Observations of the earth's surface provide important constraints on mantle dynamics. As well, our knowledge of the earth's interior comes entirely from observations made at or close to the surface. The deepest borehole penetrates only about 10 km, compared with the nearly 3000 km depth of the mantle.

Seismology provides by far the most detailed and accurate information on the structure of the earth's interior. Observed variations in the strength of gravity at the earth's surface yield complementary constraints on variations in density in the interior. The topography of the earth's surface and geographical variations in the rate at which heat conducts through the surface provide important constraints on internal dynamical processes.

This chapter focusses on observations that, for the moment, are best left in the form of their surface geography, namely topography, heat flow and gravity. The geography of the tectonic plates is also summarised here. The full interpretation and implications of these observations are intimately related to mantle dynamics, and they will be considered as part of that topic in Part 3. However, some of the significance of the observations of topography and heat flow can be appreciated more directly, and this will be demonstrated in Chapter 7.

On the other hand, although seismological observations are made at the surface, they are usually converted into graphs or images of internal structure. Therefore the presentation of seismological constraints is deferred to Chapter 5, where they will be presented in conjunction with our knowledge of the material properties of the interior.