The viscosity of magmas: General considerations concerning the viscosity of liquids

Abstract

This paper, given as an informal introduction to the symposium on the viscosity of magmas, contained a brief review of the study of the viscosity of liquids in general. Starting from the formal definition of the coefficient of viscosity the author outlined the various methods used in the measurement of this quantity, including the observation of uniform flow through capillary tubes, the rotating cylinder method, the terminal velocity of falling rigid spheres or rising bubbles in liquids, the rate of elongation of a rod, and the rate of sag of a rod supported horizontally at two points. In the last two methods coefficients of viscous traction are measured and the relation of this quantity to the coefficient of viscosity is well discussed by Trouton (Proc. Roy. Soc, v. 77, p. 926, 1906). These last two methods which are especially suitable for the measurement of the coefficients of viscosity of highly viscous liquids (for example, pitch at room‐temperature) are of especial significance in geophysical problems. The well‐known formulae connecting the rate of laminar flow of a liquid, the rate of sinking of a solid mass in a liquid medium, or the rate of rise of a gas‐bubble in a liquid all involve powers greater than unity of the dimensions of the masses under consideration. Simple calculations show that in masses of dimensions of the order of those met in geological problems important movements of matter may occur in liquids whose viscosity is 109 c.g.s. units or even greater.