The Mechanism of Surtseyan Volcanism
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J. geol. Soc. London, Vol. 140, 1983, pp. 939-944, 2 figs. Printed in Northern Ireland. The mechanism of Surtseyan volcanism B. P. Kokelaar SUMMARY: During eruption, the vent of a Surtseyan volcano is occupied by a highly mobile slurry of tephra, hyaloclastite and water. As magma ascends rapidly through the slurry, mixing occurs due to velocity shear, acceleration of the fluid-fluid contacts and mass incorporation. Consequent expansion of the admixed water causes the jetting and continuous up-rush activity that characterizesSurtseyan volcanism. There is no evidence of the widely accepted fuel-coolant interaction mechanism. Basaltic volcanic eruptions occurring at an air-water thus permanently secured the vents from the influx of interface are characterized by violent explosive activity sea water. Further Surtseyan explosive activity occur- which clearlyresults from interaction of the magma red with the temporary emergence of volcanic piles at and water. Such activity occurred during the emerg- the satellite vents of Syrtlingur (May to October 1965) ence, in 1963, of thesubaqueous volcanowhich and Jolnir (December 1965 to August 1966). During formedthe new island named Surtsey (63'18'N the activity of Surtur 1, Surtla, the most distal satellite 20"36'W), in the Westmann Island group off southern vent, built a pile to within 5 or 6m of sea level Iceland. The activity has since been termed Surtseyan. (December 1963 toJanuary 19641,with onlyminor There have been several attempts (Saemundsson1967; ejections of tephra above sea level. From an examina- Jones 1970; Bennett 1972; Tasieff 1972; Peckoveret al. tion of Surtla,Kokelaar & Durant (1983) deter- 1973) to explain the mechanism of the magma-water mined eruption processestransitional from purely interaction, but these are irreconcilable with descrip- subaqueous to truly Surtseyan. tionsand films of Surtseyan activity (Thorarinsson 1965, 1966, 1967; Thorarinsson et al. 1964; Knudsen, undated a, b). Here, the course of events and salient Characteristics of Surtseyan features of the type eruption are described, previously volcanism proposed mechanisms are critically appraised, and an alternative mechanism consistent with observations is The following observations derive primarily from the proposed. activity atSurtsey, but are applicable toother Surtseyaneruptions, suchas those atCapelinhos The Surtsey eruption (Azores, 1957-8) (Machado et al. 1962;Machado & Forjaz 1968). Theemergence of Surtsey,during the night of Surtseyan explosive activity results when water gains November 14-15 1963, was preceded by approximate- access tothe top of thevent. When water is ly 1 week of submarine volcanic activity when completely and lastingly excluded the explosions soon predominantly clastic deposits(Jakobsson & Moore become comparatively mild and due solely to escape of 1980) gradually built up from Iceland's insular shelf, volatiles from within the magma. The initial exclusion 130 m below sea level. Initial Surtseyan activity, along of water results from enclosureof the vent by a ring of a short NE-trending fissure, built an elongate islet of tephra,although suchenclosure may not result in tephraabout mediana line of submarinevents. immediateexclusion. Film of theeruption of Jolnir However, activity soon becamecentred and a (Knudsen, undated 6) shows water flooding through a horseshoe-shaped island was formed.The vent, named permeable, albeit narrow, tephra embankment. Clear- Surtur l, was almost continuously open to the sea in ly suchaccess would be less likely through older the SW and Surtseyan explosions occurred there until andior wider embankments.Temporary enclosure, January 31 1964. On February 1, a second vent, Surtur resulting in restricted access or temporary exclusion of 2, was initiated 500 mNW of Surtur 1 and, as it too water, is commonly coincident with increased effusion wasmostly opentothe sea, Surtseyan activity rate and increased violence of eruption. Where there continued. However, on April 4 1964, the growingpile is considerable marine erosion, as at Surtsey, a high of tephra persistently enclosed the vent and seawater rate of effusion is required to maintain the tephra ring was atlast completely excluded. In response, the and hence the exclusion of water. violentlyexplosive Surtseyan activity gave way to Surtseyanexplosions arecharacterized by jets of comparatively mild fountaining of incandescent mag- tephra.These are mostlyshort-lived, occurring at ma(Hawaiian activity). Lowviscosity lava, flowing various frequencies, commonly up to 1 per second, or from a small lava lake, formed a carapaceto theclastic can combine into a continuous up-rush. At Syrtlingur deposits around the southern part of the islandand (Thorarinsson 1966) the phases of continuous up-rush 0016-764C'~7/110C-0939$02.00 63 1983 The Geological Society Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/140/6/939/4887762/gsjgs.140.6.0939.pdf by guest on 23 September 2021 940 B. P. Kokelaar were preceded by increasing frequency and violence of withhyaloclastites, grading from fine atthe top to discretejets. Discrete jets are almost silent and coarseatthe base, underlain by water-enriched continuousup-rush emits a muted rumbling noise. magma and then normal magma. This sequenceis held Violentjets can be accompanied by basesurges in a condition of subdued boiling until the magma next (Moore 1967; Waters & Fisher 1971). movesupwards. Bennett considered that, when this Commonly,successive jets of tephra showa wide occurs, the resulting decrease in hydrostatic pressure variation in direction,and displayaspreading or causes part or allof thesequence to boil violently, non-spreading form (likened to cocks’ tails and cypress thus resulting in aneruption with sequentiala treesrespectively). Bombs, with black,comet-like appearance of ejecta. However, this model does not tails, shoot out of eachblack, tephra-laden jet, and explain the high frequency jets or continuous up-rush, then the whole turns grey and white as the associated as the processes it requireswould not have time to steamcondenses. At night, the columns associated occur. Also, it would be impossible to obtain a column with continuous up-rush activity can be seen to contain of ‘clean’ water in contact with the magma in a vent, alarge proportion of incandescentmaterial. Coarse becausepoorly sorted clastic deposits would always tephra can be ejected to heights of over 1 km and fine slump into such a hole, so a layered sequence could tephra can becarried far higher by convective not develop in the manner proposed. Theoretically, a processes,particularly during a continuous up-rush. graded clastic sequence could result from sorting in a However, a considerable proportion of the ejecta falls steam fluidized system within a well-defined cylindrical within thecrater rim and, by directfall, by mass- pipe, but the vent is surrounded by unstable deposits, gravity flow, or by sliding, is returned to the vent. is not well defined and, again, the process would take In the quiescent intervals during intermittent jetting more time than is generallyavailable. Whatever the activity, the vent area is mostlyoccupied by avery mechanism, it must be capable of high frequency and shallow lagoon, violentlyagitated near its centre by continuousaction. Close to sea level, magma will the release of bubbles of steam. The steam condenses always have to pass through a poorly-sorted slurry of abovethe lagoon to form a dense, white billowing tephra, hyaloclastite and water. column.Such columns are commonly generated Tazieff (1972) considered that, following an initial continuously during jetting activity. subaqueous magmatic explosion, asuccession of steam explosionsresults fromthe entrapment of steam within or beneath fragments of incandescent magma The Mechanism (someassuming an ‘umbrellashape’) as they are hurled upwards through water. However, in Surtseyan Saemundsson (1967) and Jones (1970) suggested that ventsthere is no simplemagma-water interface as Surtseyanexplosive activity mightbe dueto water envisaged and, where incandescent basaltic magma is beingdrawn intothe volcano’splumbing system, explosively eruptedinto water, the steam initially through the subaqueousflanks of the pile, by a process produced is more likely to form an envelope around akin to Venturi action. If this was so, the characteristic the fragments and the vent, and tends to isolate the activity would continue irrespective of the exclusion of magma from further explosive interaction with water. water from the top of the vent, which is not the case. In support of this postulation, at Surtla, surfaces of a Althoughthe pile is permeableand water-saturated contortedslab of basaltic spatterare bread-crusted (Jakobsson & Moore 1980), this water evidently has and agglutinated with smaller lapilli of basalt (Koke- little or no role in Surtseyan activity, and at deeper laar & Durant, 1983). levels of the pile is probably mostly isolated from the The most favoured published explanation of Surt- magma by chilled basalt lining the conduit(s). Also, in seyan activity was partly derived from an attempt to clastic deposits close to any long established conduit, solvea problem caused, coincidentally, by another continuous vaporization of water will cause precipita- eruption in the Westmann Island group, that of Eldfel tion of salts, mostly NaCl, which will markedly reduce on the island of Heimaey(Colgate & Sigurgeirsson the permeability. 1973). There, in 1973, a subaerial Strornbolian cinder Fromexamination