Basalt
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Image:BasaltUSGOV.jpg 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.
Unweathered basalt is frequently black to greenish-black in color, characterized by a preponderance of calcic plagioclase feldspars and pyroxene together with minor amounts of accessory minerals such as olivine. Basaltic cinders are often red. Glass may be present, particularly as rinds on rapidly chilled surfaces of lava flows, and is commonly (but not exclusively) associated with underwater eruptions. Amygdaloidal structure is common in relic vesicles and beautifully crystallized species of zeolites, quartz or calcite are frequently found.
Image:Columnar basalt at Sheepeater Cliff in Yellowstone-closeup-300px.JPG Image:Pillow basalt crop l.jpg
The lava flows of the Deccan Traps in India, the Columbia River Plateau of Washington and Oregon states in the United States, as well as the Triassic lavas of eastern North America are basalts. Perhaps the most famous basalt flow in the world is the Giant's Causeway on the northern coast of Ireland, in which the vertical joints form hexagonal columns and give the impression of having been artificially constructed.
Pliny used the word basalt and it is said to have had an Ethiopian origin, meaning a black stone.
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Pillow lava
Image:Boyabat.jpg When basalt erupts underwater or flows into the sea, the cold water quenches the surface and the lava forms a distinctive pillow shape, then the hot lava breaks through to form another pillow. This pillow texture is very common in underwater basaltic flows and pillow basalts are diagnostic of an underwater eruption environment when found in ancient rocks. Pillows typically consist of a fine-grained core with a glassy crust and have radial jointing. Size of individual pillows vary from 10 cm up to 6 m.
Lunar basalt
The dark areas visible on Earth's moon, the lunar mares, are plains of basalt and gabbro, and basalt Moon samples were brought to Earth by the astronauts of the Apollo program.
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.
References
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See also
- Basalt fiber
- Famous columnal basalts:
