Extrusive

When lava is above Earth's surface and a rock is formed, it is called extrusive igneous rock. When it is forming, it forms rather quickly. Sometimes it only takes a few hours. Since the rock forms so quickly, there is not enough time for big crystals to form. The crystals are usually very small. In fact, the crystals are so small you can't even see them!

Pumice

Obsidian