Basalt
Basalt is a common gray to black
volcanic rock. It is usually fine-grained due to rapid cooling of
lava on the Earth's surface. It may be
porphyritic containing larger crystals in a fine
matrix, or
vesicular, or frothy
scoria. Unweathered basalt is black, characterized by a preponderance of calcic
plagioclase feldspars and
pyroxene together with minor amounts of accessory
minerals such as
olivine.
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 or
gabbro. The crustal portions of
oceanic
tectonic plates are predominantly made of basalt.
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.
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 oxidised
iron from weathered iron-rich minerals such as
pyroxene.
‘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.
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 erupted in this way in the 19th century. Another example is the 0.2Ma
Diamond Head Tuff, Hawaii.
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 relic
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 accommodate shrinking in the vertical direction (the flow becomes less thick), it cannot easily accommodate shrinking in the horizontal direction unless cracks form. The extensive fracture network that develops results in the formation of columns. Because
hexagons fit together efficiently with no vacant space (a
tessellation), this is the most common pattern that develops.
Pentagonal,
heptagonal or
octagonal joint patterns are also known, but are less common. 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, and vice versa.
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Pillow basalts on the south Pacific seafloor |
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. 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. Size of individual pillows vary 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 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.
Glass or
obsidian may be present, particularly as rinds on rapidly chilled surfaces of lava flows, and is commonly (but not exclusively) associated with underwater eruptions.
The
lava flows of the
Deccan Traps in
India, the
Siberian Traps in
Russia, the
Columbia River Plateau of
Washington and
Oregon states 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.
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.
Ancient
precambrian basalts are usually only found in fold and thrust belts, and are often heavily metamorphosed. These are known as
greenstone belts.
Jade or
nephrite is a gem mineral found in metamorphosed basalts.
* Famous columnar basalts:
**
Giant's Causeway**
Devil's Postpile** Narooma Basalt,
Narooma, New South Wales,
Australia**
Samson's ribsPliny used the word basalt and it is said to have had an
Ethiopian origin, meaning a black stone.
The dark areas visible on Earth's
moon, the
lunar maria, are plains of basalt and
gabbro, and basalt Moon samples were brought to Earth by the
astronauts of the
Apollo program.
Lunar basalts occur in two forms;
* High
titanium (Ti) basalts, with 9-13%
TiO2, generally confined to the period 3.85 to 3.55 Giga-annum (Ga)
* Low-Ti lavas (1 to 5% TiO
2) are most common in the period 3.45-3.15 Ga (Meyer, 1987).
Lunar basalts show exotic textures and mineralogy, particularly shock
metamorphism, lack of the oxidation typical of terrestrial basalts, lack of hydration and peculiar geochemistry. Their short period of eruption is considered to be related to the swift cooling of the Moon's magma ocean after the satellite formed from a theorised impact with a
Mars-sized body.
*
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.
Pyroxene (
augite and orthopyroxene or
pigeonite), calcium-rich
plagioclase, and
magnetite are common minerals. Contains interstitial
quartz or
tridymite plus minor
olivine.
*
Olivine tholeiite has augite and orthopyroxene or pigeonite with abundant olivine. Olivine may have rims of pyroxene.
*
High alumina basalt has typically only augite with common olivine. Has greater than 17%
alumina (Al
2O
3) and less titanium than tholeites.
*
Alkaline basalt is relatively poor in silica and rich in sodium. It has augite, olivine,
feldspathoids, and may have
alkali feldspar and
phlogopite.
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;
*
Greenschist facies
*
Blueschist facies
*
Zeolite facies*
Granulite facies
*
Eclogite facies
Metamorphosed basalts are important hosts for a variety of hydrothermal
ore deposits, including
gold deposits,
nickel deposits,
volcanogenic massive sulfide ore deposits and others.
*
Basalt fiber*
Mafic rocks*
Volcanoes*
Igneous rocks*
Lava - water interface*
Pillow lava USGS*
Petrology of Lunar Rocks and Mare Basalts*
Basalt in Northern Ireland
