We can learn the most about where and how an igneous rock formed by studying its

Igneous rock (derived from the Latin word ignis meaning fire) is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or lava. Igneous rock may form with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. Igneous rock can be found on the surface in areas of current or past volcanism or in uplifted and eroded areas of past plutonic intrusions, deep underground nearly anywhere, or in areas of past or present glaciation, where igneous rock has been eroded and deposited.