Volcanic flame: source of fuel and relation to volcanic gas-lava equilibrium

Qeochimics et Cosmochimica Acts, 1973,Yol. 37,pp. 11133 to 1169. Pergamon Press. Printedin Northern Ireland

Volcanic flame : source of fuel and relation to volcanic gas-lava equilibrium* JOHN J. NAUGHTON Hawaii

Institute

(Received

of Geophysics and Chemistry Department, Honolulu, Hawaii 96822, U.S.A. 3 August

1972; accepted

in revised form

University

of Hawaii,

18 October 1972)

Abstra&--Flame formation at or near volcanic vents is a somewhat unusual phenomenon, but during October 1970 true flames were observed to issue at intervals from a series of vents in a lava tube or from spatter cones associated with the new eruptive center (Mauna Ulu) on the This situation was similar to that under which southeast rift of Kilauea volcano, Hawaii. successful volcanic gas collections have been made in the past in Hawaii and elsewhere. After several attempts two gas samples were collected successfully between flaming episodes. Analysis showed these to be chiefly water vapor with a small amount of hydrogen and carbon dioxide. The probable source of the hydrogen in the gas is explained on the basis of the high-temperature equilibria involved. The flame-forming properties of dilute mixtures of hydrogen and noncombustible gas were studied and the most dilute mixtures that would produce flames under Intermittent flaming can be ascribed volcanic conditions were measured (flammability limit). to the periodic attainment of the flammability limit for hydrogen in the volcanic gas mixture, which wss found to be achieved in one of the collected samples.

INTRODUCTION TEIE NATURE, and even the existence,

of primary flames in or near volcanic vents has been the subject of some controversy among volcanologists. Particularly puzzling is the nature and source of the fuel being burned in such flames. In the best samples of volcanic gas collected to date, there does not seem to be any combustible component present in sufficient quantity to account for the flame formation. The sporadic occurrence of this phenomenon would indicate that a peculiar set of physical circumstances is needed for its generation. Many eruptions occur with no such flaming, and many long-time observers at eruptions have not seen the process at work (DANA, 1890). In addition, the flame usually is pale and transparent, difficult or impossible to see in daylight, and is obscured easily by the more brilliant display of nearby incandescent lava. Flames over lava flows advancing through vegetation or other combustibles are common. However, the process taking place in the sterile and worked-over area of the primary vent, which presumably results from the oxidation of primary magmatic combustibles, is the subject of this discussion. JAC-GAR (lQlSa, 1918b) in his long-term observations at Kilauea made many detailed reports on flame phenomena, and made visual observations of spectra from this source. Associated with this study he described a method of locating flames in the presence of incandescent lava by observing the suspected location through a spectroscopic prism, when the flame spectrum is identifiable even in the presence of the spectral continuum from the lava. He failed to find flames associated with lava fountaining. On the other hand, PERRET (1913) claimed to note a momentary flash of flame associated with certain types of lava fountain. JAGGAR (1917) has * Hawaii Institute of Geophysics Contribution No. 513. 1163

1164

JOHN J.

NAUUHTON

ascribed an important role to surface and near-surface oxidation as a supplier of the heat needed to maintain the high temperature of an active lava lake. He found support for this theory in his observations of higher temperatures in the surface layer of the lava lake with lower temperatures below. Flame blasts from a blowing cone gave the highest temperatures observed, estimated to be in excess of 1350°C. Many of the classic collections of volcanic gas made by Shepherd and Jaggar at Kilauea in the period 1912 to 1919 (SHEPHERD, 1938) were sampled from around flaming areas in cupolas and cracks. More recently, detailed observations of volcanic flames and the analyses of gases collected from flaming vents have been reported for Niragongo in the Congo (TAZIEFF, 1966) and for Surtsey in Iceland (SIWALDASON and ELISSON, 1968). Another concern in the study of volcanic flames has been the quantitative measurement of the emission spectra from such sources with a view to an understanding of the chemistry and energetics involved. VERHOOGEN (1939) made such measurements at Nyamlagira volcano in the Congo, with later work reported by DELSEMME (1960) on Nyirangongo volcano, and by MURATA (1960) at Kilauea. Most recently, CRUIKSHANK and MORRISON (1972) have reported the fist direct evidence that hydrogen is the combustible involved in flame formation from studies of spectra obtained during a flaming episode at Kilauea. EXPERIMENTAL The phases and characteristics of the flank eruption of Kilaues volcano currently &king place with the main center of activity at Mauna Ulu (‘Growing Mountain’) have been described by SWANSON et al. (1971). The fissure which opened on the eastern apron of Mauna Ulu in July 1970 had been roofed over by October 1970, with small lava mounds and sp&ter cones marking areas where holes pierced the roof. At that time a rapidly flowing river of lava could be observed through some of these openings. Presumably at the point examined this was a portion of the lava tube which was feeding subcrustally into a lava lake in the process of filling an older pit crater (Alae). Since these openings were approachable it was suggested by D. Peterson of the Hawaii Volcano Observatory, U. S. Geological Survey, that they might offer a suitable opportunity for making direct gas collections. Accordingly, silica gel collecting tubes of the type described previously (NAUQJZTON et al., 1963) were prepared, and on October 9, 1970 a group Five samples were taken from two holes, but, entered the area to attempt gas sampling. because of the high temperatures and the tendency of the glass tubes to soften and collapse, only two samples were subsequently found to be satisfactory. The vents were not emitting gas at a noticeable pressure at the time of collection. The temperature within the main flowing tube where MU-2 (Table 1) was collected was estimated visually to be roughly llOO’C, and for the grotto where MU-4 was collected, roughly 800%. After these collections had been made an intense roaring W~EZnoted from the vents, which The collections provod to be produced by very pale flames of the character previously described. were made quite by chance, then, from the vents during a quiet period previous to this flaming episode. Later these episodes were observed to occur at irregular intervals and to persist for about a minute. Unfortunately all of our sampling tubes had been used and no collections from within the flame were possible. At a later time this entire trench area collapsed, so that no such favorable collecting opportunities were available to us again. At the site the sample tubes were sealed immediately on removal by means of fluorocarbon caps which shrink-seal on to the hot tubes. A short time later the glass of the tubes was sealed with a torch. Analysis was by gas chromatography using equipment and procedures similar to those described previously (NAUGHTON et al., 1963). The results of the analyses are listed in Table 1. From the reduced nature of the gas, incompatible with oxygen at collecting temperatures, ambient air was not part of the original gas mixture but doubtlessly ws,s taken into the

Volcanic Table

1. Analyzed

flame:

source of fuel and relation

and equilibrium

content of components volcanic gas Analyses

Sample

Hz0

4.03 *MU-2 (as analyzed) MU-2 (corrected for air 92.7 content) 88.9 *MU-4 (as analyzed) MU-4 (corrected for air content) 99.9 Calculated equilibrium composition-based on H,O and CO, 92.7 Calculated equilibrium composition-based on all active vapor components 94.0 * Also analyzed

to volcanic

gas-lava

relevant

(mole

1165

equilibrium

to combustion

in Hawaiian

%)

Ar

02

N,

CO,

CO

“/o Air

0.103

0.817

22.32

72.67

0.512

0

96.0

2.37 0.0352

0.069

4.94 0.044

0 0

11.1

0

-

H,

2.465

8535

0.0059

-

-

-

0

1.57

-

-

-

4.94

0.194

-

0.337

-

-

3.97

0.031

-

for and not found-He,

CH,,

1.55 x 10-e H&I, SO,.

cooled collecting tube during sampling. The volume (or mole) per cent composition is given for each sample, both as analyzed, and with corrections applied for ambient air, assuming both oxygen and nitrogen to be from this source. Corrections for atmospheric argon and carbon dioxide based on the nitrogen content also were applied. The precision of the analyses using the present form of gas analysis equipment is about 0.5 per cent (relative standard deviation) for major components, increasing to 10 per cent for minor components in low4 volume per cent range. As judged by the analysis of standard samples, the accuracy is approximately 1.5 per cent deviation of the mole per cent values. The sensitivity varied from 10-s (CO and SO,) to about low5 volume per cent (H,). At a flaming vent we are concerned with the combustion of preheated gases at a high temperature orifice. A search of the literature revealed no studies conducted on the characteristics of flames under conditions pertinent at a volcano. Of particular concern was the lowest concentration of a mixture of hydrogen and non-combustible gases that would produce visible flaming under these conditions. An experimental study was made of this ‘flammability limit’ by passing streams of hydrogen and helium measured by calibrated flowmeters through a silica glass tube placed in a furnace held at approximately 1OOO’C. It was found that the important controlling parameter in the determination of the flammability limit was the temperature at the jet at the end of the silica tube where combustion took place. This was measured by means of a thermocouple held adjacent to the nozzle, and could be varied by adjusting the jet position to various temperature points within the furnace. Flaming was made more visible by means of strands of silica wool which became visibly heated only within the flame, or by coating the inside surface of the silica tube with NaCl which vaporized to give the characteristic yellow sodium color to the flame. The results of the measurements of flammability limit as a function of orifice temperature are shown in Fig. 1.

The special circumstances that lead to flaming at or near volcanic vents suggest that the primary factor to be considered is the nature of the volcanic gases that provide the fuel for the flame. The analysis of gases collected around volcanoes has given highly variable results, but the conclusion is generally accepted that the composition varies with distance from the center of eruption, and with time since the inception of eruption (WHITE and WARING, 1963). The first suggestion that thjs is so was made by DEV~LLE (1857). A review of the work of others that illustrates

1166

JOEN

J. NAUGEETOM I

TOYAL 0 A A l

TEMPERATURE

AT FLAME

GAS 200 400 450 470

I

I

FLOW RATE i ml/min. ml/min. ml/min ml/min

ORIFICE,

:

“C

Fig. 1. Lowest concentration of hydrogen in a hydrogen-helium mixture which will just form a self-sustaining flame, plotted .W a function of the temperature in the region in which the flame orifice is held. Note that this is not the flame temperature.

this phenomenon and leads to the same conclusions is given by STOIBER and ROSE (1970). In our own work (F~NLAYSON et aE., 1968) we have noted a progressive depletion of gases in a cooling lava lake with time from eruption, with the inert gases, nitrogen, sulfur gases, carbon gases and water being released by the lava in the order given. This probably reflects the solubility of the gases in the fluid lava, Water is most important quantitatively at all sta,ges and the most persistent in time. Our present evaluation of the composition of the main gaseous components at the source within the lava fountain (95 */” H,O; 4 o/0CO,; 1 o/oSO,) has been obtained by infrared spectrometric measurements of such active fountains (NAUGHTON et aE., 1969). It should be mentioned that some investigators (TAZIEFF, 1971) discount the importance of water as a component of volcanic gas, but in samples taken by us in Hawaii it seems to play an important role. In the instance of the collections reported here, it is believed that primary degassing and reaction had taken place at the nearby vent, possibly at the summit of Mauna Ulu which was at a distance of about two hundred meters. The main content of permanent gases and of sulfur gas was lost during this initial stage of fountaining. At the sampling point, water and some carbon dioxide were left as the chief gaseous components dissolved in the lava, and were later exsolved to provide the gases occupying the volume of the lava tube and cupoIas above the flowing lava. The oxygen fugacity of a melt of Hawaiian tholeiite basalt has been determined by FUDALI (1965) at 1200°C to be lo- 8*2. Using this and the dissociation reaction of water at the same temperature, with the corresponding equilibrium constant, IGo = II, + $02, &*** = lo+*‘, it is possible to calculate the fugacity ratio fIEze/fHtto be about 59, At this temperature and one atmosphere total pressure, fugacities are practically equal to partial pressures. Then for the water content of the best sample (S&I-2, 92.7 per cent),

Volcanic flame:

source of fuel and relation to volcanic ga+lava

equilibrium

1167

the percentage of hydrogen in the mixture is calculated to be l-9 per cent. This is in reasonable agreement with the hydrogen content found for this sample (2.37 per cent). For the corresponding dissociation of CO, and its content in the same sample, one would expect to find a carbon monoxide content of about 0.02 per cent. These results are included in Table 1. The lack of detection of the expected amount of carbon monoxide cannot be explained completely, but may be due to the low instrumental sensitivit,y for this gas. It remains, then, to understand how a mixture with this low content of combustible gas can oxidize with the production of a flame. The study of flames even under the ideal conditions of the laboratory is difficult, and many facets of the phenomena associated with flaming are poorly understood. (Gaseous flames are divided into two general types, premixed and diffusional. Premixed flames are those in which the oxidant and combustant are mixed and are swept along to the point of combustion at the flame nozzle, as in a normally operating bunsen burner. In diffusional flames one component of the reaction (normally, air) surrounds the flame, and the mixing an,d reaction of the oxidant and combustant t&es place at the orifice within the flame, such as in a luminous bunsen flame. Cou~bustible gases have a charac~ristic self-i~ition temperature, which for lean hydrogen-air mixtures is cited to be somewhat below 700°C (GAYDON and WOLFFIARD, 1960). It is almost certain that volcanic flames of the type discussed here are of the di~usional type. The temperature within the cupola or lava tube is well above that required to initiate the reaction of any combustible with air. If air was present premixed within this region, no combustible such as hydrogen would be found in the sampled gases. Also, gas samples have been secured from within such flaming vents which were found to contain negligible amounts of the necessary oxidant, air (SIGVALDASON and ELISSON, 1968). To explain the flaming of the lean combustible mixtures of volcanic gases we must seek information on the lower flammability limits that will form a self-igniting diffusional flame under conditions likely to be found at volcanic vents. Investigations on premixed flames show that the lower combustible limit for hydrogen in hydrogenair mixtures is about 4 per cent (ZABATAKIS, 1965). From the results of the investigation on the flammability limits for hydrogen-inert gas mixtures in diffusional flames as given in Fig. 1, it can be seen that it is possible to produce flames in mixtures containing less than 1 per cent hydrogen provided the temperature at the orifice is above 750°C, just about visible redness. The lower limit for self-ignition of such mixtures is about 55O’C. This flaming behavior seems not to be influenced greatly by ohange in the velocity of gas flow through the flaming nozzle. The natural settings in which observers in our group have seen primary volcanic flames have been from cones, tubes, and grottos or cupolas which contain fresh but non-erupting lava. These may be cones or hornitoes from which lava previously has erupted, but has withdrawn leaving a still-incandescent vent. We have made many spectroscopic obse~ations on primary volcanic fountains in the ~ont.i~~~~o~ls range from the visible (450 nm) to the infra-red (16 pm) regions of the spectrum, but have never observed any emission lines (even the sodium doublet lines) that would characterize a flame. Our experience with regard to the observation

1168

JOHN J. NATJGHTON

of volcanic flames, then, is in agreement with that of JA~GAR (1918b) cited previously. In light of the measurements reported here on the low flammability limits for the combustible gas hydrogen from a heated nozzle, one can explain how a flame would be produced under the conditions observed in nature. In a molten lava stream or small lake occupying an enclosed area, which has lost most of its volatiles during the first phase of eruption and fountaining, the persistent and most soluble volatiles, water and some carbon dioxide, are left to reach equilibrium with the lava. As indicated this would produce a gaseous mixture with a hydrogen content of about 1.9 per cent by volume, which would be a flammable gas mixture provided it is vented to the air through a nozzle that is heated above approximately 750°C. This situation is likely to be achieved under the conditions noted at flaming vents. One might wonder, then, why flaming is not usually observed at the primary volcanic fountain where the gas-richest condition prevails. It should be pointed out that the gases being emitted at the fountain contain many neutral gaseous and vapor species such as nitrogen, rare gases, and halides which would dilute the content of combustibles in the equilibrium mixture, and possibly bring it to a value below the flammability limit for the fountain temperature. Oxidation would indeed take place and may play a role in increasing surface temperatures as envisioned by JAGGAR (1917), but the production of a flame plasma would not occur. We have made equilibrium calculations (NAUOHTON and LEWIS, 1972) of all possible reactive components that might be present in the primary fountain, based on infra-red measurements and volatile collections made above such fountains. The values thus calculated for the content of the important gases producing combustibles are listed in Table 1. For hydrogen the content can be seen to be lower than that calculated for the water-lava equilibrium alone. This is due to the removal of hydrogen to form other compounds in the equilibrium mixture, mainly the halogen acids. Also it should be remembered that there would be an additional reduction in absolute combustible content produced by unreactive and unmeasured volatiles such as the inert gases and nitrogen. CO?XLUSIONS From measurements of the lower limit of flammability of hydrogen-noncombustible gas mixtures when emitted to the air through a heated nozzle, it has been found that this limit is low enough to be achieved by the hydrogen content produced in the normal equilibration of water vapor with lava at high temperatures. It has been shown that such a system could exist under conditions prevailing naturally when volcanic flames are produced, and that one need not invoke exotic component’s or extraordinarily high contents of hydrogen in volcanic gas to explain their occurrence. Finally, a collection made at a tube vent between flaming episodes showed that, in fact, a flame-producing composition was present. Acknowledgements-Thanks are due to the staff of the Hawaii Volcano Observatory, U.S. Geological Survey, and particularly to Dr. DONALDPETERSONfor aid in the logistics of sample collection. Dr. J. B. FINLAYSON,Hilo College, University of Hawaii, gave invaluable aid during the gas collections, and JONG LEE performed the gas analyses, for which we are most grateful. The research was supported by the National Science Foundation under grant GA-20316.

Volcanic flame:

source of fuel and relation to volcanic gas-lava

equilibrium

1189

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