Basalt () is a common
extrusive volcanic rock. It is usually grey to black and
fine-grained due to rapid cooling of
lava at the surface of a planet. It may be
porphyritic containing larger crystals in a fine
matrix, or
vesicular, or frothy
scoria. Unweathered basalt is black or grey.
On Earth, most basalt
magmas have formed by
decompression melting of the
mantle. Basalt has also formed on Earth's
Moon,
Mars,
Venus, and even on the asteroid
Vesta. Source rocks for the partial melts probably include both
peridotite and
pyroxenite (e.g., Sobolev et al., 2007). The
crustal portions of
oceanic
tectonic plates are composed predominantly of basalt, produced from upwelling mantle below
ocean ridges.
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 are generally referred to as
diabase (also called dolerite) or
gabbro.
Vesicular basalt at
Sunset Crater, Arizona. US quarter for scale.
Types
Large masses must cool slowly to form a polygonal joint pattern.
- * MORB (Mid Ocean Ridge Basalt), 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 (slightly more enriched in incompatible elements).
- High alumina basalt may be silica-undersaturated or -oversaturated (see normative mineralogy). It has greater than 17% alumina (Al2O3) and is intermediate in composition between tholeiite and alkali basalt; the relatively alumina-rich composition is based on rocks without phenocrysts of plagioclase.
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 (
augite and
orthopyroxene or
pigeonite) 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 (liquidus) and near or below 1000 °C (solidus); 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 (e.g., Ozerov, 2000).
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% SiO
2, 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% Al
2O
3; boninites have magnesium contents of up to 15% MgO. Rare
feldspathoid-rich
mafic rocks, akin to alkali basalts, may have Na
2O + K
2O contents of 12% or more.
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 (Hofmann, 2003).
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 (normalized to atmospheric values), but to 15–24+ for ocean island basalts thought to be derived from
mantle plumes.
Morphology and textures
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 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 oxidized
iron from weathered iron-rich minerals such as
pyroxene.
‘A‘a types of blocky, cinder and breccia flows of thick, viscous basaltic
lava are common in Hawaii.
Pahoehoe 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 build up 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 cannot 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 often erroneously described as being predominantly hexagonal. In reality, the
mean number of sides of all the columns in such a structure is indeed six (by geometrical definition), but polygons with three to twelve or more sides can be observed. 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, while slow cooling is more likely to produce large columns.
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
polygonal columns and give the impression of having been artificially constructed.
An ancient 13th century religious complex, called
Nan Madol, was built on the Pacific island of
Pohnpei, using columnar basalt quarried from various locations on the island. The massive ruins remain to this day.
thumb|Columnar jointed basalt in Hong Kong, near Basalt Island and High Island Reservoir areas
thumb|S-shaped columnar basalt at the Hong Kong High Island Reservoirthumb|Basalt columns seen on Porto Santo Island.
- Notable columnar basalts:
- * Basalt Island area, Hong Kong; including High Island Reservoir area, Hong Kong SAR, China
Submarine eruptions
Pillow basalts on the south Pacific seafloor
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 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 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.
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 thus, the
oceanic crust) and seawater suggest that these interactions may lead to an application of
hydrothermal vents to the
origin of life.
Distribution
The
lava flows of the
Deccan Traps in
India, the
Chilcotin Group in
British Columbia,
Canada, the
Paraná Traps in Brazil, 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, the
Karoo flood basalt province in South Africa, the
Mount Etna in eastern
Sicily and the islands of the
Hawaii volcanic chain, forming above a
mantle plume. Basalt is the rock most typical of
large igneous provinces.
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 (present in the mineral
ilmenite), ranging from less than 1 wt% TiO
2, 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:
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. Chemical weathering of basalt minerals release
cations such as
calcium,
sodium and
magnesium, which give basaltic areas a strong
buffer capacity against
acidification. Calcium released by basalts bonds up
CO2 from the atmosphere forming
CaCO3 acting thus as a CO
2 trap. To this it must be added that the eruption of basalt itself may release huge quantities of CO
2 into the atmosphere.
See also
