Basic to ice mechanics — be it the theory of glacier flow, the response of floating ice plates to external loading, ice drifting and ice ridging, or the very practical questions of ice forces on structures — are the fundamental laws of continuum physics. These consist of the balance laws of mass, momentum, angular momentum, and energy, and indeed, there is no essential problem in glaciology in which use of one or more of these laws is not made. Not all of them would necessarily be used for answering the questions in mind. For instance, in the so-called ‘mass balance’ of a glacier, one only makes use of the law of conservation of mass. A similar situation prevails when one is investigating the response of a glacier to changes in climate. Of course, in such situations the true picture is oversimplified, and the neglect of certain physical laws might have to be bought at the expense of accuracy, or must be compensated by introducing phenomenological statements which replace the neglected balance laws. However, the simplifications in the physical picture and the replacement of certain basic balance laws by phenomenological statements often leads to drastic mathematical reductions yielding detailed physical insight that could not otherwise be obtained. As an example we mention that the distribution of stress and velocity in an ice sheet is often determinable without simultaneously searching for the temperature distribution. Alternatively, the temperature distribution may be determined independently from that of velocity. In either case, some assumptions about the neglected fields must be made, assumptions motivated by experimental observations or by some sort of plausibility arguments.
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