Radioactive age dating
These minerals are common to many igneous and metamorphic rocks.Such techniques have had an enormous impact on scientific knowledge of Earth history because precise dates can now be obtained on rocks in all orogenic (mountain) belts ranging in age from the early Archean (about 4 billion years old) to the early Neogene (roughly 20 million years old).At greater depths the temperature effect should become dominant, and response to stress should result in flow rather than fracture of rocks. Rubey, demonstrated that fluids in the pores of rock may reduce internal friction and permit gliding over nearly horizontal planes of the large overthrust blocks associated with folded mountains.
Experimental petrology also provides valuable data on the stability limits of individual metamorphic minerals and of the reactions between different minerals in a wide variety of chemical systems.
After estimating the rate of this radioactive change, he calculated that the absolute ages of his specimens ranged from 410 million to 2.2 billion years. Wasserburg applied the mass spectrometer to the study of geochronology.
Though his figures were too high by about 20 percent, their order of magnitude was enough to dispose of the short scale of geologic time proposed by Lord Kelvin. This device separates the different isotopes of the same element and can measure the variations in these isotopic abundances to within one part in 10,000.
These experiments are carried out at elevated temperatures and pressures that simulate those operating in different levels of the Earth’s crust.
Thus the metamorphic petrologist today can compare the minerals and mineral assemblages found in natural rocks with comparable examples produced in the laboratory, the pressure–temperature limits of which have been well defined by experimental petrology. Bridgman developed a technique for subjecting rock samples to high pressures similar to those deep in the Earth.