pushed underneath the continental plate, causing mountains and possibly volcanoes to form along the destructive plate boundary.

A view of the Himalayas from Gorak Shep

A view of the Himalayas from Gorak Shep

As the plates push together, the continental crust is squashed together and forced upwards. This is called folding. The process of folding creates fold mountains. Fold mountains can also be formed where two continental plates push towards each other. This is how mountain ranges such as the Himalayas and the Alps were formed.

Back to Revision Bite