On the Study of Mechanical Properties of Rocks in Earth Sciences

Abstract

The study of mechanical properties of rocks enables us to understand properly the conditions under which earth's phenomena such as earthquakes, volcanism, orogenesis and tectogenesis take place. These phenomena are believed to occur due to physical processes such as mechanical and thermodynamical as well as physico-chemical processes in the earth. It is very important therefore to elucidate the mechanical properties of the constituent material of the earth, that is, rocks.Stresses set up in the earth are mostly based on the body forces, arising from the gravitational field, and they appear through the processes of deformation of rocks. The study of mechanical properties of rocks in earth sciences is based on the solution of the broad problems associated with the earth's phenomena as mentioned above. In a wider sense, it is the aim of the study to obtain the characteristic features of rocks counteracting the stresses. These characteristic features appear in elasticity, viscosity, plasticity, creep deformation, fluidity, yield strength, and fracture. The present paper deals with an outline of the recent developments in this mechanistic investigation in connection with the field of earth sciences.Rock is the aggregate of various kinds of minerals, and its properties depend not only on the properties of minerals themselves, but on the aggregate state of the minerals, namely, the properties of mineral boundary and rock mass structure. Consequently, these properties are considered to change according to the physical condition of the rock mass or the experimental methods. In earth sciences, however, the various results achieved by the experimental methods which are adequate to the solution of the present problems are synthesized through discussion. The study is in progress so as to use these results for a quantitative understanding of the conditions under which physical processes may occur in the earth. As a matter of cource, it is necessary to investigate the irreversible processes such as melting, recrystallization, or chemical change in the earth as well as the physical properties as mentioned above for full understanding of the earth's phenomena.The present paper is divided into four sections. Section 1 is introductory, and in Section 2 is given the present state of experimental conditions for rock deformation. In Section 3 are described the rock deformation under the low stresses of short period, the elasticity and attenuation of elastic waves. The velocity of elastic waves in rocks under high pressure and the anisotropy of rocks based on the wave propagation are discussed in connection with the estimation of the rock type below a Moho-discontinuity. In Section 4 are discussed rock deformation under the stresses of long period, the general deformation, creep, ductility, strength, fracture, and viscosity. The occurrence of earthquake is especially discussed not only on the basis of the fracture of rocks, but also from the point of view of transition from brittle to ductile in rocks. The hypotheses of earthquakes are also described.Rocks are structurally and chemically complicated, heterogeneous, anisotropic, and deformable, involving various elements affecting their properties. Regarding the conditions of rocks inside the earth, the temperature range is in the order from near zero to 103 degrees, and pressures from a small fraction of a bar to 106bar, and strain rates from 103sec to 10-15sec. It is not possible to make experiment of all these cases to investigate the state of rocks with such variables in broad band and their behaviors in the field of their aggregation. Accordingly, in order to obtain the quantitative, rather than phenomenological, understanding of the earth's phenomena