Basalt is a common extrusive igneous rock formed from the rapid cooling of basaltic
lava exposed at or very near the surface of a planet or moon. Flood basalt describes the
formation in a series of lava basalt flows.
Definition
By definition, basalt is an aphanitic igneous rock with less than 20% quartz and less than
10% feldspathoid by volume, and where at least 65% of the feldspar is in the form of plagioclase.
Basalt is usually grey to black in colour, but rapidly weathers to brown or rust-red
due to oxidation of its mafic minerals into rust. Although usually characterized as "dark",
basaltic rocks exhibit a wide range of shading due to regional geochemical processes; indeed
some basalts are quite light coloured, in some cases superficially resembling rhyolite
to untrained eyes. Basalt almost always has a fine-grained mineral texture due to the
molten rock cooling too quickly for large mineral crystals to grow, although it can
sometimes be porphyritic, containing the larger crystals formed prior to the extrusion that
brought the lava to the surface, embedded in a finer-grained matrix.
Basalt with a vesicular or frothy texture is called scoria, and forms when dissolved
gases are forced out of solution and form vesicles as the lava decompresses as it reaches
the surface. 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 groundmass are generally referred to as diabase or gabbro.
In the Hadean and Archean eras of Earth's history the chemistry of erupted basalts was
significantly different from today's, due to crustal and asthenosphere differentiation
issues—so much so that there is an alternate name for this kind of basalt.
Etymology The word "basalt" is ultimately derived from
Late Latin basaltes, misspelling of L. basanites "very hard stone," which was imported from
Ancient Greek βασανίτης, from βάσανος and originated in Egyptian bauhun "slate".
The modern petrological term basalt describing a particular composition of lava-derived rock
originates from its use by Georgius Agricola in 1556 in his famous work of mining and mineralogy
De re metallica, libri XII. Agricola applied "basalt" to the volcanic black rock of the
Schloßberg at Stolpen, believing it to be the same as Pliny the Elder's "very hard stone".
Types
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. MORB, is characteristically low in incompatible
elements. MORB is commonly erupted only at ocean ridges. MORB itself has been subdivided
into varieties such as NMORB and EMORB. High alumina basalt may be silica-undersaturated
or -oversaturated. It has greater than 17% alumina and is intermediate in composition
between tholeiite and alkali basalt; the relatively alumina-rich composition is based on rocks
without phenocrysts of plagioclase. Alkali basalt is relatively poor in silica
and rich in sodium. It is silica-undersaturated and may contain feldspathoids, alkali feldspar
and phlogopite. Boninite is a high-magnesium form of basalt
that is erupted generally in back-arc basins, distinguished by its low titanium content
and trace element composition. Occurrence
On Earth, most basalt magmas have formed by decompression melting of the mantle. Basalt
commonly erupts on Io, the third largest moon of Jupiter, and has also formed on Earth's
Moon, Mars, Venus, and the asteroid Vesta. The crustal portions of oceanic tectonic plates
are composed predominantly of basalt, produced from upwelling mantle below, the ocean ridges.
Uses Basalt is used in construction, making cobblestones
and in making statues. Heating and extruding basalt yields stone wool, said to be an excellent
thermal insulator. Petrology
The mineralogy of basalt is characterized by a preponderance of calcic plagioclase feldspar
and pyroxene. Olivine can also be a significant constituent. Accessory minerals present in
relatively minor amounts include iron oxides and iron-titanium oxides, such as magnetite,
ulvospinel, and ilmenite. 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 and calcium-rich
plagioclase are common phenocryst minerals. Olivine may also be a phenocryst, and when
present, may have rims of pigeonite. The groundmass contains interstitial quartz or tridymite
or cristobalite. Olivine tholeiite has augite and orthopyroxene or pigeonite with abundant
olivine, but olivine may have rims of pyroxene and is unlikely to be present in the groundmass.
Alkali basalts typically have mineral assemblages that lack orthopyroxene but contain olivine.
Feldspar phenocrysts typically are labradorite to andesine in composition. Augite is rich
in titanium compared to augite in tholeiitic basalt. Minerals such as alkali feldspar,
leucite, nepheline, sodalite, phlogopite mica, and apatite may be present in the groundmass.
Basalt has high liquidus and solidus temperatures—values at the Earth's surface are near or above 1200 °C
and near or below 1000 °C; these values are higher than those of other common igneous
rocks. The majority of tholeiites are formed at approximately
50–100 km depth within the mantle. Many alkali basalts may be formed at greater depths,
perhaps as deep as 150–200 km. 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. Geochemistry
Relative to most common igneous rocks, basalt compositions are rich in MgO and CaO and low
in SiO2 and the alkali oxides, i.e., Na2O + K2O, 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, 5–14 wt% FeO and 14 wt% or more Al2O3. Contents of CaO are commonly near 10
wt%, those of MgO commonly in the range 5 to 12 wt%.
High alumina basalts have aluminium contents of 17–19 wt% Al2O3; boninites have magnesium
contents of up to 15% MgO. Rare feldspathoid-rich mafic rocks, akin to alkali basalts, may have
Na2O + K2O contents of 12% or more. The abundances of the Lanthanide or rare earth
elements can be a useful diagnostic tool to help explain the history of mineral crystallisation
as the melt cooled. In particular, the relative abundance of europium compared to the other
REE is often markedly higher or lower, and called the europium anomaly. It arises because
Eu2+ can substitute for Ca2+ in plagioclase feldspar, unlike any of the other Lanthanides,
which tend to only form 3+ cations. MORB basalts and their intrusive equivalents,
gabbros, are the characteristic igneous rocks formed at mid-ocean ridges. They are tholeiites
particularly low in total alkalis and in incompatible trace elements, and they have relatively flat
REE patterns normalized to mantle or chondrite values. In contrast, alkali basalts have normalized
patterns highly enriched in the light REE, and with greater abundances of the REE and
of other incompatible elements. Because MORB basalt is considered a key to understanding
plate tectonics, its compositions have been much studied. Although MORB compositions are
distinctive relative to average compositions of basalts erupted in other environments,
they are not uniform. For instance, compositions change with position along the Mid-Atlantic
ridge, and the compositions also define different ranges in different ocean basins.
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 3He/4He, are also of great value: for instance, ratios
for basalts range from 6 to 10 for mid-ocean ridge tholeiite, but to 15-24+ for ocean island
basalts thought to be derived from mantle plumes.
Source rocks for the partial melts probably include both peridotite and pyroxenite.
Morphology and textures
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 which erupts under open air forms three distinct types of lava or volcanic deposits:
scoria; ash or cinder; 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
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 columns, while slow cooling is more likely to produce large columns.
Submarine eruptions
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 associated with underwater 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. Recent research has shown that some Fe(II)-oxidizing bacteria cultured from iron-sulfide
surfaces are also able to grow with basaltic rock as a source of Fe(II). In recent work
at Loihi Seamount, Fe- and Mn- oxidizing bacteria have been cultured from weathered basalts.
The impact of bacteria on altering the chemical composition of basaltic glass and seawater
suggest that these interactions may lead to an application of hydrothermal vents to the
origin of life. Distribution
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
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 stunning
range of titanium concentrations, 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 possesses 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
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:
Blueschist facies Eclogite facies
Granulite facies Greenschist facies
Zeolite facies Metamorphosed basalts are important hosts
for a variety of hydrothermal ore deposits, including gold deposits, copper deposits,
volcanogenic massive sulfide ore deposits and others.
Weathering
Compared to other rocks found on Earth's surface, basalts weather relatively fast. The typically
iron-rich minerals oxidise rapidly in water and air, staining the rock a brown to red
colour due to iron oxide. Chemical weathering also releases readily water-soluble cations
such as calcium, sodium and magnesium, which give basaltic areas a strong buffer capacity
against acidification. Calcium released by basalts binds up CO2 from the atmosphere forming
CaCO3 acting thus as a CO2 trap. To this it must be added that the eruption of basalt
itself is often associated with the release of large quantities of CO2 into the atmosphere
from volcanic gases. Carbon sequestration in basalt has been studied
as a means of removing carbon dioxide, produced by human industrialization, from the atmosphere.
Underwater basalt deposits, scattered in seas around the globe, have the added benefit of
the water serving as a barrier to the re-release of CO2 into the atmosphere.
See also Basalt fiber
Flood basalt Igneous rocks
Mafic rocks Volcanoes
References
External links Basalt Columns
Basalt in Northern Ireland Lava–water interface
Petrology of Lunar Rocks and Mare Basalts Pillow lava USGS