The circulation of the atmosphere distributes this material over a wide region. Slowly, the ash settles back to the surface of the earth. The radioactive material in the ash layer can be dated directly. Ash flows also are a useful relative dating tool. Thus, for any one eruption, the composition of the ash layer is the same.
Researchers can identify the same volcanic event over the area of ash distribution.
If the ash layer has been dated in one area the age is good for the ash layer where ever it is found. Lava flows also are found in Antarctica, sometimes in association with glacial sediment. The lava flow contains radioactive elements that can be used to directly date the creation of the flow. This, in turn, provides information about the age of the glacial deposit. It usually is not useful to date radioactive elements from the minerals and rock fragments in sediment.