which the models are constructed to quantify the processes occurring on the
geological time scales.
Forces and response
To understand the workings of the Earth requires a knowledge of the forces
acting on the Earth and of the response of the planet to these forces. Two clas
ses of problems can be identified. In the first, the forces are well known, such as
the tide raising gravitational potential or the centrifugal force. Here the
observations of displacement or deformation determine the response function or
rheology of the planet appropriate for this particular problem (Figure 2). In the
second class of problems the deformations are observed but the forces are largely
unknown. An example of this is the motions of the continents and the spreading
of the sea floor. In this type of problem the forces can be estimated if the
response function of the Earth is known. The two classes of problems are closely
linked.
Force
Unknown
Earth
Response
Function
(Unknown)
Deformation
(Observed)
Force
(Unknown)
ll
II
ll
Earth
Response
Function
(Known)
Deformation
(Observed)
The problem is easily stated in this way but the achievement of a solution is a
different story. One reason is that the range of forces is limited. Another reason
is that the Earth as a whole, oceans, atmosphere as well as solid Earth, is often
357
Figure 2
