The most common extrusive igneous rock, characterized by low silica content, and composed essentially of plagioclase feldspar and pyroxene. It is a dark, fine-grained rock, solidified from lava erupted from fissures or craters. Submarine basalts, extruded along mid-ocean ridges, form the oceanic crust. Subaerial eruptions produce extensive flows, the largest of which form the Deccan Plateau, India, and the ColumbiaSnake R Plateau, USA. It is commonly used as a building stone and road-stone aggregate.
For the cities, see Basalt, Colorado and Basalt, Idaho.Basalt (IPA: /ˈbasɒlt, bəˈsɒlt/) is a common gray to black volcanic rock.
Basalt magmas form by decompression melting of peridotite in the Earth's mantle, a process discussed in the entry for igneous rocks.
Pliny used the word basalt and it is said to have had an Ethiopian origin, meaning a black stone.
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 (also called diabase) or gabbro.
Types of basalt
Tholeiitic basalt is relatively poor 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. Boninite is a high-magnesium form of basalt or andesite that is erupted generally in back-arc basins, distinguished by its low titanium content and trace element composition.Petrology
The mineralogy of basalt is characterized by a preponderance of calcic plagioclase feldspar and pyroxene. Because of the presence of such oxide minerals, basalt can acquire strong magnetic signatures as it cools, and paleomagnetic studies have made extensive use of basalt.
In tholeiitic basalt, pyroxene (augite and orthopyroxene or pigeonite) and calcium-rich plagioclase are common phenocryst minerals.
In high-alumina basalts, phenocrysts of feldspar commonly are bytownite in composition.
Alkali basalts typically have mineral assemblages that lack orthopyroxene but contain olivine.
Basalt has high liquidus and solidus temperatures -- values at the Earth's surface are near or above 1200°C (liquidus) and near or below 1000°C (solidus); The origin of high-alumina basalt continues to be controversial, with interpretations that it is a primary melt and that instead it is derived from other basalt types (e.g., Ozerov, 2000).
Geochemistry
Basalt compositions are rich in MgO and CaO and low in SiO2 and Na2O plus K2O relative to most common igneous rocks, consistent with the TAS classification.
Basalt generally has a composition of 45-55 wt% SiO2, 2-6 wt% total alkalis, 0.5-2.0 wt% TiO2, 4-8 wt% MgO and 14 wt% or more Al2O3.
High alumina basalts have aluminium contents of 17-19 wt% Al2O3;
MORB basalt, a tholeiite particularly low in total alkalis and in incompatible trace elements, has a flat REE pattern normalised to mantle or chondrite values.
Isotope ratios of elements such as strontium, neodymium, lead, hafnium, and osmium in basalts have been much-studied, so as to learn about evolution of the Earth's mantle. Isotopic ratios of noble gases, such as He, are also of great value: for instance, ratios for basalts range from 6 to 10 for mid-ocean ridge tholeiite (normalized to atmospheric values), but to 15-24+ for ocean island basalts thought to be derived from mantle plumes.
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;
Basalt in the tops of subaerial lava flows and cinder cones will often be highly vesiculated, imparting a lightweight "frothy" texture to the rock.
‘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. 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.
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.
Columnar Basalt
During the cooling of a thick lava flow, contractional joints or fractures form.
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.
Famous columnar basalts: Giant's Causeway Devil's Postpile Narooma Basalt, Narooma, New South Wales, Australia Samson's ribs Isle of Staffa, Inner HebridesSubmarine 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.
When pahoehoe lava enters the sea it usually forms pillow basalts.
The island of Surtsey in the Atlantic is a basalt volcano which breached the ocean surface in 1963.
Distribution
The lava flows of the Deccan Traps in India, the Siberian Traps in Russia, the Columbia River Plateau of Washington and Oregon, as well as parts of the California inner coastal ranges 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.
Ancient precambrian basalts are usually only found in fold and thrust belts, and are often heavily metamorphosed.
Lunar basalt
The dark areas visible on Earth's moon, the lunar maria, are plains of flood basaltic lava flows.
Lunar basalts differ from their terrestrial counterparts principally in their high iron contents, which typically range from about 17 to 22 wt% FeO.
Lunar basalts show exotic textures and mineralogy, particularly shock metamorphism, lack of the oxidation typical of terrestrial basalts, and a complete lack of hydration. While most of the Moon's basalts erupted between about 3 and 3.5 billion years ago, the oldest samples are 4.2 billion years old, and the youngest flows, based on the age dating method of "crater counting," are estimated to have erupted only 1.2 billion years ago.
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.
Greenschist facies Blueschist facies Zeolite facies Granulite facies Eclogite faciesMetamorphosed basalts are important hosts for a variety of hydrothermal ore deposits, including gold deposits, copper deposits, volcanogenic massive sulfide ore deposits and others.
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