J. geol. Soc. London, Vol. 141, 1984, pp. 183-186, 1 fig. Printed in Northern Ireland

Pillow and the Leidenfrost effect

A. A. Mills

SUMMARY: Field observations of the formation of pillows by hot, de-gassed 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 Kilaueaspilled into the sea after flowing 12 km down the S flank of the . 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 theliquid 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 is important to note that the Hawaiian flow studied ‘transitional’regime below TL inFig. 1 will then by Moore et al. (1973) and Moore & Tepley (1974) was begin.This descent towards andthrough the de-gassed bv travel for a long distance over land, forin Nukiyama temperature is associated with most vigor- shallow water the proposed schemes could be invalid ous boiling, often near-explosive in its violence (Witte forlavas containing much dissolved gas, its frothy et al. 1970). Stevens & Witte (1973) have coined the releaseresulting in extensive mixing, comminution, term ‘transplosion’ for the phenomenon. Implosion of andphreatic explosion. The hydrostatic pressure in theresulting vapour domes then occurs on contact waterdeeper than about 30m shouldgenerally with cold water, giving alikely source for the shock prevent such exsolution (Decker & Decker 1981). waveswhich distressed Moore’s scuba divers. Some Inconnection with extrusion of lavain the deep types of hyaloclastite could be generated during this ocean it is relevant to consider thecritical behaviour of quenching stage, and the fluidization produced by lava water. The critical pressure of pure water is 221 bars flowing beneath unconsolidated wet sediments (Koke- and its critical temperature 374°C. The figures applic- laar 1982) should probably be associated with volumi- ableto sea water are not known, but itscritical nous steam production around the Nukiyama tempera- temperature cannot be much above the valueof 414°C ture. deduced for 0.615 molal sodium chloride solution from Rapid cooling of the outside of a pillow will result in theresults of Marshall & Jones(1974). The critical a glassy layer in such a highly strained state that some pressure will be exceeded by the hydrostatic pressure of it may spa11 off later, as well as being particularly at depths greater than about 2000 m (McBirney 1963), liable to aqueousalteration (Moore 1970). Cooling butthe usual ocean bottom water temperatures of continuesradially inwards, but the low thermal 0-3”C are, of course,far removed from the critical conductivity of the thickening walls will soon slow the temperature.However, exposure to hot lava at a rate of heat transfer from the interior. It is therefore temperature well in excess of this value must result in possible to buildtube-likea structure capable of thegeneration of acontiguous filmof supercritical conveying fresh hot lava to the extrusion zone. Note high-pressure gaseous water for, by definition, liquid/ that once the outer surface has cooled belowTN it will vapourinterfaces cannot exist abovethe critical remaincooled by contact withthe surrounding temperature and so nucleated boiling is not applicable. water-nly cracking will exposeincandescent lava. Therefore, a continuous gaseousfilm will surround the As flow diminishes and then stagnates, so further slow hotlava, exactly comparable with the vapour film cooling will lead to solidification and radial columnar produced by film boiling at lower pressures. For this structure. Upstream blockage could lead to drainage reason,pillows must be common products of lava and ‘empty’ tubes. extruded from mid-ocean rifts, the spreading of plates This model for the formation of pillow lavas leads to from these centres leading to the observed abundance the connected, roughly cylindrical, interdigitated lobes of pillow lavas beneath the oceanic sediments (Ballard (with their long axes extending downslope) which are & Moore 1977). apparent in many of the best exposures (Jones 1968~; Moore 1970; Moore et al. 1971). Eventually, a pile of Further applications pillow upon pillow may be built up, contemporaneous effusionbeing reflected inthe observed mutual On this basis, the presence of superincumbent liquid sagging and distortion of the still-plastic masses (Jones water is essentialto the generation of true pillow 19686). However, the occasional underwater extrusion lavas.Therefore, any division observed in the field of hot mobile lava in an upward direction, to give a between them and the blocky, vesiculated ‘aa’ surface low-velocity fountain, could lead to individual pillows. characteristic of subaerialeffusion should mark the Thisfollows from the Rayleighinstability of ajet waterlevel at the time of eruption(Moore & Fiske causingnecking-off and separation into rounded 1969; Jones & Nelson 1970; Mapstone et al. 1975). cylinders, each insulated by its own Leidenfrost layer The LeidenfrostiNukiyama phenomenon would also andpartially buoyed up by thesurrounding water. appear important in other aspects of igneous geology Surface tension continues to act, pulling these masses such as the propagation of lengthy sills (Francis 1982), intoroughly ellipsoidal cushions (Fuller 1940) which the flow of some submarine lavas to form remarkably pile up to give formations such as those described by extensive sheets (Smith 1983), and the generation of Bailey et al. (1964). maars. These aspects will be examined elsewhere. References BAILEY,E. H., IRWIN,W. P. & JONES, D. L. 1964. the geology of westernCalifornia. Bull. &l$. Div. Franciscanand related rocks, and theirsignificance in Mines Geol. 183, 51.

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BALLARD, R. D.& MOORE,J. G. 1977. Photographic Atlas 63, 269-77. of the Mid-Atlantic Ridge RiftValley. Springer, New -, CRISTOFOLINI,R. & GIUDICE,A. L. 1971. Develop- York. ment of pillows on thesubmarine extension of recent BELL, K. J. 1967. The Leidenfrostphenomenon: a survey. lavaflows, Mount Etna, Sicily. Prof. pap. U.S. geol. Chem. eng. Prog. Symp. Ser. 63, 73-82. Surv. 750-C, 89-97, DECKER,R. & DECKER, B.1981. Volcanoes. Freeman, San - & FISKE,R. S. 1969.Volcanic substructure inferred Francisco. fromdredge samples and ocean bottom photographs. FRANCIS,E. H. 1982. and sediment-I.Emplace- Bull. geol. Soc. Am. 80, 1191-201. ment mechanism of late Carboniferous tholeiite sillsin - & LOCKWOOD, J. P.1978. Spreading cracks on pillow northern Britain. J. geol. Soc. London, 139, 1-20. lava. J. Geol. Chicago, 86, 661-71. FRIDLEIFSSON,I. B., FURNES, H. & ATKINS,F. B. 1982. -, PHILLIPS,R. L., GRIGG, R. W., PETERSON, D. W.& Subglacial volcanics-on the control of magma chemistry SWANSON,D. A. 1973.Flow of lavainto the sea, on pillowdimensions. J. Volcanol.Geotherm. Res. 13, 1969-1971, KilaueaVolcano, Hawaii. Bull. geol. Soc. 103-17. Am. 84, 537-46. FULLER,R. E. 1940.Ellipsoidal structure as thegigantic - & TEPLEY, L. 1974. Fire Under the Sea:the Origin of dispersephase of anemulsion (Abstract). Bull. geol. Pillow Lavas. 16 mm sound motion picture by Moonlight Soc. Am. 51, 2022. Productions, 2243 Old MiddlefieldWay, Mountain GEIKIE, A. 1897. The Ancient Volcanoes of Great Britain. View, California. Macmillan. London. NUKIYAMA,S. 1934. The maximum and minimum values of GOTTFRIED,B. S., LEE, C. J. & BELL, K. J. 1966. The theheat Q transmittedfrom metal to boiling water Leidenfrost phenomenon: film boiling of liquid droplets under atmospheric pressure. J. Soc. mech. Eng. Japan, on a flat plate. J. Heat Mass Transfer, 9, 1167-87. 37(206), 367-74. In Japanese;trans. BRICKLEY, S. G. HULME,G. 1974. The interpretation of lava flow morpholo- 1960. AERE Trans. no. 854. gy. Geophys. J. R. astron. Soc. 39, 361-83. SHAW,H. R. 1969. Rheology of in the melting range. JONES,J. G. 1968a.Pillow lava andpahoehoe. J. Geol. J. Petrol. 10, 51&35. Chicago, 76, 485-8. -, WRIGHT,T. L., PECK, D. L. & OKAMURA, R.1968. -1968b. Intraglacial volcanoes of the Laugarvatn region, The viscosity of basalticmagma: an analysis of field south-westIceland. Q. J. geol. Soc. London, 124, measurements in Makaopuhi lava lake, Hawaii. Am. J. 197-21 1. Sci. 266, 225-64. -& NELSON,P. H. H. 1970. The flow of basalt lava from SMITH,P. J. 1983.Extensive oceanic lava flows? Nature, air into water-its structural expression and stratigraphic London, 302, 14. significance. Geol. Mug. 107, 13-19. SNYDER, G. L.& FRASER,G. D. 1963. Pillowed lavas 11: a KOKELAAR, B. P.1982. Fluidization of wet sediments during review of selected literature. Prof. pap. U.S. geol. Surv. the emplacement and cooling of various igneous bodies. 454-c. J. geol. Soc. London, 139, 21-33. SPARKS, R.S. J. & PINKERTON,H. 1978. Effect of degassing LEIDENFROST,J. G. 1756. On the Fixation of Water in Diverse on rheology of basalticlava. Nature, London, 276, Fire. Duisburg. In German; trans. WARES, C. 1966. Znt. 385-6. J. Heat Mass Transfer 9, 1153-66. STEVENS,J. W. & WITTE, L.C. 1973.Destabilization of LIENHARD, J.H. 1981. A Heat Transfer Textbook. Prentice- vapor film boiling aroundspheres. Int. J. HeatMass Hall, New York. Transfer, 16, 669-78. MAPSTONE,N. B., ROOD, A. P. & JACKSON,N. G. 1975. TAMURA,Z. & TANASAWA,Y. 1959.Evaporation and Underwaterstudies of recentlava flows aroundSao combustion of a drop contacting with a hot surface. 7th Miguel (Azores). Geol. Mag. 112, 309-15. Symposium(International) on Combustion. Butter- MARSHALL, W. L.& JONES, E.V. 1974. Liquid-vapor critical worths, London, 509-22. temperatures of aqueous electrolyte solutions. J. inorg. WALKER, J.1977. Water on a hot skillet. . . Sci. Am. 237(2), nucl. Chem. 36, 2313-18. 12G30. MCBIRNEY,A. R. 1963.Factors governing the nature of WILLIAMS,H. & MCBIRNEY,A. R. 1979. Volcanology. submarine volcanism. Bull. Volcanol. 26, 455-69. Freeman, San Francisco. MOORE,J. G. 1970. Pillow lava in the historic lava flow from WITTE,L. C., Cox, J. E. & BOUVIER, J.E. 1970. The vapor Hualalai Volcano, Hawaii. J. Geol. Chicago, 78, 239-43. explosion. J. Metals, N.Y. 22 (Feb.), 39-44. -1975. Mechanism of formation of pillow lava. Am. Sci.

Received 23 May 1983; revised typescript received 30 August 1983. A. A. MILLS, Departmentof Geology, The University, Leicester LE1 7RH.

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