Tectonics

Programme Outline

The Structure of the Earth

The Earth is made up of four layers:

1. The inner core
2. The outer core
3. The mantle
4. The crust

Click here for more information on the structure of the Earth.

The Earth's crust is called the 'lithosphere', and is cracked and brittle like an eggshell to form several segments called tectonic plates. The plates which form the crust are not static, and have moved over millions of years. These movements have created some of the most dramatic features of our world: mountain ranges, earthquakes, volcanoes and islands.

Constructive Margins

The creation of Surtsey

In 1963, Icelandic fishermen saw ash, steam and lava shooting out of the sea. Almost overnight a new island emerged which was named Surtsey. The eruptions continued for four years. Within months, birds began to visit and plants started to grow.

The formation of Surtsey

At the bottom of the ocean: two plates had moved apart very slowly: less than 5cm a year. Molten rock called 'magma', lying below the plates, oozed up through the cracks. When it reached the surface, the magma cooled and solidified to form lava. This gradually built up until it broke through the ocean surface to form an island.

A ridge of lava and mountains has been formed where the two plates have moved apart. This ridge runs down the middle of the Atlantic Ocean and is called the Mid-Atlantic Ridge. Since Columbus set out from Spain to discover America in 1492, the Atlantic Ocean has widened by about 25m.

Destructive Margins

Where two plates move towards one another and collide, one plate is forced under the other in a subduction zone and destroyed in the mantle. This is known as a 'destructive margin'.

Volcanoes like Merapi in Indonesia are caused by the collision of oceanic and continental plates. Over millions of years heavier oceanic plate is being forced down under the lighter continental plate - creating a trench. Carrying water and debris with it, the oceanic plate breaks up under immense pressure.

As it is forced down it gets hotter and hotter, partly as a result of friction and partly because the temperature rises rapidly towards the Earth's centre.

Eventually it melts and the molten magma rises to form volcanoes some distance from the trench. Sometimes a row of these volcanoes can form an island like Java. Millions of years of eruptions have made the country very mountainous and fertile.

These volcanoes release ash and rocks that break down quickly into soil, producing fresh supplies of the minerals plants use to grow. Living in the shadow of Merapi is dangerous. The volcano often erupts and in 1994 it claimed the lives of more than 60 people.

Why do volcanoes erupt?

Molten magma from below the Earth's crust pushes up into the mountain. This causes tremors, which can be measured by seismometers. Another device called a 'tiltmeter' can actually detect how much the mountain is bulging as it fills with magma.

Magma is thick, like porridge, and when it reaches the surface it solidifies into a hot crumbly plug. Pressure on the magma builds up and bits fall off all the time. The plug, or dome, gets too big and top-heavy. Eventually, it is forced apart, gas boils out and foam rushes up. Shattered rocks and steam form a huge and heavy cloud that soon falls back to Earth, crashing down the mountain like a bulldozer. It is grey on the outside where it is cool, but inside are red-hot gas, sand and exploding rocks.

The cloud drops vast quantities of boulders and ash over the surrounding land, but when it rains it poses a new danger. Water mixes with ash into a heavy mud flow, covering fields and villages, powerful enough to carry huge boulders along with it.

In the past, volcanoes have erupted with cataclysmic effects. In 1883, the island of Krakatoa almost blew itself off the face of the Earth. Ash was propelled to a height of 80km. It blocked out the sun and plunged the region into darkness for two and a half years. It caused a tidal wave 36m high which drowned over 30,000 people. The explosion was heard in Australia, over 3,000km away.

But volcanic eruptions are not the only effect of the Earth's plates moving together.

Earthquakes at Destructive Margins

Crustal plates don't always slide smoothly one under the other. The oceanic crust can meet fierce resistance, so it slows or sticks, like a machine without oil. The pressure builds up until there is a sudden violent movement and the plate is forced forward. The pressure is released, sending shockwaves through the Earth's crust. The first earthquake is often followed by a series of aftershocks. The epicentre is immediately above the focal point on the surface, but the shockwaves are often felt far away. Mexico City was 400km from the epicentre of an earthquake which devastated the city in 1995.

But not all earthquakes result from plates moving together at destructive margins.

Earthquakes at Conservative Margins

In San Francisco, California in the USA, people are living on a time bomb. This densely populated area is frequently hit by earthquakes, and one of the largest occurred in 1906. Parts of the city were reduced to rubble, causing fires which burned for three days.

San Francisco is built on the edge of the North American Plate, next to the Pacific Plate. The boundary between the two is called the San Andreas Fault. This has been caused by the two plates sliding past each other at a rate of 5cm a year.

The fault extends for over 1000km from San Francisco, south through California. At present, Hollywood, Los Angeles, lies 650km south of the Golden Gate. Both plates are moving northwest, but the Pacific Plate is moving faster, and in 12,500,000 years Hollywood will be north of San Francisco.

Collision Zones

The Himalayas are the highest mountain range in the world, and are getting higher all the time. Every year Mount Everest grows by 1cm. These mountains are caused by the collision of two plates of continental crust.

They are called 'fold mountains' because the layers of different rocks within the Earth's crust - the strata - are folded and bent. Over time, erosion of the layers can occur at different rates, producing characteristic jagged rock formations.

The Himalayas mark the boundary between the Indian Plate and the Eurasian Plate. There, two sections of continental crust are moving towards each other.

The world's major fold mountains, volcanoes and earthquake zones all lie on or near the boundaries between the Earth's tectonic plates. All are in constant motion powered by the Earth's intense inner energy.