Geologists classify igneous rocks based on both their texture (appearance) and mineral composition. The texture of the igneous rock helps to identify a sample and describe a specimen, but also provides clues to determine where it was formed. Geologists will first look at the size of the minerals in a sample. If they can see the minerals with their naked eye then it was formed inside the Earth’s crust. This texture is called phaneritic (coarse grained) when the minerals are large and visible. Rocks like granite or gabbro have a phaneritic texture. They cooled slowly, from magma taking up to tens of years for small plutons and hundreds of thousands to millions of years for large batholiths.

Phaneritic texture in a granite

Phaneritic texture in gabbro