How do pillow basalts show that ancient lava erupted underwater?

Pillow basalts serve as a diagnostic indicator of underwater eruptions due to the specific physical processes that occur when molten lava contacts water.

When highly fluid basaltic lava enters a body of water or water-saturated sediment, the exterior of the flow undergoes rapid chilling, which creates a tough, elastic, and often glassy rind. This process forms budlike projections that resemble pillows or grain-filled sacks.12

As the internal pressure of the molten material increases, the skin of these buds ruptures, allowing fresh lava to escape and form new, interconnected lobes. This cycle of rapid quenching and budding continues, resulting in the characteristic ovoid masses known as pillow lava. Because this structure requires the immediate, high-rate cooling provided by immersion in water to maintain the distinct pillow shape, geologists use these formations to identify ancient submarine volcanic activity.123