Three rules of relative dating
Prior to the late 17th century, geologic time was thought to be the same as historical time.Archbishop James Ussher of Armagh, Ireland, 1654, added up generations from the Old Testament and determined that Earth formed on October 23, 4004 BCE.
Geologist in the 1800s worked out 7 basic principles of stratigraphy that allowed them, and now us, to work out the relative ages of rocks.Relative geologic age is established, based on such evidence as the order in which layers of sediment are stacked, with the younger layer originally on top.By using the principles of relative geologic age, the sequence of geologic events -- what happened first, what happened next, what happened last -- can be established.The units commonly used for geologic age are mega-annum (Ma) for millions of years, giga-annum (Ga) for billions of years, and kiloannum (ka) ka for thousands of years.Because these units are used according to the rules of the metric system, the M in Ma and the G in Ga must be capitalized, and the k in ka must not be capitalized.Conversely, the igneous rocks are younger than the sedimentary rocks.
Other examples of cross crutting relationships can be related to faults (fault has to be younger than the rock it is found in) and unconformities (see below).
Absolute dating places events or rocks at a specific time.
If a geologist claims to be younger than his or her co-worker, that is a relative age.
From the beginning of this course, we have stated that the Earth is about 4.6 billion years old.
How do we know this and how do we know the ages of other events in Earth history?
By carefully digging, we have found that each trash pit shows a sequence of layers.