How we know what is inside the Earth: seismic waves

0:00How do we know what is inside the Earth, if no one has ever been there?

0:05The deepest hole ever drilled is about 12 kilometres deep, but the centre is about 6,400 kilometres down.

0:14No hole has even gone through the crust, Earth's thin outer layer of rock.

0:20Instead, scientists use seismic waves: vibrations from earthquakes that travel through the Earth.

0:28Instruments called seismometers record these waves all around the world.

0:33P waves, or primary waves, arrive first. They pass through solids and liquids.

0:39S waves, or secondary waves, arrive later. They pass through solids only.

0:46S waves reach the near side, but never the far side. This is the S-wave shadow zone.

0:53S waves cannot pass through liquid, so a liquid layer must be blocking them.

0:59This is the liquid outer core. Above it is the mantle, mostly solid rock.

1:06P waves do cross the core, but they slow down and bend as they enter it.

1:11This leaves a band with almost no P waves: the P-wave shadow zone.

1:17In 1936, Inge Lehmann noticed faint P waves arriving inside this shadow zone.

1:24She explained that an inner core, where P waves travel faster, had bent them there.

1:31We now know the inner core is solid, because the pressure there is enormous.

1:36Seismic waves reveal the layers: the crust, the mantle, the liquid outer core and the solid inner core.