2. The structure and composition of the Earth

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During the last century there were huge advances in our knowledge of the Earth, although there is still much more to be discovered.

 

2.1. Methods for studying the Earth's interior

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• Core sampling and mining. With current technology, the deepest they can drill is 13km, an insignificant distance compared to the Earth´s 6371km radius. Therefore, this is not a very efficient method for learning about the Earth.
• Studying rocks. We may not be able to enter the interior of the Earth, but there are natural phenomena which move rocks from the interior to the surface. These include erosion, which breaks down the rocks on the surface and exposes other deeper rocks; and volcanic eruptions that often drag fragments of the Earth's interior to the surface.
• Studying meteorites and asteroids. These are fragments from when the Solar System was created, that did not join others to form a planet. They tell us about the original materials that formed the Earth.
• Seismic methods. These are the most important methods. They measure the seismic waves produced by earthquakes or controlled explosions. These waves travel through the Earth's interior and are recorded on seismographs, which give us information about the layers they have passed through. There are two types of seismic waves, P and S.

Waves	Origin of the name	Speed	Substances they travel through	Movement they provoke
P	Primary: they are the first to arrive to the surface.	Faster	All. They travel faster in solids than liquids.	They make the terrestrial particles move in the same direction as the wave.
S	Secondary: they arrive after the P waves.	Slower	They travel through solids not liquids.	They make the terrestrial particles move perpendicularly to the wave.

Seismic discontinuity

By studying internal P and S waves, scientists discovered the existence of seismic discontinuities. These are places in the Earth's interior where there are abrupt changes in the speed of these waves, indicating a change in the composition or the physical state of the materials they are travelling through. They show us the boundaries between different layers of the Earth.

Weblink 3: Seismic waves

2.2. The composition and dynamics of the Earth

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The study of seismic waves using seismographs revealed that the interior of the Earth is divided into layers, similar to those of an onion. Each layer can be classified according to two criteria: its chemical composition or its physical state.

• Compositional layers. These are the crust, mantle and core. They are classified in order of increasing density and are separated by seismic discontinuity boundaries.
  
  The Mohorovičić discontinuity or Moho separates the crust and the mantle. The crust is made up of less dense rocks, rich in silicon and aluminium, than the mantle, which is made up of denser rocks like peridotite and magnesium.
  
  The Gutenberg discontinuity separates the mantle and the outer core. The core is denser than the mantle and is mainly made up of iron.
• Physical layers. The lithosphere is the solid and very rigid surface that covers the Earth's surface. The asthenosphere and mesosphere layers are found under the lithosphere. They are solid, but are more flexible than the lithosphere. The endosphere consists of the core. It is mainly liquid (outer core) and the inner core is solid.

 

 

2.3. Vertical movements: isostasy

The advancement of technology and the development of new techniques have increased scientists' knowledge and understanding of why there are highs and lows along the surface of the Earth. These changes in elevation are due to an interaction between the lithosphere and the asthenosphere.

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The lithosphere, the rigid surface layer of the Earth, rests on the asthenosphere, which although it is solid, has a flexible behaviour. If a heavy weight forms on the surface of the lithosphere, it sinks into the asthenosphere, making a dent in the surface of the Earth.

These vertical movements are very slow (1cm/year) and, due to the rigidity and thickness of the lithosphere, large variations in mass are needed for them to occur. This is what happens in the formation or melting of ice caps, the deposition of large quantities of sediments and in the elevation or erosion of mountains.

2.4. The crust and the lithosphere

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The lithosphere is the rigid surface of the Earth. It includes the crust and upper mantle. It is not a continuous layer, but is broken into pieces called plates.

The graph below shows the variation in the density, temperature and melting point of the materials in the Earth's interior. As you can see, the melting point increases with depth. This is due to the effect of pressure, so iron, which melts at 1550°C on the surface, is solid in the centre of the Earth, at 6000°C.