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Chronostratigraphy is the process of using dating methods to determine rock ages.This can either be absolute dating, in which the minerals of the rocks themselves are dated, or relative dating, where the rocks are compared to other outcrops of known ages to get an approximate age.For example, zircon crystals are found in many igneous rocks and can be weathered out and re-deposited into sedimentary rocks.Because of this, if you use absolute dating methods on these detrital zircons (or bits of zircons in other rocks), you can get the oldest date for those zircons but not the absolute age of the rocks themselves.Some types of radioactive absolute dating methods include carbon dating (for more recent deposits), or potassium-argon dating.These types of dating methods give exact answers, plus or minus a thousand or a million years, depending on the dating type used.The age ranges of different chronostratigraphic methods can also limit their usefulness - carbon dating can be used on sedimentary rocks or fossils, but only for the past 50,000 years or so because it decays so quickly.
Other times, chronostratigraphy can give you only the maximum known age.
When you're trying to understand the activity of a fault zone, you need to know whether the rocks that a fault cross-cuts through are 5,000, 50,000, or 500,000 years old - that would tell you how old the fault itself was, and how likely it was to move again.
This is why geologists use stratigraphy, the study of putting rocks in order according to age and making a timeline out of geologic processes.
Depending on the characteristics of the rocks being studied, geologists may utilize one or more stratigraphy methods. For example, biostratigraphy would be useless in areas of unknown lithostratigraphy - you must first know which rocks are older than others in order to establish how biostratigraphic assemblages of fossils in rocks change through time.
Similarly, chronostratigraphy can be really useful, but because most methods are limited to igneous rocks, they aren't useful in areas of metamorphic or sedimentary rocks.