Now, here’s the key: igneous rocks only form when that melt cools and solidifies. This is where things get personal for the rock. If the magma cools slowly, deep underground, it forms large, visible crystals. Think of granite countertops—those speckles are crystals that had millions of years to grow.
If the lava cools fast, say, when it hits ocean water or air, you get tiny crystals. Sometimes, you get no crystals at all, just glass, like obsidian. Obsidian is basically volcanic glass—sharp enough to make a surgeon jealous. Lily’s porous rocks had air bubbles trapped in the rush, proving they cooled on the go.
Granite Rocks What Is Granite Rock And How Is It Formed Igneous Rocks
Two Flavors: Intrusive vs. Extrusive
Geologists split igneous rocks into two families based on where they cool. Intrusive rocks form inside the Earth, from magma. They’re slow-cooked, coarse-grained, and often look chunky. Extrusive rocks form on the surface, from lava. They’re quick-frozen, fine-grained, and sometimes full of holes from escaping gas—like a volcanic sponge.
Guess which one Lily brought home? You got it—extrusive, because it erupted. The deep stuff never sees sunlight unless erosion uncovers it. So, most volcanic rocks you walk on are the extroverts of the rock world.
