Interior of the Earth

How we learn about the inside of the earth, how earthquake waves reveal its layers, and the volcanoes and landforms that magma makes.

Key ideas

• Direct sources are mines, deep drilling and volcanic material; indirect sources are meteors, gravity, magnetic surveys and earthquake waves.
• The focus is where an earthquake starts inside the earth; the epicentre is the point on the surface right above it.
• P-waves are fastest and arrive first, passing through solids and liquids; S-waves travel only through solids; surface waves arrive last and do the most damage.
• S-waves cannot cross the liquid outer core, which creates the shadow zones and shows the core’s nature.
• Layers: crust (thin under oceans, thick under continents), mantle down to 2,900 km with the asthenosphere at its top, then a liquid outer core and a solid inner core of nickel and iron.
• Volcano types: shield (fluid basalt, low), composite (explosive, layers of lava and ash), caldera (collapsed), flood basalt (like the Deccan Traps) and mid-ocean ridge.
• Magma that cools inside the crust forms batholiths, laccoliths, lapoliths, phacoliths, sills and dykes.

Remember

Watch out

• Saying S-waves pass through the outer core: they stop because it is liquid.
• Mixing up the focus (underground) and the epicentre (on the surface).
• Confusing a sill (horizontal) with a dyke (vertical wall).
• Giving the lag as the S time alone instead of S time minus P time.

Worked example