The Earth's outer core has a radius of about 3,500 km. It consists of iron and nickel. The inner core is more than 5,000 °C hot but in solid state due to the high pressure. The outer core is liquid and somewhat cooler. The Earth's core is surrounded by the mantle. Its solid but slightly ductile (malleable) rock is composed of magnesium and iron-rich silicate minerals. The upper part of the mantle is 1000-1400 °C hot. On this so called asthenosphere floats the lithosphere. It is 100-200 km thick and is com¬posed of the uppermost, solid layer of the mantle and the Earth's crust, which is the solid, brittle surface of the planet. The oceanic crust is approximately 8 km thick, in some places even up to 20 km. The continental crust is 35 km thick in average. Under large mountain ranges such as the Himalayas or parts of the Andes, it is seven up to 70 km thick.
The high temperatures inside the earth are largely caused by the decay of radioactive iso-topes. They also stem partially from the residual heat dating from the formation of the Earth. The steep temperature gradient between the interior of the Earth and the Earth's surface causes heat flow. As this so-called thermal convection takes place in malleable material, it causes displacements. Due to the material and resulting density differences in the various layers of the Earth, this process is very complex. The convection in the mantle drives the movement of the tectonic plates. Lithospheric plates "float" on the viscous asthenosphere layer. The position and shape of the continents and oceans have changed constantly as a result.
At the oceans mid-ocean ridges, new sea floor is constantly generating. At this spreading zone, the Earth crust is torn apart. At the subduction zones it is eventually absorbed.
Text: Christina Bonanati,GEOMAR Helmholtz Centrre for Ocean Research Kiel