4 rules of relative dating
Simply stated, each bed in a sequence of sedimentary rocks (or layered volcanic rocks) is younger than the bed below it and older than the bed above it.This law follows two basic assumptions: (1) the beds were originally deposited near horizontal, and (2) the beds were not overturned after their deposition.If a geologist claims to be 45 years old, that is an absolute age.Superposition: The most basic concept used in relative dating is the law of superposition.The nuclear decay of radioactive isotopes is a process that behaves in a clock-like fashion and is thus a useful tool for determining the absolute age of rocks.
Basically, he deduced that rocks were laid down in vertical order just as sediments are laid down today, under water, with new on top of old. Today we know that some strata start out tilted, but nevertheless this principle enables us to easily detect unnatural degrees of tilt and infer that they have been disturbed since their formation.Particularly useful are index fossils, geographically widespread fossils that evolved rapidly through time.Crosscutting Relationships: Relative ages of rocks and events may also be determined using the law of crosscutting relationships, which states that geologic features such as igneous intrusions or faults are younger than the units they cut across.Geologists generally know the age of a rock by determining the age of the group of rocks, or formation, that it is found in.The age of formations is marked on a geologic calendar known as the geologic time scale.
Inclusions: Inclusions, which are fragments of older rock within a younger igneous rock or coarse-grained sedimentary rock, also facilitate relative dating.