| Popis: |
The rotation of the Earth is not regular. It changes on virtually every timescale we know in both position of the rotation axis and rotation rate. Even in our daily lives we sometimes experience the consequences of such changes, such as the second that is subtracted or added to clocks at the beginning of a new year. Although this second is not much more than a curiosity for most of us, the rotational changes it implies can influence our lives in a more fundamental sense. There are indications that the emergence of the great ice ages some two million years ago was triggered by a gradual shift of the rotation axis over the Earth's surface, combined with wandering of the continents and associated changes in ocean currents. The geological record shows that times during which there was wide-spread glaciation have been followed by times during which virtually no glaciation occurred, and vice versa, several times. The fact that we are currently living in an interglacial period of a 100,000 year glaciation cycle is also attributable to changes in the position of the rotation axis. Changes in insolation resulting from the periodic rotational changes due to the gravitational interaction between the Earth and the other members of the solar system trigger the onset and decay of the large polar ice sheets. This so-called Milankovitch cycle is responsible for the relatively mild climates we have now, instead of the severe glacial circumstances that governed the planet some 10,000 years ago. Rather than worrying about foregoing temperatures, we are troubled by an enhancement of the greenhouse effect and concomitant sea-level changes by fossil fuel combustion. The causes of these rotation changes are as diverse as the timescales on which the changes in rotation take place. Grossly, they can be divided into two categories: astronomical and geological. The effects they induce differ mainly in the way the rotation axis shifts. Extra-terrestrial causes generally induce shifts of the rotation axis with respect to the distant stars, but not with respect to the Earth's surface. Terrestrial causes generally induce shifts of the rotation axis with respect to the Earth's surface, but not with respect to the distant stars. For an observer in outer space it looks in this latter case as though the Earth were shifting as a rigid unit underneath its rotation axis, which remains fixed in position with respect to him. For an Earthbound observer, however, the polar axis seems to wander with respect to a fixed point on the ground. Some of the driving mechanisms are established with high certainty, of others we know little at the moment or virtually nothing, like the Chandler wobble. Two of the signatures for which the driving mechanisms are thought to be known are the present-day secular shift in the direction of Canada (that is, on the northern hemisphere) with a magnitude of some 10 meters per 100 years, and the satellite-derived non-tidal secular change in the second degree harmonic of the gravitational potential field. Basically all theories which have been published during the last decade relate both these signatures exclusively to post-glacial rebound and recent changes in the ice - water distribution over the Earth's surface. The main theme of this thesis is that this is not correct. It is shown that also various tectonic processes are contributing to these changes in rotation and gravitational potential field. |
