The large scale structure of the earth is caused by geodynamic processes which are explained using energetic, kinematic and dynamic descriptions. While "geodynamic processes" are understood to include a large variety of processes and the term is used by earth scientists quite loosely, the methods of their description involve well defined fields. Energetic descriptions are in­ volved with distribution of energy in our planet, typically expressed in terms of heat and temperature. Kinematic descriptions describe movements using velocities, strains and strain rates and Dynamic descriptions indicate how stresses and forces behave. As structural and metamorphic geologists we document in the field only the consequences of geological processes. The underlying causes are much harder to constrain directly. However, it is absolutely crucial to understand these causes or: "driving forces", if we are to explain the tectonic evolution of our planet. This book deals with the dynamic description of geological processes. Our descriptions relate causes and consequences - tectonic processes with field observations. In many cases, we will use equations as a concise form to describe processes and observations in nature. As we will be dealing mostly with large scale tectonic questions, the observations that we shall use are also on a large scale. For example, we shall use observations on the elevation (Fig. 1.1, 1.2) and heat flow of mountain ranges, the thickness of continents and the water depth of the oceans.