Earth Science: Origins and Geology – Reconstruction
In order to reconstruct how and when the continents migrated into their current positions and the route they are now following, researchers have developed measuring procedures to observe the traces of rock “memory.”
There are traces of ferrous silicates almost everywhere in the lithosphere. When molten rock is cooling in the crystallization phase, the magnetic iron minerals arrange themselves in the direction of the existing magnetic field, much like a com- pass needle. Measuring the angle of the minerals, known as the inclination, can determine where a continent formed and which movement it has since taken.
Geophysicists are able to retrace more than just large-scale plate movements across geological time. Thanks to more sophisticated procedures, they can also measure current changes in position, although these only amount to a few inches per year. At locations where the earthquake risk is particularly high, for example, near the San Andreas Fault in California, they have set up a network of measuring points.
With laser geodesy instruments, changes in angle and distance between individual points can be precisely determined. However, the function of geodetic networks is tightly confined due to air pollution and the Earth’s curvature.
GPS: global positioning system
Another procedure for calculating plate movement is the GPS measuring method. Most commonly used in navigation systems for automobiles, it consists of 24 satellites, each transmitting a distinct signal. As every point on Earth can always receive a signal from at least four satellites at once, its exact position can be determined nearly to a fraction of an inch based on the differences in signal transit time.
Thus, the various gaps between individual reference points measured over a year can be used to determine the movements of the continental massifs. The same principle governs radio astronomical measuring procedures; however, in this case, the radiation from pulsars or quasars is analyzed. Specialized laser satellites are used to measure vertical changes at the edges of the plates.
BASICS
GRAVIMETRY measures changes in gravitational force.
SEISMOGRAPHY analyzes the duration, form, and strength of earthquake vibrations.
GROUND-PENETRATING RADAR “illuminates” the topmost layers of the Earth with electromagnetic waves.
MAGNETOTELLURICS is a method of imaging the Earth’s subsurface by using natural magnetic fields.