EARTH STRUCTURE AND ORIGINS 423 variation in gravitational pull imposed by the rotation of the Earth causes Earth tides There is also a smaller contribution to Earth tides by the Sun Internal Configuration What we know of the internal configuration of the Earth comes from the following sources: study of surface rocks and rocks brought up by eruptive processes (kimberlite diatremes are particularly important with respect to the mantle as they sample regions within it that would otherwise be unknown to us directly (see Volcanoes; Igneous Rocks: Kimberlite); analogies with meteorites (in particular between nickel–iron meteorites and the core, and between carbonaceous chondrites and the mantle); instrumental measurements (geophysical), especially seismology (Figure 3); and geochemical research (relating to the Earth, other planetary bodies, and meteorites) (Figure 4) The outermost is the crust, beneath it is the mantle (divided into an upper mantle and lower mantle), and at the centre is the core (divided into the inner core and outer core) The plate-tectonics paradigm also accepts the existence of a lithosphere, which is not synonymous with the crust, but is a solid and rigid zone extending into the upper part of the mantle and bounded below by a transition zone This separates the strong elastic layer from the weak and ductile asthenosphere, which is solid but very slowly convecting The crust is bounded below by a discontinuity at which the velocity of seismic P waves increases abruptly; this is the Mohorovicic discontinuity and forms the lower boundary of the zone of isostatic adjustment (see Earth: Mantle; Crust) The properties of the various concentric zones of the Earth are summarized in Table Magnetic Field As a result of these approaches it is accepted that the Earth is made up of a series of concentric shells, each with its own physical and chemical properties The Earth’s magnetic field is environmentally critical because it forms a protective shield that blocks out solar radiation and harmful incoming particles It has the character of a dipole, but has, throughout geological time, been subject to reversals, with the poles Figure Earthquake energy travels down and out from the focus as shown, but at each internal boundary part of it is re flected or refracted and returns to the surface Times of travel of the energy waves and their behaviour have given us much of the information that we have about the depths of the boundaries and the nature of the material forming the concentric shells of the Earth (reproduced from Van Amdel TJ (1994) (eds.) New Views on an Old Planet, 2nd edn Cambridge: Cambridge University Press, with permission from the author and publisher) Figure The configuration of the Earth’s internal zones as deduced from the four indirect sources listed in the text (repro duced with permission from C M R Fowler (2000) Mantle convec tion, plumes, viscosity and dynamics In: Hancock PL and Skinner BJ (eds.) Oxford Companion to the Earth, pp 649 652 Oxford: Oxford University Press)