In this chapter, we will introduce a few basic concepts that will be used frequently throughout this book. First, we review some commonly used classifications of glaciers by shape and thermal characteristics. Then we consider the mathematical formulation of the concept of conservation of mass and, associated with it, the condition of incompressibility. This will appear again in Chapters 6 and 9. Finally, we discuss stress and strain rate, and lay the foundation for understanding the most commonly used flow laws for ice. Although a complete consideration of these latter concepts is deferred to Chapter 9, a modest understanding of them is essential for a fuller appreciation of some fundamental concepts presented in Chapters 4-8.

A note on units and coordinate axes

SI (Système International) units are used in this book. The basic units of length, mass, and time are the meter (m), kilogram (kg), and second (s) (MKS). Temperatures are measured in Kelvins (K) or in the derived unit, degrees Celsius (°C). Some other derived units and useful conversion factors are given on p. ⅹⅶ.

In comparison with the earlier glaciological literature, one of the most significant changes introduced by use of SI units is that from bars to pascals as the principal unit of stress. The bar (= 0.1 MPa ≈ 1 atmosphere) was a convenient unit because stresses in glaciers are typically ∼1 bar.