Earth's Interior

The Earth's crust is a thin hard outer shell of rock. Under the crust, there is a deep layer of hot soft rock called the mantle. The crust and upper mantle can be divided into three layers according to their rigidity:

• the lithosphere,
• the asthenosphere,
• the mesosphere.

Beneath the mantle is a core of hot iron and nickel. The outer core is so hot (4500°C - 6000°C) that it is always molten. The inner core is even hotter (up to 7000°C) but it stays solid because the pressure is 6000 times greater than on the surface.

The inner core contains 1.7% of the Earth's mass, the outer core 30.8%; the core - mantle boundary 3%; the lower mantle 49%; the upper mantle 15%; the ocean crust 0.099% and the continental crust 0.374%.

fig. 3 - The main layers that form the Earth.

Our knowledge of the Earth's interior comes mainly from studying how earthquake waves move through different kinds of rock.

Analysis of how earthquake waves are deflected reveals where different materials occur in the interior. S (secondary) waves pass only through the mantle. P (primary) waves pass through the core as well. P waves passing through the core are deflected, leaving a shadow zone where no waves reach the far side of the earth.

The speed of earthquake waves reveals how dense the rocky materials are. Cold, hard rock transmits waves more quickly than hot, soft rock.