1.2 Continuity equation
Our qualitative, intuitive understanding of continuity can, indeed, be transformed into a quantitative mathematical formalism. This formalism is widely used in numerical geodynamic modelling in form of a continuity equation which describes the conservation of mass during the displacement of a continuous medium. Let’s write this equation and try to understand its structure in detail.
The first thing that we have to learn is that the form of the mass conservation equation (and many other time-dependent conservation equations) can be either Eulerian or Lagrangian depending on the nature of a geometrical point for which thisequationiswritten.TheEuleriancontinuityequationiswrittenforanimmobile, or fixed point in space; it has the form: