Relative dating of rock strata
The Auckland Islands lie approximately 375km south of Stewart Island on the Campbell Plateau.These islands were formed by multiple geological events.With this in mind geologist have long known that the deeper a sedimentary rock layer is the older it is, but how old?Although there might be some mineral differences due to the difference in source rock, most sedimentary rock deposited year after year look very similar to one another.However determining the actual, or absolute, age of strata (for example, 3.5 million years old) is often difficult since the age of a fossil cannot be determined directly.The most useful tool in dating strata is radiometric dating of materials.The correlational studies described so far allow scientists to estimate the relative ages of strata.If stratum B lies above stratum A, B is the younger of the two.
Relative time can not determine the actual year a material was deposited or how long deposition lasted; it simply tell us which events came first.
This process is called stratigraphy, and a branch of it, called sequenced stratigraphy seeks to determine the order or sequence in time in which rocks were laid in the available space –or accommodation in geologist terms- The principles stated before are Superposition, Faunal succession, Crosscutting relationships, and Inclusions.
It is common that geologists come across gaps in the geologic record, for which they use “index fossils” to define and identify geologic periods, which helps relatively date rocks ages.
This means that a quartz sandstone deposited 500 million years ago will look very similar to a quartz sandstone deposited 50 years ago.
Making this processes even more difficult is the fact that due to plate tectonics some rock layers have been uplifted into mountains and eroded while others have subsided to form basins and be buried by younger sediments.