Become a backyard geologist—in one week

Igneous rocks are Earth’s original rocks. They form when magma (molten rock within the Earth) or lava (at the surface) cools to stone. The composition of the rock that melts and where that melt later cools determines the type of igneous rock that forms. 

To identify igneous rocks, start with a hand-size specimen. Look for interlocking crystals with no alignment or layering, just a jumble of minerals grown together. While this is a general rule of thumb for igneous rocks, the rocks found within a given category can vary dramatically in appearance.  

Let’s take a closer look at three specific igneous rocks.  

Where they’re found: Most igneous rocks form at tectonic plate boundaries, where volcanoes erupt lava and magma cools below the surface. But they can also be found far beyond these zones, exposed in mountain ranges or scattered across older parts of continents where erosion has revealed what once lay deep underground. 

Want to experience more wonder? National Geographic’s Wonder List features playful prompts and activities that turn everyday moments into wonder-filled discoveries—for families, anywhere, every day.