In water-cooled lavas measured cooling rates range from 0.0035 to 0.5 K/min, and up to 25 K/min in pyroclastic obsidian. These cooling rates indicate that glass forms from initially erupted melt over timescales of minutes to days or even months. Intrusive igneous rocks form at some depth within the crust due to slow cooling of magma, resulting in formation of large crystals. Individual crystals can be seen with the naked eye. These rocks are termed texturally as coarsely crystalline or phaneritic.