Pillow lavas

Pillow lavas are lavas 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 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.

Pillow lavas are commonly of basaltic composition, although pillows formed of komatiite, picrite, boninite, basaltic andesite, andesite or even dacite are known. In general the more intermediate the composition, the larger the pillows, due to the increase in viscosity of the erupting lava.

They occur wherever mafic to intermediate lavas are extruded under water, such as along marine hotspot volcano chains and the constructive plate boundaries of mid-ocean ridges. 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. Pillow lavas and the related sheeted dyke complexes form part of a classic ophiolite sequence when a segment of oceanic crust is obducted onto continental crust.

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. Pillow lavas are used generally to confirm subaqueous volcanism in metamorphic belts.

...
Wikipedia