J. geol. Soc. London, Vol. 141, 1984, pp. 183-186, 1 fig. Printed in Northern Ireland Pillow lavas and the Leidenfrost effect A. A. Mills SUMMARY: Field observations of the formation of pillows by hot, de-gassed lava entering the seahave proved that their generation involves theunderwater 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. Pillow lavas, with their relatively smooth surface and characteristicrounded form, are probably the most abundantvolcanic rocks on Earth (Williams & McBirney 1979). As long ago as 1897 Geikie suspected them to be a product of the interaction of lava with water,but controversy over all aspects of their formationcontinued until recent years (Snyder & Fraser 1963). Then, from 1969-73, repeated lava flows from the E rift zone of Kilauea spilled into the sea after flowing 12 km down the S flank of the volcano. This provided the long-awaited opportunity for J. G. Moore and other diver-geologists (Moore et al. 1973; Moore & Tepley1974; Moore 1975) to observe and film pillows being formed underwater. They reported I II that in situ formation of pillows occurredwhen I I, cracking of the cool crust of an active flow allowed the I 11 periodic extrusion of glowing lava. Growth then took TB TL TN place by axial extension, longitudinal swelling, or the budding of fresh lobes from a weak point in the wall. Temperature of surface Scarpsand corrugations might beinduced in the growingsection (Moore & Lockwood1978). This persistence of incandescent lava underwater for some FIG. 1. The generalized boiling curve seconds points to the involvement of the Leidenfrost effect (Leidenfrost 1756). surface and spreads out or, more commonly, assumes a piano-convex shape.The liquid is warmed by convection and evaporates slowly and quietly from its The Leidenfrost effect free surface: this behaviour is represented by section a-6 . This phenonenon will be familiar to anyone who has At the boiling point TBtiny vapour bubbles begin to spilt a little water on a hot stove and noted how the appear at nucleated sites in the liquidisolid interface, fluid draws itself intoconvex droplets which, sup- probably where surface imperfections or cavities trap ported by a layer of vapour, roll freely about the hot microscopicbubbles of a gaseousphase (Walker surface and take a remarkably long time to evaporate. 1977). Thenumber of thesesites, and consequently Quantitative examination of the effect may therefore therate of evaporation,increases with temperature take the formof a plot of the lifetime of small drops of along section 6-c until, by the time c is reached, the constant initial volume versus the temperature of the lens-shaped droplet has been replaced by a violently underlyingsurface (Tamura & Tanasawa 1959). All boiling irregular mass. The rapid vaporization occur- commonliquids give a graph of thegeneral form ring at a multitude of nucleatedsites produces a shown in Fig. 1. hissing sound. When the temperature of the surface is below that However, a comparatively small increase to d in the corresponding to the normal boiling pointof the liquid temperature of the heated surface exerts a profound at the ambient pressure, the droplet either wets the effect. The liquid gathers itself into flattened globules Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/141/1/183/4888059/gsjgs.141.1.0183.pdf by guest on 01 October 2021 184 A. A. Mills which, supported by a cushion of their own vapour, the hot surface, and the purity and bulk temperature glideabove the hot surface. This move into a of the liquid itself. Typicalvalues for pure water in non-nucleated film-boiling regime,with its accom- contactwith hot metallic surfaces at atmospheric panying dramatic fall in the rate of evaporation due to pressure lie inthe range 15Ck20O"C for TN and the low thermalconductivity of thethin filmof 20Ck300"C for TL. The values applicable to hot lava, vapour, constitutes the Leidenfrost effect (Bell 1967). seawater, and high hydrostatic pressures will require For comparison, liquid water at 100°C has a thermal experimental assessment. These factors would appear conductivity of 6.8 X 10-3 J cmlcm2 se&, while to elevate TN and TL considerably for the geological water vapour at the same temperature has a thermal situation. conductivity of 2.3 X 10-4 in the same units-a factor of 30 less. It will be seen that the maximum rate of evaporation Application to molten lava (minimumlifetime; point c) is associatedwith a temperature which mayconveniently be called the Eveninthe absence of reliable datathat are Nukiyamatemperature TN afterdiscovererits corrected for geological factors, we may still account (Nukiyama 1934). The temperature corresponding toa qualitatively for field observations on the interaction minimum rate of evaporation d is alreadyconven- between lava and water. The subaqueous extrusion of tionallyknown as theLeidenfrost temperature TL. incandescentmaterial at a temperature of about The transitional section c-d of the curve is unstable 1100°C is made possible by the immediate formation and hard to define experimentally, but above TL the of a temporarylow-conductivity sheath of water levitatedspheroids can be remarkably stable and vapour, produced by the film-boiling regime character- reproducible along curve d-e. A very large tempera- istic of hot-surface temperatures well in excess of the ture increase-typically hundreds degrees-isof Leidenfrost temperature. Moore (1975) and Moore necessary before the overall rate of evaporation again Lockwood (1978) neglected this phenomenon in their approachesthat observed at the normal nucleated mathematical model which, in the absence of the low boiling point TB.let alone that at TN. conductivitysheath detailed here, led to the pre- diction of a 'hairline' incandescent crack in the lava surface.When the surface is abovethe Leidenfrost temperature, the bulk of the surrounding water is not Film boiling around hot solids extensively heated,but it will benoted that the immersed in liquids glowing lavasurface is nevertheless in contactwith superheated steam. Dissolution of the latter is known to reduce substantially the viscosity of a silicate melt Thepreceding observations have been made upon (Sparks & Pinkerton 1978) so giving the appearance of relativelysmall spheroids. Somewhat larger volumes an 'elastic skin'. of liquid form a vibrating flattened pool separated from Moltenmaterial in an active flow isconveyed by the hot surface by the excess pressure in a vapour film lava tubes (see below), and extrusion will occur only about 0.1 mm thick(Gottfried et al. 1966);but whenthe pressure (e.g. hydrostatic head from an otherwise the boiling curveis simply elevated vertically elevated source) exceeds the viscous resistance within as a whole. Volumes exceeding a millilitre tend to be thetube. The viscosity of lava variesgreatly with disruptedinto lesser masses by vapourbreaking temperature (Shaw 1969), and it is consistent with its throughthe surface of thepool at various points. non-Newtonianrheology (Shaw et al. 1968; Hulme Addition of still more liquid finally produces a stage 1974) thatthe extrusion frequently occurs in surges where a hot solid is immersed in, or otherwise totally (Moore1975). It isalso likely thatthe pressure covered by, a cooler liquid. This is a common situation requiredto maintain the flowin a lengtheningtube in industrywhenever liquids have to be evaporated will eventually exceed the strength of its wall nearer by hotsurfaces, steam coils. or electricheaters. the source, leading to the 'budding' of a new pillow at Nukiyama'sexperimental technique was, for this that weak point. Composition and temperature will be reason, actually based on measurement of the changes important factors controlling the viscosity, hence the in heat flux exhibited by an electricallyheated wire lengthand diameters of pillows (Hulme 1974; Frid- immersed in water. His work has been confirmed and leifsson et al. 1982). extended by numerous workers (Lienhard 1981). It is For film boiling to persist, the incandescent surface alsorelevant to the quenching and tempering of of the lava must supply energy at a rate sufficient to metals, and to several areas of nuclear reactor safety maintainthe vapour film andthe multitude of tiny research. bubbles which rise from it (Moore 1975) as a result of The Nukiyama and Leidenfrost temperatures of any instabilities:radiation will bethe most important given liquid are not constants, being affected by such mode of heattransfer above 500°C. Unlessreple- factors as pressure, composition and physical state of nished by freshlava, the temperature of thehot Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/141/1/183/4888059/gsjgs.141.1.0183.pdf by guest on 01 October 2021 lavas and the Leidenfrosteffect thePillow and lavas 185 surface will thereforedecrease-albeit more slowly Influence of dissolved gases than if it werein direct contact with liquidwater- and the depth of water untilit eventually fallsbelow thecorresponding Leidenfrosttemperature. Contact quenching in the It