The Onion Structure of the Earth
When defining the character of a planet, we would like to know to what degree heavy materials have been separated from light materials (by "gravitational segregation"), and whether this resulted in a layer of water (perhaps still present as ice on some planets). On Earth, the heaviest matter, represented by the metals nickel and iron, is in the core of Earth. The radius of the core is about 3485 km, and the volume is 30% of the solid Earth. The weight represents 32.5%. The fact that our planet has an iron core is reflected in a fairly strong magnetic field with two well-defined magnetic poles that are rather closely tied to the rotational poles.
The internal structure of the Earth. (Source: USGS)
The thick layer around the iron core is called the mantle. It makes up two thirds of the volume of the home planet, and about 67% of its weight. The mantle consists of iron- and magnesium-rich silicates. This material is rather similar to the familiar black rock found in association with mid-ocean volcanoes (basalt). The Hawaiian Islands, for example, are essentially made of mantle rock (except of course for the reef rock surrounding each island which is made by marine organisms).
The mantle is covered by a thin shell called the crust. The oceanic crust is about 5 to 10 km thick . It differs but little from the mantle material, except for showing the effects of reaction with seawater. (Also, it carries sediments on top.) The continental crust is between 25 and 50 km thick, and has an enormous variety of rocks, depending on circumstances and history. Largely, we deal with schist and gneiss and granitic rocks, in addition to a veneer of sediments (shales, sandstones and limestone rocks).
Compared with the oceanic crust, the continental crust has more silica, sodium and potassium in its rocks, and less iron and magnesium. The differences in the chemistry of oceanic crust and continental crust are results of convection. In essence, the continents acquire that portion of material from the down-going slab (in the subduction zone) that is more easily mobilized through heating and through the addition of water.
It is not known exactly how the planetary matter segregated into layers of heavy and light solids, and into water and gases. Did some of it happen during the accretion of primordial material, with heavy particles coming in earlier than light ones? Or was it all achieved through large-scale melting after Earth had collected most of its mass? It is interesting to note that among the rocky debris moving around the Sun, and coming to us as meteorites, we find both metallic and stony pieces, suggesting that segregation went on early in the history of the solar system.
Presumably, some combination of processes related to assembly and processes producing segregation within Earth was at work. Apparently, for segregation to occur, there must be a minimum size, and this is certainly true for generating convection in the interior. We would expect convection to be favorable for segregation between core and mantle. Convection also results in two fundamentally different kinds of crust, with considerable variability. The fact that we have a variety of crustal materials must be ascribed to chemical segregation involving water and resulting from physical convection depending on heat.
For sufficient availability of heat, and for availability of water, the size of the planet is an important factor, as we have discussed. Thus, the onion structure of Earth is a result of the fact that our planet is large enough to undergo gravitational segregation.