How are the textures of extrusive and intrusive igneous rocks different?

Answer 1

Extrusive igneous rocks have come to the surface when cooling down. Intrusive igneous rocks stay within the earth. This difference is important.

When a magma comes to the surface, it's called a lava. On the surface, the lava will be subject to cooling by wind and water. This will happen fast. When a magma stays inside the earth, it will cool down considerably slower, because it has to radiate it's heat to the rest of the earth. This doesn't go fast because while water and air can flow, rock can't.

The texture of an igneous rock has everything to do with the cooling speed. As a magma or a lava cools, it forms crystals. When the cooling happens relatively fast, these crystals do not have a lot of time to grow together, so you will get a lot of small crystals. When the cooling is very slow, the crystals will have more time to grow, and the crystal size will be a lot bigger.

Take for instance granite. If you've ever seen it, you know you can see the crystals with the naked eye, because they are very large. Basalt, on the other hand has a very fine structure, you can't see the crystals without magnifying equipment like a microscope.

So there you have it: an extrusive igneous rock will have a very fine texture, with small crystal sizes, an intrusive igneous rock will have a very coarse texture, with large crystal sizes.