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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/2/615/3428899/i0016-7606-84-2-615.pdf by guest on 28 September 2021 616 D. A. SV/ANSON 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. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/2/615/3428899/i0016-7606-84-2-615.pdf by guest on 28 September 2021 PAHOEHOE FLOWS, K.ILAUEA VOLCANO, HAWAII 617 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.