Structure of the earth

The crust is the outermost solid layer and is brittle.

• Oceanic crust is thinner: about 5 km on average.
• Continental crust is about 30 km, thicker under major mountains: up to 70 km in the Himalayan region.

The mantle lies below the crust, from the Moho discontinuity down to 2,900 km.

• Its upper part is the asthenosphere ('astheno' means weak), to about 400 km. It is the main source of magma that reaches the surface in eruptions.
• The lithosphere is the crust plus the uppermost mantle, 10-200 km thick.
• The lower mantle, below the asthenosphere, is solid.

The core begins at the core-mantle boundary, 2,900 km deep; earthquake wave speeds revealed it.

• The outer core is liquid; the inner core is solid.
• It is made of very heavy material, mostly nickel and iron, so it is called the 'nife' layer.

Example (new): if the earth were a large boiled egg, the thin shell would be the crust, the white the mantle and the yolk the core: the crust is very thin compared with the whole.