Pillow lava
Encyclopedia
Lava
Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...
s that contain characteristic pillow-shaped structures that are attributed to the extrusion of the lava under water, or subaqueous extrusion. Pillow lavas in volcanic rock
Volcanic rock
Volcanic rock is a rock formed from magma erupted from a volcano. In other words, it is an igneous rock of volcanic origin...
are characterized by thick sequences of discontinuous pillow-shaped masses, commonly up to one metre in diameter. They form the upper part of 'Layer 2' of normal oceanic crust
Oceanic crust
Oceanic crust is the part of Earth's lithosphere that surfaces in the ocean basins. Oceanic crust is primarily composed of mafic rocks, or sima, which is rich in iron and magnesium...
.
Composition
Pillow lavas are commonly of basaltBasalt
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...
ic composition, although pillows formed of komatiite
Komatiite
Komatiite is a type of ultramafic mantle-derived volcanic rock. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content...
, picrite, boninite
Boninite
Boninite is a mafic extrusive rock high in both magnesium and silica, formed in fore-arc environments, typically during the early stages of subduction. The rock is named for its occurrence in the Izu-Bonin arc south of Japan...
, basaltic andesite
Basaltic andesite
Basaltic andesite is a black volcanic rock containing about 55% silica. Minerals in basaltic andesite include olivine, augite and plagioclase. Basaltic andesite can be found in volcanoes around the world, including in Central America and the Andes of South America. Basaltic andesite is common in...
, andesite
Andesite
Andesite is an extrusive igneous, volcanic rock, of intermediate composition, with aphanitic to porphyritic texture. In a general sense, it is the intermediate type between basalt and dacite. The mineral assemblage is typically dominated by plagioclase plus pyroxene and/or hornblende. Magnetite,...
or even dacite
Dacite
Dacite is an igneous, volcanic rock. It has an aphanitic to porphyritic texture and is intermediate in composition between andesite and rhyolite. The relative proportions of feldspars and quartz in dacite, and in many other volcanic rocks, are illustrated in the QAPF diagram...
are known. In general the more intermediate the composition, the larger the pillows, due to the increase in viscosity
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
of the erupting lava.
Occurrence
They occur wherever mafic to intermediate lavas are extruded under water, such as along marine hotspotHotspot (geology)
The places known as hotspots or hot spots in geology are volcanic regions thought to be fed by underlying mantle that is anomalously hot compared with the mantle elsewhere. They may be on, near to, or far from tectonic plate boundaries. There are two hypotheses to explain them...
volcano
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...
chains and the constructive plate boundaries
Divergent boundary
In plate tectonics, a divergent boundary or divergent plate boundary is a linear feature that exists between two tectonic plates that are moving away from each other. Divergent boundaries within continents initially produce rifts which produce rift valleys...
of mid-ocean ridge
Mid-ocean ridge
A mid-ocean ridge is a general term for an underwater mountain system that consists of various mountain ranges , typically having a valley known as a rift running along its spine, formed by plate tectonics. This type of oceanic ridge is characteristic of what is known as an oceanic spreading...
s. As new oceanic crust is formed, thick sequences of pillow lavas are erupted at the spreading center fed by dykes
Dike (geology)
A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...
from the underlying magma chamber
Magma chamber
A magma chamber is a large underground pool of molten rock found beneath the surface of the Earth. The molten rock in such a chamber is under great pressure, and given enough time, that pressure can gradually fracture the rock around it creating outlets for the magma...
. Pillow lavas and the related sheeted dyke complex
Sheeted dyke complex
A sheeted dyke complex or sheeted dike complex is a normal component of an ophiolite, a piece of oceanic crust that has been emplaced within a sequence of continental rocks. The dykes act as feeders for the overlying sequence of extrusive rocks typically pillow lavas, together forming layer 2 of...
es form part of a classic ophiolite sequence when a segment of oceanic crust is obducted
Obduction
Obduction is the overthrusting of continental crust by oceanic crust or mantle rocks at a convergent plate boundary. It can occur during an orogeny, or mountain-building episode....
onto continental crust
Continental crust
The continental crust is the layer of igneous, sedimentary, and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial due to more felsic, or granitic, bulk composition, which lies in...
.
The presence of pillow lavas in the oldest preserved volcanic sequences on earth, the Isua
Isua greenstone belt
The Isua Greenstone Belt is an Archean greenstone belt in southwestern Greenland. The belt is aged between 3.7 and 3.8 Ga, making it among the oldest rock in the world. The belt contains variably metamorphosed mafic volcanic and sedimentary rocks...
and Barberton greenstone belt
Greenstone belt
Greenstone belts are zones of variably metamorphosed mafic to ultramafic volcanic sequences with associated sedimentary rocks that occur within Archaean and Proterozoic cratons between granite and gneiss bodies....
s, confirms the presence of large bodies of water on the Earth's surface early in the Archean
Archean
The Archean , also spelled Archeozoic or Archæozoic) is a geologic eon before the Paleoproterozoic Era of the Proterozoic Eon, before 2.5 Ga ago. Instead of being based on stratigraphy, this date is defined chronometrically...
. Pillow lavas are used generally to confirm subaqueous volcanism in metamorphic
Metamorphism
Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...
belts.
Pillow lavas are also found associated with some subglacial volcano
Subglacial volcano
A subglacial volcano, also known as a glaciovolcano, is a volcanic form produced by subglacial eruptions or eruptions beneath the surface of a glacier or ice sheet which is then melted into a lake by the rising lava...
es at an early stage of an eruption.
Formation
They are created when magmaMagma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...
reaches the surface but, as there is a large difference in temperature between the lava and the water, the surface of the emergent tongue cools very quickly, forming a skin. The tongue continues to lengthen and inflate with more lava, forming a lobe, until the pressure of the magma becomes sufficient to rupture the skin and start the formation of a new eruption point nearer the vent. This process produces a series of interconnecting lobate shapes that are pillow-like in cross-section. The skin cools a lot faster than the inside of the pillow, so it is very fine grained, with a glassy texture. The magma inside the pillow cools more slowly, so is slightly coarser grained than the skin, but still classified as fine grained.
Use as a 'Way-up' criterion
Pillow lavas are used as way-upWay up structure
A way up structure, way up criterion, or geopetal indicator is a characteristic relationship observed in a sedimentary or volcanic rock, or sequence of rocks, that makes it possible to determine whether they are the right way up or have been overturned by subsequent deformation...
criterion in geology. There are three key ideas that can be used as part of this, and that a pillow lava will show if it is the correct way-up:
- VesicleVesicular textureVesicular texture is a volcanic rock texture characterised by a rock being pitted with many cavities at its surface and inside. The texture is often found in extrusive aphanitic, or glassy, igneous rock...
s will be found towards the top of a pillow as the gas will be less dense than the surrounding rock. - The pillow structures will show a convex upper surface.
- The pillows will have a tapered base downwards as they have moulded to the underlying pillows during their formation.