Sedimentary rock typically is layered, with the layers derived from different periods of sediment accumulation. Almost any place where the forces of erosion - or free online dating sites nsw crews - have carved through sedimentary rock is a good place to look for rock layers stacked up in the exposed rock face. These rock layers formed from sediments deposited in a lake.
Photo courtesy of Rod Benson, www. When you look at a layer cake, you know that the layer at the bottom was the first one the baker put on the plate, and the upper ones were added later. In the same way, geologists figure out the relative ages of fossils and sedimentary rock layers; rock layers, and the dating a man while he is going through a divorce they contain, toward the bottom of a stack of sediments are older than those found higher in the stack.
Correlations are especially reliable when they include index fossils, which are fossils with a limited age range. Good index fossils are easily recognized and widely distributed around the world, so that they aid both local and international correlations. Absolute dating complements relative dating by providing a specific not necessarily precise chronological age for a given specimen, such as "50 million years before present. These methods are based on the known, regular decay of certain radioactive elements isotopes into other isotopes or "daughter products.
Many people have heard of the Carbon 14 method and assume that fossils are dated by it. Actually, in most cases the C method is useful on organic material less than about 50, years old, which includes many human remains and artifacts, but excludes most different types of fossil dating. Absolute dating of fossils requires other dating methods such as the potassium-argon or rubidium-strontium methods, which involve isotopes with slower decay rates longer "half-lives".
For example if you have a fossil trilobite and it was found in different types of fossil dating Wheeler Formation. The Wheeler Formation has been previously dated to approximately million year old, so we know the trilobite is also about million years old. Scientists can use certain types of fossils referred to as index fossils to assist in relative dating via correlation.
Index fossils are fossils that are known to only occur within a very specific age range. Typically commonly occurring fossils that had a widespread geographic distribution such as brachiopods, trilobites, and ammonites work best as index fossils. If the fossil you are trying to date occurs alongside one of these index fossils, then the fossil you are dating must fall into the age range of the index fossil. Sometimes multiple index fossils can be used. In a hypothetical example, a rock formation contains fossils of a type of brachiopod known to occur between and million years.
Different types of fossil dating same rock formation also contains a type of trilobite that was known to live to million years ago. Since the rock formation contains both types of fossils the ago of the rock formation must be in the overlapping date range of to million years. Studying the layers of rock or strata can also be useful.