MeMoVolc report on classification and dynamics of volcanic explosive eruptions

Abstract

Classifications of volcanic eruptions were first introduced in the early twentieth century mostly based on qualitative observations of eruptive activity, and over time, they have gradually been developed to incorporate more quantitative descriptions of the eruptive products from both deposits and observations of active volcanoes. Progress in physical volcanology, and increased capability in monitoring, measuring and modelling of explosive eruptions, has highlighted shortcomings in the way we classify eruptions and triggered a debate around the need for eruption classification and the advantages and disadvantages of existing classification schemes. Here, we (i) review and assess existing classification schemes, focussing on subaerial eruptions; (ii) summarize the fundamental processes that drive and parameters that characterize explosive volcanism; (iii) identify and prioritize the main research that will improve the understanding, characterization and classification of volcanic eruptions and (iv) provide a roadmap for producing a rational and comprehensive classification scheme. In particular, classification schemes need to be objective-driven and simple enough to permit scientific exchange and promote transfer of knowledge beyond the scientific community. Schemes should be comprehensive and encompass a variety of products, eruptive styles and processes, including for example, lava flows, pyroclastic density currents, gas emissions and cinder cone or caldera formation. Open questions, processes and parameters that need to be addressed and better characterized in order to develop more comprehensive classification schemes and to advance our understanding of volcanic eruptions include conduit processes and dynamics, abrupt transitions in eruption regime, unsteadiness, eruption energy and energy balance.

Appendix A

Examples of descriptions and classifications of volcanic eruptions

Eruption classification needs to be fit for purpose (e.g. scientific understanding, hazard/risk assessment, communication with public, civil defence institutions and scientific community) and clear and simple enough to promote accurate transfer of knowledge and scientific exchange. It might vary depending on whether the classification is based on direct observations (i.e. real time) or on volcanic deposits (i.e. post-eruption). In particular, in real time, classification should be based on quantitative observations of phenomena (Table 3), while, for post-eruption descriptions, classification should be based on the quantification of volcanic products and deposit-derived parameters (Table 4). Here, we present some concrete examples developed by workshop participants. For two eruptions (i.e. Montserrat, 17 September 1996; Etna, 12 January 2011), we provide both types of descriptions (real time and post-eruption).

Examples of real-time descriptions

Gas piston event at Pu’u ‘O’o, Hawaii (23 February 2002)

Basaltic lava flow from vent at foot of Pu’u ‘O’o south wall begins at 19:59 and extends 100 m east by 20:15 (5 m wide proximally). A bulk volume flow rate of 0.26 m³ s⁻¹ for the lava flow was derived based on an emplacement duration of 16 min, which can be converted into a MER value of 414 ± 219 kg s⁻¹ by using the vesicle-corrected density of Harris et al. (1998) (i.e. 1590 ± 840 kg m⁻³). Continuous spattering at vent was observed throughout emplacement. Spattering transits to bubble bursts at 20:41. Bursts increase in frequency to more than 1 per second by 20:45. At 20:45, bubble bursting and lava emission terminated by onset of gas jet with loud roar to 25(?) m. Waning gas jet until 20:15. Vertical blue gas jet with few diffuse, small (cm-sized) incandescent particles. Spatter-bubble-jet cycle recommences; next jet at 21:16. It was classified as gas piston event type “c” according to Marchetti and Harris (2008). Gas flux was not measured.

Montserrat, West Indies (17 September 1996)

A major phase of lava dome collapse began at 11:30 am on the 17 September 1996, continued for 9 h and waned after 8:30 pm. The explosive eruption began at 11:42 pm and had finished by 00:30 am on 18 September. Seismic energy on the RSAM record peaked at about midnight and then declined exponentially. A vertical plume was intercepted by a commercial jet at 11.3 km, which is associated with a dense rock equivalent (DRE) discharge rate of magma of 1300 m³ s⁻¹ (based on Sparks et al. 1997). Assuming a constant discharge rate over the whole 48-min duration, a DRE volume of about 3.7 × 10⁶ m³ was obtained. From weather satellite images (Satellite Analysis Branch of NOAA/NESDIS), plume transport was both to the west and to the east by regional trade and antitrade winds with a maximum speed at tropopause of 17 m s⁻¹. Pumice and lithic lapilli fell widely across southern Montserrat. Classified as small-moderate based on plume height and MER according to Bonadonna and Costa (2013).

Etna, Italy (12 January 2011)

The eruption began with intermittent bubble explosions with increasing frequency and intensity from the evening of 11 January to 21:40 GMT of 12 January and intermittent fountains from 21:40 to 21:50 GMT (first phase). From 21:50 to 23:15 GMT, a transition to sustained fountains was observed with a peak magma jet height of 800 m and tephra plume height 9 km (second—paroxysmal—phase); a lava flow was also observed in the evening of 12 January. Small intermittent bubble explosions were again observed from 23:15 to 23:30 GMT, and low-intensity effusive activity and irregular low-frequency bubble explosions were observed up to 04:15 GMT (third phase).

Examples of post-eruption descriptions

Montserrat, West Indies (17 September 1996; fully described by Robertson et al. (1998))

On 17 September 1996, the Soufriere Hills Volcano started a period of dome collapse involving about 12 × 10⁶ m³ (DRE) of andesitic lava. A peak plume height of 14–15 km was derived based on the largest pumice clasts (from the model of Carey and Sparks 1986). The height estimate indicates a DRE discharge rate of magma of 4300 m³ s⁻¹ (based on Sparks et al. 1997). Wind speed averaged over plume rise was about 6–8 m s⁻¹. An approximate DRE volume of andesitic tephra fallout of about 3.2 × 10⁶ m³ was derived assuming a peak discharge rate of 4300 m³ s⁻¹ and an exponential decay of discharge rate with a decay constant of 12 ± 3 min. Magma water content was of 2.5–5 %. Ejecta consists of moderate (density = 1160 kg m⁻³) to poorly (density = 1300 to 2000 kg m⁻³) vesicular juveniles, dense non-vesicular glassy clasts (density = 2600 kg m⁻³), breccias cut by tuffisite veins and hydrothermally altered lithics (mean density = 2480 kg m⁻³). A maximum launch velocity of 180 m s⁻¹ is estimated for 1.2-m diameter dense blocks ejected to 2.1-km distance by using projectile models (Fagents and Wilson 1993; Bower and Woods 1996). Based on plume height and magma discharge rate, the explosive eruption can be classified as small-moderate to sub-Plinian based on plume height and MER according to Bonadonna and Costa (2013).

Etna, Italy (12 January 2011—paroxysmal phase; fully described by Calvari et al. (2011), Andronico et al. (2014a) and Viccaro et al. (2015))

Sustained fountains of potassic trachybasaltic magma occurred between 21:50 to 23:15 GMT on 12 January 2011 that were associated with a peak magma jet height of 800 m, a tephra plume height 9 km and the emplacement of a lava flow. A mass of erupted tephra fallout of 1.5 ± 0.4 × 10⁸ kg was derived averaging values obtained from the method of Pyle (1989), Fierstein and Nathenson (1992), Bonadonna and Houghton (2005) and Bonadonna and Costa (2012) (without considering the cone fraction), and a MER of 2.5 ± 0.7 × 10⁴ kg s⁻¹ was obtained dividing the erupted mass by the duration of the paroxysmal phase (100 min). The total grain size distribution peaked at −3 ϕ with a range between −5 and 5 ϕ was derived applying the Voronoi Tessellation of Bonadonna and Houghton (2005). Winds were blowing with almost constant direction from the NNE and intensity of 16, 15, 86 and 95 knots, at 3, 5, 7 and 9 km a.s.l. (http://weather.uwyo.edu/). It was classified as violent Strombolian based on Walker (1973) and small-moderate based on plume height and MER according to Bonadonna and Costa (2013).

Vesuvius, Italy (plinian phase of the AD 79 Pompeii eruption; fully described by Carey and Sigurdsson (1987) and Cioni et al. (1992, 1995, 1999))

The tephra-fallout deposit associated with the AD 79 Pompeii eruption consists of two main units, compositionally zoned and south-easterly dispersed, intercalated with PDC deposits in proximal areas. Deposit density for both units is 490 kg m⁻³ in proximal area (<20 km, Mdphi < −2) and 1020 kg m⁻³ in distal area (>20 km, Mdphi > −1). A polymodal cumulative total grain size distribution was derived based on the integration of isomass maps of individual size categories and on the method of crystal concentration of Walker (1980). Mode values of individual grain size populations are −2.8, −0.8 and 5 ϕ, respectively.

White pumice fallout: simple, massive, reversely graded, bearing accidental lithic fragments (mainly limestone and marbles) from the volcano basement and cognate lithics (mainly lava) (wt% lithics averaged over the whole deposit = 10.3). Magma composition = K-phonolite; 10–15 vol% phenocrysts; peak plume height = 26 km (based on the method of Carey and Sparks 1986); MER = 8 × 10⁷ kg s⁻¹ (derived from plume height applying the model of Sparks 1986); tephra volume = 1.1 km³ (applying the method of Fierstein and Nathenson 1992); wind direction = N145; wind speed = 28 m s⁻¹ (based on the method of Carey and Sparks 1986); maximum measured thickness = 120 cm at 10 km from vent. Classified as Plinian based on the diagram of Walker (1973).

Grey pumice fallout: simple stratified pumice-rich deposit with four ash-bearing, plane to cross laminated, PDC beds interlayered (wt% lithics averaged over the whole deposit = 11.8). Magma composition = K-tephritic phonolite; 16–20 vol% phenocrysts; peak plume height = 32 km (based on the method of Carey and Sparks 1986); MER = 1.5 × 10⁸ kg s⁻¹ (derived from plume height applying the model of Sparks (1986)), tephra volume = 1.8 km³ (applying the method of Fierstein and Nathenson 1992); wind direction = N145; wind speed = 31 m s⁻¹ (based on the method of Carey and Sparks 1986); max measured thickness = 160 cm at 10 km from vent. Classified as Plinian based on the diagram of Walker (1973).