Here’s the cheat sheet. You don’t need a geology degree for this. If a rock has visible crystals that you can see without a magnifying glass, it probably cooled underground. That’s intrusive igneous rock.
If the rock looks glassy, dark, or full of tiny holes like Swiss cheese, it probably cooled on the surface. That's extrusive igneous rock.
Basically, the speed of cooling is everything. Slow = big crystals. Fast = smooth or bubbly.
Why it’s cooler than you think
Think about this: every time you walk on a granite sidewalk, you’re walking on something that was once hotter than a pizza oven. It was liquid fire that became solid under your feet.
And obsidian? Ancient humans used it to make razor-sharp knives and arrowheads. Even today, surgeons use obsidian scalpels because they can be sharper than steel. A rock from a volcano, used in an operating room. Wild, right?
So the next time you see a rocky cliff or a pebble on the ground, give it a second glance. It might have a story to tell, born in the deep, fiery heart of the Earth.
And now you know the definition. It’s not just a rock. It’s Earth’s coolest frozen moment.
