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

Digitale Tijdschriftenarchief Stichting De Hollandse Cirkel en Geo Informatie Nederland

Lustrumboek Snellius | 1990 | | pagina 379