Basalt

Basalt is a common gray to black volcanic rock. It is usually fine-grained due to rapid crystallization as lava on the Earth's surface. It may be porphyritic containing larger crystals in a fine matrix. Basalt in the tops of subaerial lava flows and cinder cones will often be highly vesiculated, imparting a lightweight "frothy" texture to the rock. The term basalt is at times applied to shallow intrusive rocks with a composition typical of basalt, but rocks of this composition with a phaneritic (coarse) groundmass should generally be referred to as diabase or gabbro. The crustal portions of oceanic tectonic plates are predominantly made of basalt.

Types of basalt

• Tholeiitic basalt is relatively rich in silica and poor in sodium. Included in this category are most basalts of the ocean floor, most large oceanic islands, and continental flood basalts such as the Columbia River Plateau. Pyroxene (augite and orthopyroxene or pigeonite), calcium rich plagioclase, and magnetite are common minerals. Contains interstitial quartz or tridymite plus minor olivine.
• Olivine tholeiite has augite and orthopyroxene or pigeonite with abundant olivine. Olivine may have rims of pyroxene.
• High alumina basalt has typically only augite with common olivine. Has greater than 17% alumina (Al₂O₃) and less titanium than tholeites.
• Alkaline basalt is relatively poor in silica and rich in sodium. It has augite, olivine, feldspathoids, and may have alkali feldspar and phlogopite.