DONALD A. SWANSON U.S. Geological Survey, Menlo Park, California 94025

Pahoehoe Flows from the 1969-1971 Mauna Ulu Eruption, Kilauea Volcano, Hawaii

Note: This paper is dedicated to Aaron and Elizabeth portant papers of Macdonald (1953) and Waters on the occasion of Dr. Waters' retirement. Wentworth and Macdonald (1953) outline the basic details of how basalt flows move, and this paper attempts to build on their work. ABSTRACT Three types of chemically similar pahoehoe SHELLY PAHOEHOE flows were observed to form during the 1969- The summit fissure of Mauna Ulu (Fig. 1), a 1971 Mauna Ulu eruption. (1) A cavernous new shield on the east rift of Kilauea, con- type called shelly pahoehoe, characterized by tained an active lava lake during much of its fragile gas cavities, small tubes, and buckled first two years of activity (Swanson and fragments of surface crust, was deposited when others, 1971; Duffield, 1972). The lava often gas-charged lava welled out of the source fissure took part in a rise-fall cycle that was ap- with little or no accompanying fountaining. (2) parently gas-driven. In the simplest instance, A comparatively smooth-surfaced, dense type, gases exsolved from the lava were trapped in a characterized by surface channels and only a volatile-rich layer of melt beneath the solid few large cavities, formed from voluminous crust that capped the lake. The expanding flows of partly degassed fallout away from the gases gradually lifted the crust and melt up- foot of lava fountains more than 100 m high. ward several meters during a 10- to 20-min (3) A relatively dense type, characterized by interval, during which time virtually no fumes hummocky surfaces with abundant low tumuli were emitted. The gases finally broke through and overlapping pahoehoe toes and lobes, the lake crust and escaped, generating a dense formed when largely degassed lava issued from fume cloud, vigorous spattering, and low foun- tubes after flowing underground for several taining. The escape of gas caused a reduction in kilometers or more. Shelly pahoehoe is rarely the volume of the lake, usually by 1 to 2 X found in the geologic record, but the other two 104 m3, and caused the surface of the lake to types occur commonly. These three types of lower within 1 to 2 min to the level that it pahoehoe, which are completely intergrada- maintained before the gas expansion cycle tional, can be related qualitatively to the rela- began. Escape of gas and consequent lowering tive gas content and mode of flowage of the of the lake surface could be artificially trig- lava. The present surface of Kilauea is under- gered by dropping rocks or other foreign lain mostly by hummocky, tube-fed pahoehoe. material through the surface crust when the gas pressure was critically high. INTRODUCTION The column of gas-inflated lava sometimes Three general types of tholeiitic pahoehoe rose quietly to the lip of the vent and over- flows that are almost identical chemically ex- flowed down a slope of 1° to 3°, producing a cept for volatile content were observed to form cavernous type of flow called shelly pahoehoe during the 1969-1971 Mauna Ulu eruption by Jones (1943) and Wentworth and Mac- (Swanson and others, 1971) at Kilauea Volcano donald (1953). Two completely intergrada- (Fig. 1). Recognition that each of these three tional varieties of shelly pahoehoe flows were pahoehoe types develops by distinct processes produced, depending on the local relief at the and under different conditions aids in under- rim of the fissure. When relief was more than standing flow mechanics and in interpreting about 1 m, the lava spilled out of the vent in older basalt flows. For example, these principles several narrow flow tongues confined to the provide clues on how the subaerial part of topographic lows. These narrow tongues, which Kilauea Volcano was constructed. The im- sometimes merged a few meters or tens of

Geological Society of America Bulletin, v. 84, p. 615-626, 13 figs., February 1973 615

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meters away from the vent if relief became smoother, advanced downslope along an ir- regular or lobate front and produced a kind of shelly pahoehoe that is here called the "amoe- boid variety." When the local relief at the rim was less than about 1 m, the lava advanced slowly as a crusted sheet flood along a broad front often as wide as several tens of meters, giving rise to a kind of shelly pahoehoe that is here termed the "sheet-flood variety." This variety sometimes changed downslope into the amoeboid variety when underlying surface re- lief became rougher or when flow velocity de- creased, but the reverse was not observed. In the ideal case, each variety of shelly pahoehoe Figure 1. Index map showing areas discussed in has its own set of characteristic structures. text. The stippled pattern denotes the area covered by Slight variations in eruptive mechanism and new lava during the 1969-1!'71 Mauna Ulu eruption. near-vent relief, however, caused many flows The heavy dashed lines in the eastern part of the new to develop structures transitional to both lava show die location of the tube system described in varieties, and classification of such flows as one text. variety or the other is impossible and meaning- For several minutes after amoeboid flows less. stopped moving, expanding gas bubbles within the stagnant lava in undrained toes floated up- Amoeboid Variety ward and collected beneath the relatively im- The gas-charged lava of amoeboid flows permeable crust, creating a gas cavity (Fig. 3). normally advanced away from the fissure in Most of the cavities are 30 to 50 cm deep and slowly moving lobes and toes a few tens of comprise at least 50 percent of each pahoehoe centimeters thick that merged and overlapped roe. These cavities are dome shaped, conform- to produce a coherent flow. Much of the ing to the outer surface of the toes, and the flowage took place within small tubes a few surface crust is normally less than 5 cm thick tens of centimeters in diameter that were and cannot support much weight. The presence formed by the successive budding of small of these fragile gas cavities and associated pahoehoe toes as described by Macdonald drained tubes results in treacherous footing; (1953) and Wentworth and Macdonald (1953). walking across a shelly pahoehoe flow is much When supply was interrupted, partial down- like walking on huge egg shells. slope draining of lava from these small tubss The formation of such gas cavities in the left hollow, very fragile toes (Fig. 2) whose amoeboid variety of shelly pahoehoe was ob- solid crust is only a few centimeters thick. served many times, and it was possible to trace Some gas was lost continuously from the lava the development of a cavity by poking an during flowage, but this quantity was gen- object such as the point of a rock hammer erally small, judging from the minor amount of detectable fumes. The solidified crust that formed within a few seconds after exposure of the molten lava to air effectively trapped a relatively large proportion of the gas, allowing the lava to remain in a highly inflated state. Probing with a hammer into a freshly budding toe sometimes induced noticeable de- gassing of the lava, a process that was both audible and visible as the toe deflated. Lava samples collected on the hammer at such times and rapidly quenched in air have specific gravities of 1.2 to 1.5, compared to about 2.8 Figure 2. Small tubes; formed by successive budding for degassed basaltic lava of this eruption; thus of toes in amoeboid variety of shelly pahoehoe at Mauna the flowing lava in the toes was inflated abou'; Ulu. Direction of flow was away from observer, down a 50 percent by its expanding gases. slope of 2 to 3 degrees about 100 m from source fissure.

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through the thin crust of a stagnant, cooling after movement stopped. Such flows could be toe. Probing showed that no cavity was present observed to deflate over several minutes, but immediately after lava in the toe stopped nonetheless they usually displayed at least a moving. A few minutes later, however, a cavity partly shelly nature when solidified. In other had formed beneath the now slightly thicker flows, some gases were lost during flowage when and upbowed crust. The transitional stages the crust cracked or when small open channels were similarly probed, and it was sometimes developed as a toe was breached; the resulting possible to induce deflation of a toe if the pahoehoe is less cavernous than typical shelly hammer hole stayed open, depending on the pahoehoe. Some flows on relatively steep slopes thickness and rigidity of the crust. developed most of their cavities through drain- The observed process of cavity formation ing of tubes. Quite often, active pahoehoe generally did not alter the total volume of the lobes advanced over stagnant cooling ones, and toe. Sometimes, however, the pressure of ex- the underlying flow was crushed or, if still panding gas within the toe was sufficient to very hot, even remobilized to some extent. The bulge the somewhat plastic crust as much as 10 weight of the new flow, together with any cm (more commonly 1 to 5 cm) upward like a remobilization, caused the large gas cavities to balloon, thus enlarging the volume of the in- collapse and smear out, leaving only an intricate ternal cavity by an estimated 5 to 10 percent platy parting bordered by a scoriaceous zone to and accentuating the domal shape of the toe. mark the former presence of a gas cavity. In Such inflation took place over an interval of 5 general, however, the shelly nature of near-vent to 10 minutes when the crust of the toe was 1 amoeboid flows, caused by the two processes of to 3 cm thick. This inflation was observed for tube drainage and gas separation and accumula- only a few toes and perhaps indicates that the tion, is easily identifiable. crust in these toes was unusually impermeable. Direct observations also showed that prob- Sheet-Flood Variety ably 25 to 50 percent of the cavities in amoe- The sheet-flood variety of shelly pahoehoe boid shelly pahoehoe flows developed by de- spilled out of the fissure and advanced down- gassing of the cooling lava, not by draining of slope in a slowly moving (usually 0.5 to 1 lava out of the toe. For example, many toes at m/sec) front sometimes as wide as 30 m and as the downslope end of shelly pahoehoe flows are high as 1.5 m. Solidified crust covered the up- cavernous (Fig. 3) and obviously could not per surface of such flows and continued to grow have drained anywhere; Macdonald (1953, p. during movement; at times, this crust was 175) made a similar observation on Mauna rafted away from the vent, and at other times, Loa. Also, the process of ballooning of some it grew downstream while rooted to the crust toes during gas separation was clearly visible capping the fissure. Except when flowage was and could not have resulted from simple drain- sufficiently vigorous to rupture the floating ing. crust, this crust formed a complete envelope There were many variations within this gen- over the front of the flow that was usually eral scheme. Some flows or parts of flows lost between 50 and 100 cm high (Fig. 8a). As the much of their gas through cracks in the crust flow advanced, the relatively flexible crust was pushed and rolled over the front and overridden by the internal fluid lava, like the movement of tractor treads. Eventually the crust that formed on top of the flow was deposited as the basal layer of the flow by this process. Some- times the crust would crack or tear where it was warped around the flow front, and molten lava would pour onto the ground surface and form a small toe (Fig. 8A of this paper and color photographs in Schmincke, 1971). Such toes were then normally covered by the slowly advancing sheet flood, and the toes sometimes collapsed beneath the added weight. The in- Figure 3. Toe of amoeboid variety of shelly pahoe- terior of these flows, revealed in cross sections hoe at downslope end of flow near summit of Mauna exposed on walls of craters and fissures that Ulu. Broken roof of toe shows hollow interior. Small bucket is 10 cm high. developed after the flows were observed to

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form, is comparatively dense and shows wispy internal structures that mark the former crust of the overridden toes (Fig. 4). The dense in- terior grades upward into the normal, cavern- ous flow (Fig. 5). Such collapsed, dense interiors may also form in amoeboid flows but were much more common in the thicker sheet-flood variety at Mauna Ulu. The surface crust locally buckled away from the underlying fluid lava during movement, producing a cavity between the lava and crust that was usually transverse to the direction of flowage (Fig. 6). The resulting folds and warps were commonly asymmetric, or even imbricate, Figure 4. Collapsed base of sheet-flood variety of so that the direction of movement was pre- shelly pahoehoe, showing wispy internal structures, served (Fig. 6). Elsewhere, the buckling im- overlies platy basalt formed as several successive sheet parted a smoothly billowed, or rolled, appear- floods drained down a slope of 2 to 3 degrees. Exposure ance to the moving crust with no evidence of is on a near-vertical wall of an elongate crater that asymmetry. Such billows typically had a wave formed during the 1969-1!'71 Mauna Ulu eruption. length of 1 to 2 m, were several meters long The lava was erupted from a fissure a few tens of transverse to the direction of flowage, and meters away and flowed toward left and observer. The created a relief of several tens of centimeters on loose block in upper left is about 20 cm long. the surface of the flow. Similar rolls of crust developed along the sides of the flow as it widened. Other buckled areas were char- acterized by less regular jumbles of crustal fragments, some of which were a meter or more high. Small surface channels 1 to 3 m wide formed locally as liquid lava poured to the surface through cracks in the folded crust. A crust quickly formed on this sluggish lava and became wrinkled into large festoons convex downslope as slow movement continued for several minutes (Fig. 7). Large gas cavities similar to those already described were also Figure 5. Collapsed basal part of sheet-flood variety formed, but they were chiefly confined to toes of shelly pahoehoe grading upward into cavernous and lobes along the sluggish flow margins, upper part. Exposed on wall of collapse crater at Mauna where movement was identical to that of Ulu. The elliptical hole near center of photograph is 10 amoeboid flows. to 15 cm long. Direction of flowage toward left and observer. The sheet-flood type of shelly pahoehoe normally lost much gas through cracks in the thin layer of surface crust after it stopped moving, and the flows were commonly observed to deflate by 25 to 50 percent during the 20 to 45 min usually required for the degassing (Fig. 8). Cavities produced by the buckling of crust during movement were preserved during the over-all deflation. The resulting solidified flow had a comparatively flat, stable surface over the relatively degassed solidified lava, on which were superimposed numerous fragile buckles of crust. Areas of folded and buckled crust nor- Figure 6. Buckled crust on sheet-flood variety of mally comprised from one-third to two-thirds shelly pahoehoe, 50 in from source vent at summit of of the total surface area of the deflated flow, but Mauna Ulu. Direction of flowage was right to left. the entire surface of some flows was billowed Note asymmetry of bucklcs, indicative of flowage and extremely treacherous to walk on. direction.

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move evidence of their existence. Shelly flows that collapse beneath an overriding flow while still hot leave a comparatively dense rock with wispy, discontinuous layers marking the old cavities (Fig. 4); such structures may well be preserved in the geologic record and should be a criterion for proximity to the source vent. SMOOTH-SURFACED PAHOEHOE PRODUCED FROM HIGH FOUNTAINS Nine episodes of lava fountains more than 100 m high (maximum of 540 m) took place at Figure 7. Festooned, ropy crust on sheet-flood variety of shelly pahoehoe several tens of meters from Mauna Ulu between June and December 1969 source vent on flank of Mauna Ulu. Direction of (Swanson and others, 1971). The fountains flowage was left to right. were gas driven, and billowing fume clouds rising above them attested to the relatively General Comments large quantity of escaping gas. As the molten Neither variety of shelly flow at Mauna Ulu advanced more than a few hundred meters from the source fissure. The extent of shelly flows was not primarily limited by cooling of the flowing lava, for measurements with an optical pyrometer through holes in the crust of toes at the distal margin indicated temperatures similar to those at the vent itself (1,160° to 1,165°C). Instead, the flows stopped moving because the feeding lava column withdrew back down the vent, and the hydrostatic head was then insufficient to rupture the chilled skin on the toes at the leading margin of the flow. The slope of the ground surface varied from essentially flat, in which case the flow formed its own gradient of less than 1 degree to several degrees. These distances and slopes should not necessarily be considered as limiting factors in the formation of shelly pahoehoe, although on slopes steeper than 4° to 5°, the lava tends to "run out," depositing only a thin, relatively dense, noncavernous layer (Fig. 4). Flows whose velocities exceeded about 5 m/sec, be- cause of steep slopes or high rate of supply, were denser and less cavernous than typical shelly pahoehoe. The higher velocity inhibited crustal formation and broke pre-existing crustal plates, so that a high proportion of gases could escape. Flows of this type resemble those Figure 8. Deflation ot downslope end ot stieet-ilood variety of shelly pahoehoe. A. flow is barely moving, produced from low fountains, as described in a and liquid lava (light) oozes through cracks in crust at following section. two places in front of observer. The flow stopped Shelly pahoehoe is not commonly found in moving a few minutes after photo A was taken, and ancient rocks for several reasons. These flows photo B, 20 to 30 minutes later, shows that the flow deflated as it cooled and lost gas. Compare especially the are probably confined to localities very near the part of the flow behind the observer in A with that in source vent, so that their representation in the front of the observer in B (observer in photo B stands a stratigraphic record is undoubtedly very scant. few steps behind his position in A). Note how level of The cavernous nature of the flows makes them lava near source vent (background) also lowered, in very fragile, and the processes of burial, com- part by degassing and in part by draining into fissure paction, and diagenesis probably go far to re- when feeding stopped. Photographs by W. A. Duffield.

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spatter fell back to the ground, it collected most of the typical features (except tumuli) into wide flows that moved rapidly away from described by Wentworth and Macdonald (1953, the vent. Observers were unable to determine p. 33 to 45). Most or' these structures and if any lava was fed directly into the flows textures, such as wide surface channels, levees, from the eruptive fissure without first foun- and a relatively smooth upper surface, differ taining into the air, because the dense fallout from those of shelly pahoehoe and probably are obscured the rim of the fissure. Certainly, related to different volume rates of supply and however, the fallout supplied most of the lava other fluid dynamic factors, such as velocities in each flow. of flowage and channel width. In contrast to Spatter on the margin of the fountains shelly pahoehoe, fountain-fed pahoehoe con- visibly cooled, but the interior of each fountain tains very few large cavities for two principal apparently lost little heat. Optical pyrometer reasons. First, fountain-fed flows had a different temperatures of the interior of fountains,1 and mode of flowage; they advanced chiefly as of the resulting flows, were always about the rapidly flowing rivers, rather than as small, same (1,160° to 1,165°C) as those observed at slowly moving toes and lobes. Second, the other times in the active lava lake from a relative gas content of the two types of flow distance of only a few meters. differs; fountain-fed flows lost a considerable The fountain-fed lava flows moved as broad amount of their original gas content during rivers, building channels, levees, distributaries, fountaining. Consequently, toes and lobes and meanders. Surges in velocity or rate of along the sluggish margins of fountain-fed supply caused the normally 1- to 3-m-deep flows lack cavities, except where lava has clearly channels to overflow, and at times floods of drained downslope out of them, whereas large lava 100 or more meters wide and as much as 4 gas cavities are abundant in shelly pahoehoe. m deep rushed at velocities of at least 15 km/hr Also, samples of fountair.-fed flows are in gen- for several hundred meters down the gentle eral less vesicular than those of shelly pahoehoe. slopes away from Mauna Ulu. A thin solidified Flows generated by fountains less than about crust, commonly broken by narrow cracks into 100 m high cannot be so neatly categorized. slabs several meters on a side, normally floated They show all transitions to both shelly and on the lava rivers. This crust, a poor heat con- dense pahoehoe, probably because of an inter- ductor (Robertson and Peck, 1969), impaired mediate degree of degassing during the low cooling and enabled the lava rivers to remain fountaining. Such flows are common within 1 very fluid for many hundreds of meters. to 2 km of the active Mauna Ulu vents. Volume rate of extrusion was comparatively large, always between 160,000 m3 and 500,000 DENSE, HUMMOCKY PAHOEHOE m3 per hour; this rate was 10 to 50 times the PRODUCED FROM LAVA TUBES eruption rate of shelly pahoehoe. Flows with An intricate system of tubes carried pahoehoe large volume that traveled 3 to 6 km from the lava from the vent as far as the ocean (Fig. 1), vent changed into aa across a transition zone a about 11.5 km away, during part of the 1969- few hundred meters wide. When fountaining 1971 Mauna Ulu eruption (Swanson and others, stopped, the pahoehoe and aa flows continued 1971). The tubes were formed by crusting-over to advance downslope but at an ever-de- of surface channels and by the growth and creasing rate, and the transition zone between coalescence of pahoehoe toes and lobes, with or pahoehoe and aa migrated upstream as much as without subsequent burial by younger flows 1 to 2 km. Finally, momentum was overcome (Wentworth and Macdonald, 1953, p. 45). Lava by cooling and friction, and the movement poured from the source fissure through one ceased. Some aa flows continued to move for as tube 100 to 150 m long into a crusted lava lake long as one day after fountaining stopped. in Alae Crater, which borders the southeast Pahoehoe produced by the fountaining had margin of Mauna Ulu (F:g. 1). Another tube served as the outlet to the lake (Swanson and 1 A correction of +50°C was made for measurements Peterson, 1972), carrying lava farther down- of the interior of the fountains in order to correct for slope and eventually, through a complex sys- absorption in the 300 to 500 m observation path (Wright tem of distributaries, intD the sea, where a and others, 1968). From distances of only a few meters, lava "delta" was formed (Moore and others, optical pyrometer measurements agree within severa'. 1973). Collapsed portions Df the roofs of some degrees with those obtained by thermocouples. Repeated measurements with the pyrometer, a disappearing tubes allowed direct observation of flowing filament type, give a reproducibility of 5°C or better. lava at many places (Fig. 9), and electro-

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magnetic surveys further defined the route of Hilina Fault System (Fig. 1), and thin pahoe- part of the tube system (Anderson and others, hoe spread out at the base of the 350-m-high 1971). cliffs. On September 21, the tube system was The tube system developed over the course completed, as pahoehoe lava entered the ocean. of several months, and the process of develop- The master tubes continued to carry lava ment was important in governing the type of almost continuously until mid-May 1971 at an pahoehoe that issued from the tubes. The main approximately constant rate, and they deep- tube leading from Alae Crater was probably ened to as much as 15 m by erosion of softened formed in October 1969, but voluminous flow- or partly melted wallrock. As the tubes grad- age of lava within it did not take place until ually deepened, the level of lava flowing in early August 1970 (Swanson and others, 1972; them correspondingly lowered, thereby with- Swanson and Peterson, 1972). This tube ex- drawing support from the roof and upper walls. tended about 2.5 km southward from the cra- Occasionally, parts of the weakened roof a few ter, and lava that first emptied from it onto the meters wide collapsed to form a window that ground surface in August 1970 changed to aa exposed flowing lava to the cooler air. Crust after a few hundred meters of surface flowage. immediately began to form on the surface of Gradually, however, the pahoehoe tube system the lava beneath the window as heat was lost extended itself, in part by a continuing flow of to the atmosphere. This crust gradually grew lava through interconnecting pahoehoe toes downstream and inward from the walls of the and in part by crusting over of small surface tube to cover the flowing lava, eventually channels. The formation of the crust slowed healing the tube in a few hours or days, de- pending on the size of the window. In this cooling, so that hot, relatively fluid pahoehoe manner, different levels or tiers of solidified encroached on and eventually covered the lava developed within tubes beneath old win- slightly older aa flows. As each pahoehoe lobe dows (Fig. 9). As many as three different tiers, broke onto the surface from the end of the new each a few tens of centimeters above the other tube and advanced downslope, it changed and resulting from different collapse events, gradually to aa that was covered shortly after- could be seen in a single master tube when the ward by still newer pahoehoe that emerged at level of flowing lava was low and the overlying, the surface from the ever-lengthening tube older tiers had partly caved in. It is important system. This process—-pahoehoe changing to aa to realize that these tiers are not continuous only to be covered by slightly younger pahoe- along the entire length of the tube; instead hoe as the tube system advanced—was repeated they formed only directly beneath a window in over and over again throughout the next several the roof of the tube, as the system attempted to weeks. By early September 1970, the tube sys- regain thermal equilibrium. tem extended over the steepest cliffs in the A complicated system of small, distributary tubes branched from the master tubes. These distributaries carried lava for intervals of a few days, until they became clogged for various reasons. The distributary tubes remained com- paratively small but were the main source for growth of the pahoehoe field laterally from the master tubes. In fact, probably most of the tube-fed pahoehoe was added directly to the surface through the anastomosing distributar- ies. The gradient of the tubes, both master and distributary, varied from less than 1° to more than 30°, averaging 3° to 4°. The velocity of flow within the tubes varied accordingly, averaging 2 to 3 km/hr. The volume rate of Figure 9. Lava (light color in center of photo) flow- supply to the master tube from Alae Crater ing in tube about 4 km south of Mauna Ulu, April 1971. 4 3 People sit on rubble from collapsed roof of tube. After was usually about 1.3 X 10 m /hr but varied 4 3 collapse, the flowing lava crusted over, forming the from zero to as much as about 3.4 X 10 m /hr smooth shelf in center of photograph. Direction of (Swanson and Peterson, 1972). flow is away from observer. The tubes provided such efficient thermal

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insulation that the flowing lava maintained temperatures equal to, or only slightly less than, the eruption temperature of 1,165° + 5°C. For example, a traverse from Alae Crater to a point near the ocean revealed that the highest optical pyrometer temperatures of lava visible in each window of the tube system were 1,150° to 1,155°C, with no recognizable ten- dency for progressive cooling downslope. Such efficient heat retention can be predicted from the low thermal conductivity of hot solidified basalt (Robertson and Peck, 1969; Murase and McBirney, 1970), because loss of heat by other processes (radiation, gas transfer, conduction, Figure 10. Hummocky appearance of tube-fed and convection) was minimal in the mostly pahoehoe 3 km from ocean. Man stands on top of one enclosed tube system. tumulus. Lava flowing in the tubes constantly evolved relatively dense (Fig. 11), quite unlike un- gas because confining pressure was lower than collapsed shelly pahoehoe flows (Figs. 4, 5). it had been prior to eruption. The effect of the Flat-topped uplifts 1 to 2 m high and 10 to continuous gas loss could easily be observed, for 15 m wide occur in places where the tube visible bubbling of the lava was vigorous close gradient was very slight. Such uplifts probably to the source and almost nonexistent far down- formed in a manner similar to the tumuli but stream in the tubes. Also, near-vent lava perhaps were localized over underground pools emitted much more fume than did lava near of lava rather than a single tube. Sag structures the distal end of the tube system. For example, similar in size and shap: to inverted flat-topped heavy fume made it difficult to breathe on the uplifts likewise occur in areas of low gradient downwind side of flowing lava near Mauna and presumably reflect: subsidence of the crust Ulu, but breathing was easy in a similar situa- of a shallow underground lava pool when the tion a few kilometers from the vent. The pool drained. normal loss of gas with time from lava in the Numerous thin lobes and toes of pahoehoe tubes was doubtless hastened by agitation and issued from the mouths of tubes, through friction on the walls of the tube, even though flowage was apparently laminar wherever seen. The tube-fed pahoehoe has a hummocky ap- pearance that distinguishes it from either shelly or fountain-fed pahoehoe. This hummocky sur- face is partly caused by the presence of tumuli (Fig. 10); tumuli are absent or very rare in other pahoehoe types. Most of the tumuli are 2 to 4 m high, 4 to 8 m in diameter, and have the general characteristics described by Went- worth and Macdonald (1953, p. 45). In places, the surface of a tube-fed flow is crowded with tumuli; in other places, there are only a few tumuli per square kilometer. The tumuli are not restricted to flat terrain, and some formed on slopes as steep as 5 degrees. These tumuli ap- parently formed above clogged distributary tubes. When lava continued to feed into these Figure 11. Cross section of several prehistoric tube- tubes, the roof of the tube at a weak point fed flows exposed in old sea cliff. Photo taken from bowed upward into a dome, which generally surface of lava del a formed when Mauna Ulu tube-fed cracked along its crest and slopes as it was up- flows entered the ocean. The lensoid appearance of the prehistoric flows is caused by overlapping filled lava lifted. Lava from the tube commonly was ex- tubes or low tumuli. The Mauna Ulu tube-fed flows truded through the cracks. Cross sections of would probably have a similar cross section. Compare tumuli and associated pahoehoe lobes and toes with the shelly pahoehoe in Figures 4 and 5. Cliff is show that the hummocky, tube-fed flows are about 4.5 m high.

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cracks in the roofs of tubes, or through cracks because of the heterogeneous distribution of near the bases and on the sides of tumuli and vesicles. The surface glassy rind of tube-fed flat-topped uplifts. These lobes and toes over- flows tends to be thicker (several millimeters) lapped as they advanced by budding, making and much less vesicular than that of either the ground surface between tumuli quite ir- shelly or fountain-fed types. Furthermore, the regular and hummocky on a small scale. The surface of the glassy rind commonly is not surface irregularity created by the small, over- striated or filamented, as are most other lapping pahoehoe toes is similar in degree to pahoehoe flow surfaces, because such structures that of shelly pahoehoe, but the toes are solid, are typically produced by the walls of broken not cavernous (Fig. 12A). Many of these solid and unbroken bubbles that streak out during toes have shapes that closely resemble some flowage (Wentworth and Macdonald, 1953, p. pillows (Fig. 12A), and it might be difficult 35, and personal observations). Elongate under some circumstances to distinguish such vesicles perpendicular to the chilled rinds com- toes from subaqueous pillows (Jones, 1969). monly occur; they are most abundant near the The hummocky, tube-fed pahoehoe is com- bottom of toes and lobes. These elongate paratively dense and contains fewer gas vesicles resemble pipe vesicles but do not cavities, both large and small, than does the penetrate the outer surface of the glassy skin fountain-fed type. This observation is obvious and hence could not have formed by entrap- on an over-all scale but is hard to quantify, ment of air or steam. Such elongate vesicles in Kilauean flows are more abundant in hum- mocky pahoehoe than in other types of flows. As a general conclusion, the tube-fed pahoe- hoe at a distance of several kilometers from the vent was distinctly poorer in gases than were the other pahoehoe types, although its tempera- ture was the same or only slightly lower. As a result, viscosity of the tube-fed pahoehoe must have been somewhat higher, probably ac- counting for the thick glassy rinds and, in part, for the general surface irregularity. Pahoehoe that issued from the tubes at inter- mediate distances (2 to 5 km) from Alae Crater has textures transitional to those of smooth- surfaced, fountain-fed pahoehoe and has struc- tures transitional to those of shelly pahoehoe. An attempt was made to measure the rate of A gas loss during flowage in the tube system. Samples of molten lava were collected from the eruptive fissure just before the lava spilled out into a shelly pahoehoe flow and from several places in the tube system between Alae Crater and the ocean (Table 1). The samples were collected by dipping a small bucket or hammer into the lava, and the specific gravity was determined for the glassy, quickly chilled parts of each specimen. This specific gravity yields percent vesicularity using Figure 7 in Peck and others (1966, p. 642) and provides an approximate measure of the content of ex- solved gas in the flowing lava. The data in Table 1 are few but support qualitative observations and clearly demon- Figure 12. A. Pillowlike pahoehoe toes fed from strate the loss of gas in the distance from the lava tube after 9 km of flowage. Note shiny, glassy vent to the ocean. Along a best-fit line through skin. B. Interior of largest toe in A is vesicular but not these data, vesicularity drops about 10 percent cavernous, as in shelly pahoehoe; dime gives scale. per 4 km flowage, but this rate indicates only

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TABLE 1. SPECIFIC GRAVITIES OF TUBE-FED MOLTEN LAVA SAMPLES

Distance from Specific Vesicularity Sample vent (km) gravity (percent) Comment

1 0 <1 to 1.49 bu to 70 Dipped from sum- mit fissure 2 4.5 1.73 42 Collected through window in tube 3 10 1.84 38 Surface ooze fed by tube 4 12 2.48 18 Collected where lava emerges from tube at coast'ire

a trend because of data scatter and slightly different modes of collecting. Also, the final 2 km of flowage was across almost flat terrain, so that velocity greatly declined, allowing more time for loss of gases than would otherwise be indicated by the short distance of travel. The flowing lava also lost volatile sulfur, chlorine, and water in a way consistent with the vesic- ularity data (D. A. Swanson and B. P. Fabbi, unpub. data). Many small tongues a few centimeters thick of tube-fed pahoehoe are only slightly vesicular (Fig. 13), and the ends of toes and lobes of such B tongues are commonly so dense (sp gr = 2.8 +) and glassy that they resemble resinous obsidian. Figure 13. A. Man's left foot rests on dense, highly degassed pahoehoe tongue; his right foot rests on Such tongues seldom advanced more than 10 m normal tube-fed pahoehoe. Gentle slope of large flat- from their point of emergence and probably topped uplift in. background. The dense pahoehoe compose less than 10 percent of the total surface issued from a cri ck in the surface crust of the normal area of new hummocky flows. They show a tube-fed flow to right oi: photograph. B. Cross section complete transition into the more common of toe on which hammer point in photo A rests. Note type of hummocky, tube-fed flow. The thick glassy rind about 1.5 cm thick, comparatively dense glassy skin on many of these tongues is folded interior, and small gas cavity in center. The toe has a and festooned (Fig. 13), and flowing lava of this flat basal surface and domed sides, a typical shape. type is markedly more viscous than that flow- ing in the tubes. These viscous tongues toes, and so this glass is not simply a normal, normally issued from cracks on the sides of chilled margin. Probably the thickness of the tumuli or flat-topped uplifts all along the tube glassy rind was controlled largely by increased system, but they were far more common on the viscosity because of gas loss, not necessarily by flat near the ocean, where lava in small tubes lowered temperature within the feeding tubes. could reasonably have pooled, stagnated, and lost gas before emerging to the surface. Thus TRANSITION TO AA FLOWS the dense flows probably formed when lava Macdonald (1953) attributed the higher that had been stored and largely degassed was viscosity of aa relative to pahoehoe flows of the forced onto the surface by increased hydro- same composition to cooling, loss of dissolved static pressure within the tube system. Optical gas, and greater degree of crystallization. pyrometer temperatures of such iava flowing on Cooling, with attendant crystallization and in- the surface were lower by 20° to 40° than that creased viscosity, was apparently more im- of lava flowing in tubes, suggesting that the portant than loss of gas in governing the change largely degassed lava was also cooler; however, from tube-fed pahoc ioe lava to aa during the the speed with which the lava visibly cooled 1969-1971 Mauna Ulu eruption. The evidence made the pyrometer readings unsatisfactory. of this influence is the relatively abrupt change The dense glass contains less than 5 pcrcent from pahoehoe to aa, through a transitional crystals even as far as 4 cm inside some small stage called slabby pahoehoe (Wentworth and

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Macdonald, 1953), within several hundred tinuous nature of the degassing process. Details meters after emergence from the feeding tube, of structural and textural differences described even many kilometers from the source vent. in the Mauna Ulu flows should not be extrap- The increased rate of cooling caused by radia- olated to flows of other compositions, but the tion and conduction to the air, not the loss of importance of gas content, together with mode gases, probably induced the rapid transition, of flowage, in determining the structure of because a large proportion of gases had already pahoehoe flows and pillows (Jones, 1969; been lost by the time the fluid pahoehoe lava Moore, 1970) is generally valid. left the feeding tube. The Mauna Ulu aa is These observations can be applied to a more highly crystalline than the pahoehoe, a geologic problem at Kilauea to explain the relation characteristic of aa flows in general prehistoric, but at least partly Hawaiian age (Macdonald, 1953). This crystallinity further (later than 700 to 900 A.D.), pahoehoe fields suggests the primary importance of lower along the south coastal region of the volcano. temperature, not gas loss (which inhibits Fault scarps several hundred meters high crystallization), in governing the fluidity separate the nearest possible vent areas on (Shaw, 1969, p. 516). In fact, Shaw's work Kilauea's east and southwest rifts from the shows that fluidity greatly decreases with in- south coast, yet the widespread lava field below creased degree of crystallinity, and this rela- the scarps is mostly dense pahoehoe with many tion suggests that whether lava will produce tumuli. The presence of pahoehoe below the pahoehoe or aa is largely dependent on the scarps has been puzzling, for most surface degree of crystallinity reached by the lava flows of pahoehoe would change to aa after while it is still flowing. cascading over such cliffs, as Macdonald (1953) A similar argument can be made for the and many others have observed. Coastal change from fountain-fed pahoehoe lava to aa. pahoehoe can be interpreted in a few places to Most of the fountain-fed pahoehoe flows from drape over some of the cliffs, but this relation is Mauna Ulu changed into more highly crystal- generally unclear. Thus the possibility has re- line aa only about 3 to 6 km from the vent, and mained that some of this coastal pahoehoe it is improbable that such flows would have could have been faulted down to its present lost significantly more gas than tube-fed pahoe- level, although there are no Hawaiian legends of hoe flows that traveled 11 km at a much slower such catastropic structural events. rate of movement, even taking into account the Ancient tube systems like those active during gases lost during fountaining. Decrease in the Mauna Ulu eruption could account for the fluidity caused by cooling and accompanying extensive coastal pahoehoe, and its similarity to greater degree of crystallization during surface the dense, hummocky pahoehoe fed by the flowage thus seems to be the chief reason for Mauna Ulu tubes is strong evidence for this this transition, at least for the Mauna Ulu explanation. The extensive "regional pahoehoe flows. It should be emphasized that flows of flows" mapped by Walker (1969) in the Ka'u pahoehoe with much greater volume than those Desert doubtless had a similar tube-fed origin. from Mauna Ulu commonly extend consider- In fact, most of the present surface of ably farther downslope on Mauna Loa before Kilauea is underlain by pahoehoe with struc- changing into aa (Macdonald, 1953), probably tures which indicate that it flowed through because the large volume flow retains heat more tubes away from the source vents. Except for efficiently and the steeper slopes increase the the 1969-1971 Mauna Ulu eruption, however, velocity of flowage. historic eruptions have not given rise to significant tube systems, probably because APPLICATION OF OBSERVATIONS their short durations, generally high volume TO GROWTH OF KILAUEA rates of eruption, and small total eruption Observations made during the 1969-1971 volumes have not been appropriate for the Mauna Ulu eruption demonstrate that differing development of tubes. These observations sug- modes of flowage and gas contents govern three gest that the most significant, mountain- general types of pahoehoe. There are complete building rift eruptions at Kilauea during pre- transitions between shelly, smooth-surfaced historic time probably were more nearly like (fountain-fed), and hummocky (tube-fed) the post-1969 part of the 1969-1971 Mauna pahoehoe in those features that can be related Ulu eruption than like other historic eruptions to gas content, as is expected from the con- in terms of long duration, mild but continuous

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rate of eruption, and large volume of output son, D. W., and Swanson, D. A., 1973, Flow of (Swanson and others, 1971). The eruptive rate lava into the sea, 1969-1971, Kilauea Volcano, of the 1969-1971 Mauna Ulu eruption prob- Hawaii: Geol. Soc. America Bull., v. 84, no. 2, ably approximated more closely the rate of p. 537-546. magma supply from the mantle than did other Murase, Tsutomu, and McBirney, A. R., 1970, historic flank eruptions (Swanson, 1972), Thermal conductivity of lunar and terrestrial igneous rocis in their melting range: Science, further suggesting that the 1969-1971 eruption v. 170, p. 165-167. was more "typical" than any other historic Peck, D. L„ Wright, T. L., and Moore, J. G., 1966, flank eruption. Crystallization of tholeiitic basalt in Alae lava lake, Hawaii: Bull. Volcanol., v. 29, p. 629- ACKNOWLEDGMENTS 656. I gratefully acknowledge the comments and Robertson, E. G., and Peck, D. L., 1969, Thermal observations of many geologists who visited the conductivity of vesicular basalt: Am. Geophys. Hawaiian Volcano Observatory during the Union Trans., v. 50, no. 4, p. 339. Schmincke, H.-U., 1971, Die Dynamik lebender 1969-1971 Mauna Ulu eruption. I especially Vulkane: Bild der Wissenschaft, v. 8, no. 3, p. thank D. W. Peterson and W. A. Dufield of 214-227. the observatory staff for fruitful discussion, and Shaw, H. R., 1969, Rheology of basalt in the Peterson, H. R. Shaw, and R. L. Christiansen melting rar.ge: Jour. Petrology, v. 10, p. 510- for beneficial reviews. J. B. Judd helped, make 535. dynamic observations of shelly pahoehoe, and Swanson, D. A., 1972, Magma supply rate at R. T. Okamura made the specific gravity de- Kilauea Volcano, 1952-1971: Science, v. 175, terminations. I am indebted to all staf mem- p. 169-170. bers of the Hawaiian Volcano Observatory for Swanson, D. A., and Peterson, D. W., 1972, Partial draining and crustal subsidence of Alae lava their assistance during those three exciting but lake, Kilauea Volcano: U.S. Geol. Survey hectic years. Prof. Paper 800-C, p. 1-12. This paper is dedicated to Aaron Waters, Swanson, D. A., Duffield, W. A., Jackson, D. B„ whose stimulating teaching, challenging ideas, and Peterson, D. W., 1972, The complex and critical insight spurred my interest in filling of Alae Crater, Kilauea Volcano, volcanic rocks. Hawaii: Bull. Volcanol., v. 36. Swanson, D. A., Jacksan, D. B., Duffield, W. A., REFERENCES CITED a.id Peterson, D. W., 1971, Mauna Ulu eruption, Kilauea Volcano: Geotimes, v. 16, Anderson, L. A., Jackson, D. B., and Friscb>necht, no. 5, p. 12-16. F. C., 1971, Kilauea Volcano: Detection of shallow magma bodies using the V.!,F and Walker, G. W., 1969, Geologic map of the Kau ELF induction methods: Am. Geophys. Union Desert quadrangle, Hawaii: U.S. Geol. Survey Trans., v. 52, no. 4, p. 383. Geol. Quad. Map GQ 827, scale, 1:24,000. Duffield, W. A., 1972, A naturally occurring model Wentworth, C. K., ar.d Macdonald, G. A., 1953, of global plate tectonics: Jour. Geophys. Re- Structures and forms of basaltic rocks in search, v. 77, p. 2543-2555. Hawaii: U.S. Geol. Survey Bull. 994, 98 p. Jones, A. E., 1943, Classification of lava surfaces: Wright, T. L., Kinoshita, W. T„ and Peck, D. L„ Am. Geophys. Union Trans, for 1943, pt. 1, 1968, March 1965 eruption of Kilauea Volcano p. 265-268. and the formaticn of Makaopuhi lava lake: Jour. Geophys. Research, v. 73, no. 10, p. Jones, J. G., 1969, Pillow lavas as depth indicators: 3181-3205. Am. Jour. Sci., v. 267, p. 181-195. Macdonald, G. A., 1953, Pahoehoe, aa, and block lava: Am. Jour. Sci., v. 251, p. 169-191. MANUSCRIPT RECEIVED BY THE SOCIETY JANUARY Moore, J. G., 1970, Pillow lava in a historic lava 12, 1972 flow from Hualalai Volcano, Hawaii: Jour. REVISED MANUSCRIPT RECEIVED APRIL 24, 1972 Geology, v. 78, no. 2, p. 239-243. PUBLICATION AUTHORIZED BY THE DIRECTOR, U.S. Moore, J. G., Phillips, R. L., Grigg, R. W., Peter- GEOLOGIC/1.L SURVEY

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