Geologists love to overcomplicate things, but here’s the cheat sheet. Intrusive rocks form inside the Earth, from magma. They’re like the slow-cooker meals of geology—takes ages, but you get nice big crystal chunks.
Extrusive rocks form outside, from lava. These are the microwave dinners—fast, hot, and you end up with something fine-grained or even glassy. You’ve seen them as basalt cliffs or that weird pumice stone you use on your heels. (Come on, you know you’ve tried it.)
Here’s the funny part: both start as the same goopy, red-hot mess. The only difference is the cooling speed and location. You take magma, stick it underground for a million years—boom, granite. You spray it into the air as lava on a Tuesday—boom, you get scoria or that bubbly pumice that floats.
5.1.1 How do rocks form? – Earth & Space Science
Real-Life Signs You’re Looking at Igneous Rock
Next time you’re at a park or on a mountain, look for rocks with visible crystals—that’s a dead giveaway for intrusive rocks like diorite. If it’s smooth, dark, and almost glassy, like those black river pebbles, you’re likely holding a piece of extrusive basalt.
And if it’s full of holes? That’s from trapped gas bubbles escaping the lava as it cooled—like the rock version of fizzy soda going flat. Yes, rocks can be bubbly. I’m not making this up.
A fun test: take a piece of pumice. Drop it in water. It floats! That’s because it’s basically a frozen foam of lava. Try doing that with a piece of granite—it’ll sink like your motivation on a Monday morning.
