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The Earth is made up of four distinct layers:

• The crust is the outer layer of the earth. It is a thin layer between 5-70 km thick. The crust is the solid rock layer made of large slabs called plates. These form both the ocean floor and the continents.
• The mantle is the largest section, containing 84% of the Earth’s volume. It has a diameter of 2,900 km. The mantle is mostly formed of solid rock, but it can deform slowly transferring heat and material.
• 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.
• 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.

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.

There are two different types of crust: continental crust: which carries land, and oceanic crust, which carries water.

Continental plates are usually quite thick (between 35 to 70 km) compared to the much thinner oceanic plates (between 5 and 10 km).

The oceanic rocks are usually much younger than the continental rocks as new plate material is still being made at theseplate boundaries.

The oceanic rocks are usually made from basalt, while continental rocks are made from granite.

Continental and oceanic plates all fit together to form the outer crust of the planet. Eight major plates are named on the diagram below.

The Earth's crust is broken up into pieces called plates.

These plates move due to convection currents in the mantle.

Heat from the core makes magma in the mantle rise towards the crust. As the hot current nears the crust, it begins to cool and sink back towards the core. As the magma sinks, it drags the plates across the surface of the Earth.

The movement of the plates, and the activity inside the Earth, is called plate tectonics.

The point where two plates meet is called a plate boundary.

Where convection currents move plates apart, constructive plate boundaries (margins) are formed.

Where convection currents push plates together, destructive plate boundaries (margins) are formed.

Plate tectonics cause earthquakes and volcanoes.Earthquakes and volcanoes are most likely to occur either on or near plate boundaries.

Constructive plate margins are where tectonic plates pull apart due to convection currents in the mantle. Most constructive plate boundaries are found on ocean floors.

As the plates move apart (very slowly), magma rises from the mantle to fill the gap.

This is also accompanied by earthquakes.

When the magma reaches the surface, it cools and solidifies to form a new crust of igneous rock. At constructive plate boundaries, mid-ocean ridges will be formed.

A mid-ocean ridge is an underwater mountain chain of volcanoes built up after repeated eruptions at a constructive boundary, when the sea floor spreads apart, e.g. the Mid-Atlantic Ridge.

Some of these undersea volcanoes become so large that they erupt out of the sea to form volcanic islands, e.g. Surtsey and the Westman Islands near Iceland.

At destructive plate margins, subduction zones and ocean trenches will be formed.

At a 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. As they move together, the oceanic plate is forced underneath the continental plate.

The point at which this happens is called the subduction zone.

When the oceanic plate is forced below the continental plate it melts to form magma and earthquakes are triggered. The plate melts due to friction and due to the heat from the mantle.

The magma collects to form a magma chamber.

This magma then rises up through cracks in the continental crust.

As pressure builds up, a volcanic eruption may occur causing composite cone volcanoes (e.g. Nevado del Ruiz in Colombia).

A major example of this plate movement is along the coastline of South America (at the Nazca and South American plate boundary).

At the point where the plate is forced down, there will be a deep gap under the sea which is known as an ocean trench. The deepest ocean trench is the Mariana trench found in the western Pacific Ocean.

Fold Mountains, such as the Andes, are also found at destructive plate boundaries. They are formed due to the pressure of the two plates colliding, which forces the continental crust to crumple and push upwards along the western coastline of South America.

Collision zones are found at a special form of destructive boundary. They form where two continental plates are forced together by the convection currents in the upper mantle. At collision zones fold mountains will be formed.

• Where an area of ocean separates two plates, sediments will settle onto the ocean floor. Over millions of years these deep sediment layers are compressed, squeezing out water and forming sedimentary rock.
• When the two continental plates move towards each other, the layers of sedimentary rock on the sea floor become compressed, crumpled and are folded upwards.
• Eventually this folded sedimentary rock may rise above sea level as a range of fold mountains .
• The peak of Mt Everest (Sagarmatha), the world’s highest mountain is five miles above sea level. It is made of limestone rock that originally formed on the ocean floor that once lay between India and South Asia.
• Where the rocks are folded upwards, they are called anticlines. Where the rocks are folded downwards, they are called synclines.

At conservative plate margins, fault lines will be formed.

These are plate boundaries where two plates are either slipping past each other in opposite directions or at different rates in the same direction.

The plates do not move past each other smoothly.

There will often be a build-up of friction or energy that is ultimately released as an earthquake.

A good example of this is along the San Andreas Fault where the Pacific and the North American plates are sliding past each other.