These rocks contain the mineral zircon, which contains both the elements thorium (Th) and uranium (U) in its crystal structure.Over very long periods of time, uranium decays to lead.For example, the half-life of potassium (K) is about 1.3 billion years.One of the most popular methods in absolute age dating is the use of carbon 14 (carbon atoms with 14 neutrons in the nucleus).Through several decades of intense study, paleontologists know which fossils are older than others, so by comparing the fossils in one rock outcrop to another, we can relatively tell which one was deposited first, or which is older.
The half-life of uranium with 238 neutrons is about 4.47 billion years, and uranium with 235 neutrons has a half-life of 704 million years.
But how do we talk about time, and how do we know how old our rock formations are?
The geologic timescale is the most common way geologists organize and communicate major periods of the Earth’s past.
Fossils are contained within sedimentary rocks, which are rocks that are formed over time from the accumulation of sediment, such as in a lake, ocean basin, or river.
When those sediments become buried over time and eventually re-exposed, either in outcrops (exposed rocks) or road cuts, we can examine the rocks for its fossil assemblage.