DEFINITION OF PILLOW LAVAS

A morphological type of lava that forms

under water, typically in mafic lava flows. They have a wide

variety of shapes and sizes, ranging from simple pillow

shapes that drape over underlying pillows, to long complex

tubes that branch and splay. Overlying pillows tend to fill in

any depressions that develop on or between underlying pillows,

so typically have an apical region that points downward,

and an upward-pointing convex surface. Most pillows

are several inches to three–six feet (a few tens of cm to 1–2

m) in cross section and may be similar sized or larger along

their long axes. A thin fine-grained chill margin that is glassy

in young pillows develops as the lava next to the seawater

cools quickly, with coarser grained parts forming in the pillow

center where the magma cooled more slowly. Pillow

lavas form in many tectonic settings, the most abundant of

which is in the upper volcanic layer of oceanic crust. Piles of

pillows in oceanic crust and in pieces of oceanic crust thrust

onto land (ophiolites) may be up to a few miles (2–3 km)

thick. Pillow lavas are also common in submarine sections of

island arc systems, hot spot volcanoes such as Hawaii, and in

other submarine volcanic settings.

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