Granite is a well-known type of plutonic igneous rock, but there are many others as well.Geologists distinguish these types of rock based on their chemical and mineralogical composition.
Plutonic rocks on the other hand cool very slowly, on the order of a million years or more for some deeply buried and insulated magmas.
The mineral grains in these rocks can grow very large and are readily distinguished in hand samples.
In metamorphic rocks, new minerals form that are more stable at higher temperatures and pressures.
Sometimes the minerals segregate into distinct bands.
The boxes on the next few pages present a brief tutorial in rocks, minerals, and radioactivity.
Certain minerals, such as zircon and monazite, which form as common trace constituents in igneous rocks, have crystal structures which can accommodate varying amounts of the naturally occurring radioactive elements, uranium and thorium.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.Sedimentary rocks are secondary in formation, being the product of precursor rocks (of any type).Igneous rocks form from molten material, and are further subdivided into two main categories, the volcanic rocks which form from lava extruded at or near the surface; and plutonic rocks which form from magma, deep within the crust.Both types of igneous rocks comprise a mixture of different minerals.