When they break and engulf chunks of sedimentary rocks, it's called stoping. The original rock layers around subsidence areas are called wall rocks and the layers that xenoliths came from are called parent rocks.
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.
Using the basic ideas of bracketing and radiometric dating, researchers have determined the age of rock layers all over the world.
This information has also helped determine the age of the Earth itself.
Stratigraphy is the study of sedimentary rock layers.
According to the law of superposition, as long as an area remains undeformed by outside forces, the deeper you go down through the layers of rock, the older they are.
Metamorphic rock is formed by great pressure far below the Earth's surface.
Scientists have also made improvements to the standard radiometric measurements.
The most widely known form of radiometric dating is carbon-14 dating.
This is what archaeologists use to determine the age of human-made artifacts. The half-life of carbon-14 is only 5,730 years, so carbon-14 dating is only effective on samples that are less than 50,000 years old.
For example, by using a laser, researchers can measure parent and daughter atoms in extremely small amounts of matter, making it possible to determine the age of very small samples [source: New Scientist].
Sedimentary rocks form from soil and silt carried and deposited by moving water.