Basalt facts for kids

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BasaltUSGOV
Basalt
Giants causeway closeup
Large masses cool slowly to form a polygonal joint pattern.

Basalt is a common grey to black extrusive volcanic rock. It is usually fine-grained because of its rapid cooling of lava on the Earth's surface.

Unweathered basalt is black or grey. Basalt blocks are often used for roads and pavements (sidewalks). It is extremely hard-wearing.

On Earth, most basalt magmas have formed by melting of the mantle. Basalt has also formed on Earth's Moon, Mars, Venus, and even on the asteroid Vesta.

The crustal portions of oceanic tectonic plates are composed mostly of basalt, produced from upwelling mantle below mid-ocean ridges.

Basalt can be made into fiber. Basalt fiber is often used as a safe replacement for asbestos.

Morphology and textures

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An active basalt lava flow

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

Subaerial eruptions

Basalt that 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 oxidized iron from weathered iron-rich minerals such as pyroxene.

ʻAʻā types of blocky, cinder and breccia flows of thick, viscous basaltic lava are common in Hawaii. Pāhoehoe is a highly fluid, hot form of basalt which tends to form thin aprons of molten lava which fill up hollows and sometimes forms 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 buildup of volcanic gases. Hawaii's Mauna Loa volcano erupted in this way in the 19th century, as did Mount Tarawera, New Zealand in its violent 1886 eruption. 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 relict vesicles and beautifully crystallized species of zeolites, quartz or calcite are frequently found.

Columnar basalt
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Columnar jointed basalt in Turkey
See also: List of places with columnar basalt

During the cooling of a thick lava flow, contractional joints or fractures form. If a flow cools relatively rapidly, significant contraction forces build up. While a flow can shrink in the vertical dimension without fracturing, it can't easily accommodate shrinking in the horizontal direction unless cracks form; the extensive fracture network that develops results in the formation of columns. The topology of the lateral shapes of these columns can broadly be classed as a random cellular network. These structures are predominantly hexagonal in cross-section, but polygons with three to twelve or more sides can be observed. The size of the columns depends loosely on the rate of cooling; very rapid cooling may result in very small (<1 cm diameter) columns, while slow cooling is more likely to produce large columns.

Submarine eruptions

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Pillow basalts on the south Pacific seafloor
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Outcrop of a pillow basalt, Italy
Pillow basalts

When basalt erupts underwater or flows into the sea, contact with the 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. The size of individual pillows varies from 10 cm up to several meters.

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 Ocean 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.

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

Pillow basalt is also produced by some subglacial volcanic eruptions.

Life on basaltic rocks

The common corrosion features of underwater volcanic basalt suggest that microbial activity may play a significant role in the chemical exchange between basaltic rocks and seawater. The significant amounts of reduced iron, Fe(II), and manganese, Mn(II), present in basaltic rocks provide potential energy sources for bacteria. Some Fe(II)-oxidizing bacteria cultured from iron-sulfide surfaces are also able to grow with basaltic rock as a source of Fe(II). Fe- and Mn- oxidizing bacteria have been cultured from weathered submarine basalts of Loihi Seamount. The impact of bacteria on altering the chemical composition of basaltic glass (and thus, the oceanic crust) and seawater suggest that these interactions may lead to an application of hydrothermal vents to the origin of life.

Distribution

Parana traps
Paraná Traps, Brazil

Basalt is one of the most common rock types in the world. Basalt is the rock most typical of large igneous provinces. The largest occurrences of basalt are in the ocean floor that is almost completely made up by basalt. Above sea level basalt is common in hotspot islands and around volcanic arcs, specially those on thin crust. However, the largest volumes of basalt on land correspond to continental flood basalts. Continental flood basalts are known to exist in the Deccan Traps in India, the Chilcotin Group in British Columbia, Canada, the Paraná Traps in Brazil, the Siberian Traps in Russia, the Karoo flood basalt province in South Africa, the Columbia River Plateau of Washington and Oregon.

Many archipelagoes and island nations have an overwhelming majority of its exposed bedrock made up by basalt due to being above hotspots, for example, Iceland and Hawaii.

Ancient Precambrian basalts are usually only found in fold and thrust belts, and are often heavily metamorphosed. These are known as greenstone belts, because low-grade metamorphism of basalt produces chlorite, actinolite, epidote and other green minerals.

Lunar and Martian basalt

Lunar Olivine Basalt 15555 from Apollo 15 in National Museum of Natural History
Lunar olivine basalt collected by Apollo 15.

The dark areas visible on Earth's moon, the lunar maria, are plains of flood basaltic lava flows. These rocks were sampled by the manned American Apollo program, the robotic Russian Luna program, and are represented among the lunar meteorites.

Lunar basalts differ from their terrestrial counterparts principally in their high iron contents, which typically range from about 17 to 22 wt% FeO. They also possess a wide range of titanium concentrations (present in the mineral ilmenite), ranging from less than 1 wt% TiO2, to about 13 wt.%. Traditionally, lunar basalts have been classified according to their titanium content, with classes being named high-Ti, low-Ti, and very-low-Ti. Nevertheless, global geochemical maps of titanium obtained from the Clementine mission demonstrate that the lunar maria possess a continuum of titanium concentrations, and that the highest concentrations are the least abundant.

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.

Basalt is also a common rock on the surface of Mars, as determined by data sent back from the planet's surface, and by Martian meteorites.

Alteration of basalt

Metamorphism

Basalt structures
Basalt structures in Namibia

Basalts are important rocks within metamorphic belts, as they can provide vital information on the conditions of metamorphism within the belt.

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

Uses

Basalt is used in construction (e.g. as building blocks or in the groundwork), making cobblestones (from columnar basalt) and in making statues. Heating and extruding basalt yields stone wool, said to be an excellent thermal insulator.


Basalt Facts for Kids. Kiddle Encyclopedia.