illustrated in Figure lb.
The rock represents one extreme example of the geological record. From
Western Australia, it contains the oldest known terrestrial material, a 4.2 billion
year old mineral, zircon. The rock reveals that crust was already being created
and destroyed very early in the Earth's history; that many of the processes
occurring in more recent geological time were already occurring then. After
about 3.1 billion years ago, the sediments hosting these minerals - the remnants
of earlier geological cycles - were buried to a depth of more than 15 km and
subjected to temperatures in excess of 500 °C. Later, it found its way back to the
surface where it has remained for perhaps the past billion years. A remarkable
history in itself, and a remarkable achievement that the record contained in the
rock can be read today. But it does not reveal the forces that were at work,
shaping the early minerals, burying them to great depths and then returning them
to the surface, not in single events, but as part of sequences of tectonic events
spanning long time intervals. This is where the other recorder of deformation
plays a role, the geodetic instrument measuring the short duration episodes of
deformation. The radio-telescope, satellite laser ranging system, or GPS receiver,
measures the "instantaneous" deformations of the Earth; the deformations that
occur on short durations when viewed on these geological time scales. But they
provide us with a measure, albeit indirect, of the forces at work and of the
response of the planet to these forces. These are the very building blocks upon
356
Figure la
Figure lb