ABSTRACT
The ear th is roughly spherical with an average radius of a round 6400 km. Its inner s t ructure was determined from the propaga t ion of ea r thquake waves. One sector th rough the Ear th ' s center is shown in Figure 2 .1 . The Ea r th consists of three spherical shells of quite different physical propert ies . The outer shell is a thin crust of thickness varying from a few kilometers to a few tens of kilometers; the middle shell is the mant le , abou t 2900 k m thick, and the M o h o (Mohorovicic) discontinuity is its interface with the crust; the innermost shell contains the core, of radius approximately 3500 km. The crust is made of various types of rock, differing in composi t ion and thickness in its oceanic and cont inental par ts . The crust in the cont inenta l pa r t consists of two layers, granitic in the outer and basalt ic in the inner layer, with total thickness abou t 30-40 km, but reaching 70 k m under high moun ta ins , such as in the Qing-Zang pla teau of west China. The crust under the oceans is basalt ic only, with n o granitic deposit , with a thickness of only abou t 5 km. The mant le consists mainly of comparat ively uni form ul t rabasic olivine rock; its outer 40-70 k m shell together with the crust is usually referred to as the l i thosphere, directly under which is a layer of soft viscoelastic as thenosphere a few hundreds of ki lometers in thickness. The wave velocity in the as thenosphere is obviously lower than those in its neighbor ing rocks, pe rhaps due to its viscoelastic, or creep, proper ty under high tempera ture and confining pressure. The l i thosphere and as theno sphere together form the upper mant le . Below, the lower mant le extends a further 1900 km or so. The core consists of outer and inner cores. Because it is found tha t no transverse wave can propaga te th rough the outer core, it is agreed tha t the outer core is in a liquid state.
