Igneous Textures

Gases are going to be in the magma. If you see bubbles, they will settle into porous vesicles in a process known as vesiculation. If the pressure is great enough, it will escape. If not, it will produce a frothy, light-weight stone. Vesiculation can occur if crystallization happens in the stone. It can also happen if there is sudden decompression of the magma. The latter is referred to as the first boiling. Crystallization explosivity is called second boiling. Thin magma explodes more readily than thicker magma. Mafic is low in crystals, thin, and more likely to just spill out of the earth. Felsic magma is so thick that, if any activity happens, it will be explosive.

IGNEOUS TEXTURES

Igneous rocks will have different textures. These are use to describe the rocks and classify them. The six textures you should know about are these:

• Aphanitic – this is rock that you see when the magma cools so quickly that they don't ever form large crystals. They are often extrusive rocks. You will see this texture in andesite, rhyolite, and basalt.

• Phaneritic – this means that the crystals are visible to some degree. You will be able to see them with the naked eye. Gabbro, diorite, and granite are all phaneritic stone.

• Glassy – this is also called vitreous. These happen when crystals can't occur and glass is made without crystals in them. Obsidian is an amorphous glassy black rock.

• Pegmatitic – this is rock that happens when minerals get very large – up to several meters in size. Figure 28 is pegmatitic because it’s basically a crystal of some kind without much basalt or gabbro stone: