What are igneous rocks?
Geologists sometimes call igneous rocks primary rocks because they form from molten material that originates deep in the Earth's crust.
Igneous rocks can be distinguished from other rocks because they are made of a mosaic of mineral crystals, usually without layers. The crystals in igneous rocks are usually welded together.
Igneous rocks occur in many different structures, both underground (intrusive) and on the surface (extrusive).
Intrusive igneous rocks form underground in large masses called batholiths, and relatively small intrusions, for example, sills and dykes.
Extrusive igneous rocks build volcanoes. These may have large rocky craters or be mountains of ash and dust.
Igneous rocks are the best rocks for radiometric dating because the crystals in them formed at a definite time in the past. These crystals may not have altered since their formation, so an accurate date can often be obtained. Radiometric dating is most accurate if the rocks used are fresh and unweathered.