Mafic Plinian volcanism and ignimbrite emplacement at Tofua volcano, Tonga
Tofua Island is the largest emergent mafic volcano within the Tofua arc, Tonga, southwest Pacific. The volcano is dominated by a distinctive caldera averaging 4 km in diameter, containing a freshwater lake in the south and east. The latest paroxysmal (VEI 5–6) explosive volcanism includes two phases of activity, each emplacing a high-grade ignimbrite. The products are basaltic andesites with between 52 wt.% and 57 wt.% SiO2. The first and largest eruption caused the inward collapse of a stratovolcano and produced the ‘Tofua’ ignimbrite and a sub-circular caldera located slightly northwest of the island’s centre. This ignimbrite was deposited in a radial fashion over the entire island, with associated Plinian fall deposits up to 0.5 m thick on islands >40 km away. Common sub-rounded and frequently cauliform scoria bombs throughout the ignimbrite attest to a small degree of marginal magma–water interaction. The common intense welding of the coarse-grained eruptive products, however, suggests that the majority of the erupted magma was hot, water-undersaturated and supplied at high rates with moderately low fragmentation efficiency and low levels of interaction with external water. We propose that the development of a water-saturated dacite body at shallow (<6 km) depth resulted in failure of the chamber roof to cause sudden evacuation of material, producing a Plinian eruption column. Following a brief period of quiescence, large-scale faulting in the southeast of the island produced a second explosive phase believed to result from recharge of a chemically distinct magma depleted in incompatible elements. This similar, but smaller eruption, emplaced the ‘Hokula’ Ignimbrite sheet in the northeast of the island. A maximum total volume of 8 km3 of juvenile material was erupted by these events. The main eruption column is estimated to have reached a height of ∼12 km, and to have produced a major atmospheric injection of gas, and tephra recorded in the widespread series of fall deposits found on coral islands 40–80 km to the east (in the direction of regional upper-tropospheric winds). Radiocarbon dating of charcoal below the Tofua ignimbrite and organic material below the related fall units imply this eruption sequence occurred post 1,000 years BP. We estimate an eruption magnitude of 2.24 × 1013 kg, sulphur release of 12 Tg and tentatively assign this eruption to the AD 1030 volcanic sulphate spike recorded in Antarctic ice sheet records.
KeywordsTofua Mafic Plinian Tonga High-grade Ignimbrite Caldera
Recognition of the importance and large scale of explosive volcanism at mafic-intermediate centres has increased markedly in the last few decades (Williams 1983; Foden 1986; Sigurdsson and Carey 1989; Freundt and Schmincke 1995; Torres et al. 1995; Allen 2005; Palladino and Simei 2005). This is particularly because mafic eruptions are now recognised as extremely important climate forcing events because they inject large volumes of sulphur- and halogen-rich volcanic gases into the upper atmosphere (Thordarson and Self 2003; Witter and Self 2007). Reconstruction of explosive mafic eruptions from their deposits is often difficult: evidence of fragmentation style is poorly preserved when activity at this scale produces highly welded ignimbrite but is well preserved when it generates Plinian fallout. Triggering mechanisms hypothesised for highly explosive mafic eruptions commonly include water–magma interaction or basaltic intrusion into/replenishment of a storage chamber. Early successions produced by caldera-forming eruptions of this style are commonly characterised by an abundance of hydrothermally altered lithic/country rock clasts. Subsequent eruption progression and dynamics depend on the mechanism and pace of caldera collapse and resulting modification of vent geometry (Druitt and Sparks 1984; Rosi et al. 1996; Scandone 1996; Roche and Druitt 2001). Two main end-member modes of caldera collapse have been identified, associated with what have been referred to as overpressured and underpressured calderas (Marti et al. 2009). In the case of chamber overpressure, arcuate ring faults form prior to the onset of eruption in response to localised stresses, and early in the eruption a small volume of magma may be ejected through these restricted arcuate cracks, producing a fire-fountaining phase during which marginal, degassed magma is displaced by hydraulic action. Subsequent failure of the chamber roof overlying a shallow magma body rapidly decreases magma pressure, triggering paroxysmal volatile exsolution and expansion that drives a climatic pyroclastic-flow forming stage via collapse of a high Plinian eruption column or spill-over of dense, low and hot plumes (Branney and Kokelaar 1992; Allen 2005). In contrast, underpressure calderas form by subsidence along ring faults during chamber recovery following an earlier eruptive phase (Marti et al. 2009). Studying the nature of volcanic deposits and their distribution within the associated volcanic successions provides a means of reconstructing eruption characteristics whilst distinguishing among products of discrete eruptive phases (e.g. Palladino and Simei, 2005).
Examination of Tofua successions was carried out in the early 1970s as a series of exploratory studies (Bauer 1970; Baker et al. 1971; McReath 1972). Following subsequent advances in work on classification, nomenclature (Sparks et al. 1973; Druitt and Sparks 1984) and emplacement mechanisms (Branney and Kokelaar 1992; Calder et al. 2000) of explosive pyroclastic deposits, it is pertinent to reassess the mafic volcanic deposits of Tofua. Also, new information is needed to improve our understanding of local eruption hazards in Tonga, and potentially to identify events that have had a global impact on atmospheric chemistry. Accordingly, we present the results of field mapping, facies analysis, geochemistry and volatile budgets of Tofua volcano, Tonga, southwest Pacific. Here, we recognise two successions of variably welded ignimbrites representing paroxysmal VEI 5–6 eruptions, including a caldera-forming event within the last 1,000 years. The stratigraphic exposure afforded on Tofua offers a unique opportunity to constrain eruption triggers, dynamics and timing of these highly explosive mafic caldera-forming eruptions—events at large scales that, until now, have remained overlooked within the Tofua Arc.
Following Bauer (1970), the four main chronological sub-divisions are retained with the Hokula Froth Lava, re-named as the vesicular platform lava and the Kolo formation now being interpreted to comprise the caldera-forming Tofua pyroclastic flow deposits and the younger Hokula flow interpreted as pyroclastic flow deposits from a later event associated with continued caldera deepening. Radiocarbon dating of a carbonised log recovered from the base of the Tofua ignimbrite sequence on the west coast, 200 m south of Hota’ane, gave an age of 970 ± 50 year BP (Shotton and Williams 1971) corresponding to a calendrical age range of AD 1025–1210 (using OxCal: Bronk Ramsey 2008).
Three post-caldera cones form an intra-caldera construct near its northern margin. Two extinct cones have been breached to the south by blocky lobate lava flows that terminate at the Lofia crater lake. Pyroclastic deposits from the currently active Lofia cone clearly form a mantle of tephra over the northwest caldera, reflecting the direction of prevailing low-level (<3 km) winds (Fig. 1). This sequence is up to 3 m thick along the main dispersal axis and consists of sets of accretionary lapilli beds in thinly bedded surge and fall deposits, punctuated by frequent bomb-sag structures and lithic blocks up to 15 cm in diameter. SiO2 in whole-rock samples associated with Tofua's explosive caldera eruptions ranges between 52.45 wt.% and 56.37 wt.% (Caulfield et al. 2008). A single lava flow of pyroxene-bearing dacite (64.97 wt.% SiO2), in the Hamatau Formation of Bauer (1970), intercalated with pre-caldera lavas is the only evidence of the development of significantly evolved magmas on Tofua.
Stratigraphy of the caldera pyroclastic sequence
The Tofua ignimbrite is by far the most voluminous unit: offshore observation of outcrops along the island’s perimeter show it is distributed in a radial fashion around the majority of the island with an unknown volume deposited offshore. Accessible, representative sections are located in the intra-caldera summit cliffs (sections 1 and 2) and on the northwest coast (‘Rope’ section 4). The deposit shows gradational lithofacies, both vertically and laterally, expressed by variations in degree of welding, colour and clast size/type. This dominantly high-grade (heavily sintered) deposit is characterised by common cauliform light-coloured pumice and darker scoria clasts. Fluctuation in fragmentation related external cooling may be responsible for both pumice and scoria examples. Alternatively, variable porosity of clasts may result from different degrees of welding compaction—early development of chilled margins on cauliform pumices conceivably reduced further compaction. Intervening deposits consist of a coarse, reversely graded lapilli tuff, along with distinctive bedded lapilli fall and surge units up to 0.5 m thick preserved in the northern caldera rim (section 3) and within 20 m long lenses (section 4) in parts of the northern coast. The upper Hokula ignimbrite unit is up to 14 m thick and best exposed in the northern fissure zone (section 3) and on the north coast (sections 4, 6, 7). This unit forms an almost continuous sheet-like outcrop from Hokula for over 2.5 km along shore to the west. East of Hokula the deposit is unwelded. From base to top, the unit is internally stratified showing a gradation in colour, degree of welding and clast concentration/type with no evidence for unconformities. It contains up to 25% lithics and displays a much greater lateral variation in degree of welding than the underlying Tofua ignimbrite. This lateral (and vertical) variability in welding of the pyroclastic deposits is common throughout all of the examined sections. High-grade welding is typically associated with the sites of thickest deposition. The degree of welding, however, varies considerably between sites and with stratigraphic height along any one continuous section over lateral distances of 20–50 m.
Typically, juvenile matrix material is ash in both ignimbrites. Loss of primary pore space is common and greatest in heavily sintered zones. Essential lapilli and bombs are moderately to highly vesicular in the Tofua ignimbrite with diffuse dense margins containing a high density of fine cracks, suggestive of rapid exterior cooling of ductile juvenile material (Cole et al. 2005). In the upper Hokula ignimbrite lapilli and bomb-sized pyroclasts are rare (<5%), finely vesicular and contain abundant angular lithic fragments. Lithic clasts are angular to sub-rounded aphyric blue-grey lava fragments 4–8 cm in length. Spatter clasts are found within ≤100 m of the caldera rim, immediately beneath the proximal Hokula deposits. Clasts are elongate and >30 cm long, with increasing vesicularity towards clast centres.
Proximal facies (caldera rim)
Three sections were measured in the northern fissure zone (Fig. 3). Here, deposits overlie an unknown thickness of intra-caldera breccia, and thus post-date the Tofua ignimbrite. The breccia is composed of 40% each of variably coloured hydrothermally altered lithic clasts and poorly vesicular scoria lapilli, both set in a partially lithified orange ash matrix. Clasts are angular, up to 6 cm diameter, and display roughly the full range of silica content known from the island. The central and eastern sections contain an approximately 30-cm thick bed of unconsolidated tephra comprising juvenile scoria lapilli with similar vesicularity to those found in the underlying breccia. Additionally, the western section includes 1.5 m of alternating surge and ash fall beds forming an intervening deposit between the intra-caldera lithic breccia and the continuous scoria lapilli bed. Rootless lava flows and welded agglutinates (samples TF522, TF521, Fig. 3) within the proximal spatter deposits are common and show highly opposing dip directions. Within the upper 1–2 m, individual ropey spatter clasts are visible. In contrast, deposit/layer interiors are welded with thicker sections showing the highest grades due to increased loading and heat retention, producing highly domainal deposits (cf. Carey et al. 2008). Clast cores are composed of coalesced and elongate vesicles with iridescent interiors. The degree of vesicularity decreases towards the outer margins, which comprise glassy rims. Angular fragments of aphanitic lava and dense non-juvenile scoria (up to 12 mm) are included in the spatter. Non-juvenile fragments are identified by their lower vesicularity and blocky shapes (e.g. Heiken 1972). They are very similar to clasts observed in spatter deposits from the mafic Siwi pyroclastic sequence, Tanna, Vanuatu (Allen 2005). Large lithic blocks (up to 2 m in diameter) are found lodged within the spatter deposits. This lithic-rich zone comprises a series of spatter mounds forming a distinctive hummocky topography. Capping the succession, and ubiquitous in the north of the island are a pair of mantling lithic breccias and underlying ash beds (samples TF1277, TF1276, Figs. 3 and 4b), similar to the breccia found at the top of section 1. Clasts are angular, average 6–8 cm in long axis and range from uniform aphyric blue grey lava to orange/red/white coarse crystalline hydrothermally altered rock. Figure 3 is a composite section representing all the deposits of the northern fissure zone.
A south facing slump scar, identified by a marked change in slope angle, is located north of the Lofia cone (Fig. 1). This structure may be the footwall of a buried fault. The fault scar spans almost 800 m east of the Lofia cone; however, exposures are best at its western extent where steep sections lack spatter cover. Numerous bread-crusted bombs litter the ground surface with its ubiquitous cover of medium scoria lapilli. The spatter encrusted Lofia cone is the most likely source of these blanketing deposits. The underlying section is composed of two fine ash/lithic breccia pairs, separated by up to 4 m of inter-bedded, poorly sorted ash and scoria. The lithic clasts are of the same hydrothermally altered types found in the caldera rim volcanic breccias in section 3; here, the maximum clast size reaches up to 50 cm. The section is capped by 2 m of agglutinated spatter, within which individual clast margins are still visible.
Medial facies (northern coast)
Above the shear zone, the Hokula unit is visible, distinguished from the Tofua ignimbrite by a much greater vertical and lateral variation in the development of welding and a marked increase in lithic clast content. The basal 2 m is composed of fine-grained ash-rich pyroclastic deposits with common fine (2–4 cm) scoria and lithic lapilli distributed throughout. Within this portion of the unit, a localised clast-rich lens composed of accessory blocks outcrops at a high angle to the surrounding horizontal flow fabric (Fig. 5b). This is fines-poor and contains rounded scoria blocks up to 30 cm in diameter. Above 2 m a series of fine-grained, undulose ignimbrite flow units are distinguished by sharp depositional boundaries, topped by a fines-poor, block and lapilli-rich horizon. The upper flow unit consists of 4 m of red/brown intensely welded ignimbrite, containing grey scoria clasts and a zone enriched in sub-rounded lithic clasts at its base (Fig. 2). Here, the depositional facies again is inferred to comprise a series of overlapping lobes, represented by intercalated lenses each up to 2.5 m thick (Fig. 5b). A well-developed eutaxitic flow band is located approximately 60 cm from the top of the unit (Fig. 5c). Imbricated elongate fiamme (up to 20 cm long) are visible, displaying feathered terminations. The unit is capped by black vesicular lava originating from the lowermost flow front found on the northwest flank (Fig. 1).
East of Hokula (section 7), the pyroclastic flow unit is unwelded and lies atop the vesicular platform lavas. At the contact, the upper 2 m of lava comprises a ‘rubbly’ zone of vesicular blocks (up to 40 cm across), inferred to be an autobrecciated ‘aa’ cooling crust. The base of the Hokula deposit shows coarse-tail grading and is white/orange in colour. This lower section contains the largest sub-rounded lithic blocks (up to 50 cm) and most abundant rounded scoria clasts (up to 30 cm). Immediately above, the deposit grades into a poorly sorted unwelded deposit with a clearly visible scoria-rich zone. The upper 5 m grades into red and finally black immediately beneath the vegetation cover (Fig. 7b). Along this section of coastline, the Tofua ignimbrite does not outcrop and there is also a lack of corresponding deposits within the caldera rim above (rim area west of section 1, see Fig. 1).
Distal facies (eastern islands)
Faulting has been the major structural control on the evolution of Tofua. Arcuate caldera-bounding faults define the caldera margins in the north and northeast (section 3) and south of the island. The southern fissure zone largely occupies the southeast quadrant of the caldera (Fig. 1). The fissure zone is a vast expanse of spatter deposits comprising four individual spatter cones and agglutinate dikes arranged in a northwest–southeast en-echelon pattern. A ubiquitous layer of scoria lapilli forms the uppermost deposit on the hangingwall block. Spindle bombs up to 25 cm long litter the scoria cover. A steep cliff runs the length of the fissure zone forming a footwall, with the fissure zone itself located atop the hanging wall block. The degree of displacement increases to the south, as evidenced by the thickening of the cliff fault-scarp exposure in the same direction. This arrangement suggests scissor-like normal faulting, hinged at the most northerly extent of the fissure zone. Smaller scale structures run parallel to the main fault in the north and northeast, forming outside and concentric to the caldera faults (northern fissure zone). These structures were identified by Bauer (1970) as conduits for magma eruption. The proximal accumulation of a large volume of spatter has produced a hummocky topography topped by the pair of mantling lithic breccias. The style and stratigraphic level of deposits suggests that the spatter of the southern fissure zone and the rootless lava/agglutinate of the northern spatter zone are coeval. Magma displacement due to slumping/caldera deepening in the south produced an extensive spatter field site and likely coincided with eruption of a similar, but smaller volume of magma in the north through pre-existing caldera bounding faults.
Elsewhere on Tofua Island, the distinction and correlation of the two ignimbrite units is possible using field observations and geochemistry (Fig. 8). A full set of geochemical analyses from Tofua will be presented elsewhere. As shown in Fig. 8, the Tofua and Hokula data plot in distinct major element compositional fields (fields 1 and 4, respectively). A bulk Hokula ignimbrite sample TF746M from locality 6 has almost identical geochemistry (field 4) to the capping ignimbrite unit found above the Tofua ignimbrite section sampled in the northeast of the caldera wall, 06TF69 (section 1). A deposit with the same composition is not found in section 2; here, the caldera walls are composed entirely of the Tofua ignimbrite. However, the "ramp" of unwelded ignimbrite ramped up against the eastern caldera margin is the lithic rich, proximal equivalent of the unwelded coastal Hokula ignimbrite found east of Hokula (section 7, Fig. 7b). Section 2 marks the easterly extent of the mantling breccia deposits, confirming that the Hokula ignimbrite was cooler and less widespread in the east. The remainder of the caldera wall ignimbrite is correlative with the Tofua unit of the Rope section (Figs. 2 and 8). Importantly, at the contact between the Tofua and Hokula units, geochemical reversals are apparent. In proximal sections, the base of the Hokula unit comprises mafic laminated ash fall (base Hokula, field 2, samples TF1277, TF1276) overlain by relatively evolved basaltic-andesite spatter deposits (intervening spatter, field 3, samples TF522, TF521), before returning to more mafic compositions typical of this unit (field 4). This evidence suggests eruption of chemically distinct magma batches in close succession during the early stages of the second (Hokula) event.
Sample TF1063 is the ash matrix of the lithic breccia found in the slump scar (south of section 3, see Fig. 1) and is geochemically identical to that found in the equivalent deposit from the summit sequence (section 3, Fig. 3). Although texturally diverse, the vast majority of juvenile clasts are chemically identical to their host matrix, e.g. the light-coloured cauliform clasts (06TF56) seen in the dark, intensely welded ignimbrite (06TF57) of the ‘Rope’ section 4 (Fig. 2). This is interpreted to reflect the mingling of a small volume of magma previously quenched (via an external water source) with hot juvenile pyroclasts. Indeed their cauliform shape, the development of internal vesicle gradients, and a lack of compaction even in a high-grade matrix of identical chemical composition, all favour quenching in the shallow conduit. These features may record the interaction of spatially distinct magmatic regimes during formation of pyroclastic flows that produced the ignimbrite deposits. Lithics in the ignimbrite, believed to be fragments of pre-caldera lava, form a linear compositional trend spanning ∼4 wt.% SiO2 (Fig. 8). The pre-caldera dacite plots on the same trend (not shown in Fig. 8), extending the range in SiO2 of pre-caldera lavas to 65 wt.%. Such a range indicates an extended period of differentiation, inferred to have occurred during the cone-building phase. Post-caldera deposits plot at the primitive end of the Tofua array, forming a parallel tend, but over a much smaller range of SiO2. The limited spread in SiO2 may reflect the time elapsed since caldera formation.
Samples Ha3-3a and Ha3-3b, from different levels in the same major shower bedded unit on Ha’afeva island (section 8) are chemically indistinguishable from the Tofua (caldera-forming) ignimbrite (field 1, Fig. 8). This unit correlation is supported by the overlapping age ranges obtained from pollen separate/14C dates for organic material recovered from the base of each unit. Furthermore, the tephras are typically shower-bedded in cores and exposures, showing either variations in syn-eruptive wind conditions or, more likely, variations in eruption intensity, consistent with a collapsing and reforming column (Fierstein and Hildreth 1992). There is no evidence in the distal tephra for significant intra-eruption pauses. The thickness (0.5 m) and relatively coarse grain size of the fall units east of the volcano imply eruption columns of sufficient height to enter the higher-level westerly wind streams, i.e. above 3 km in altitude (Reid and Penney 1982). Current modal high-level wind speeds are found to be ∼24 m/s (Reid and Penney 1982); sieving of well-sorted distal fall deposits reveals a median clast size of 4 mm (−2 phi). The 1886 basaltic Plinian fissure eruption of Tarawera provides an analogue model for release heights calculated using horizontal displacement distances, average wind speed and pyroclast fall times (Walker et al. 1984). Applying this model to the deposits of Tofua, we infer a minimum release height of ∼12 km for the caldera-forming column.
Major element concentrations and volatile contents of melt inclusions, composition of melt inclusion hosts and whole-rock major element data for Tofua (06TF70) and offshore (Ha3-3) tephras
Melt inclusion name
Melt inclusion host
Mg#host (near melt inclusion)a
The lowermost unit of the ‘Rope section’ (section 4) is the oldest and largest outcropping ignimbrite on the island, and is probably the deposit of the caldera-forming ‘Tofua’ eruption. At this location, two distinct cooling packages are clearly distinguishable, separated by heavily sheared intervening units. These units comprise intervening fall and surge deposits (Fig. 5a), giving evidence for variations in mass eruption rate and style and possibly also indication of a pause in the eruption sequence. Structural evidence suggests major slip motion between the ignimbrites occurred on the upper surface of the Tofua ignimbrite either during or immediately following the eruption of the second (Hokula) ignimbrite. Tofua has an average flank slope of 35°. Striations at field section 4 imply a SE directed sliding motion along a shallow dip angled in the same direction (Fig. 4d). Grooves preserved on the upper surface of the older ignimbrite imply that it had cooled to a state of near solidity. The cooling time calculated for the Tofua ignimbrite is one day using the AshPac computational model (Miller and Riehle 1994; Riehle et al. 1995). This result is based on an eruption temperature of 900°C and an average deposit thickness of 15 m. A lack of weathering or soil deposits suggests the interval was no longer than a few decades (cf. Chadwick et al. 1999). Fracturing of these intervening units may have occurred due to collapse of the underlying Tofua ignimbrite into the caldera and/or slippage of the overlying Hokula deposit in response to post-Hokula chamber recovery.
The Hokula unit is made up of multiple intercalated lenses that are readily distinguishable in outcrop (Fig. 5b), separated by localised segregations of co-ignimbrite lithic breccia (e.g. Freundt and Schmincke 1985; Druitt and Bacon 1986). The intercalated lenses, each up to 2.5 m thick, show that discrete depositional units overlap and were deposited one at a time. We correlate the two mantling breccia/ash pairs on the present caldera rim (Fig. 4b) as the vent-proximal equivalents of the Hokula ignimbrite. These match geochemically, lithologically and geographically by being confined to the north and east caldera rim. The lithic clast fraction shows an average size decrease away from the vent whilst intercalated ignimbrite flow units are further evidence for pulsatory eruptions. Offshore tephras sharing the same geochemistry as the Hokula unit have not been identified, implying that it represents a smaller scale of activity than the caldera-forming Tofua event.
Both the Tofua and Hokula ignimbrites form single individual cooling units comprising several sub-units bounded by discordant contacts (Fig. 2) and lack evidence for significant intra-event time breaks. Layered, variably welded ignimbrite units are believed to form by progressive aggradation during sustained passage of a single particulate flow with properties varying over time (Branney and Kokelaar 1992). Here, however, we see evidence for multiple lobes and pulses. Changes in source dynamics through time may be reflected in the facies variation recorded within discrete ignimbrite units. For example, that the upper layer of the Tofua ignimbrite (section 4, Fig. 2) displays an enrichment in rounded scoria bombs and lapilli, may reflect an increase in abrasion in response to a rise in the suspended load. The influence of topography is clearly seen in the mid-level of the Hokula Ignimbrite where a localised clast rich lens outcrops at a high angle to a series of fine-grained discordant welded beds. In detail, the clast-rich lens abuts and thickens away from the contact with the adjacent welded deposit documenting the variable, and in this case indeterminable, azimuth of flow (Fig. 5b). A thin bed of imbricated fiamme is located in the upper part of the Hokula deposit (Fig. 5c), providing evidence for the pervasive loss of primary vesicularity due to welding. Moreover, an extremely high-grade band of welded Tofua ignimbrite of variable thickness spans a 1.5 km stretch of the coastline west of section 4. This represents an extremely high-grade zone in which large amounts of internal heat were retained within the ignimbrite body, which thickened in response to the underlying topography (Fig. 5d). These features suggest the extent of welding was dependent upon local topography rather than being vent controlled; notably, the most densely welded zones are not in proximal sections.
The depositional lithofacies of the coastal tuff cone buried by the ignimbrites suggests it was formed by pyroclastic density currents and fall deposits from explosive phreatomagmatic eruptions (cf. Chough and Sohn 1990; Sohn 1996). The Tofua and Hokula ignimbrites were deposited from either high energy, highly mobile flows that were unable to deposit on or/and surmount the obstacle of the tuff cone (Fisher et al. 1993), or were emplaced by low-energy flows pinching out by the summit of the underlying tuff cone (Valentine et al. 1992; Giordano et al. 2002). Associated offshore fall units suggest high column heights during at least part of this eruption sequence, supporting the highly mobile alternative. High-grade welding and high mobility indicate rapid emplacement with little heat loss, implying a hot flow consistent with the mafic/intermediate pyroclast composition and high mass-ejection rates. Welding and compaction has resulted in a significant loss of primary vesicularity in juvenile clasts. For the Hokula unit, the absence of an associated fall unit on distal islands, its restricted distribution, high variability in degree of welding, and structure comprising multiple small lobes indicate that it may, in contrast, have been emplaced with relatively low energy from low, dense plumes spilling over the northern caldera rim. This extremely domainal welding is similar to the ‘local welding’ style described in the 1875 eruption deposits of Askja volcano (Carey et al. 2008). These authors identify the availability of supplied heat and insulation as the primary controls on local welding. This style of welding suggests that the Hokula phase was cooler than the Tofua phase with a lower degree of lateral heat transfer.
The proximal mantling breccias probably formed from near vent explosive fall-out (e.g. Walker 1985), forming a coarse veneer of vent- and conduit-derived clasts ripped up during the early stages of the second explosive phase. The slump scar breccia/ash pairs likely represent equivalent intra-caldera “ramp” deposits, which are inferred to be necessarily thicker because they must occupy the space created by the syn-eruptive subsidence of the caldera floor.
Identification of two distinct stages to the eruption sequence makes a piecemeal collapse scenario for Tofua volcano highly improbable (cf. Branney and Kokelaar 1994). The two vent proximal mantling breccia/bedded ash successions, intercalated coastal flows and clast rich lenses forming the upper unit suggest that the Hokula ignimbrite was emplaced via intermittent collapse of an unstable column.
Comparison with other ignimbrites from caldera-forming eruptions
Common characteristics of intermediate (broadly andesitic) ignimbrites include: a significant proportion of both lithic and juvenile scoria clasts, evidence for involvement of external water (e.g. chilled clast margins), spatter deposits and a variable degree of welding extending to extremely high grade facies (Mellors and Sparks 1991; Robin et al. 1994a).
The mid-fifteenth century Kuwae caldera, Vanuatu, was produced by a series of submarine phreatomagmatic explosions. These phreatomagmatic phases are represented by dominantly basaltic and dacitic units composed of poorly vesicular blocky vitric lapilli and blocks with well-developed glassy chilled margins (Robin et al. 1994b). This feature, combined with only rare development of welding under very thick deposit piles (>30 m) may indicate that this pyroclastic regime was overall cooler and wetter than that on Tofua. In contrast, more recent work on proximal deposits finds no evidence for a high intensity (VEI 6) eruption, implying dominantly submarine activity (Nemeth et al. 2007). Preserved vesicular juvenile material within the Tofua deposits is restricted to rounded cauliform pumices, requiring more rapid cooling histories involving external water. Further diversity is seen in the pyroclastic deposits of Gran Canaria, Canary Islands (Ignimbrite D) that is inferred to be the deposit of a drier system (Kobberger and Schmincke 1999). The highly peralkaline rhyolite would have had a similar viscosity to the more mafic compositions considered here and so serves as a useful comparison. Pervasive rheomorphism within the unit recorded a large component of gravitational remobilisation (Kobberger and Schmincke 1999), a feature not seen in the Tofua deposits. The Siwi pyroclastic sequence, Tanna island, Vanuatu, is composed of basaltic andesites and andesites. It contains only isolated welded packages and a much greater volume of accidental material producing highly heterogenous deposits (Allen 2005). Both the Siwi and Tofua sequences contain vent proximal spatter agglomerate, interpreted as ‘fountain’ deposits.
Eruption dynamics and trigger
The occurrence of localised geochemical reversals either side of the Tofua/Hokula contact suggests tapping of different levels of a stratified chamber and/or magmatic mingling as the major explosive phases proceeded. Indeed, Kennedy et al. (2008) have demonstrated that rotation and tilting of a caldera roof block as it sinks can produce a stirring effect, resulting in the eruption of compositionally and physically distinct magmas in close succession (Fig. 10c). A strong component of tilting is in agreement with the large volume of eroded roof block material found in the mantling lithic breccia deposits on the northern rim. Stratigraphic relationships of the Tofua/Hokula contact are crucial in identifying a likely eruption trigger for the second explosive phase. The basal Hokula ignimbrite tapped a distinct, incompatible element depleted magma, then intervening spatter deposits tapped what remained of the Tofua magma. Finally, the Hokula magma was erupted (Fig. 10d). Eruption of Tofua-like spatter during block slumping, immediately followed by the Hokula phase requires that the distinct (more mafic) Hokula magma had already been emplaced beneath the caldera. Thus, it is entirely possible that the second, caldera deepening, event was triggered by mafic chamber recharge. The post-caldera volcanics extend to lower silica and higher magnesium than any of the juvenile material from preceding explosive phases (Fig. 8). This suggests post-caldera replenishment of mafic magma, which subsequently differentiated to produce a resurgent trend similar to that of the pre-caldera lavas.
Eruption magnitude and sulphur release
Based on caldera dimensions (width 4 km, height 0.5 km) we estimate that the volume of juvenile magma erupted could have been as much as 8 km3. Despite the mafic composition of eruptive products, this is comparable to the volume of the 1883 Krakatau eruption of 10.1 km3 DRE (Mandeville et al. 1996) and more than an order of magnitude greater than the 0.3 km3 DRE ejected during the 1980 Mt. St. Helens eruption (Carey et al. 1995). A VEI value of 5–6 is obtained combining our estimates of magnitude (total volume) and intensity (column height) (cf. Parfitt and Wilson 2008). An estimate of volatile emission during an eruption can be made by comparing the volatile contents of melt inclusions with the volatile concentrations measured in matrix glasses of the erupted magma (Witter and Self 2007). A clinopyroxene hosted glass melt inclusion within sample Ha3-3 (distal caldera-forming tephra, section 8, Fig. 2) yielded the highest offshore H2O content of 3.19 wt.%. The same inclusion contains 833 ppm S (Table 1). Fresh matrix glass from a Tofua tephra is found to contain 30 ppm S (Keller et al. 2008). Using the method of Sharma et al. (2004), taking a vesicle-free crystal mass fraction of 0.35 and 2.24 × 1013 kg as the mass of erupted magma, we estimate a mass of 12 Tg of S. This is equivalent to 14 Tg SO2 loading taking an aerosol conversion efficiency of 60% (e.g. Bluth et al. 1992). Thus, the petrologic budget suggests that degassing of the silicate melt alone resulted in significant aerosol release. Exsolution of sulphur from a magma is however strongly influenced by the presence of CO2-H2O rich vapour in the melt, in which it is highly compatible (e.g. Keppler 1999).
The column height and the mafic, volatile rich composition of this magma imply that the paroxysmal Tofua ignimbrite eruption was almost certainly a climate-forcing event, indicating that it should be recorded in ice-core records. We infer that this event may be the source of either the AD 1030 or AD 1257 volcanic sulphate spikes recorded in Antarctic ice sheet records (Budner and Cole-Dai 2003; Zou et al. 2006) on account of their associated southern hemisphere cooling signals (Oppenheimer 2003). Based on our estimates of erupted mass and sulphur release, the caldera forming eruption cloud of Tofua would have emitted 520 ppm S per unit mass of erupted magma. The AD 1257 eruption is believed to have produced 130 ± 30 Tg of S (Oppenheimer 2003 and references therein). If the Tofua eruption was the source of the “great mid-13th century eruption”, its magnitude would have needed to have been 2.5 × 1014 kg, an order of magnitude greater than predicted. It seems more likely therefore that the AD 1030 date, previously correlated with the 5 × 1013 kg, 10 Tg eruption of Baitoushan volcano, Northern China (Zielinski 1995), may have originated in the southern hemisphere.
In the last 1,000 years, Tofua Island has undergone two phases of highly explosive activity, emplacing two high-grade ignimbrites, each comprising a series of flow units. The thick, relatively coarse nature of fall units found on islands to the east of Tofua imply that the first of these eruptions produced a plume high enough to enter the westerly stratospheric wind streams at an altitude of ∼12 km. This explosive event is estimated to have had a VEI of 5–6, and given its mafic composition it likely had an important atmospheric impact. The first and largest event was of highest intensity, likely triggered by development of a H2O saturated dacitic magma. Subsequent interaction of the rising body of magma with external water may have enhanced explosivity. The second event accompanied caldera-deepening as a result of chamber instability, likely accentuated by further recharge of a chemically distinct magma depleted in incompatible elements. The caldera-forming Tofua ignimbrite is of evolved basaltic-andesite composition. The dominantly mafic Hokula ignimbrite displays chemical anomalies in basal parts of vent-proximal northern sections, likely recording the concurrent eruption of a distinct primitive magma delivered by recharge, and one with more-evolved composition similar to that of the Tofua ignimbrite. It is possible therefore that a portion of the magma from the Tofua event was retained in the crust and was erupted during the onset of the second explosive phase.
Evidence of direct magma–water interaction is recorded only by frequently cauliform clasts in the earliest part of the eruption sequence. The main explosive eruptive phases were moderately fragmental comprising pyroclasts that were largely hot and highly mobile, resulting in elongate ignimbrite deposits. The intensely welded basal section of ignimbrite overlying a coastal tuff cone shows that the Hokula ignimbrite was also hot and mobile. The lower content of dense juvenile clasts containing angular lithic fragments, and a volumetrically greater proportion of lithics in the Hokula unit, suggests eruption of magma through pre-existing conduits and recycling of pyroclasts. The degree of welding of both ignimbrite deposits was mostly dependent on depositional thickness, in turn controlled by local topography. The thickest sections are very strongly welded and preserve extremely little primary pore space. The recognition of these young events in the Tofua arc demonstrates a higher degree of hazard to local communities than previously considered. All of the age estimates for this major eruption (once calibrated), overlap with ages of several major explosive eruptions recorded as sulphate spikes in Antarctic ice cores. Based on evidence presented here, we propose the AD 1030 date as the most likely candidate. Tofua may, then, represent the “smoking gun” for one of the acid peaks in the ice-core records of this era.
JTC wishes to thank Kelepi Mafi and the Tongan Survey for sampling permission and logistical assistance, Graham Smith for invaluable help with fieldwork and the crew aboard the yacht Bliss for essential coastal sampling and photography during the field season. Oded Navon is thanked for helpful discussion and editorial comments. Detailed and constructive reviews provided by Armin Freundt, Jacopo Tacceucci and James White helped greatly in improving the clarity and scope of the manuscript. ST acknowledges the support of an ARC Federation Fellowship. SJC acknowledges the support of FRST Contract MAUX0401 “Learning to live with volcanic risk”. The analytical data were obtained using instrumentation funded by DEST Systemic Infrastructure Grants, ARC LIEF, NCRIS, industry partners and Macquarie University. This is contribution 719 from the Australian Research Council National Key Centre for the Geochemical Evolution and Metallogeny of Continents (http://www.gemoc.mq.edu.au).
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