In this chapter we look at the structure, chemical composition and physical state of the earth's interior, and especially of the mantle. The internal structure of the earth defines the mantle, and hence the arena of mantle convection. There is also important structure within the mantle. The physical state and chemical composition of the mantle determine the properties of the mantle that permit mantle convection to occur and that control its form.

The primary internal structure of the earth is defined by its main layers, and most of this chapter is about the layers and their inferred composition. We finish with a summary of threedimensional structure, which has been increasingly well-resolved over the past decade or so and which has recently become clear enough to begin to relate directly to mantle convection.

The internal structure is determined mainly from seismology (e.g. [1]). With the main layers so determined, composition is inferred from three main sources: field occurrences of rocks inferred to come from the shallower parts of the earth; igneous rocks thought to be formed from magma derived from melting the mantle; and laboratory measurements of the physical properties of minerals and rocks.

The term physical state here includes not just, for example, whether the material is solid or liquid, but also the particular assemblage of mineral phases in which each part of the mantle exists. This mineralogy depends on chemical composition, temperature and pressure. Pressure-induced changes are of particular importance. They define the main internal mantle structure and they have the potential to substantially modify mantle convection. These potential dynamical effects arise from the interaction of temperature and composition with the pressure-induced phase transformations, so we will need to look at those aspects in some detail.