Why can layers of volcanic ash found between other rocks layers often serve as good geologic time markers?

Answer 1

Firstly, ash (pulverised rock; so only produced by explosive volcanic eruptions) is extremely fine material, and so can be suspended in the atmosphere for a very long time. Depending on prevailing wind direction, this means that the ash can be spread over a very wide geographical area, and in the case of the very large eruptions, the entire world. This is one reason why ash layers are very useful in determining worldwide sequence stratigraphy. Secondly, every volcano around the world has a unique chemical composition of magma, due to the nature of the processes of magmatic differentiation and assimilation, and can even vary from one eruption to another at the same volcano. This means the ash that is ejected from the volcano when it erupts also preserves this unique chemical composition in the rock record. By chemically analysing these layers by mass spectrometry, X-ray diffraction and other techniques, the layers can be identified as being the same (or different) deposits in different locations around the world where they may appear similar in either hand specimen or even when examined under a petrographic microscope. This allows these deposits to be correlated in different sequences around the world and can be used as a method for relative dating of rocks. Also, by analysing ash deposits close to volcanoes and comparing them to the layers found in the geological record around the world, these layers can be traced back to a specific region, volcano, or, in some cases, even specific eruptions, which can make them useful as a potential absolute dating technique.

Answer 2

nothing