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Although it is not always possible to date a geologic event or surface on an absolute time scale, it may be possible to establish the order in which events occurred by the traditional methods of superposition and cross-cutting relationship among various geologic units.
Material units that were deposited on other units clearly postdate the units on which they lie.
Absolute age dating determines the "calendar" time at which a rock, surface, or feature formed; relative age dating determines the order-but not the time-of formation. If the rocks have remained as closed isotopic systems, it is possible to calculate their age by measuring the amount of radiogenic isotopes relative to the amount of stable isotopes now present.
In practice, this procedure requires an accurate assessment of the initial abundances of the isotopes produced in the radioactive decay.
On the other hand, most solid bodies in the solar system display a record of accumulated impact cratering on their surfaces.
The total number of craters recorded by a surface is a measure of its age.
Sizes and the size distribution can be estimated using various remote sensing techniques.
] One of the major goals of planetary exploration is to determine the surface histories of the solid planets and satellites.
Surface histories tell us how these bodies evolved through time and provide information on the probable causes for observed differences.
Crater densities indicate that the present rates of formation of large craters on each of the terrestrial planets and on the Moon are approximately within a factor of 2 of the present cratering rate on Earth.
Ten kilometer diameter craters are produced on Earth at the rate of ~2 X 10 (Shoemaker et al., 1979).