Earth is a dynamic planet which is constantly changing. The outer shell is made up of thin, rigid plates that move relative to each other. The discovery of these plates lead to the theory of plate tectonics, which was formulated during the early 1960's. Scientists use plate tectonics to explain many geological events such as earthquakes and volcanic eruptions as well as mountain building and the formation of the oceans and continents.

The Earth's structure

If we could slice through the planet we would be able to see that the Earth is made up of layers. On the outside is the thin, cold rocky crust which we live on, but as you go deeper into the Earth, the temperature gets hotter and the rock becomes partially molten. Right in the centre of the Earth is a white-hot core of solid iron and nickle.

Types of plate margin

Conservative Margins
Conservative margins are commonly defined by shallow earthquakes. Most are found on the ocean floor, but they can also occur on land. The San Andreas fault zone, for example, slices through two thirds of the length of California. The fault is about 1300km long, and in places tens of kilometres wide. The Pacific and North American plates have been grinding horizontally past each other for 10 million years, at an average rate of about 5cm/yr. Conservative margins are so called because they neither destroy or create crust; instead, the Earth's crust is conserved.

The San Andreas fault zone, North America

Destructive Margins
If by magic we could pull a plug and drain the Pacific Ocean, we would see a number of long, narrow curving trenches thousands of kilometres long cutting into the ocean floor. Trenches are the deepest parts of the ocean floor and are created by subduction. The Marianas Trench, for example, plunges deeper into the Earth's interior (nearly 11,000m) than Mount Everest, the world's tallest mountain (about 8,864 m), rises above sea level.

Subduction processes (where one plate is pulled down below another, into the mantle) also result in volcanic activity. Earthquakes and rapid uplift of mountain ranges are common in such regions (e.g. The Andes). The deepest part of the subducting plate breaks into smaller pieces. These become locked in place for long periods of time before suddenly moving to generate large earthquakes, often accompanied by uplift of the land by as much as a few metres.

Mount St Helens


Mount Pinatubo

Constructive margins
As hot magma rises due to convective processes in the mantle, the crust is cracked and pushed apart by the cooling magma, and new ocean crust forms. As a result an underwater mountain range builds up. These submerged mountain ranges are called the mid-ocean ridges and total approximately 60,000km in length. Major features of the mid-ocean ridges are underwater volcanoes and hydrothermal vents (extremely hot water eruptions rich in dissolved minerals).

Left: A vent on a mid ocean ridge, known as a "black smoker".
The plume of black "smoke" is actually hot water rich in dissolved metals and minerals

Collision Zones
When two continents collide, neither is subducted because the continental rocks are relatively light. Instead, the crust tends to be pushed upward or sideways. The collision of India into Asia 50 million years ago resulted in the Himalayan mountain range. The Himalayas, towering as high as 8,854m above sea level, form the highest continental mountains in the world, and the neighbouring Tibetan Plateau, at an elevation of about 4,600m, is higher than most peaks in the Alps.

Continents through time
The continents have changed position dramtically over the past 200 million years. Driven by the heat of the Earth, the plates have moved around the Earth's surface from a single mass 200 million years ago, to the pattern of continents we see today.

Earthquakes and volcanoes active today show us that plate tectonics are still active, and that the continents are still moving. In a few million years' time the position of the continents will again be different, either driven further apart by mid ocean spreading ridges, or pulled together by subduction.

The changing position of the continents over time

This web page is based on a poster created by Ivo Grigorov, a postgraduate student at the National Oceanography Centre, Southampton with the help of pupils of St Anne's School, Southampton. Visit our website at www.noc.soton.ac.uk
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