Why can volcanic ash layers be useful for both relative and absolute dating?

Volcanic ash layers are highly valued in geochronology because they serve as both effective stratigraphic markers for relative dating and reliable sources for absolute dating.1

Utility for Relative Dating

Volcanic ash functions as a key bed or marker horizon because it is deposited nearly instantaneously across vast geographic areas. Because these layers settle over such wide regions, they allow geologists to correlate disparate sedimentary environments that might otherwise be difficult to link. By identifying and tracing these distinctive ash units, researchers can establish the relative temporal order of geologic events across different locations.123

Utility for Absolute Dating

Volcanic ash layers are also ideal for absolute dating because they often contain unaltered mineral crystals suitable for radiometric analysis. Geochronologists use techniques such as potassium-argon and argon-40–argon-39 dating to measure the radioactive decay of isotopes within these minerals. By providing precise numerical ages for these ash layers, scientists can calibrate the geologic time scale and determine the absolute age of adjacent fossil-bearing strata.1245