Glassy

Obsidian

Natural glass produced by the cooling of molten lava too quickly to permit crystallization (view)

Frothy

Pumice

A light, porous, glassy lava (view)

Scoria

Porous cinderlike fragments of dark lava (view

Aphanitic

(fine grain) too small to see without magnification

Rhyolite

A fine-grained extrusive volcanic rock, similar to granite in composition and usually exhibiting flow lines (view)

Andesite

A gray, fine-grained volcanic rock. Andesites contain crystals composed primarily of plagioclase feldspar and one or more of the minerals pyroxene (clinopyroxene and orthopyroxene) and lesser amounts of hornblende. At the lower end of the silica range, andesite lava may also contain olivine. Assoc. with composite volcanoes. (view)

Basalt

 A hard, dense, dark volcanic rock composed chiefly of plagioclase, pyroxene, and olivine, and often having a glassy appearance.

assoc with ocean crust and shield volcanos (view)

Komatiite

 Intermediate

Dacite

A light gray volcanic rock containing a mixture of plagioclase and other crystalline minerals in glassy silica, similar in appearance to rhyolite

Diabase

A dark-gray to black, fine-textured igneous rock composed mainly of feldspar and pyroxene and used for monuments and as crushed stone.

Phaneritic

coarse grain

Granite

A common, coarse-grained, light-colored, hard igneous rock consisting chiefly of quartz, orthoclase or microcline, and mica (view)

Diorite

Any of various dark, granite-textured, crystalline rocks rich in plagioclase and having little quartz. (view)

Gabbro

A usually coarse-grained igneous rock composed chiefly of calcic plagioclase and pyroxene. (view)

Peridotite

Any of a group of igneous rocks composed mainly of olivine and various pyroxenes and having a granitelike texture. (view)