Basalt

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Basalt

Basalt is a common gray to black volcanic rock. It is usually fine-grained due to rapid cooling of lava on the Earth's surface. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black, characterized by a preponderance of calcic plagioclase feldspars and pyroxene together with minor amounts of accessory minerals such as olivine.

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 dolerite or gabbro. The crustal portions of oceanic tectonic plates are predominantly made of basalt.

Morphology and textures

The shape, structure and texture of a basalt is diagnostic of the way it erupted and where it erupted - whether into the sea, in an explosive cinder eruption or as creeping pahoehoe lava flows, the classical image of Hawaiian basalt eruptions.

Subaerial eruptions

Basalt which erupts under open air (that is, subaerially) forms three distinct types of lava or volcanic deposits: scoria, ash or cinder; breccia and lava flows.

Basalt in the tops of subaerial lava flows and cinder cones will often be highly vesiculated, imparting a lightweight "frothy" texture to the rock. Basaltic cinders are often red, coloured by oxidised iron from weathered iron-rich minerals such as pyroxene.

‘A‘a types of blocky, cinder and breccia flows of thick, viscous basaltic magma are common in Hawaii and other basalts which erupt slightly cooler. Pahoehoe is a highly fluid, hot form of basalt which tends to form thin aprons of molten lava which fill up hollows and form lava lakes. Lava tubes are common features of pahoehoe eruptions.

Basaltic tuff or pyroclastic rocks are rare but not unknown. Usually basalt is too hot and fluid to build up sufficient pressure to form explosive lava eruptions but occasionally this will happen by trapping of the lava within the volcanic throat and build up of volcanic gases. Hawaii's Mauna Loa erupted in this way in the 19th century. Another example is the 0.2Ma Diamond Head Tuff, Hawaii. Maar volcanoes are typical of small basalt tuffs, formed by explosive eruption of basalt through the crust, forming an apron of mixed basalt and wall rock breccia and a fan of basalt tuff further out from the volcano.

Amygdaloidal structure is common in relic vesicles and beautifully crystallized species of zeolites, quartz or calcite are frequently found.

Columnar basalt is formed by cooling of a thick lava flow, which forms contractional joints or fractures. If a flow cools relatively rapidly, significant contraction forces build up. While a flow can accommodate shrinking in the vertical direction (the flow becomes less thick), it cannot easily accommodate shrinking in the horizontal direction unless cracks form. The extensive fracture network that develops results in the formation of columns. Because hexagons fit together efficiently with no vacant space (a tessellation), this is the most common pattern that develops. Pentagonal, heptagonal or octagonal joint patterns are also known, but are less common. Note that the size of the columns depends loosely on the rate of cooling; very rapid cooling may result in very small (<1 cm diameter) columns, and vice versa.

Submarine eruptions

Pillow Basalts
When basalt erupts underwater or flows into the sea, the cold water quenches the surface and the lava forms a distinctive pillow shape, through which the hot lava breaks to form another pillow. This pillow texture is very common in underwater basaltic flows and is 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 several metres.

When pahoehoe lava enters the sea it usually forms pillow basalts. However when a'a enters the ocean it forms a littoral cone, a small cone-shaped accumulation of tuffaceous debris formed when the blocky a'a lava enters the water and explodes from built-up steam.

The island of Surtsey in the Atlantic is a basalt volcano which breached the ocean surface in 1963. The initial phase of Surtsey's eruption was highly explosive, as the magma was quite wet, causing the rock to be blown apart by the boiling steam to form a tuff and cinder cone. This has subsequently moved to a typical pahoehoe type behaviour.

Glass or obsidian may be present, particularly as rinds on rapidly chilled surfaces of lava flows, and is commonly (but not exclusively) associated with underwater eruptions.

Distribution

The lava flows of the Deccan Traps in India, the Siberian Traps in Russia, 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. Other famous accumulations of basalts include Iceland and the islands of the Hawaii volcanic chain, forming above a mantle plume.

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.

Ancient precambrian basalts are usually only found in fold and thrust belts, and are often heavily metamorphosed. These are known as greenstone belts. Jade or nephrite is a gem mineral found in metamorphosed basalts.

Famous columnar basalts:
- Giant's Causeway
- Devil's Postpile
- Narooma Basalt, Narooma, New South Wales, Australia
- Samson's ribs

Pliny used the word basalt and it is said to have had an Ethiopian origin, meaning a black stone.

Lunar basalt

The dark areas visible on Earth's moon, the lunar maria, are plains of basalt and gabbro, and basalt Moon samples were brought to Earth by the astronauts of the Apollo program.

Lunar basalts occur in two forms;

High titanium (Ti) basalts, with 9-13% TiO₂, generally confined to the period 3.85 to 3.55 Giga-annum (Ga)
Low-Ti lavas (1 to 5% TiO₂) are most common in the period 3.45-3.15 Ga (Meyer, 1987).

Lunar basalts show exotic textures and mineralogy, particularly shock metamorphism, lack of the oxidation typical of terrestrial basalts, lack of hydration and peculiar geochemistry. Their short period of eruption is considered to be related to the swift cooling of the Moon's magma ocean after the satellite formed from a theorised impact with a Mars-sized body.

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.

Metamorphism

Basalts are important rocks within metamorphic belts, as they can provide vital information on the conditions of metamorphism within the belt. Various metamorphic facies are named after the mineral assemblages and rock types formed by subjecting basalts to the temperatures and pressures of the metamorphic event. These are;

Metamorphosed basalts are important hosts for a variety of hydrothermal ore deposits, including gold deposits, nickel deposits, volcanogenic massive sulfide ore deposits and others.

References

External links

Igneous rocks | Volcanology

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