It was only in the early part of the 20th century, when isotopic dating methods were first applied, that it became dating age in california to discover the absolute ages of the rocks containing fossils. In most cases, we cannot use isotopic techniques to directly date fossils or the sedimentary rocks they are found in, but we can constrain their ages by dating igneous rocks that cut across sedimentary rocks, or volcanic ash layers that lie within sedimentary layers.
Isotopic dating of rocks, or the minerals in them, is based on the fact that we know the decay rates of certain unstable isotopes of elements and that these rates have been constant over geological time.
Biotite and hornblende are also commonly used for K-Ar dating. An important assumption that we have to be able to make when using isotopic dating is that when the rock formed none of the daughter isotope was present e. If we dated a number of individual grains in the sedimentary rock, we would likely get a range of different dates, all older than the age of the rock.
Radiocarbon dating can be used on sediments or sedimentary rocks that contain carbon, but it cannot be used on materials older than about 60 ka. Using the decay curve shown on this graph, estimate the age of the rock.