The Formation of Deep-Sea Limu O Pele

Bull Volcanol (2001) 63:482Ð496 DOI 10.1007/s004450100165

RESEARCH ARTICLE

Doris Maicher á James D. L. White The formation of deep-sea Limu o Pele

Received: 13 October 2000 / Accepted: 3 July 2001 / Published online: 8 September 2001 © Springer-Verlag 2001

Abstract Deep-sea sheet hyaloclastite consists mostly Introduction of sand-sized blocky and splinter-shaped shards, but also contains subordinate mm- to cm-sized thin, curved, and An intensive investigation of sheet hyaloclastite was un- wrinkled plates and sheets of sideromelane. This latter dertaken in 1995 (Batiza et al. 1996; Maicher et al. type of shard, termed Limu o Pele, has been observed 1996). These thinly bedded hyaloclastites consist of forming subaerially on Kilauea by entrapment of water sand-size sideromelane clasts, and are located high on in flowing lava followed by expansion of steam to form the flanks and summit areas of Seamount Six, at large bubbles which burst into thin fragments. Deep- 12¡45′N, 102¡35′W on the Cocos Plate. Individual bod- marine limu has been inferred, from comparative mor- ies of sheet hyaloclastite are localised and small phological studies and assessment of physical bubble- (<200 m2, <4 m3 dense rock equivalent), and occur to- forming conditions, to form in a similar way but with the gether with pillow talus, knobbly fist-sized lava frag- increased ambient pressure and the higher viscosity of ments and thin (<10 cm) sheet lava. Extensive volatile water reducing bubble expansion. Differing mechanisms degassing at the vent to drive submarine fire-fountaining of heat transfer and rates of magma chilling also modify (Smith and Batiza 1989) does not seem to have occurred, the limu-forming process in the deep sea. This paper as Seamount Six hyaloclastites are non-vesicular, super- evaluates a variety of deep-sea limu-forming processes saturated with CO2 and show only a slight degassing and develops a new and quantitatively supported model, trend of H2O (Dixon 2000, personal communication). based on observed limu-forming processes and criteria Based partly on earlier work by Lonsdale and Batiza derived from dive samples and observations at Seamount (1980), and Batiza et al. (1984), Maicher et al. (2000) Six, Cocos Plate. It is inferred that water and/or water- developed a model which met the following observation- saturated sediment was trapped in extremely thin, fluid al constraints: (1) sheet hyaloclastite occurs on flat sum- and rapidly advancing lava flows by various processes. mit plateaux of seamounts, which does not allow for ex- Bubble formation might also occur during small-scale tensive gravitational reworking; (2) sheet hyaloclastite magma-fountaining driven by magmatic volatile exsolu- locally forms a continuous, uninterrupted blanket super- tion and extreme vent constriction or during collapse of imposed on lava flows; (3) sedimentary characteristics of pillows and rapid drainage of the magma, but we found the sheet hyaloclastite indicate some sort of lateral cur- no deposits clearly resulting from these processes. rents; (4) stretched folded shards, occasionally with the folded limbs of a shard welded together, are present and Keywords Lava bubble-wall fragments á Hyaloclastite á thought to indicate a dynamic environment in which the Sheet lava á Heat transfer á Water entrapment á shards did not come into immediate direct contact with Submarine explosions á Fragmentation liquid water; and (5) jigsaw-fit puzzle shards, formed by in-situ shattering due to thermal granulation of magma, are present, suggesting emplacement of still-hot fragments with virtually no subsequent movement. Fragmentation to sand-size and finer fragments dur- Editorial responsibility: J. Gilbert ing eruption requires energy to break the erupting magma apart, from expansion of either exsolving gases, boil- D. Maicher á J.D.L. White (✉) ing water, or from thermal stresses imposed during rapid Geology Department, University of Otago, P.O. Box 56, Dunedin, New Zealand quenching of hot melt (Heiken and Wohletz 1985). e-mail: [email protected] The latter mechanism produces sand-sized, blocky Tel.: +64-3-4799009, Fax: +64-3-4797527 equant shards, the most common shard population of Se- 483 amount Six hyaloclastites. The typical signature of ex- interaction under high (relative to atmospheric) con- solving magmatic gas is vesiculation, yet there are virtu- fining pressures. ally no vesicles in the associated hyaloclastite fragments. Instead, a significant proportion of the hyaloclastite fragments consists of tiny, thin, wrinkled plates of glass Observed shoreline limu formation which appear to be the broken walls of isolated bubbles an order of magnitude larger than typical vesicles. Limu o Pele at Kilauea consists of translucent basaltic These platy shards are here termed “limu”, because of sheets with fluidal surfaces and fractured sheet edges, their morphological resemblance to stretched glass plates the latter indicating post-burst brittle comminution of of so-called “Limu o Pele” (Hon et al. 1988; Keszthelyi sheets (Wright et al. 1992, their Fig. 119, p. 122). The 1995, personal communication; Hon 1998, personal sheets contain abundant tiny gas vesicles (<0.1-mm tube communication). Their formation has been observed in diameter), which are highly flattened and elongated due littoral settings at Kilauea (Kjargaard 1990; Wright et al. to stretching of the sheets during bubble growth (Hon et 1992; Mattox and Mangan 1997), on shallow marine la- al. 1988). The glassy limu flakes are centimetre sized in va flows (Tribble 1991), and during laboratory experi- major dimensions and vary in thickness from several ments (Zimanowski 1998, personal communication). Re- millimetres to much less than one millimetre. The curva- cently, limu fragments have been discovered along the ture of the shards is very slight because of the large bub- sediment-covered Mid Atlantic ridge at 800 and 1,700 m ble diameter. They are often associated with Pele’s hair below sea level (bsl) (Fouquet et al. 1998; Barriga 1998, (Wright et al. 1992; Mattox and Mangan 1997), repre- personal communication), on Loihi seamount at senting thin strands of stretched magma. 1,150Ð1,950 m bsl (Clague et al. 2000), and on the Kilauean limu forms from very fluid (viscosity ca. Gorda Ridge at 3,200 m bsl (Clague 2000, personal com- 200 Pa s at 1,150 ¡C, Shaw 1969), basaltic pahoehoe la- munication). va which has flowed 8Ð11 km down to the coastline Kilauean lava bubbles can reach diameters of 5Ð10 m (Kjargaard 1990; Heliker et al. 1993; Mattox 1994). in less than two seconds (Mattox and Mangan 1997), and Limu bubbles develop from this largely degassed lava bubble shapes resemble dome-like bulges and irregular (Swanson and Fabbi 1973) where (1) surf is splashed on half-spheres or light bulb-shaped blowups (all hereafter open-channel pahoehoe lava, (2) buried lava tubes are called “bubbles”). They are similar in appearance to bub- fractured by lava delta collapse, or (3) bubbles expand bles emerging on vent lava surfaces during strombolian out of shallow marine lava channels. Limu also form eruptions (Blackburn et al. 1976). Observations at Kil- during (4) experiments producing interactions of magma auea, however, clearly show that limu bubbles can form with water. Fragmentation of limu bubbles at Kilauea by entrapment of water and subsequent water vapour ex- starts with tearing of magma at the top of a bubble, about pansion. Formation of limu bubbles by in-situ exsolution two seconds after the bubble first starts to grow. Bubbles of magmatic volatiles is considered unlikely because, for may become perforated before bursting to eject spatter a magma at constant pressure, vesicle growth by exsolu- and limu sheets high into the air. tion of volatiles is on the order of millimetres per day (e.g. Sarda and Graham 1990), too slow to be effective in producing limu bubbles on lava flow surfaces. Open-channel lava

Swift-flowing pahoehoe lava in broad open channels de- Why are deep-sea limu important? velops standing waves along curved sides of the channel or where flowing against obstacles. The waves have 1. Limu represents an expansion of gas and fragmenta- steep to overhanging faces upstream, thus trapping water tion of magma which can occur without involvement where surf splashes onto the lava. After initially boiling of magmatic volatiles and at depths from subaerial to for some seconds on the lava surface, the water expands deep marine, yet they are apparently uncommon Ð to vapour within the lava, repeatedly producing streams why? of delicate decimetre-scale lava bubbles (Hon et al. 2. Limu formation is inferred to occur to at least the crit- 1988) and jetting of steam through the lava (Kjargaard ical depth of seawater (2,980 m bsl, Bischoff and 1990). Rosenbauer 1988), but only in the absence of explosive magmatic fragmentation or explosive hydrovolcanic magma-water interactions of shallow subaque- Lava delta and partial bench collapse ous setting, i.e. Surtseyan eruptions. Therefore, limu shards may be useful indicators of paleodepth and Collapses of lava deltas fracture active lava tubes near or paleoenvironment. above sea level. Water is sometimes entrapped in roofed- 3. Limu is only locally present on the seafloor at appro- over lava streams which pour down the cliff into the priate depths. Its occurrence is related to specific vol- ocean (Griggs, in Wright et al. 1992). In the gap between canological conditions which are the focus of this pa- the crust and the flowing lava the trapped water is heat- per and which illuminate processes of magma-water ed, and steam blasts and bubbles emanate (Kjargaard 484 1990). For limu to form, vigorous magma-water inter- sion to vapour of water entrapped in the melt stretches mingling in the surf zone is required (Thordarson and and strains the melt to form dome-shaped, thin-walled Self 1991; Jurado-Chichay et al. 1996). High lava flow melt bubble shells. The “slow” heating of enclosed water rates may result in tephra jets 10s of metres high, formed observed in experimental runs emphasises the passive by the interaction of lava and seawater. Angular fine la- nature of the process. Where water is trapped underneath pilli and ash with subsidiary spatter bombs and limu lava, it vaporises and water vapour rises through the la- sheets are produced by waves crashing onto and disrupt- va. The rise velocity, evaluated with Archimedes’ princi- ing the stream of melt. ple of buoyancy (Eq. 9 below) and Stoke’s formula for Partial bench collapses drop active lava tubes into the the drag on a sphere, is on the order of 10s of m sÐ1 sea, allowing seawater to seep into the tube along the (strongly depending on lava viscosity), and thus rise collapse headwall. Mild explosions form lava bubbles times of bubbles are comparable to the timescales of ob- several metres inland from the shoreline. The most vig- served limu formation. orous interactions produce continuous “littoral foun- Limu-forming lava is confined to tubes or open chan- tains” closer to the shoreline, often preceded by bubble nels, with no limu formation observed during advance of bursts (Mattox and Mangan 1997). pahoehoe tongues over water-saturated sediment at the shoreline, nor during violent, littoral cone-forming seawater interaction with ‘a’a lava (Jurado-Chichay et al. Shallow marine limu 1996). Bubble formation sometimes alternates with more violent magma-water interactions, e.g. littoral fountains Scuba divers in Hawaii observed submarine channelised and tephra jetting in lava delta collapses. These can be lava streams at <50 m bsl, emerging from shallow tubes seen as a spectrum of increasing intensity of interaction. at low benches in the lava delta (Tribble 1991). Rapidly expanding lava bubbles 50Ð100 cm in diameter formed repeatedly at particular spots along the length of the sub- Lava cooling and crust formation aqueous lava flows, commonly along cracks in the lava crust (Tribble 1991). Hon et al. (1988) observed limu Steam will rise through low viscosity magma until it bubbles resulting from such processes rising from the reaches the viscous cooled skin which invariably develops water 1Ð5 m offshore. at the surface of lava. During emplacement of a lava flow, the skin tears and reheals (Kilburn 1993; Stasiuk et al. 1993), and inflation, deflation or flexure of the flow Experimentally produced limu shards cracks the outer crust where formed, allowing cracks to penetrate inwards into partially molten, visco-elastic crust Limu formation has also been observed during laborato- (Turcotte and Schubert 1982). Bubbles can emerge from ry experiments (Zimanowski 1998, personal communica- the lava at these sites, stretching only freshly exposed tion). Water injected into remelted and almost fully de- magma of relatively low viscosity to form the bubble skin. gassed alkali basalt and nephelinite commonly forms vi- At the moment of disruption to form limu, bubble walls olent fuel-coolant interactions (FCIs; Zimanowski et al. are still plastic and above welding temperature, as indicat- 1991, 1997), and a fraction of water not involved direct- ed by spatter formation and tack welding respectively. ly in the FCI, but fully enclosed in the melt, is somewhat more slowly heated and expands to form melt-walled bubbles of 5Ð10 cm diameter before bursting. Controls on bubble size

Limu bubbles at Kilauea range in size from tens of centi- Critical aspects of observed limu metres to several metres in diameter, and the final bubble size is achieved in about two seconds. The factors con- From the above observations, learning how deep-sea limu trolling limu bubble size seem to be the quantity of water forms seems likely to require an understanding of (1) involved, steam expansion and pressurisation, and a how bubbles are initiated and grow at depth, (2) how sub- cooling-induced, time-dependent increase in melt viscos- sea lavas cool and develop a crust, and (2) what controls ity. Shard thickness depends on the thickness of any ini- the size of bubbles which form at depth. Our understand- tial surface skin, and on the balance between bubble ex- ing of these aspects for subaerial eruptions is summarised pansion versus melt cooling rates. below, followed by a detailed treatment of deep-sea limu. Deep-sea limu Initiation and growth of bubbles Most deep-sea limu shards are slightly curved, like bee- Limu-forming hydrovolcanic interactions are mildly ex- tle wings or bits of a broken light bulb (Fig. 1a). The plosive and appear, from the described examples in Ha- curvature is even and smooth, though not necessarily waii, to require entrapment of water in magma. Expan- constant. Some shards are wrinkled and folded tightly 485

Fig. 1aÐd Deep-marine formed limu sheets. a Gently curving 1239. c Limu shard in thin section, folded back and welded to it- limu sheet with smooth tension-moulded upper surface and frac- self (arrow); no. 3012-1246. d Nearly complete, wrinkled limu ture-bound sides; dark patches result from spalling of palagonite bubble, preserved as alteration crust of shard only; no. 3011-1203 rim; no. 3014-1239. b Slightly wrinkled limu sheet; no. 3014-

Table 1 Morphological and compositional characteristics of deep-marine limu from Seamount Six and subaerially, shallow-marine and experimentally produced limu shards

Property Deep-marine limu Observed limu

Shard thickness 0.05Ð0.43 mm, average 0.14 mm <1 mm to several cma, d Shard size mm2 to 1 cm2 mm2 to several cm2 Colour and Clear brown, opaque rims Subaerial: clear brown to greenish, opaque rims around translucency around phenocrysts phenocrystsd; experimental: translucent-patchy opaque, brown Material Glass Subaerial and experimental: glass Crystals 0Ð5% plagioclase, very rarely quench Subaerial: very few olivinesb; experimental: none crystallites Vesicles Rarely tiny, elongate Subaerial: ca. 20Ð30%, highly stretchede; experimental: none Composition Tholeiitic basalt, hawaiite Subaerial and shallow marine: ocean island tholeiite; experimental: alkali basalt and nephelinite Water content of Tholeiite 0.214 wt%; hawaiite 0.695 wt%c Subaerial: 0.05Ð0.1 wt%b associated lava Bubble diameter 0.16Ð7 cm, average 2.2 cm Subaerial: several cm, up to 5Ð10 ma, b; shallow marine: 0.5Ð1 mf; experimental: 5Ð10 cme a Mattox and Mangan (1997) d Sample from Kilauea b Hon (1998, personal communication) e Zimanowski (1998, personal communication and sample) c Dixon (1998, personal communication) f Tribble (1991; no samples collected) 486 (Fig. 1b, d). The limbs of some folded limu fragments in Table 1. Figure 2 shows the limu shard thickness and are tack-welded to one another (Fig. 1c), but separate the original limu bubble size extrapolated from evenly shards are never welded to one another. Horizons of limu curved shards. occur throughout hyaloclastite deposits on Seamount The formation of bubbles is, potentially, possible by Six, and limu constitutes up to 25% of the shard popula- either exsolution of magmatic volatiles or expansion of tion. A comparison of morphological and petrological entrapped water. The former process is very unlikely be- characteristics between small platy limu shards in sea- cause diffusive growth of magmatic vesicles, under iso- floor hyaloclastite and subaerial limu fragments is given baric conditions (Sarda and Graham 1990), requires on the order of months to reach the dimensions of Seamount Six limu bubbles. The remainder of this paper provides a quantitative demonstration that deep-sea limu formation can occur by processes equivalent to those active subaerially near the shoreline. Since limu fragments are not ubiquitous in deep-sea hyaloclastite, factors favouring limu formation at Seamount Six and other known sites are also evaluated and their implications for other seafloor eruptions discussed.

Environment of limu formation on Seamount Six

Limu-bearing sheet hyaloclastite was formed at depths of 1,400Ð2,000 m bsl. Seamount slopes up to 15¡ are covered with lava talus and pillow tubes, and alternate with subhorizontal areas tens of square metres in extent of pahoehoe and sheet lava flows, sediment fields and hyaloclastite sheets. Planktic foraminiferal ooze, mostly Globigerina, is the main modern sediment on Seamount Six, ubiquitously infilling surface depressions. A very similar environment is likely to have existed during hyaloclastite formation on top of the Seamount Six edifice Fig. 2aÐb Curvature and thickness of 102 limu shards measured from thin section projections to a extrapolate original bubble di- some 10s to 100s of thousand years ago. ameter. b Bubble diameter versus limu shard thickness. Shards re- The limu-bearing hyaloclastite is associated with present 30Ð80¡ of the bubbles, given as angle of circumference sheet lavas (Fig. 3) which are (Maicher et al. 2000):

Fig. 3aÐd Critical features of thin lava flows associated with limu-bearing hyaloclastites. a Hand specimen of very thin, glassy, contorted sheet lava with empty vugs (arrow); stippled line shows original horizontal surface; no. 3013-1338. b Sheet lava covered with thick manganese crust, prominent vug filled with Globigerina ooze (arrow); no. 3016-1016. c Thin section of disrupted lava; tachylitic to the right, vug enclosing some hyaloclastite shards; no. 3013-1033. d Dis- turbed base of cryptocrystalline lava; cavity formerly filled with Globigerina ooze (arrow A); trails from the base con- necting with vesicles (arrow B); no. 3013-1215 487 jumbled, ropy or smooth, very thin (2Ð10 cm), and inferred to have had a very low viscosity (≅100 Pa s); glassy to cryptocrystalline, with sparse clusters of plagioclase phenocrysts; distributed randomly on gentle slopes and often rest- ing on sediment or lava rubble; in places contorted and disrupted, protruding above the hyaloclastite surface (Fig. 3a); have open vugs or vugs containing sediment (Fig. 3b) or hyaloclastite.

Fully subaqueous water entrapment mechanisms

Interpretations of how water was entrapped to form limu and hyaloclastite are constrained by the characteristics listed above of the invariably associated sheet flows, and Fig. 4aÐe Submarine sheet lava flowing onto a sediment-filled by the observations that (1) deposits lie on gently slop- pond. aÐc Thin flow, supported by sediment, covers a small pond ing, yet rugged terrain with ubiquitous sediment in topo- and displaces some material, developing an irregular base; or d graphic re-entrants, and (2) hyaloclastite is distributed sediment failure after fluidisation beneath lava, lava sinks into the patchily, and there are no systematic variations in hyalo- sediment and continues to flow beneath it, continuously fluidising and vaporising the wet sediment (arrow), sediment entrapment of clastite grain size, bed thickness or deposit thickness initially supporting sediment, e sediment entrapment in flow vugs across the 301-m vertical and 1.75-km2 horizontal zone near the flow front (arrow) of investigated sites. Sediment may be an important factor in water entrapment. Trapping of water beneath advancing lava flows such as Globigerina ooze, pore water drains freely away requires extreme lava fluidity and flow rates (see below) from the point of loading and the load stress acts only on because liquid water has no yield strength (Dorsey the particles. Moore (1962) calculated the bearing 1940). Lava flows can, however, be fully to partially strength b for surface samples of silts and silty sands supported by sediment (Moore et al. 1973), and we infer from an ocean depth of ca. 100 m and determined a that entrapment of water in overridden Globigerina ooze range of b=6.2Ð13.1 kPa. A lava flow exerts a pressure was an important process at Seamount Six. This is based Pm given by on the ubiquitous presence of sediment beneath hyaloclastite and associated thin lavas, and on observations of (1) lava-base irregularities (Fig. 3d) which are interpreted to record incipient, small-scale penetration and rise of onto the sediment per unit area A. Vm is the volume of la- ρ steam entraining Globigerina ooze into magma, with ti- va, g is the acceleration due to gravity, and m is the den- ny bubbles preserved in the lava. Steep flow bands wrap sity of lava. Established values for parameters used in around the bubbles and extend downwards into enclos- this and the following calculations are given in Table 2. ing lava. The bands are thought to be sutures formed as Following Eq. (1), the mass of lava flows up to 50 cm the melt closed-over behind rising bubbles. thick can be supported by such sediment. Bearing- strength measurements for loose Globigerina ooze are not available, but the well-sorted ooze would likely have Sediments beneath lava flows a similar or somewhat higher compressive strength under static load because of the absence of a lubricating, de- To evaluate the likelihood of sediment entrapment be- formable fine-grained matrix. Bearing strength is strictly neath advancing lava flows at Seamount Six (Fig. 4), we applicable to static loads only, and emplacement of lava consider the mechanical behaviour of unconsolidated is associated with some shearing of the substrate. Defor- sediment. Critical factors controlling the mechanical be- mation of the sediment associated with drainage of pore haviour of sediment which is loaded and heated by em- water, however, is controlled by the internal friction, co- placement of a lava flow are (1) static sediment bearing hesion and load imposed irrespective of the applied strength, (2) ability to drain expanding pore fluids, and stresses (Moore 1962). (3) framework toughness (e.g. Duffield et al. 1986).

Pore-fluid pressurisation, expansion and migration Static sediment bearing strength If pore fluids cannot migrate upwards through a lava A static load imposed onto sediment is carried by the flow (or laterally beyond the flow), addition of the lava’s elastic yield strength and, in a fully drained sediment mass at the sediment surface results, before any signifi- 488 Table 2 Definition of notationsa

Parameter Description Value Reference

Basalt (m) k (m2 sÐ1) Thermal diffusivity 3.8×10Ð7 Kennedy and Holser (1966) s (N mÐ2) Surface tension 0.35 (av.) Dragoni (1993) Ð1 Ð1 × 3 cp (J kg K ) Heat capacity 1.15 10 Murase and McBirney (1973) Tg (K) Glass transition temperature ca. 730 Ryan and Sammis (1981) K (J mÐ1 sÐ1 KÐ1) Thermal conductivity 1.06×10Ð4 Touloukian et al. (1981) ε Lava emissivity 0.9 Griffiths and Fink (1992) ρ (kg mÐ3) Density 2,700 Mattox and Mangan (1997)

Water (w) k (m2 sÐ1) Thermal diffusivity 1×10Ð7 Griffiths and Fink (1992) Ð1 Ð1 × 3 cp (J kg K ) Heat capacity 4.18 10 Griffiths and Fink (1992) K (J mÐ1 sÐ1 KÐ1) Thermal conductivity 5.32×10Ð5 Dorsey (1940) α Coefficient of thermal expansion 1×10Ð2 Bischoff and Rosenbauer (1985) β (PaÐ1) Compressibility 4×10Ð10 Freeze and Cherry (1979) η (Pa s) Viscosity 3.3×10Ð4 Delaney (1982) ρ (kg mÐ3) Density 1,000

Sediment (s) b (Pa) Bearing strength 6.2Ð13.1×103 Moore (1962) k (m2 sÐ1) Thermal diffusivity 4×10Ð7 (estim.) Jaeger (1959); Godinot (1987) K (J mÐ1 sÐ1 KÐ1) Thermal conductivity 8.8×10Ð5 Kennedy and Holser (1966) β (PaÐ1) Compressibility 1×10Ð10 Delaney (1982) κ (m2) Permeability 10Ð13 Wasp et al. (1977) φ Porosity 0.69 T (K) Temperature 1 S (J sÐ1 mÐ2 K4) Stefan-Boltzman constant 5.67×10Ð8 a Subscripts and other notations: m magma, bub bubble, s sedi- diffusivity (m2 sÐ1), p pressure (Pa), B buoyancy force (N), F heat ment, conv convective, rad radiation, vap vapour, h flow thickness flux (W mÐ2), Q thermal energy (J), V volume (m3), δ Boundary Ð1 σ (m), m mass (kg), r radius (mm), v velocity (m s ), w hydraulic layer thickness, v vertical stress cant transfer of heat, in an instantaneous load-induced ∆ σ increase in pore pressure Pv due to vertical stress v imposed by lava as given by Duffield et al. (1986):

(2)

σ Vertical stress v is for a 5-cm-thick lava flow (average thickness of sheet lava associated with limu-bearing hya- β loclastite), compressibility of the sediment is s and that β φ of the water is w, and the estimated total porosity is 0.69 for the dive-sampled Globigerina ooze. Heat transfer following emplacement of lava onto sediment causes thermal expansion of pore fluids and vaporisation at subcritical pressures (<298 bar or 2,980 m b.s.l., Fig. 5). Strong local pressure gradients, with pore pressure exceeding hydrostatic pressure, develop and lead to disruption of the sediment matrix. Pore pressure increase is greatest when the lava is emplaced rapidly (Delaney Fig. 5 Temperature-pressure diagram for water, showing critical 1982). The minimum contact temperature Tcontact can be points of freshwater and seawater, pressure range of Seamount Six calculated, using the equations given by Jaeger (1959): (shaded), and equal steam expansion for freshwater (solid lines); lines for seawater (stippled) are estimated. For seawater, the critical point is at ~407 ¡C and 298 bar (Bischoff and Rosenbauer 1988) (3) in which the temperature of the magma Tm=1,100 ¡C, the An initial minimum contact temperature between lava temperature of the sediment Ts=1 ¡C, K=thermal con- and seawater-saturated ooze (3.2% NaCl) is calculated as ductivity, and k=thermal diffusivity of the magma and about 600 ¡C, or for a contact with seawater alone as ca. sediment. 550 ¡C. This implies that, at any magma temperature 489 above the ca. 730 ¡C glass transition temperature of ba- over an overhanging or very steep ledge, projecting over saltic magma (Ryan and Sammis 1981), a vapour film water or sediment before touching down; (2) advancing will form at the interface (at subcritical local pressures). lava bridges gaps in talus; (3) lava overrides and/or bur- The value of 600 ¡C for the contact temperature with rows into sediment, and heated wet sediment rises into sediment is at best a first approximation of actual ther- lava; and (4) upper-surface lava flow vugs enclose sedi- mal behaviour, because pore-fluid flow away from the ment. site of heating is neglected. Nevertheless, for all situa- tions any increase in temperature is accompanied by 1. A vertical or very sharp drop, such as the steep side of ∆ some further increase in pore pressure Pp (after Duf- a talus block, can act as a ramp for fast-flowing lava field et al. 1986): (0.2 m sÐ1). We infer that detachment and movement away from a ramp is largely driven by flow momen- tum, analogous to a ballistic projectile. From funda- (4) mental equations of motion, neglecting effects of magma viscosity and flow continuity, it is calculated where C is a dimensionless measure for the expansion of that the front of a 10-cm-thick lava flow leaving a 20- water under ambient pressure of 0.93, and the hydraulic cm-high ramp would advance no more than 4 cm be- diffusivity w and the compressibility β of the water/ fore landing to entrap overridden water and/or sedi- β β φ β φ sediment mixture are given by = w+[(1- ) s/ ], ment. Such a fast-flowing and highly fluid lava may, κ φη β κ η w= /( w ) with =absolute permeability, and =dy- however, spread and “unfurl” as it moves from a solid namic viscosity of water. Heat transfer by thermally in- ramp substrate into water, possibly slowing down its duced convection is negligible, and conduction is domi- vertical fall and allowing the lava to cover a larger nant (Delaney 1982; Francis 1982). The pressure effects horizontal distance. This is particularly true if vapour on the viscosity of water can be disregarded under the forms beneath a thin unfurling sheet. With this mech- pressure regime considered (Sverdrup et al. 1942). anism, either water may be entrapped during fall, or a ∆ ∆ ∆ The total pore pressure increase is P= Pv+ Pp= pocket of sediment covered and enclosed. Steep flow ~100 kPa, matching well with experimental data from fronts of older flows and talus blocks, ubiquitously Delaney (1982) which indicate that virtually all rocks associated with hyaloclastite, may provide appropri- with permeabilities >10Ð13 m2 undergo pressure increases ate ramps for small flows or flow tongues. of less than 1 MPa. The pore-fluid pressure is now sig- 2. Where lava “caterpillars” across a gap between talus nificantly higher than the overburden pressure of fragments, the size of the gap it can span depends on 1.35 kPa exerted by lava of 5 cm thickness [following the flow’s velocity and the viscosity of the lava. Low Eq. (1)]. The most likely consequences of the high pres- viscosity, fast-flowing Seamount Six lavas may have sure gradient are either uplift of the whole lava layer or been able to span gaps of several centimetres width. “injection” of the water vapour-sediment mixture into 3. Unlike water, unconsolidated, cohesionless deep-sea the lava above (Fig. 3d), provided that no breach of con- Globigerina ooze possesses a finite bearing strength, finement occurs. and can support the weight of a lava flow up to 50 cm thick (Eq. 1). The bases of some sampled lava flows are irregular, possibly due to localised loss of strength Framework toughness in the sediment and the presence of rising steam. Two mechanisms for the rise of heated sediment into the The emplacement of a lava flow changes the drainage lava are envisaged, (1) buoyant rise of steam under conditions by blocking the exchange of pore water with unconfined conditions, and (2) pressurised upward the water column along the lava-sediment contact expansion of heated sediment. Sealing of a sediment (Godinot 1987). Emplacement is often accompanied by pond beneath lava (Fig. 4aÐc) creates confined, un- liquefaction and fluidisation of underlying sediment due drained conditions within the enclosed water-sedi- to rapid and uneven loading resulting in collapse, de- ment mixture, provided that the base of the pond is watering and heating (Schmincke 1967; Kokelaar 1982; impermeable. White and Busby-Spera 1987). Expansion and migration of pore fluids may lead to rapid pore-fluid convection or Complete failure of the sediment can occur where (1) a local along-contact fluid flow which disrupts the sedi- lava flow emplaced on a sediment surface exceeds the ment framework and reduces or destroys the sediment’s bearing strength of the sediment, and the accompanying yield strength. heating and loading causes fluidisation and thus loss of strength of the sediment, or (2) a lava flow dropping from a ramp shears the ooze. In each instance, the lava Submarine water entrapment mechanisms will sink into the sediment, possibly enclosing some Globigerina ooze (Fig. 4d). Entrapment of water and/or water-saturated sediment The lava may continue to flow along the base of the may, based on dive observations, lead to full confine- sediment pond, and flow-top vugs, described below, pro- ment of the entrapped water if (1) a stream of lava pours vide some observational evidence for another style of 490 sediment entrapment during flowage beneath (fluidised) is heated well above the boiling point. The rate of va- sediment. porisation for the superheated water is so great that vapour expansion can briefly and unstably proceed to the 4. Extremely glassy, thin, ropy pahoehoe flows from Se- point of producing lower pressure in the vapour than the amount Six exhibit tight, closed folds up to 15 cm confining pressure exerted by the surrounding liquid long at upper lava flow surfaces (Fig. 3a, b). The (Wohletz and McQueen 1984). This implies that “the folds form so-called sheet flow vugs which enclose critical point of water is not necessarily a limiting factor calcareous mud, foraminifers and/or hyaloclastite. in vapour explosions” (Wohletz 2001). Such folds, some with walls <1 mm thick, indicate Vaporisation occurs either on reaching the homogene- extremely low viscosity lava (<100 Pa s). Capture of ous nucleation temperature at which all water vaporises the sediment in the flow folds is possible only where instantaneously, or when the system is perturbed by an ex- lava has flowed beneath sediment (Fig. 4e). ternal trigger (Wohletz 1986). It is not yet clear whether such superheat vapour expansion plays a role in deep-sea Other possible scenarios include entrapment of water by limu formation, but it does provide a possible mechanism collapse of a lava-tube roof, as suggested by Clague et for producing steam expansion at depths greater than the al. (2000). Mingling of rapidly moving magma with wa- critical pressure. Without strong confinement, however, ter after failure and drainage of pillow tubes, and small- heat energy dissipates in the ocean, preventing sufficient scale lava fountaining at a restricted vent (Maicher et al. build-up of superheated water to initiate and sustain sig- 2000) may provide additional mechanisms of entrapping nificant explosions (Wohletz and McQueen 1984). water within clots of melt. As stated by Kokelaar (1982), full confinement of water during magma-sediment mingling, which would al- Comparison of physical properties of freshwater low pressurisation, is unlikely to be common. The high and seawater percentage of limu shards in Seamount Six hyaloclastites indicates, however, the existence of one or several mech- Most physical properties of freshwater and seawater are anisms of water entrapment and successful pressurisation very similar (e.g. Dorsey 1940; Sverdrup et al. 1942; which led to limu bubble formation. We infer that thin Bischoff and Rosenbauer 1985). Dissolved components lava flows which covered sediment, including those and suspended particles do, however, have some influ- which subsequently flowed beneath sediment, formed ence on the physical behaviour of water (Wohletz 1986), most limu at Seamount Six. Folding of both upper and such as a lessened heat capacity for particle-bearing wa- lower sheet flow surfaces into vugs was an important ter, which allows increased temperature rise of a mixed process of entrapment. system upon heating. The greater heat conductivity of particles can, in contrast, reduce the rate at which water in a mixture is raised to boiling temperatures (White Physics of submarine limu formation 1996). Seawater itself is 9% less thermally conductive than pure water (Dorsey 1940). Despite these differ- Here the thermodynamic behaviour of water is reviewed ences, steam from seawater can be considered to be pure and the physical forces acting on a limu bubble forming H2O, with NaCl effectively partitioned between liquid under deep-marine conditions assessed. Information is and vapour (Bischoff and Rosenbauer 1988). drawn from experimental, physical and engineering studies of heat transfer, lava flow emplacement and sill intrusion. Heat transfer and magma cooling

The mechanism of heat transfer and the cooling rate de- Behaviour of water and water vapour under pressure pend critically on the medium into which a magma is erupted. Air is a poor heat conductor, and heat is lost Water in the deep sea experiences hydrostatic pressure slowly by radiation and convection, with the cooling la- resulting from the overlying water column. The pressure va surface remaining plastic for some time. Under sub- range considered here is 140Ð200 bar (14Ð20 MPa), pro- marine conditions heat transfer is more rapid, occurring portional to water depths of 1,400Ð2,000 m. Steam ex- by conduction at direct magma-water contacts or across pansion, the driving motor of limu bubble formation, is a steam film at the interface. Subsequently, heat is re- strongly pressure dependent. Under high pressures, ex- moved by convection of the overlying water column. A pansion due to simple heating of seawater to the boiling steam film sharply reduces the rate of heat transfer point (liquid/vapour phase boundary) is progressively re- (Walker 1977; Mills 1984), and the main mechanism of duced until the critical point of water is reached (Fig. 5). heat transfer across the vapour film is steam convection Rapid heating of water at the contact with subaqueou- below 500 ¡C (Walker 1977). Above 500 ¡C, radiation is sly extruding magma is commonly not simple, however, the predominant heat-transfer process (Mills 1984). and may involve passage through a non-equilibrium Griffiths and Fink (1992) calculated the contact tem- thermodynamic state of superheating in which the water peratures and thickness of solidified crust of basaltic 491 melt at 1,150 ¡C in contact with seawater of 7 ¡C. Con- vection is considered to be the dominant mechanism for heat transfer at a vapour-free magma/water interface, with also a small component of radiative heat transfer. Movement with time of solidus isotherms for basalt at 1,100 ¡C and glass transition temperature Tg at 730 ¡C (Ryan and Sammis 1981) are illustrated. The latter is the physically relevant temperature for quick cooling of magma to form hyaloclastite or to halt growth of limu bubbles. Calculations of vapour-free heat flux (Griffiths and Fink 1992; Gregg and Chadwick 1996) from the surface of deep submarine lava flows combine convective Fig. 6 Heat fluxes under subaerial and submarine conditions; note Fconv and radiative Frad fluxes. The effect of radiation is that scale of heat flux is logarithmic very small, e.g. it decreases solidification times by 4% (Griffiths and Fink 1992). Frad and Fconv are given by The calculations show that the heat flux across the va- (5) pour film from the hemisphere of a 2.2-cm-diameter bubble is an order of magnitude lower than the combined (6) radiative and convective heat flux from the same surface area during direct magma-water contact. Figure 6 illus- with a lava emissivity ε of 0.9, a constant γ close to 0.1, trates heat-flux variations over the temperature interval Stefan-Boltzman constant S and heat capacity of water from 1,150 to 730 ¡C. Heat fluxes from magma covered with a vapour film take an intermediate position between cp. The coefficient of thermal expansion rises non-linear- ly with temperature (Bischoff and Rosenbauer 1985) and those of subaerial and deep-marine lavas with no vapour a value close to the critical point, α=1×10Ð2, is taken formed. At initially high temperatures, the heat flux (and thus, cooling) is comparable to subaerial fluxes. here. The total heat flux is Ftot=Frad+Fconv.

Cooling of a magma surface through a vapour film Bubble expansion and penetration A vapour film forms as erupting lava contacts seawater The scenarios of sediment entrapment outlined above re- (Moore et al. 1973), and inhibits rapid heat transfer quire either (1) vaporisation of fully entrapped water to (Walker 1977; Wohletz 1986) because the thermal con- form bubbles in lava, or (2) penetration of steam, formed ductivity of steam is 1/30th that of water. The heat flux beneath a lava flow, upwards into the lava. Both cases from lava to the steam film, mainly by conduction, con- are discussed in the following section. trols total heat flux (Wohletz 1983). Heated water is dis- persed into the water column by turbulent convection (Griffiths and Fink 1992), preventing significant heating Bubble expansion of the water adjacent to the lava (e.g. Moore et al. 1973; Tribble 1991). The basal contact of a lava with solid rock Water trapped and heated in a lava flow will try to ex- is dominated by conductive heat transfer, whereas a pand against the confining pressure exerted by the mag- magma-wet sediment contact is likely to involve a steam ma and the overlying water column. Expansion requires film (cf. Kokelaar 1982), similarly to the upper magma- that the gas pressure within a heated water drop of a giv- water contact. A vapour film, as outlined by Sparks en size exceeds the confining pressure of surface tension (1978) and Wohletz (1983), acts as an insulator by re- and pressure of the overlying water column plus that of ducing heat fluxes: the lava flow, and the resistance to growth resulting from the viscosity of magma. These inter-related factors are (7) rapidly changing functions of time. Adaptation of bubble growth equations (McBirney and Murase 1971; Sparks with the radius of a melt sphere r1, and the radius of a 1978) leads to melt sphere plus the surrounding vapour film r , mea- 2 (8) sured from the centre of the sphere. Limu bubbles are not solid melt spheres but shells of melt surrounding a where Ph is the hydrostatic pressure plus the pressure of steam-filled sphere. Only the outer 0.8 mm (Sparks the overlying lava flow, Pσ the surface tension term 1978) of spheres is involved in the heat transfer process, which gives the pressure to balance the surface tension however, and many limu shards are of this order of of the liquid-gas interface, and Pη is the pressure re- thickness. The thickness of the vapour film is taken to be quired to expand against the viscous resistance of the 1/10 the diameter of a melt sphere (Wohletz 1983, and magma. The second and third terms are the same for following Eq. (11), this paper). subaerial and subaqueously formed bubbles, which are 2 492

Fig. 7aÐd Process of submarine bubble formation aÐb in an un- A bubble of 2.2-cm diameter will experience a buoyancy crusted lava flow, and cÐd in a lava flow with a brittle crust. a En- force of 0.1 N, which corresponds to a buoyancy pres- trapped sediment underneath lava flow is fluidised, vaporised, and pressurised pore fluids bulge upwards from the substrate-lava in- sure of 111 Pa. Application of Stoke’s law shows that an terface into lava, driven by buoyancy force and balanced by vis- updoming area, with a maximum diameter of 10 cm, is cous drag force of lava and visco-elastic surface skin. Note en- sufficiently buoyant to force the basal visco-elastic skin trained sediment. b Diapiric rise of steam bulge to stretch the vis- upwards into the lava flow (Fig. 7). co-elastic layers into a limu bubble skin. A steam film protects the forming bubble. c A lava flow with a rigid, brittle upper surface crust and visco-elastic sublayer with steam trapped and ponding beneath the crust (either individual steam bubble or larger steam Bubble breaching lava flow surface reservoir), in which crack formation occurs, penetrating the brittle crust downwards into the visco-elastic sublayer. d Steam bulge What force drives a steam bubble out of a lava flow? breaches through crack, stretching the visco-elastic sublayer into a limu bubble with a protecting steam film The buoyancy force of a lighter medium within a denser one causes the upward movement, which is accompanied by a small (for thin lava) pressure decrease. At the upper and 4 orders of magnitude, respectively, smaller than the surface of unchilled lava, a steam bubble will experience first term. Under isobaric conditions, no decrease in the a slight drop in confining pressure due to the much lower viscosity of water reducing resistance to growth. Howev- first term, Ph, occurs. Hence, the confining pressure, two orders of magnitude higher for submarine Seamount Six er, a bubble rising to the top of a lava flow typically en- conditions than subaerially, is the dominant component counters a surface crust. If it is a brittle crust, the bubble of the total pressure which the expanding steam has to needs to either break through solid basaltic material or overcome. breach through liquid magma along a crack in the crust. The tensile strength of basaltic material, some 210Ð460 MPa at point of failure (Kennedy and Holser Steam penetration into lava 1966, p. 277), is several orders of magnitude higher than the stresses exerted by an individual limu bubble (fol- Water-saturated sediment overridden by lava is immedi- lowing from Eqs. 8, 9). Bubble escape through a solid ately and increasingly pressurised. If the weakest point crust is unlikely. of confinement is the overriding lava, it may be penetrat- As outlined in the shallow-marine limu discussion, a ed by the steam, especially if the buoyancy force of the trapped steam bubble underneath a crust can emerge gas exceeds the drag force exerted by melt viscosity. The freely when encountering a newly formed crack in the crust, effectively breaching through uncrusted lava. A buoyancy force of a bubble Bbub depends on the density ρ Ð3 comparison of the buoyancy force of a steam bubble bub=30 kg m and volume Vbub of the bubble, and it is equal to the weight of lava displaced by the bubble, ac- (Eq. 9) with the surface tension of the lava-gas inter- cording to Archimedes’ principle (after Tipler 1982). face of 0.35 N mÐ2 shows that the surface tension of uncrusted lava is insufficient to prevent bubble emer- (9) gence. 493 Bursting of bubbles energy heats, over a surface area of 7.6 cm2, a water mass of 0.087 g. The specific volume of steam at 700 ¡C Crystal-free, basaltic magma behaves as a Newtonian flu- and 200 bar is 21 cm3 gÐ1 (Kennedy and Holser 1966). id. A transformation to non-Newtonian behaviour occurs Therefore, a corresponding steam film has a thickness of under high strain rates, where the melt crosses the glass about 2.3 mm, which is a tenth of the bubble diameter. transition and deforms in a brittle rather than ductile fash- This calculation has only limited validity, as it as- ion (Sparks and Pinkerton 1978; Webb and Dingwell sumes that no heat loss from the film to the water occurs, 1990; Bottinga 1994). The stresses required for such strain that no heat is used to compensate for adiabatic cooling rates are on the order of 108 Pa (Webb and Dingwell during steam expansion in the interior of the limu bub- 1990), and are unlikely to be generated during limu bubble, and that all available energy is used to form steam. ble expansion. In a subaqueous environment, bubble These effects, however, might be balanced by the also bursting can also be caused by rapid quenching and ther- neglected effects of additional heat supply from the main mal shock granulation during direct magma-water contact lava body and contemporaneously formed blocky shards, after steam film collapse. Plates formed in this way, how- as well as incremental bubble growth (initial heat losses ever, are unlikely to show the wrinkling or tack-welding from a smaller surface area would be lower). of shards observed in these limu, both of which require plastic-state bursting at elevated temperature and a protecting steam film. Furthermore, for wrinkling to take Deep-sea limu formation model place, the steam film possibly needs to be retained long enough to allow surface tension reshaping of the plates. Critical physical features of deep-marine limu fragments The preserved limu edges are sharply broken and and their formative processes are: show no signs of necking. It is not certain whether this brittle-state fragmentation represents part of the primary 1. Limu formation seems to be restricted to low viscosi- breaking up of the limu bubbles or is the product of sub- ty (<200 Pa s), basaltic magma and especially to thin, sequent breakage during later chilling, impacts during glassy sheet lava flows. transport, deposition (unlikely to be a significant grain 2. Limu formation is possible only in uncrusted fresh la- modifier in this context), or compaction (unlikely, be- va, either at the flow front or along cracks. cause adjacent clasts rarely show jigsaw-fit margins). 3. Limu bubble expansion is driven by entrapped water- Nevertheless, the absence of observed necking at limu saturated sediment of varying quantity, either beneath margins among the thousands of fragments studied here a lava flow or within surface folds (vugs). suggests that final bubble fragmentation was a brittle 4. Hydrostatic pressure is the factor controlling steam fragmentation phenomenon, and we infer that it is most expansion, as well as the pressurisation of entrapped plausibly related to aqueous chilling very closely follow- pore fluids. ing bubble bursting. 5. Steam forms over the whole cooling interval from the eruption temperature to the glass transition temperature of basaltic magma; the rate of steam formation Energy balances and proportional expansion are unaffected by pressure. To determine the amount and thickness of a vapour film 6. Convective heat transfer across a vapour film is shielding a limu bubble from the surrounding seawater, roughly equivalent to subaerial heat transfer rates at the thermal energy Q of the magma must be calculated. initial eruptive temperatures. If all heat in a limu skin is used to vaporise a volume of 7. The time span for submarine bubble cooling (1Ð2 s) is seawater, the energy balance can be calculated (after Tip- adequate to allow bubble expansion prior to cooling ler 1982): to the glass transition temperature (10) 8. Bubble bursting occurs when quench shattering occurs by cooling below the minimum contact tempera- × Ð4 The mass of magma mm=3.1 10 kg, and cooling is ture of steam formation and/or stretching rates exceed over a temperature interval of 376 ¡C (1,100Ð724 ¡C, the ductile-brittle threshold. minimum magma-water contact temperature, Eq. 3). The output amounts to 130 J per average limu bubble (2.2-cm Based on these points and by analogy with subaerial ob- diameter, 0.14-mm melt-wall thickness). This energy is servations, individual deep-sea limu are inferred to form available to form a steam film at the interface. Equa- as follows. Sediment is entrapped by lava underneath tion (10) can be rewritten as flows which bridge seafloor irregularities or travel over sediment (Fig. 4a, b), or by envelopment during lava- (11) flow burrowing (Fig. 4d, e). Heating and loading of pore fluid in the sediment lead to sediment fluidisation and The temperature difference between the boiling point of pressurisation, driving sediment particles and a steam seawater under 200 bar and the initial temperature of bulge upwards into, or lifting up, the lava. Steam pene- seawater on Seamount Six is 359 ¡C. Thus, the available tration into the lava results from rise through uncrusted 494 lava by upward stretching of the basal and upper lava flow skins (Fig. 7a, b), or from steam entrance through cracks in base-crusted lava (Fig. 7c, d). Limu bubbles rise through and emerge from a lava flow by stretching of the surface skin. During bubble expansion, the outer surface of the fragile skin is protected by a vapour film of relatively low heat transfer properties. Bubble expansion ceases either when the maximum degree of steam expansion is reached, or after sufficient melt cooling, or as a result of the bubble bursting. Energy balance calcu- Fig. 8 Position of limu formation sites in both the active, uncrust- lations suggest vapour-film thickness lies within the ed lava flow front and through newly formed cracks in older crust range inferred from experimental data (Wohletz 1983). After bubble emergence and bursting, the lava surface is steam reservoir has sufficient buoyancy force and kinetic either preserved in its contorted shape, or heals, poten- energy to break through a solid crust formed on a lava tially allowing renewed limu formation. After bubble flow. The force is just enough to overcome the surface burst, individual limu plates are either chilled “instant- tension of hot, fresh magma. It is concluded that limu ly”, preserving the gentle curvature of the original bub- bubbles most likely breach either fresh, uncrusted lava of ble (Fig. 1a), or retained in a steam envelope on time- the flow front or along newly formed cracks in thick- scales long enough for modification of the shard curva- ened, brittle crust (Fig. 8). ture and tack welding. Limu bubble formation is probably restricted to basaltic melts extruded at high temperatures and low viscosi- ties. Thin submarine sheet lavas are inferred to extrude Discussion with high discharge rates (Bonatti and Harrison 1988; Holcomb et al. 1988) and represent flows which advance The presence of fluidal bubble-wall shards in Seamount with relatively little crust formation (Griffiths and Fink Six and other hyaloclastites has posed an enigma for 1992). many years. Exsolution of magmatic volatiles has been Limu in sheet hyaloclastite deposits, consisting most- shown to be unlikely to expand such bubbles. Recent ob- ly of cogenetic blocky, non-vesicular glassy shards and servations of similar shards forming subaerially and in in association with geochemically coeval lava flows, are laboratory experiments demonstrate that bubbles can be inferred to indicate short transport and deposition near blown in magma through interaction with water, and the site of formation. Observed ocean currents are un- give us confidence in suggesting mechanisms based on likely to redistribute hyaloclastite shards significantly entrapment of water and water-bearing sediment. The (Maicher et al. 2000). mechanisms are developed from subaerial observations Several models for the formation of limu-bearing and modified to fit Alvin dive observations and recov- sheet hyaloclastite are developed. (1) Deep-marine bub- ered samples, and are supported by the absence of limu- ble formation might be closely associated with sup- bearing hyaloclastite on the sediment-free East Pacific pressed tephra jetting activity, based on observations Rise (Batiza et al. 1998). during lava delta collapse in Kilauea, our favoured mod- Thermodynamic, physical and rheological parameters el. Alternatively, (2) mingling of water with rapidly of bubble formation show that stretching of a lava skin moving melt during small-scale magma fountaining driv- depends crucially on the time available to stretch the en by magmatic volatile exsolution and extreme vent skin before it has cooled down to the glass transition constriction probably leads to magma-water interaction temperature. Cooling during direct magma-water contact producing limu and blocky shards, which would explain is very fast, too fast to allow bubble expansion. In con- how water is partially degassed without preserved vesi- trast, the cooling rates of magma across a vapour film cles. (3) Similarly to (1), collapse of pillows and rapid are much lower, similar to subaerial cooling rates at high drainage of the magma may lead to limu-bearing sheet magma temperatures. We conclude that the timeframe of hyaloclastite formation. (4) Water entrapped in sheet la- 1Ð2 s provided by magma cooling through a vapour film va flow vugs, though potentially producing limu bub- is long enough to blow a limu bubble (by analogy with bles, is unlikely to be related to any further fragmenta- subaerial bubble-formation times). tion of the melt. The total quantity of steam involved in limu bubble The occurrence of limu seems to be restricted to ei- formation is difficult to assess because it is not clear ther subaerial settings where lava meets the sea, or sub- whether each limu bubble forms singly or as multiple aqueously to a depth range of about 800 to 3,200 m b.s.l. bubbles as observed at Kilauea (Kjargaard 1990). In the Limu shards are unknown from deposits formed in shal- second case, a much larger steam reservoir, either a low subaqueous Surtseyan-type eruptions. steam accumulation beneath the surface crust of the lava At Kilauea, limu-forming lava is mostly degassed, flow (Fig. 7c, d), or a continuously produced and mobile and limu bubbles in laboratory experiments are produced layer of steam from ponded sediment would be required. by using re-melted, almost totally degassed magma. Se- Neither an individual limu bubble nor a reasonably sized amount Six hyaloclastites are partly degassed (water), 495 partly supersaturated (CO2) and erupted at a depth level Dorsey NE (1940) Properties of ordinary water-substance. Am at which volatile exsolution is considered to be ineffec- Chem Soc Monogr Ser, Reinhold Publishing Company, New York tive in inducing magmatic fragmentation (Fisher and Dragoni M (1993) Modelling the rheology and cooling of lava Schmincke 1984). Restriction of bubble formation at flows. In: Kilburn CRJ, Luongo G (eds) Active lavas. UCL greater depth is explained by higher confining pressures Press, London, pp 374 suppressing or prohibiting steam expansion. Duffield WA, Bacon CR, Delaney PT (1986) Deformation of poorly consolidated sediment during shallow emplacement of In shallow subaqueous eruptions, however, magma- a basalt sill, Coso Range, California. Bull Volcanol 48:97Ð107 water interactions are highly explosive (e.g. Fisher and Fisher RV, Schmincke H-U (1984) Pyroclastic rocks. Springer, Schmincke 1984; Wohletz 1986), and eruption violence Berlin Heidelberg New York is apparently enhanced by strong exsolution of magmatic Fouquet Y, Barriga F, Charlou JL, Elderfield H, German CR, volatiles, e.g. Kokelaar (1986). Steam formation, expan- Ondréas H, Parson L, Radford-Knoery J, Relvas J, Ribeiro A, Schultz A, Apprioual R, Cambon P, Costa I, Donval JP, sion and collapse are possibly too vigorous for a steam Douville E, Landuré JY, Nromand A, Pellé H, Ponsevera E, film to act as a protecting shield for the growing bubble, Riches S, Santana H, Stephan M (1998) FLORES diving instead fragmenting any incipient limu bubbles. cruise with the Nautile near the Azores Ð first dives on the In summary, subaqueous limu bubble formation oc- Rainbow field: hydrothermal seawater/mantle interaction. Int Ridge-Crest News 7:24Ð28 curs in the absence of vigorous magmatic fragmentation Francis TJ (1982) Thermal expansion effects in deep-sea sedi- and explosive magma-water interaction, in the presence ments. Nature 299:334Ð336 of protective steam films. The presence of limu shards in Freeze RA, Cherry JA (1979) Groundwater. Prentice Hall, New hyaloclastite deposits can be seen as a paleoenvironment York Godinot A (1987) Intrusion of Miocene dikes into wet tephra, and paleodepth indicator. Kaipara Harbour, Northland New Zealand. PhD Thesis, Uni- versity of Auckland Acknowledgements This work presents results of the PhD Thesis Gregg TKP, Chadwick JWW (1996) Submarine lava-flow infla- of DM. Many thanks go to Rodey Batiza for cruise leadership, fi- tion: a model for the formation of lava pillars. Geology nancial support, fruitful discussions and constructive comments 24:981Ð984 which helped considerably to develop the ideas presented. The au- Griffiths RW, Fink JH (1992) Solidification and morphology of thors appreciate manuscript reviews by L. Wilson and S. Lane. submarine lavas: a dependence on extrusion rate. J Geophys The research was funded by NSF grant OCE 90-00193 to Rodey Res 97:19729Ð19737 Batiza. Additional support from the University of Otago Research Heiken G, Wohletz K (1985) Volcanic ash. University California Grant MFK-B70 to J.D.L.W. and an Otago Postgraduate scholar- Press ship to D.M. are gratefully acknowledged. Heliker CC, Mattox TN, Mangan MT, Kauahikaua J (1993) Kil- auea Volcano update: eleven-year-long eruption continues. EOS 74(43):644 References Holcomb RT, Moore JG, Lipman PW, Belderson RH (1988) Volu- minous submarine lava flows from Hawaiian volcanoes. Geol- Batiza R, Fornari DJ, Vanko DA, Lonsdale P (1984) Craters, cal- ogy 16:400Ð404 deras, and hyaloclastites on young Pacific seamounts. J Geo- Hon K, Heliker C, Kjargaard JI (1988) Limu o Pele: a new kind of phys Res 89:8371Ð8390 hydroclastic tephra from Kilauea Volcano, Hawaii. Geol Soc Batiza R, Becker N, Bercovici D, Coleman T, Gorman T, Head III Am Abstr Prog 20(7):112Ð113 JW, Holloway L, Karsten J, Kelly A, Keszthelyi LP, Maicher Jaeger JC (1959) Temperatures outside a cooling intrusive sheet. 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