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Relative dating biostratigraphy

For example, paleo magnetism is a method of chronostratigraphy that looks at how particles are aligned with magnetic north throughout an outcrop.Because magnetic north has changed through time, this gives you a barcode of magnetic north that can be aligned with other areas of known magnetic north and age.Though carbon dating is an exception, most types of chronostratigraphy are limited to igneous rocks and are hard or impossible to use in areas without volcanism.Not all methods of chronostratigraphy involve radioactive elements.Because fossils evolved slowly through time, the presence or absence of certain fossils, called indicator taxa, can tell a geologist what time period they are looking at.This method is not as precise as chronostratigraphy because it provides a range of ages during which the fossils lived, not an exact date.This problem is called biogeography, and can make using NALMAs difficult in unique ecosystems where indicator taxa didn't live, or places where indicator taxa lived for much longer than they did anywhere else.

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Because rocks are not made at a steady pace through time, lithostratigraphy can tell you which rock is oldest, but it cannot tell you specifically how old a rock may be.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.Because you are comparing your data to known areas, this is a form of relative, rather than absolute, dating.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.Lithostratigraphy works using the Law of Superposition, which is a fancy way of saying 'the oldest stuff is on the bottom.' Of course, not every piece of stratigraphy is nicely layered - rocks are folded, transported, twisted, and cross-cut by faults and dikes, which can make understanding which is the oldest a little bit more complicated than simply going from the bottom up.A particular challenge of lithostratigraphy is the presence of unconformities, areas where the rocks have a gap in time between layers.Depending on the rock type, this could involve dating the minerals and elements inside the rock (chronostratigraphy), or it could involve looking at the life forms in the rock (biostratigraphy), or looking at where the rock is in relation to other rocks (lithostratigraphy).For many rocks, a combination of multiple methods is used to help anchor a section of outcrop in the geologic timeline.

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