THE RESTLESS EARTH

Plate Tectonics

Plate Tectonics

The earth’s crust is unstable, especially at plate margins. Learn about the distribution of plates, contrasts between continental and oceanic plates, and destructive, constructive and conservative plate margins.

The earth is a ball-shaped planet 12,756 kilometres (7,926 miles) in diameter. At its centre lies the inner core, which is a huge ball of molten rock. Above this there are a number of other layers (outer core, mantle, upper mantle and crust, or ‘lithosphere‘). This outer crust is of two principal types: continental and oceanic.

The continental crust is composed of igneous, metamorphic and sedimentary rocks and is usually between 30 and 50 kilometres (18.6 to 31 miles) thick. The oceanic crust is just 5 to 10 kilometres (3 to 6 miles) thick but is generally composed of denser rock. The lithosphere is broken up into ‘tectonic plates‘ that ‘float’ on top of the mantle, which itself is up to 2,900 kilometres (1,800 miles) thick. This movement has been taking place since the earth formed billions of years ago, and is a never-ending process.

During the age of the dinosaurs, the earth looked very different. Seen from space 100 million years ago, the land was split into two huge supercontinents, Gondwana and Langaea. Over millions of years, these separated and floated to form the continents we recognise today.

Where two plates are moving together, this is known as a destructive plate margin or convergent boundary. Along these margins volcanoes and earthquakes are common. The Himalayas, which includes the earth’s highest mountain (Everest), is forming due to the moving together of two continental plates. This process is still occurring and the Himalayas are getting higher.

Constructive plate margins are areas where two plates are moving apart and new oceanic crust is generated. The Rift Valley in East Africa is one example of a constructive plate margin. Conservative plate margins occur when two plates are neither moving towards each other nor moving apart, such as the San Andreas Fault.

Unique landforms occur at plate margins. Learn about location and formation of fold mountains, ocean trenches, composite volcanoes and shield volcanoes.

When two plates move towards each other, sometimes (as in the Himalayas) they push upwards. But other times their edges ‘fold’ together, creating another sort of mountain range know as fold mountains. One example of this is the Jura Mountains in the Alps, a range that includes part of France, Germany and Switzerland.

Ocean trenches are formed when two plates move together under the ocean and one is pushed downwards. This can result in very deep areas of water. The Mariana Trench in the Pacific is over 11 kilometres (6.8 miles) deep. The film director James Cameron recently became only the third person to descend to the bottom of the trench. If he had stepped outside his submarine he would have been crushed by the water.

Composite volcanoes (Strato volcanoes) are conical in shape and are the result of many layers of rock forming over millions of years. Krakatoa and Vesuvius are both composite volcanoes. Shield volcanoes are often found along plate margins and erupt much more frequently. They can be huge: anything up to 6 kilometres (4 miles) in diameter and 600 metres (half a mile) high.

People use these landforms as a resource and adapt to the conditions within them. Read a case study about one range of fold mountains and discover the ways in which they are used in farming and in tourism.