eruptive and passive degassing of sulphur dioxide at nyiragongo ...

S. A. Carnbeen ephemeral and hence datasets comparable to thatpresented here for Nyiragongo are scarce. Lava lakes atErta ‘Ale (Ethiopia) and Erebus (Antarctica) have beenstudied for many years but estimated magma fluxes forthe past few decades at these systems, and also atPu’u’O’o (Kilauea, Hawaii) in 1991, are significantlylower than those calculated for Nyiragongo in 2002-03(Table 3). The phonolitic lava lake at Erebus, discoveredin 1972, is perhaps the best analogue for the alkalinelake at Nyiragongo although the highly evolved compositionand low oxygen fugacity of the Erebus magmapromote low sulphur concentrations in surface lava(Kyle et al. 1994). Emissions of SO 2have been measuredannually at Erebus since 1983 using correlationspectrometry (cospec), but published emission ratesare 2-3 orders of magnitude lower than estimated SO 2fluxes at Nyiragongo and total SO 2production for 1983-1991 was only ~0.2 Mt (Kyle et al. 1994). Discharge ofSO 2from the Erta ‘Ale lava lake, estimated at 0.58 kg s -1(~50 tons day -1 ) in 1973 (Le Guern et al. 1979), is similarto that at Erebus. The Erebus data do, however, indicatesignificant (10-fold or more) diurnal variations in SO 2flux from the lava lake that may reflect variable convectionrates or plume ‘puffing’ (Kyle et al. 1994). This mayalso be a feature of degassing at Nyiragongo but isimpossible to detect in the daily toms SO 2data. Continuouslong-term measurements of SO 2emissions fromErebus are precluded as field campaigns are limited tothe austral summer (Kyle et al. 1994).Whilst active lava lakes are relatively rare, ultimatelythey are no more than the exposed caps of magmacolumns that are present at shallow depths in the conduitsof many open-vent basaltic volcanoes. Large and/or persistent heat and volatile emissions delivered byconvecting magma are a common feature of thesevolcanoes (e.g., Francis et al., 1993, Kazahaya et al. 1994;Allard, 1997). Degassed volumes and magma fluxes attwo archetypal conduit convection systems, Izu-Oshimaand Miyakejima (Japan), are also given in Table 3 andthey are of the same order as the recent Nyiragongodata. Since erupting in 2000, Miyakejima has been oneof the largest contemporary volcanic SO 2emitters, withobserved fluxes averaging 42 kt day -1 between Septemberand December 2000 (Shinohara et al. 2003; ep tomsdetected SO 2plumes from Miyakejima in late Augustand September 2000). Fluxes have diminished progressivelysince then to current values of ~4-9 kt day -1 (gvn2003b) and we may expect a similar waning trend in theNyiragongo data provided the system is not rechargedwith fresh magma. Plume altitudes at Miyakejima aretypically under 2 km (gvn 2003b) and hence ep toms hasdetected its SO 2emissions much less frequently than atNyiragongo.From a volcanic hazards perspective it is important tounderstand the fate of the large volume of magmadegassed at Nyiragongo since 2002. Assuming that themagma supplying the observed gas flux rises close tothe surface by convection, after degassing the magmamust descend and be accommodated within the system(Kazahaya et al. 1994; see Harris et al. [1999] for adiscussion of several models). Tazieff (1977) surmisedthat 140×10 6 m 3 of magma could have been accommodatedby endogenous intrusion into the 20-km longfracture system that developed during the 1977 eruption.The estimates in Table 2 indicate that a minimumof 0.22 km 3 additional magma may have been degassedat Nyiragongo since 2002, but the 2002 fracture systemis of considerably greater extent than that developed in1977 (gvn 2002b). However, there is currently no additionalevidence signifying the mode of accommodationof the degassed magma. Further insight into the locationand depth of magma storage beneath Nyiragongomay be gained by measuring ground deformation inthe region (e.g., using synthetic aperture radar interferometry[Insar]).The exceptional SO 2emissions from Nyiragongo alsohave implications for fluxes of other volcanic gases.Compositions of eruptive gases in 1959 and 1972 arenotably rich in CO 2and relatively SO 2-poor (Le Guernet al. 1977, Gerlach 1980), and if the current eruptivegases are of similar composition CO 2fluxes could be upto 7 times greater than the SO 2flux, although someproportion of magmatic CO 2is degassed diffusivelythrough the volcanic edifice as evidenced by the CO 2-rich zones or ‘mazukus’ common in the Virunga area.Sampling of current eruptive gases emitted fromfumaroles in the crater of Nyiragongo also point tovery high halogen concentrations in the gases (e.g.,HF/SO 2ratios of 10-62; D. Tedesco, personal communication,2003). Accurate determination of SO 2emissionsis therefore essential to an assessment of thecomplete volcanic gas budget at Nyiragongo, whichwill permit a more rigorous appraisal of the environmentaland health hazards associated with the currentdegassing activity.Finally, it is perhaps worthwhile to consider the possibilityof explosive activity at Nyiragongo. The edificehas the morphology of a stratovolcano rather than ashield volcano, and Tazieff (1977) notes the existence of180 m-thick, fine-grained melilite-rich pyroclasts exposedin the crater wall. These deposits may be ofphreatomagmatic origin, since such activity clearlyplayed a role in the 1977 eruption and possibly also inthe 17 January 2002 eruption, although Tazieff (1977)attributes the deposits to explosive activity. Given thecurrent elevated gas fluxes, if the system should revertto closed-system degassing then more explosive activitymay be a hazard. Continued surveillance of SO 2emissionsat Nyiragongo is certainly warranted.AcknowledgementsSome of the ideas presented in this paper arose frominformative discussions with Patrick Allard. Mark Gray(L-3 Communications, eer Systems Inc.) developed thesoftware used to generate Figure 1. The remarks ofthree reviewers resulted in significant improvements tothe paper.ReferencesAllard P. (1997). Endogenous magma degassing and storage at MountEtna. «Geophys. Res. Lett.», 24, 2219-2222.10