Structure of the earth's crust and upper mantle

Abstract

A division of the three main structural elements of the earth, viz., crust, mantle and core, into zones is mainly based on seismological data. Most geophysicists today strongly support the hypothesis of a crust consisting of three layers (sediment, “granite” and “basalt”). In addition, various types of crust can be distinguished. The “basalt” layer is an indispensable part of all types of crust. It was the first to be formed and was produced everywhere. The “granite” layer is probably of later origin. The Mohorovic̆ić discontinuity is still of a doubtful nature. There is no certainty whether it is a boundary between rocks differing in chemical composition, or a phase boundary. Various hypotheses exist on the composition and state of the upper mantle. Certainly the low velocity zone is one,of the features and plays a certain part in the production of crustal structural eleents. This zone extends to a depth of about 300 km after which there is a gradual rise of wave velocities to the core, up to 2,900 km. The upper mantle is inhomogenous in structure. This relates to differencs in properties in the low velocity zone, and also to differences in the composition of the upper mantle as between oceans and continents. A distinction is to be made between primary inhomogeneities, which are closely related to the formation of the earth as a planet, and secondary ones which have played the main part in the tectonic activity of the earth. If it is assumed that there is a flow of mantle material of fixed density from crustal areas that are sinking to ones that are rising, then depressions should show strong negative gravitational anomalies because of mass deficiencies. No such anomalies occur within platform depressions. This means that there is a compensation by excess masses lying underneath, represented by zones of consolidation in the mantle. This consolidation is the cause of the sinking. Arguments for an expansion of material apply to uplifted parts of the crust. Thus, consolidation and expansion of mantle material are the processes responsible for vertical movements, as is clear from the gravitational fields. The flow theory must be rejected.