Field observations of the formation of pillows by hot, de-gassed lava entering the sea have proved that their generation involves the underwater exposure and movement of incandescent material. This is made possible by the Leidenfrost effect, the phenomenon whereby film boiling replaces the usual nucleated boiling above a certain temperature, thereby forming an insulating sheath of vapour around any sufficiently hot body immersed in a liquid. Only on cooling below a certain temperature (defined here as the Nukiyama temperature) will rapid heat exchange occur between water and hot, gas-free lava. Near the surface, this could produce phreatic explosions and extensive clouds of steam.

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