Potassium isotope dating
When burial pressures and temperatures get too great, the rocks melt completely, becoming new igneous rocks.
As igneous rocks cool, mineral crystals form following a specific sequence.
Metamorphic rocks represent alterations of precursor sedimentary, igneous, or other metamorphic rocks.
Through the cycles of burial, folding, faulting, and subduction of crustal plates, rocks get pushed and dragged down to depths where - under heat and pressure - changes take place.
Discoloration haloes in younger rocks tend to be smaller and less intense than in older rocks, indicating that the zone of crystal damage increases with time.
From these observations early attempts were made to use the dimensions of haloes as an age dating technique.
Granite is a well-known type of plutonic igneous rock, but there are many others as well.