1.3 The core
The average density of Earth is 5,515 kg/ m3, making it the most dense planet in the Solar system. Since the average density of surface material is only around 3000 kg/m3, we must conclude that denser materials exist within the core of the Earth. In its earliest stages, about 4.5 billion (4.5×109) years ago, the Earth was mostly molten, and as a result gravity would have caused denser substances to sink towards the center in a process called planetary differentiation, while less dense materials would have migrated to the crust. As a result, the core is largely composed of iron (80%), along with nickel and silicon; while other dense elements, such as lead and uranium, are either too rare to be significant or tend to bind to lighter elements and thus remain in the crust (see: felsic materials).
The core is divided into two parts, a solid inner core with a radius of ~1250 km and a liquid outer core extending beyond it to a radius of ~3500 km. The inner core is generally believed to be solid and composed primarily of iron and some nickel. Some have argued that the inner core may be in the form of a single iron crystal. The outer core surrounds the inner core and is believed to be composed of liquid iron mixed with liquid nickel and trace amounts of lighter elements. It is generally believed that convection in the outer core, combined with stirring caused by the Earth's rotation (see: Coriolis forces), gives rise to the Earth's magnetic field through a process known as the dynamo theory. The solid inner core is too hot to hold a permanent magnetic field (see: Curie temperature) but probably acts to stabilise the magnetic field generated by the liquid outer core.
Recent evidence has suggested that the inner core of Earth may rotate slightly faster than the rest of the planet, by ~2° per year ( Comins DEU-p.82).
1.4 Mantle
Earth's mantle extends to a depth of 2,900 km. The pressure, at the bottom of the mantle, is ~1.4 M atm (140 G Pa). It is largely composed of substances rich in iron and magnesium. The melting point of a substance depends on the pressure it is under. As there is intense and increasing pressure as one travels deeper into the mantle, the lower part of this region is thought solid while the upper mantle is plastic (semi-molten). The viscosity of the upper mantle ranges between 1021 and 1024 Pa·s, depending on depth [2]. Thus, the upper mantle can only flow very slowly.
Why is the inner core thought solid, the outer core thought liquid, and the mantle solid/plastic? The melting point of iron rich substances are higher than pure iron. The core is composed almost entirely of pure iron, while iron rich substances are more common outside the core. So, surface iron-substances are solid, upper mantle iron-substances are semi-molten (as it is hot and they are under relatively little pressure), lower mantle iron-substances are solid (as they are under tremendous pressure), outer core pure iron is liquid as it has a very low melting point (despite enormous pressure), and the inner core is solid due to the overwhelming pressure found at the center of the planet.
1.5 Crust
The crust ranges from 5 to 35 km in depth. The thin parts are oceanic crust composed of dense ( mafic) iron magnesium silicate rocks and underlie the ocean basins. The thicker crust is continental crust which is less dense and composed of ( felsic) sodium potassium aluminium silicate rocks. The crust-mantle boundary occurs as two physically different events. Firstly, there is a discontinuity in the seismic velocity which is known as the Mohorovicic discontinuity or Moho. The cause of the Moho is thought to be a change in rock composition from rocks containing plagioclase feldspar (above) to rocks that contain no feldspars (below). The second event is a chemical discontinuity between ultramafic cumulates and tectonized harzburgites which has been observed from deep parts of the oceanic crust that have been obducted into the continental crust and preseved as ophiolite sequences.
