Pillow lavas are

lava Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a Natural satellite, moon onto its surface. Lava may be erupted at a volcano or through a Fissure vent, fractu ...

s that contain characteristic pillow-shaped structures that are attributed to the extrusion of the lava underwater, or ''subaqueous extrusion''. Pillow lavas in

volcanic rock Volcanic rocks (often shortened to volcanics in scientific contexts) are rocks formed from lava erupted from a volcano. Like all rock types, the concept of volcanic rock is artificial, and in nature volcanic rocks grade into hypabyssal and me ...

are characterized by thick sequences of discontinuous pillow-shaped masses, commonly up to one meter in diameter. They form the upper part of Layer 2 of normal oceanic crust.

Composition

Pillow lavas are commonly of

basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...

ic composition, although pillows formed of komatiite, picrite, boninite, basaltic andesite,

andesite Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predomina ...

dacite Dacite () is a volcanic rock formed by rapid solidification of lava that is high in silica and low in alkali metal oxides. It has a fine-grained (aphanitic) to porphyritic texture and is intermediate in composition between andesite and rhyolite. ...

or even

rhyolite Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture (geology), texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained matri ...

are known. In general, the more

felsic In geology, felsic is a grammatical modifier, modifier describing igneous rocks that are relatively rich in elements that form feldspar and quartz.Marshak, Stephen, 2009, ''Essentials of Geology,'' W. W. Norton & Company, 3rd ed. It is contrasted ...

the composition (richer in silica - resulting in an

Intermediate composition In igneous petrology, an intermediate composition refers to the chemical composition of a rock that has 51.563 wt% SiO2 being an intermediate between felsic and mafic compositions. Typical intermediate rocks include andesite and trachyandesite ...

), the larger the pillows, due to the increase in

viscosity Viscosity is a measure of a fluid's rate-dependent drag (physics), resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for e ...

of the erupting lava.

Occurrence

They occur wherever lava is extruded underwater, such as along marine hotspot

volcano A volcano is commonly defined as a vent or fissure in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most oft ...

chains and the constructive plate boundaries of

mid-ocean ridge A mid-ocean ridge (MOR) is a undersea mountain range, seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading ...

s. As new oceanic crust is formed, thick sequences of pillow lavas are erupted at the spreading center fed by dykes from the underlying

magma chamber A magma chamber is a large pool of liquid rock beneath the surface of the Earth. The molten rock, or magma, in such a chamber is less dense than the surrounding country rock, which produces buoyant forces on the magma that tend to drive it u ...

. Pillow lavas and the related sheeted dyke complexes form part of a classic

ophiolite An ophiolite is a section of Earth's oceanic crust and the underlying upper mantle (Earth), upper mantle that has been uplifted and exposed, and often emplaced onto continental crustal rocks. The Greek word ὄφις, ''ophis'' (''snake'') is ...

sequence (when a segment of oceanic crust is thrust over the

continental crust Continental crust is the layer of igneous, metamorphic, and sedimentary rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as '' continental shelves''. This layer is sometimes called '' si ...

, thus exposing the oceanic segment above sea level). The presence of pillow lavas in the oldest preserved volcanic sequences on the planet, the Isua and Barberton greenstone belts, confirms the presence of large bodies of water on the Earth's surface early in the

Archean The Archean ( , also spelled Archaean or Archæan), in older sources sometimes called the Archaeozoic, is the second of the four geologic eons of Earth's history of Earth, history, preceded by the Hadean Eon and followed by the Proterozoic and t ...

Eon. Pillow lavas are used generally to confirm subaqueous volcanism in

metamorphic Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causi ...

belts. Pillow lavas are also found associated with some subglacial volcanoes at an early stage of an eruption.

Formation

They are created when

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...

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 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 as fine grained.

Use as a way-up criterion

Pillow lavas can be used as a way-up indicator in geology; 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: # Vesicles are found towards the top of a pillow (because the gas trapped as part of the rock is less dense than its solid surroundings). # The pillow structures show a convex (rounded) upper surface. # The pillows might have a tapered base downwards, as they may have molded themselves to any underlying pillows during their formation.

Gallery

File:Pillow lava at Oamaru.jpg, Pillow lava near

Oamaru Oamaru (; ) is the largest town in North Otago, in the South Island of New Zealand, it is the main town in the Waitaki District. It is south of Timaru and north of Dunedin on the Pacific Ocean, Pacific coast; State Highway 1 (New Zealand), Sta ...

, New Zealand File:Temagami greenstone belt pillow lava.jpg, Weathered Archean pillow lava in the Temagami Greenstone Belt of the

Canadian Shield The Canadian Shield ( ), also called the Laurentian Shield or the Laurentian Plateau, is a geologic shield, a large area of exposed Precambrian igneous and high-grade metamorphic rocks. It forms the North American Craton (or Laurentia), th ...

File:ItalyPillowBasalt.jpg, Pillow lava formations from an

sequence, Northern

Apennines The Apennines or Apennine Mountains ( ; or Ἀπέννινον ὄρος; or – a singular with plural meaning; )Latin ''Apenninus'' (Greek or ) has the form of an adjective, which would be segmented ''Apenn-inus'', often used with nouns s ...

, Italy File:Pillow basalt outcrop from Baras, Rizal, Philippines (closer view).jpg, Pillow basalt from Baras, Rizal Province, Philippines

References

External links

NeMO Explorer, NOAA - Contains link to video of pillows being formed
{{DEFAULTSORT:Pillow Lava Structural geology Volcanology sv:Lava#Kuddlava