Pillowlavas

Pillowlavas, commonly referred to as pillow lavas, are lava formations that develop when molten rock erupts underwater or flows into seawater. Rapid cooling by surrounding water causes the lava to fragment into rounded, pillow-like structures, hence the name. They are most often basaltic in composition, though they can occur across a range of basaltic to andesitic chemistries depending on tectonic setting.

Texture and structure: Each pillow typically has a thin, glassy rind formed by quenched surface lava, encasing a more plastic, sometimes vesicular interior. The rims are usually fine-grained or glassy, while the cores may contain phenocrysts. Pillows range from a few centimeters to several meters in diameter and are often stacked in beds produced by successive eruptive pulses. When eruptions fragment violently, pillow lavas can mix with hyaloclastites, forming brecciated deposits.

Distribution and context: Pillow lavas are characteristic of submarine volcanic environments, including mid-ocean ridges, back-arc basins, seamounts, and eruption centers beneath lakes. They are also preserved in ophiolites, which expose ancient submarine lava sequences on continental crust. Their occurrence can indicate past water depth and marine conditions at the time of eruption.

Scientific significance: Studying pillow lavas helps reconstruct submarine eruption dynamics, magma supply under water, and the tectonic history of ocean basins. They provide insights into magma–seawater interactions, cooling rates, and the formation of volcanic glass, contributing to petrology, geochronology, and plate tectonics research.