⇒ This is the internal structure of the Earth
⇒ The outside of the Earth is the solid crust - the crust is very thin with a maximum depth of around 50km
⇒ Under the crust we have the mantle - this is a solid, even though parts of the upper mantle can flow very slowly
⇒ Under this we have the outer core (this is a liquid) and the inner core (which is solid)
⇒ There is no way for scientists to directly observe the interior of the Earth
⇒ How do scientists know that the interior of the earth has this structure? The answer is to do with earthquakes
⇒ An earthquake happens due to a sudden movement between the tectonic plates in the Earth's crust
⇒ This causes seismic waves which carry energy away from the earthquake
⇒ These seismic waves then pass through the Earth and they can be detected by seismometers in different countries
⇒ The patterns of these waves gives us information about the interior of the Earth
⇒ There are two main types of seismic waves: P Waves and S Waves
P Waves
⇒ P waves are longitudina waves. They can pass through both solids and liquids
⇒ P waves travel faster than S waves
S Waves
⇒ S Waves are transverse waves (to remember this, remember the word 'transverse' contains the letter S)
⇒ S waves can only travel through solids
⇒ Imagine an earthquake takes place at a point on the Earth (the top of the drawing above, at the epicentre)
⇒ Seismometers on the Earth's surface can now detect the seismic waves after they have passed through the Earth
⇒ On the left shows S Wave patterns
⇒ On the right shows P wave patterns
⇒ Scientists have measured seismic waves from thousands of earthquakes
⇒ They have used these to work out the thickness of the crust and mantle and outer and inner cores