Absolute dating vs relative dating fossils
Igneous rocks form from eruptions of lava or magma.
Metamorphic rock is formed by great pressure far below the Earth's surface.
The age of formations is marked on a geologic calendar known as the geologic time scale.
Development of the geologic time scale and dating of formations and rocks relies upon two fundamentally different ways of telling time: relative and absolute.
If a geologist claims to be younger than his or her co-worker, that is a relative age.
If a geologist claims to be 45 years old, that is an absolute age.
Dating the ash layers above and below a sedimentary rock layer to determine its age is called bracketing.
Radiometric dating uses the decay of unstable isotopes -- atoms with specific electrical charges -- to calculate something's age.
Absolute dating places events or rocks at a specific time.
Inclusions are useful at contacts with igneous rock bodies where magma moving upward through the crust has dislodged and engulfed pieces of the older surrounding rock.
Gaps in the geologic record, called unconformities, are common where deposition stopped and erosion removed the previously deposited material.
One way to find the age of a xenolith or subsidence area surrounded by volcanic debris is to correlate its layers with the layers of wall or parent rocks.
Stratigraphy is the study of sedimentary rock layers.