Pillow lavas arelavas that contain characteristic pillow-shaped structures that are attributed to the extrusion of the lava underwater, orsubaqueous extrusion. Pillow lavas involcanic rock are characterized by thick sequences of discontinuous pillow-shaped masses, commonly up to one meter in diameter. They form the upper part ofLayer 2 of normal oceanic crust.
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Pillow lavas are commonly ofbasaltic composition, although pillows formed ofkomatiite,picrite,boninite,basaltic andesite,andesite,dacite or evenrhyolite are known.[1][2][3][4][5] In general, the morefelsic the composition (richer in silica - resulting in anIntermediate composition), the larger the pillows, due to the increase inviscosity of the erupting lava.
They occur wherever lava is extruded underwater, such as along marinehotspotvolcano chains and theconstructive plate boundaries ofmid-ocean ridges. As new oceanic crust is formed, thick sequences of pillow lavas are erupted at thespreading center fed bydykes from the underlyingmagma chamber. Pillow lavas and the relatedsheeted dyke complexes form part of a classicophiolite sequence (when a segment of oceanic crust isthrust over thecontinental crust, thus exposing the oceanic segment above sea level).
The presence of pillow lavas in the oldest preserved volcanic sequences on the planet, theIsua andBarbertongreenstone belts, confirms the presence of large bodies of water on the Earth's surface early in theArchean Eon. Pillow lavas are used generally to confirm subaqueous volcanism inmetamorphic belts.
Pillow lavas are also found associated with somesubglacial volcanoes at an early stage of an eruption.[6][7]
They are created whenmagma 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.[8] The skin cools much faster than the inside of the pillow, so it is very fine-grained, with a glassy texture. The magma inside the pillow cools slowly, so it is slightly coarser-grained than the skin, but it is still classified asfine grained.
Pillow lavas can be used as away-up indicator in geology;[9] that is, study of their shape reveals the attitude, or position, in which they were originally formed. Pillow lava shows it is still in its original orientation when:
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