Intrusive Rocks.

intrusive rocks harden slowly beneath the surface of the earth, and often form large mineral crystals within the rock. Granite is a good example of an intrusive rock.Porphyritic intrusive rocks have large crystals embedded in a matrix of smaller crystals. Pegmatite is the only porphyritic rock on the Science Olympiad list.Extrusive rocks harden quickly during a volcanic eruption and are usually smooth-grained. Basalt is the most common form of extrusive rock.
|IgneousName |Classification|Description |
|Andesite |Extrusive |Usually blackish-brown, sometimes greenish. Papier-mache look.Contains less than 5% quartz. |
|Basalt |Extrusive |Very dark, often black. Often contains phenocrysts of feldspars, olivine, and other dark minerals |
|Diorite |Intrusive |Dark gray to blackish gray, mottled. Evenly speckled with dark and light minerals, salt-and-peppery look.|
|Gabbro |Intrusive |Gray or light green, very coarse-grained. |
|Granite |Intrusive |Crystals of feldspar (pink or red), mica (dark brown or black), and quartz (clear pink, white, or |
| | |black).Coarse-grained. |
|Obsidian |Extrusive |Shiny black.Volcanic glass, has a conchoidal fracture (see explanation of cleavage and fracture below) Be|
| | |careful |
|Pegmatite |Intrusive |Same composition as granite but has very large, usually light crystals. |
|Pumice |Extrusive |Very light gray. Also volcanic glass, but very light and bubbly. Only rock that floats. |