of a
thin section of basalt from
Bazaltove, Ukraine 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,
ulvöspinel, 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 tholeiitic basalt 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 tholeiitic basalts 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 disagreement over whether it is a
primary melt or derived from other basalt types by fractionation.
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 their
TAS classification. Basalt contains more silica than
picrobasalt and most
basanites and
tephrites but less than
basaltic andesite. Basalt has a lower total content of alkali oxides than
trachybasalt and most basanites and tephrites. Basalt generally has a composition of 45–52
wt% SiO2, 2–5 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 (MgO) contents of up to 15 percent. 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 (REE) 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. Mid-ocean ridge basalts (MORB) and their intrusive equivalents, gabbros, are the characteristic igneous rocks formed at mid-ocean ridges. They are tholeiitic basalts 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. Mid-ocean ridge basalts have been subdivided into varieties such as normal (NMORB) and those slightly more enriched in incompatible elements (EMORB).
Isotope ratios of
elements such as
strontium,
neodymium,
lead,
hafnium, and
osmium in basalts have been much studied to learn about the evolution of the
Earth's mantle. Isotopic ratios of
noble gases, such as 3
He/4He, are also of great value: for instance, ratios for basalts range from 6 to 10 for mid-ocean ridge tholeiitic basalt (normalized to atmospheric values), but to 15–24 and more for ocean-island basalts thought to be derived from
mantle plumes. Source rocks for the partial melts that produce basaltic magma 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—for example, whether into the sea, in an explosive
cinder eruption or as creeping
pāhoehoe 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.
Aā 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 pāhoehoe eruptions. Basaltic
tuff or
pyroclastic rocks are less common than basaltic lava flows. 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 in Northern Ireland basalt in
Turkey , Russia 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. These structures, or
basalt prisms, 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 The character of submarine basalt eruptions is largely determined by depth of water, since increased pressure restricts the release of volatile gases and results in effusive eruptions. It has been estimated that at depths greater than , explosive activity associated with basaltic magma is suppressed. Above this depth, submarine eruptions are often explosive, tending to produce
pyroclastic rock rather than basalt flows. These eruptions, described as Surtseyan, are characterised by large quantities of steam and gas and the creation of large amounts of
pumice.
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
pāhoehoe lava enters the sea it usually forms pillow basalts. However, when
aā enters the ocean it forms a
littoral cone, a small cone-shaped accumulation of tuffaceous debris formed when the blocky
aā 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 fluid, 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 pāhoehoe-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. == Distribution ==