Nowadays our understanding of the solid Earth is subtle and satisfying, although the plume controversy shows that there are always new insights coming along. In particular, the idea is now firmly established that the different parts and layers of the Earth interact. Even the slow growth of the inner core has effects on the surface. Perhaps the most striking idea of the twentieth century is subduction. People have always known about volcanoes, so they had to accept that material was emerging at the Earths surface from below'. But subduction means that it is also vanishing again, and in continent-sized bites.

But even this big story misses the point. All this activity has wrhat the literary critics would term a meta-narrative, and it is called differentiation. Look at the Earth today. Solid iron (mostly) at the middle. Liquid iron plus other elements on top of it. On top of that, rock. On top of that, water in the shape of oceans, and above that, air. Every time a volcano emits material to the Earths surface, it is joining in the process of putting lighter elements on top of denser ones and so “differentiating” the Earth a little more. Despite all that crust that is recycled over time, the overall story is one of gradual differentiation.

This process has been going on throughout the history of the Earth and began when it was new, when the core and mantle separated out as the heaviest element present in abundance, iron, sank to the centre. The process is continuing as the inner core grows.

Differentiation is also going on in the Earths outer layers. Thus the crust has more light elements such as potassium and calcium than you find in the mantle. And the sorting process continues today as lighter elements of the mantle are preferred for turning into basalt at the Earths surface. So although the rocks you see around you in the Earths crust have an average density of about 2.7 grams per cubic centimetre, the Earth as a whole has a density of about 5.5.

The atmosphere itself started out as the product of “outgassing”, the gradual release of gas trapped in rocks, a process still going on today each time a volcano emits gas. Some of the gas that must have been present in the early Earth was too light to be held by the planets gravitation and has escaped into space, so helium is rare in the atmosphere. As we have seen, the helium used by todays divers and balloonists is extracted from natural gas and is a fossil remnant of the early Earth still being differentiated out. Hydrogen, the only gas that is even lighter, tends not to escape because it is too reactive, and gets involved in forming water molecules. Water vapour, carbon dioxide, nitrogen and other gases emerge from volcanoes and would have done so in greater abundance earlier in the history of the Earth.

Differentiation is not the whole story. For example, uranium is dense but its chemistry means that it is concentrated in the crust and mantle, not the core, making it available for bomb-makers and reactor-builders but also supplying heat to the upper layers of the Earth as it decomposes through natural radioactivity. Another exception is water. The sheer amount of the stuff in the oceans is the clue. It is widely thought that at least some of it must have come from comets, abundant in the new solar system, raining down on the early Earth.

Differentiation is a continuing process. Early in the Earths history it was at its most active, forming the structure we know today, from core to atmosphere. But on a more modest scale, it is still happening every time a volcano erupts.