Lesson 1 - Structure of the Earth

Layers of the Earth

Shrinking Earth Theory

This early theory said that the Earth started as a hot molten ball of rock which, as it cooled over time, began to shrink and form 'wrinkles' on its surface - a little like when custard becomes cold! These ridges that formed made the mountains we know today.

This theory was gradually disproved because it could not explain why mountains formed in distinct ranges around the planet, such as The Himalayas and The Andes. If Shrinking Earth Theory was correct, there would be mountains evenly spread all over the surface of the planet.

YouTube video on Shrinking (contracting) Earth Theory

Wegener's Theory of Continental Drift (from BBC GCSE Bitesize)

Alfred Wegener proposed the theory of continental drift at the beginning of the 20th century (1912). His idea was that the Earth's continents were once joined together, but gradually moved apart over millions of years.

Wegener’s evidence for continental drift was that:

• 1. The same types of fossilised animals and plants are found on the east coast of South America and west coast of Africa
• 2. The shape of the east coast of South America fits the west coast of Africa, like pieces in a jigsaw puzzle
• 3. Matching rock formations and mountain chains are found in South America and Africa

This 'super-continent' that once existed was known as Pangea.

But it took a long time for the idea to become accepted by other scientists. Wegener couldn't explain HOW the super-continent broke apart into pieces and spread apart to give the land masses that we know of today.

Plate Tectonics

It was later discovered that the Earth's lithosphere (crust and uppermost part of the mantle) was split into sections that float on top of the mantle. These sections are known as tectonic plates. The seven major plates are the African plate, Antarctic plate, Eurasian plate, Indo-Australian plate, North American plate, Pacific plate and South American plate. There are also many smaller plates. Sometimes, depending on where you look and in which book, you will see anywhere between 7 and 9 major plates being quoted. You don't need to know the exact number.

These plates are moving in different directions. The area where two plates meet is called a 'plate boundary'.

These plates move because of convection currents that set up in the Earth's mantle. Some metals in the Earth's crust are radioactive and when they decay they generate heat. This heat rises up and causes the mantle to heat up and behave like a gloopy, viscous liquid. As the hot mantle rock rises, it cools down and changes direction as it moves towards the crust. This then pushes the plate in one direction.

If two plates move towards each other, this is known as a convergent plate boundary. Mountain ranges and volcanoes often occur here. The friction caused by the plates rubbing together forms magma which can rise to the surface and cause volcanic eruptions!

If two plates move away from each other, this is known as a divergent plate boundary. The most famous example of this type of boundary is right down the middle of the Atlantic Ocean. Here, the Eurasian plate is moving away from the North American plate. Yes, this means Europe is moving away from North America! As they move apart, magma seeps upwards and forms new rock in the space between the plates.

Sometimes plates can move in an almost sideways manner past each other. A famous example is the San Andreas Fault in California! This is probably the most famous and most visited fault line (plate boundary) in the world.

Tectonic plates are always moving slowly (at a rate of several seconds per year), but occasionally a huge release of energy can cause them to suddely jolt past each other. When this happens, an earthquake occurs! Because scientists cannot accurately predict or sense these sudden releases of energy, it makes predicting earthquakes a VERY difficult business!

Earthquakes themselves can be devastating, causing millions of pounds worth of damage and loss of life, but secondary problems can also occur when major earthquakes strike;

The sudden tremor, if near the sea, can cause a large shockwave in the water in the ocean and this causes a fast moving wave that can travel huge distances. When in the open ocean, this wave is barely visible, but as it moves towards land and into shallower water, the wave height increases and moves rapidly on to land, causing mass damage. This type of wave is known as a tsunami.

1. On Boxing Day 2004, a powerful 9.1 magnitude earthquake struck off the coast of Sumatra. The resulting tsunami was so powerful that wave heights were increased all over the world, but the devastation caused in Asia was unprecedented. An estimated £7 billion worth of damage was caused from the tsunami with a reported death toll of 230,000 people.

2. On the 11th March 2011, the east coast of Japan was hit by an 800 km/h, 10m high tsunami, caused by a 9.0 magnitude earthquake. 18,000 people were killed, £175 billion worth of damage was caused and the Fukushima nuclear power plant began leaking radioactive steam.