Introduction

Before describing models of plume behavior, it is important to review the fluid characteristics of mantle plumes. Whereas a fluid is a substance that can undergo an unlimited amount of deformation, a solid will undergo only limited deformation before it breaks. Another distinction is that many solids will deform a certain amount for a given force and then return to their original shape when the force is removed. In contrast, a fluid will keep deforming as long as the force is applied and will stop deforming when the force is removed, but it will not return to its original shape. A Newtonian fluid is a material whose rate of deformation is proportional to the applied force. In the mantle, flowage occurs in response to stresses, which result in strain of the mantle rocks. The proportionality between stress and strain rate is expressed as the viscosity of a fluid. For a viscous fluid undergoing very slow flow, as in the mantle, driving forces are in balance with viscous resisting forces, and the mantle can usually be considered to be an incompressible fluid. Whether all or only some of the mantle behaves as a Newtonian fluid is a subject of considerable uncertainty and debate.