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A Short Primer on how Geological Times are Crystal Clear

Mass spectrometer.

How can we know when something that long ago actually did take place? By looking at a long-range time clock which nature has built into certain minerals and elements.
This slide shows a mass spectrometer, which allows precise measurements of isotopes contained in minerals.

Geochronology is a special branch of geochemistry, related to isotope physics, that involves determining the time of formation of rocks, minerals and fossils. Geochronologists examine materials ranging in age from a few years to billions of years old, mostly utilizing the principle that radioactive isotopes present at the "birth" of a mineral will decay at a certain fixed rate. Measurement of relative abundances of the "parent" and "daughter" isotopes can determine the age of the rocks.

One example of the use of radiogenic isotopes is the uranium-lead (U-Pb) method of dating zircons.

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