Igneous rocks are rocks formed when molten rock cools and hardens. The word comes from the Latin word for fire, which makes sense: igneous rocks are born from the fiery, melted rock deep inside the Earth.

When volcanoes erupt, they spew out molten rock called lava. When that lava cools and solidifies, it becomes igneous rock like basalt. Some igneous rocks form above ground this way, cooling quickly and forming small crystals. Other igneous rocks form deep underground, where molten rock called magma cools very slowly over thousands of years. This slow cooling creates larger crystals, producing rocks like granite, which you might see in kitchen countertops or monuments.

Igneous is one of the three main types of rock. The other two are sedimentary rocks (formed from layers of sediment pressed together over time) and metamorphic rocks (rocks that have been changed by heat and pressure). If you've ever held a piece of volcanic rock or seen the smooth, speckled surface of granite, you've touched igneous rock: stone that was once liquid fire deep in the Earth.