Small-volume silicic eruptions are common across a range of settings and exhibit the diverse eruption styles shown in Table 1. Relative to large-volume calc-alkaline eruptions, the ascent dynamics of often alkali-rich small-volume eruptions are relatively under-studied. Small-volume silicic eruptions typically occur as late-stage activity associated with larger volcanic features, e.g. calderas (Cole et al. 1995; Hernando et al. 2019), shield volcanoes (Shea et al. 2017) and flank fissures (Katsui and Katz 1967; Pistolesi et al. 2015) (Table 1). The evolution and eruption of these small-volume magmas may be governed by the low flux of incoming eruptible melt and crustal heterogeneity (Chamberlain et al. 2019). Scarcity of eruptible melt has been attributed to long-term, low magma-production rates, such as at Ascension, and/or significant reservoir depletion by earlier activity, such as often leads to caldera formation (Table 1).

Table 1 Comparison of explosive small-volume silicic eruptions from the literature

Direct observations and inferences from preserved deposits show that small-volume silicic eruptions are capable of intense short-lived explosive phases with few precursory signals, important transitions in eruption style and pulsatory activity capable of generating pyroclastic density currents (PDCs) (Table 1). Thus, a critical challenge for volcanic hazard assessment is to recognise the fingerprint and drivers of such variability in eruptive style through the interrogation of remaining deposits; set against challenges from the poor preservation (Preece et al. 2021) or minimal exposure (e.g. Hernando et al. 2019) typical of these eruptions, a consequence of low initial eruptive volumes, air fall offshore (for small ocean islands) and destruction by subsequent eruptions and erosion. To complement stratigraphic investigations of erupted deposits, studies of juvenile clast texture have provided key insights into eruption dynamics of larger silicic eruptions (e.g. Polacci et al. 2003; Alfano et al. 2012; Preece et al. 2014). Such textures represent an integrated history of melt-gas-crystal-strain interactions in ascending magma (Houghton and Wilson 1989), for the most part preserved at the point of fragmentation. Microscopic changes to vesicle textures can occur post-fragmentation, particularly where viscosities are low, and vesicles well connected (Gardner et al. 2017). However, this study focusses on bulk vesicularity and macro-scale clast textures in hand specimen. Post-fragmentation modifications should, therefore, have a minimal impact on the dominant clast textures and resulting interpretations discussed here. For more detailed quantification of vesicle textures, e.g. vesicle size distributions (Shea et al. 2010), these modifications must be accounted for; however, this is beyond the scope of this study. The bulk properties of deposits identified from differing stratigraphic horizons in the field can be used to interpret the drivers of changing eruption dynamics such as the development of heterogeneous pressure gradients, shear zones or plug formation and destruction (e.g. Schipper et al. 2013; Saubin et al. 2016; Shea et al. 2017). Analysis of juvenile clast texture is thus a powerful tool for reconstructing ascent and eruption dynamics of un-observed eruptions.

Trachytic melts have a lower viscosity than their calc-alkaline equivalents (Whittington et al. 2001; Giordano et al. 2004) which influences bubble structure and outgassing. Investigations of clast texture are thus particularly important for reconstructing and understanding the dynamics of these lesser-studied eruptions. Experimentation, modelling and textural studies have recently improved our understanding of the physical properties and ascent dynamics of alkali-rich melts (Giordano et al. 2004; Arzilli and Carroll 2013; Fanara et al. 2013; Shea et al. 2017). The idea that trachytic melts are “kinetically reactive” facilitating complex ascent and eruption dynamics is gaining traction. The basis for this is their low viscosity, high H2O diffusivity, rapid degassing and vesiculation and tendency to crystallise microlites rapidly in response to perturbation (Shea et al. 2017; Deniel et al. 2020).

To date, reconstruction of small-volume alkali-rich eruptions has largely focussed on longer-term volcanic histories rather than individual episodes (Buck et al. 1981; Mahood and Hildreth 1986; Rotolo et al. 2007; Singer et al. 2008; Vezzoli et al. 2009; Hutchison et al. 2016; Sbrana et al. 2018). In this paper, we present new insights into the eruptive transitions and dynamics of a small-volume explosive-effusive trachytic eruption, using unusually well-preserved and exposed deposits on Ascension Island, South Atlantic. We combine detailed eruption stratigraphy and mapping of field relationships with analysis of individual and bulk clast textures to reconstruct transitions in eruption styles and understand their drivers and the resulting implications for future activity in this and similar settings.

Geological background

Ascension Island is an 8 km by 12 km intra-plate ocean island located ~90 km west of the Mid-Atlantic Ridge, 7°56′ S of the equator. The volcanic edifice is approximately 3800 km3 (Harris 1983), 1% of which is sub-aerial (Nielson and Sibbett 1996). Volcanism initiated ~5–6 Ma at the ridge, and the oldest sub-aerial rocks are ~1 Ma (Weaver et al. 1996; Jicha et al. 2013). Whereas some alkali-enriched ocean islands such as Pantelleria have a distinct “Daly Gap” (Neave et al. 2012), Ascension does not. Ascension’s volcanic and plutonic rocks each record a full suite of alkali basalt-trachytic-rhyolitic compositions: explosive and effusive examples occur across the entire volcanic suite (Nielson and Sibbett 1996; Chamberlain et al. 2019). Trachytic and rhyolitic rocks are concentrated in the central felsic complex (CFC) and the Eastern Felsic Complex (EFC), where the Echo Canyon (EC) edifice is located; see maps in Figs. 1 and 2 (Nielson and Sibbett 1996; Kar et al. 1998; Chamberlain et al. 2016). This spatial restriction of silicic rocks was investigated by Chamberlain et al. (2019) and attributed to the presence of a series of nested silicic plutonic bodies in the underlying crust, which acted as a density filter stalling basalts until they evolved to lower density silica-rich compositions. Current data suggest that Ascension Island is typical of a low volume magmatic flux ocean island setting (Chamberlain et al. 2019). Here we estimate the averaged magmatic output rate (calculated from total volume of volcanic pile (Harris 1983) and age of volcanism (Weaver et al. 1996; Jicha et al. 2013)) on Ascension as approximately 0.63×10−3 km3/yr, much less than that of Hawaii, 0.426 km3/yr (Robinson and Eakins 2006). Ascension has only erupted three times in the last 5,000 years—basaltic lava flows in the N-NW of the island, two dated at ~500 years and one at ~1.6 ka using 40Ar/39Ar dating (Preece et al. 2018). In contrast, while closer in size to Ascension than Hawaii, Sao Miguel in the Azores has erupted 57 times in the last 5000 years: Sao Miguel is much younger—~0.9 Ma (Jeffery et al. 2016)—and considerably more active than Ascension. The oldest rocks on Ascension are silicic lavas from the CFC dated at 1 Ma (Jicha et al. 2013). More than 70 explosive felsic eruptions have occurred across the CFC and EFC in the last 1 Myr (Preece et al. 2021) but preservation is often poor; few eruptions can be traced back to a source vent (Chamberlain et al. 2016). The EFC is the younger of the two complexes (Jicha et al. 2013) and is dominated by a series of trachytic lava flows, rhyolitic domes, pumice falls and PDC and ignimbrite deposits (Figs. 1 and 2) (Atkins et al. 1964; Nielson and Sibbett 1996; Chamberlain et al. 2016, 2019). Younger basaltic eruptions have produced several flows and scoria cones around NE-Bay.

Fig. 1
figure 1

Simplified geological map of Ascension Island, adapted from Chamberlain et al. (2016) showing distribution of mafic and felsic effusive and explosive products, key infrastructure and key deposits. 1 Georgetown, 2 Two Boats Village, 3 Travellers Hill Royal Air Force station, 4 US air force base, 5 Wideawake Airfield. AL, Ariane Lava flow; EFC, Eastern felsic complex; CFC, Central Felsic Complex. Extent of EFC and CFC is denoted by dashed red ellipses. Black rectangle denotes location of Map 2. Inset: Location of Ascension Island relative to the Mid-Atlantic Ridge in the South Atlantic.

Fig. 2
figure 2

Geological map of the Echo Canyon (EC) deposits—black box in Fig. 1. OBP is orange-brown pumice. Distal pyroclastic density current deposits (PDC) mark furthest extent of EC deposits north of the Ariane lava flow (AL). White dashed lines mark footprint of valleys EC, Echo Canyon; RC, Rainbow Canyon; and SB, Spire Beach. Geology overlaid on Google Satellite image for Ascension Island (Google Earth 2006). EC deposits are overlain by AL, which is likely sourced from Devil’s Cauldron (DC).


Stratigraphy and sampling

Stratigraphy of the EC volcanic edifice was determined during two 7-week field seasons in 2018 and 2019/2020. Stratigraphic logs through eruption deposits were carried out at localities AI18-04, 22 and 23 in 2018 and AI19-02, 46 and 13 in 2019 (Fig. 2). Locations for stratigraphic logging of deposits were chosen based on exposure, accessibility and relationship to overall geometry and deposit extent. A geological map of the region (Fig. 2) was produced using data from field observations, photographs and Google Earth imagery (Google Earth 2006).

The EC edifice is divided into pumice cone and post-pumice cone deposits: the pumice cone is divided into three parts, lower, middle and upper, based on lithologic characteristics. The lower, middle and upper parts of the cone were sub-divided into 12 units comprising one or more beds. Each unit was distinguished from stratigraphic neighbours via either componentry and juvenile clast texture or the identification of a repeated sequence of distinct lithologic characteristics. Average maximum clast size was calculated by measuring the 5 largest clasts in outcrop and taking the average long axis. Modal clast size was qualitatively estimated in the field based on careful examination of the wider outcrop, and modal clast size values quoted henceforth are qualitative estimates only as a full grainsize study was not completed.

Six units from the pumice cone were sampled, one from each unit in the lower cone, one from the middle cone and one from the upper cone—at AI18-04, AI18-22 and AI19-02, respectively (Fig. 2). The sampling targeted each eruptive phase identified using lithologic observations. Sampling of the middle and upper cone was restricted due to outcrop accessibility in the steep cliffs of Echo Canyon and Rainbow Canyon. Bulk samples of explosive products of the cone and post-cone units containing clasts of 16–32 mm were collected by excavating a region up to 10 cm thick, extending laterally across the unit until enough material was acquired for density measurements and whole rock analyses. Samples of Units 1–4 and 9 were collected from Echo Canyon at localities AI18-04 and AI18-22, respectively. Unit 11 was sampled at AI19-52 in Rainbow Canyon, the lavas at AI18-20 and AI18-33 and the post-cone pumice fall (the orange-brown pumice, OBP) at AI18-19, as shown in Fig. 2.

Density measurements and textural categorisation

In the laboratory, bulk samples were passed through an 8-mm sieve. Clasts in the >8-mm size fraction were ordered by size and each axis measured to remove clasts > 32 mm and <16 mm. From the remaining clasts, the 100 largest were selected following Shea et al. (2010); for the OBP, all 16–32-mm clasts were selected. Clasts which have undergone significant post-fragmentation textural alteration often have expanded cores and dense rims, fluidal morphologies and breadcrust exteriors (Polacci 2005). None of the clasts used in producing bulk vesicularity distributions (BVDs) exhibited breadcrust exteriors or fluidal morphologies. Clasts of the same morphology, colour and size as those selected for density measurements from each unit were broken open to check for internal textures indicative of significant post-fragmentation vesicle expansion, none were found. Analysed clasts from the cone deposits lacked evidence of thermal alteration, e.g. discoloration or formation of rims, and care was taken to select the freshest looking clasts from the OBP where such discoloration was more common. Density of the 16–32-mm juvenile clasts was determined using the Archimedes principle following the suspension method detailed in Houghton and Wilson (1989) and Shea et al. (2010). For Units 1, 2, 3, 4, 6 and 11, 100 clasts were measured. For the OBP, 43 clasts were measured as the bulk sample acquired contained a high proportion of clasts > 32 mm diameter. Bulk vesicularity was calculated using a dense rock equivalent value of 2.51 g/cm3 for Units 1–11 and 2.55 g/cm3 for the OBP. Dense rock equivalent values were calculated using the method of Lange and Carmichael (1987) from whole rock compositions attained using X-ray fluorescence (XRF) analysis (see section below for details of XRF method). Measured clasts were categorised based on pervasive vesicle textures observed in hand specimen.

Geochemical analyses

Pumice clasts and lava samples were prepared for X-ray fluorescence (XRF) analyses following Chamberlain et al. (2016). Samples were then crushed, and an aliquot of each sample milled in a tungsten carbide container before XRF analyses were carried out on a Bruker-AXS S4 Pioneer at the University of East Anglia. For major elements, analyses of multiple international standards yielded accuracies ≤ ± 0.18 wt% when compared with certified values.

Trace element concentrations were measured by ICP-mass spectrometry after HNO3+HF digestion of 0.1000 g of sample powder at ~180 °C and 200 psi for 30 min, evaporation to dryness and dissolution in 100 ml of 4 vol.% HNO3. Measurements were done with a PE SCIEX ELAN-5000 spectrometer (rhodium internal standard). Precision, as determined from international standards, was better than ±2% and ±5% for concentrations of 50 and 5 ppm, respectively. Raw major and trace element data are given in the supplementary data.

Deposits and products of the Echo Canyon eruption

The EC eruption deposits have an 40Ar/39Ar age of 59 ± 4 ka (Preece et al. 2021), crop out over ~3.4 km2 and consist of a partially preserved and exposed proximal pumice cone, pumice fall deposits, a lava dome and a brecciated lava (Figs. 2, 3 and 4). There is no evidence for a significant time break between the EC deposits (e.g. paleosols or erosional surfaces), and additionally, all the eruption products have a unique phenocryst population; we therefore consider them to be products of a single eruption. The EC phenocryst population comprises 3–8-mm diameter feldspar and clinopyroxene macrocrysts (Appendix 2). Pumice clasts typically contain < 3% feldspar microlites from < 10 to 750 μm, with the exception of some clasts within the uppermost pumice fall (see below and Appendix 2 Fig. 22). The geometry of the volcanic edifice, apparently elongated towards NW, results from both the paleo-topography and prevailing wind direction (Fig. 5). The pumice cone has a maximum preserved thickness of 50 m (Fig. 4A), and it thins rapidly to the E-SE (Fig. 4A) and W-NW (Fig. 5B and C). Juvenile clast descriptions are given in Fig. 6 and Appendix 1 Fig. 13, and stratigraphic and geometric relationships between the EC deposits are shown in Figs. 2, 3, 4 and 5. Generalised descriptions and interpretations of the EC deposits are given below and in Figs. 7 and 8. Full stratigraphic and petrographic descriptions of the EC deposits are given in Appendices 1 and 2.

Fig. 3
figure 3

Composite log of the Echo Canyon (EC) eruption deposits. Beds represented by a solid fill are clast-supported; beds with a gradient fill are variably matrix and clast supported as detailed in Appendix 1. Dashed grey lines indicate stratigraphic relationships between distal, generalised sequence and ultra-proximal deposits. Dashed black lines indicate contact not always exposed. Clast types and key bed characteristics are detailed in the key. Unit numbers as referred to in text are marked in black; asterisks indicate sampled units. Maximum exposed thickness of cone deposits is 50 m.

Fig. 4
figure 4

Overview photos of Echo Canyon (EC) eruption deposits. A View into Echo Canyon showing dissected cone, relates to Fig. 5a box 1. Black dashed boxes L1, L2 and L3 show logged sections. Lower, middle and upper cone, Unit 12 and orange-brown pumice (OBP) correspond to Fig. 3. B section at Spire Beach (see Fig. 5D), white arrows from circles show slump directions, with distal pyroclastic density current deposits (PDC), the dome, lava and OBP indicated by shaded outlined regions. C View of EC deposits looking SSE from NE Bay coastal path. Valleys EC and Rainbow Canyon (RC) are marked. D Relationship between distal PDC deposits of EC eruption, felsic pyroclastics and coastal lavas, taken from the NE-Bay lower coastal path. E View into western face of RC showing relationships between the OBP, cone deposits and felsic pyroclastics. EC Units logged at 6-m scale bar location.

Fig. 5
figure 5

Interpretive cross-sections through the Echo Canyon (EC) eruption deposits corresponding to lines of sections A–D on inset. OBP is orange-brown pumice; “upper”, “mid” and “lower” refer to lower, middle and upper cone from Fig. 3. A Section down Echo Canyon to the coast with underlying felsic pyroclastics and mafic lavas projected below the land surface (dashed black line). B Section from EC to Spire Beach (SB) shows rapid thinning of pumice cone deposits; locations of the three canyons that cut the EC deposits are marked; Rainbow Canyon (RC), EC and SB. C Approximated more proximal section W-E showing dome to the east and thinning of cone stratigraphy beneath the Ariane Lava (AL) to the west. D Spire Beach section showing how OBP mantles pyroclastic density current (PDC) deposits, lava and dome. Debris falls outlined in grey dashed lines with arrows showing direction of motion. Inset map: shows orientations of cross-sections with EC deposits annotated onto satellite imagery of NE-Bay from Google Earth. Dashed lines denote extent of key valleys dissecting EC deposits, EC, RC and SB.

Fig. 6
figure 6

Description of five pumice clast textural types identified in the Echo Canyon deposits. H.S. indicates in hand specimen, // is parallel to elongation orientation, ⊥ indicates perpendicular to elongation direction. Superscripts: 1Bulk vesicularity as measured from at least 100 clasts of each textural group. Black scale bars represent 1 cm. Red boxes 1–5 are backscattered electron images of representative vesicle textures from each identified pumice type, boxes are 1mm2. SEM images 1–4 are collected on a FEI Quanta 650 FEG at the Natural History Museum, London, UK, and image 5 collected on a Zeiss Gemini sigma 300 VP at the Zeiss facility in Cambridge, UK (see Appendix 3 for method outlining rationale for clast selection).

Fig. 7
figure 7

AC Lower pumice, pumice falls. A Units 1–4 of the Echo Canyon deposit (EC). B General characteristics of clast-supported pumice falls and C ash beds of Unit 2. D Middle Cone, transition zone, with varying clast size and ash content of Unit 6. E Upper cone units above log site (L2 in Fig. 4) showing lower part of proximal pyroclastic density current (PDC) deposits in EC. F Stratified and massive PDC deposits at Rainbow Canyon (RC), tool 30 cm for scale. G Obsidian deformed around pumice clasts in stratified PDC deposits at RC. H Lava and pumice clasts in Unit 12, hammer 30 cm for scale. I Upper part of Unit 12, highly altered. J Brecciated lava, fresh parts are dark, altered are pale pink. K Obsidian bomb within the orange-brown pumice (OBP).

Fig. 8
figure 8

Generalised vertical section from Fig. 3, red arrows and asterisks mark stratigraphic locations where fine ash was identified in outcrop; the ash beds of Unit 2, Units 5–10 of the middle cone and Unit 11. Maximum and modal lithic and pumice clast data estimated in the field are shown. The grey-shaded region represents data for beds in Units 5–10. Estimated modal and average maximum lithic and clast size are smallest in middle and upper cone units. Lithic clast proportions increase in middle and upper units relative to the lower cone. Bulk vesicularity distributions for each sampled unit are presented, and the number of clasts analysed for each unit is quoted in brackets next to the unit number/name. Abundance of pumice clast textures are displayed as a percentage of analysed clasts for each Unit. Numbers 1–5 in the key correspond to the five main textural types identified: type 1, micro-vesicular equant; type 2, macro-vesicular equant; type 3, micro-vesicular elongate; type 4, macro-vesicular elongate; type 5, dense.

Pumice cone stratigraphy

The pumice cone is here divided into three parts: the lower, middle and upper cone. The lower cone is composed of massive, clast-supported pumice fall deposits (Units 1–4, Fig. 7A and B and Fig. 8) and is subdivided into 4 units (Appendix 1), one of which contains three thin ash beds (Fig. 7A and C). The juvenile material varies up through the lower cone: lower units are characterised by micro- and macro-vesicular pumice with equant vesicles; sheared pumice clasts are most abundant in Unit 3 and dense clasts most abundant in Unit 4 (Figs. 6 and 8). A conspicuous layer of lithic clasts containing syenite and granite blocks marks the boundary between Units 3 and 4.

The middle cone is more heterogeneous than the lower cone Fig. 8 and comprises interbedded matrix- and clast-supported beds (Units 5–10, Fig. 7D) of variable grainsize. Clast-supported beds contain angular pumice clasts, whereas the matrix-supported beds contain rounded pumice clasts. These are interpreted as pumice fall deposits and PDC deposits, respectively. Juvenile clasts are dense (Fig. 6). Erosional surfaces have not been observed between units of the lower and middle cone.

The upper cone comprises a sequence of matrix- and clast-supported PDC deposits containing sub-angular to rounded pumice clasts (Unit 11, Figs. 3, 4E and 7E and G). Juvenile clast textures are summarised in Fig. 6 and Appendix 1, Fig. 14 and abundances of types 1–5 displayed in Fig. 8. The upper cone is distinguished by the appearance of obsidian and obsidian breccia clasts (Appendix 1, Fig. 14) that constitute 10 % of the unit. Distal outcrops of Unit 11 occur at distances of 2 km from the cone and reach ~30 m thick. The uppermost unit in the cone (Unit 12) is a coarse-grained clast-supported, proximal fall deposit composed of pumice, spatter—some of which exceed 70 cm in diameter—and dense juvenile clasts that closely resemble the overlying lava flow. Unit 12 drapes an unconformity on top of the PDC deposits of the upper cone and some of the middle cone, a possible result of erosion by PDCs that did not leave any deposits, or collapse, reflecting the proximity of the likely vent to this location.

Post-cone lavas and pumice fall

Two trachytic lavas post-date the pumice cone. The lower lava is a 30–40-m-thick, variably altered and brecciated trachyte lava dome, 3–4 times thicker than the overlying lava (see Figs. 2, 4B and 5B–D). It contains distinctive 3–8-mm feldspar and clinopyroxene macrocrysts (3–5%) often associated with vesicles up to 1 cm. Groundmass crystallinity is heterogeneous in thin section with regions up to 70% feldspar microlites. The overlying lava is a 7.5–10-m-thick brecciated porphyritic trachyte lava flow of similar composition to the older lava but with lower overall groundmass crystallinity—up to 30% feldspar microlites. Feldspar and clinopyroxene macrocrysts comprise 3–5% of the lava. The lava has a vesicularity of up to 10 percent, with vesicle diameters up to 0.5 cm. Sub-parallel, discontinuous layering is defined by lenses of un-altered lava and brecciated regions altered to clays. The uppermost unit, the OBP, is a massive, coarse-grained, clast-supported pumice fall deposit that thickens from 3 to 10 m over 450 m from the top of the cliff above AI18-04 towards the SE to locality AI18-23 and mantles underlying topography (Figs. 3, 4A, C, E and 5a, c, d). It is composed of angular pumice (~95 vol. %) and obsidian (~5 vol. %) lapilli and blocks (Fig. 7K). In thin section, pumice clasts contain discrete microlite-rich regions within homogenous glass (Appendix 2, petrography).

Vesicularity and vesicle texture variations

Bulk vesicularity distributions (BVDs) and the abundance of each textural type (Fig. 6) in the pumice population of each sampled unit are displayed relative to their stratigraphic position in Fig. 8.

From Units 1–3, BVDs progressively narrow about a mode of 75–80%. Clasts in Units 1 and 2 are both dominated by macro-vesicular equant vesicles, and < 5% are sheared. In Unit 3, 31% of clasts are sheared, and the BVD is the narrowest of all the units. Whereas Unit 4 has similar lithologic characteristics to Unit 3, the pumice population is texturally distinct, and the BVD is broader: 64% of clasts fall outside the modal range—70–75%. The proportion of dense clasts increases by 24%, and sheared and micro-vesicular equant clast abundance is halved.

The abundance of dense clasts peaks at 36% in Unit 9. Vesicle texture heterogeneity is reflected in the BVD, which is broad with 37% of clasts with a vesicularity lower than 65%, compared to none in Unit 3. In Unit 11, the five textural types are more evenly represented and the BVD narrower than Unit 9; modal vesicularity is 70–75%. The proportion of dense clasts is 22% lower than in Unit 9, but 43% of clasts are sheared. In the OBP, modal vesicularity increases and broadens to 70–80%, and 16% of clasts have a vesicularity of < 65%. Less than 10% of clasts are sheared, and 23% are dense.

Whole rock compositions and viscosity

Whole-rock compositions of bulk samples from representative units are shown in Fig. 9. All juvenile material is trachytic: between Units 1 and 9, K2O + Na2O variation is <0.5 wt% and SiO2 <1 wt%. The lavas are ~3 wt% poorer in silica with the same range in alkalis. The overlying OBP composition sits between the lavas and the cone pumices. There was no significant difference in composition between textural groups from the same unit (see supplementary data). All the EC units have a peralkalinity index (PI) of 1–1.04 [PI = Molar (Na2O + K2O)/Al2O3]. Generally, major and trace element data for the proximal cone pumices reflect a slightly more evolved composition. Cone units have lower Al2O3, MgO, Sr, Eu and Ba than the post cone units; see Fig. 9. Proximal cone pumices have Zr concentrations > 1080 ppm, whereas the lava, dome and OBP have Zr concentrations <900 ppm.

Fig. 9
figure 9

Whole-rock major and trace element data of pumice and lavas from the Echo Canyon (EC) deposits. Proximal pumices are from the lower-upper cone. In C, F, G, H and I, blue crosses represent maximum analytical error for each element, ± 0.18 for SiO2, ±0.03 for MgO, ±0.04 for Al2O3 and ± 0.1 for K2O. A Total alkalis-silica diagram showing EC deposits plot in the trachyte field with proximal pumices slightly more evolved than the lavas and orange-brown pumice (OBP), symbols are larger than analytical error. B Ternary plot of total Fe + MnO against MgO and total alkalis. Calc-alkaline (Ca), alkaline (Alk) and tholeiitic (Th) fields denoted. C Plot of Al2O3 vs SiO2. D Trace element data normalised against normal mid-ocean ridge basalt (NMORB) showing proximal pumices depleted in Ba and Sr relative to the lavas and OBP. Trace element data is from LA-ICPMS analyses apart from Zr, from whole rock XRF data, normalisation values from Hofmann (1988). E Trace elements normalised to chondrite showing proximal pumices are depleted in Eu relative to the lavas and OBP, normalisation values from McDonough and Sun (1995). F Plot of Ba vs SiO2. Barium from La-ICPMS and SiO2 from whole-rock XRF analysis. G Plot of Zr vs SiO2. H Plot of MgO vs SiO2. I Plot of K2O vs SiO2. C, F, G, H and I show proximal pumices are compositionally more similar to each other than to the lavas and OBP.

Whole-rock major element data was used to calculate estimated melt viscosity across a range of H2O concentrations (Table 2). Samples are only mildly peralkaline so the model of Giordano et al. (2008) was applied. For comparison, calculated viscosities of a typical calc-alkaline basalt, dacite and rhyolite from Le Maitre (1976) and peralkaline rhyolite from Clarke (2020) are orders of magnitude higher (Table 2).

Table 2 Calculated viscosity for EC units and comparison magma compositions

Several previous studies indicate Ascension Island magmas are wet: 2–4 wt% for a trachyte and 8 wt% for a mingled basalt-rhyolite scoria-pumice fall (Kar et al. 1998; Chamberlain et al. 2016, 2019, 2020). We do not currently have volatile data for the EC eruption much less, volatile contents captured in different erupted glasses. So we have calculated here the melt viscosity of four sampled EC units across a range of volatile concentrations. At 4 wt% (representing a likely pre-eruptive water content), EC melt viscosity of the four sampled units ranges from 103.3–103.6 Pas at a reasonable eruptive temperature of 900°C. This is only one order of magnitude greater than a basalt erupting at 1000°C with 1 wt% water (Table 2), but with lower water contents or following degassing, viscosity would rise; our calculations demonstrate that this is highly unlikely even then to generate melt with viscosities equivalent to normal rhyolite.


Stratigraphy, juvenile clast textures and bulk vesicularity distributions (BVDs) are discussed below and used to interpret eruption dynamics, place constraints on explosivity and understand the influence of magma composition on the progression of the EC eruption.

Stratigraphy and eruptive style

The EC eruption shows multiple transitions in eruptive style: initial violent Strombolian–sub-Plinian activity progressed to a stable Plinian column (lower cone), which underwent transient collapse forming PDCs (middle and upper cone). An effusive phase followed (lava and dome), before a final, short-lived explosive phase (OBP). The deposits are linked to a single eruptive period through the absence of erosional surfaces between explosive and effusive phases, deposit geometry and the presence of a distinct phenocryst population not seen in other Ascension Island deposits (Appendix 2). The slightly less evolved compositions of the post-cone units may reflect slight compositional zonation of the magma storage region resulting in initial eruption of lower density, more evolved compositions as is often seen for trachytic melts (Chamberlain et al. 2016; Jeffery et al. 2018).

Cone deposit similarities in estimated modal lithic and pumice clast size, within each unit and abundance of pumice and lithic clasts > 64 mm, indicates they are proximal (cf., Cas and Wright 1996). Deposits thin rapidly, lack matrix and have modal bulk vesicularity >70%. Characteristics of Units 1 and 2 indicate proximal air fall, similar to Strombolian pumice cones described by Houghton et al. (1985) on Mayor Island, New Zealand (Table 1). The thickness, lack of stratification and low lithic clast abundance in Units 3 and 4 reflect progression to an established, stable plume with little input of conduit material (cf., Cioni et al. 2008, 2015). Conversely, the graded boundary between the lower and middle cone suggests waning eruption intensity and increased column instability, often associated with pulsatory sub-plinian activity (Cioni et al. 2015; Andrews et al. 2018). Thin, interbedded units in the middle cone containing angular clasts and high proportions of lithic fragments record short-lived convective plumes (cf., Cioni et al. 2008; Scott et al. 1996; Houghton et al. 2004) during the transition to the main PDC-forming phase. In the upper cone, the dominance of metres thick, stratified ash rich beds containing lenses of pumice clasts records a significant phase of partial/total column collapse which generated PDCs (cf., Scott et al. 1996; Cole et al. 1999; Houghton et al. 2004; Cioni et al. 2008; Andrews et al. 2018). Obsidian and obsidian breccia clasts record fragmentation of accumulations of degassed melt on the conduit walls and formation of tuffisite veins—an additional degassing pathway (Heap et al. 2019).

A transition to effusive activity produced a dome and brecciated lava with groundmass crystallinities of 30–70% and vesicularities < 5%, indicating significant degassing prior to effusion (cf., Deniel et al. 2020). The OBP represents the final, volumetrically subordinate explosive phase of the EC eruption. Angular pumice clasts contain discrete microlite-rich regions which may represent incorporation of accumulated degassed melt from the conduit margins (cf., Mitchell et al. 2019). Evidence for persistence of conduit accumulations of densified melt is provided by obsidian bombs. A late-stage explosive magmatic eruption may be linked to shallow pressurisation from the closure of degassing pathways beneath the flow and dome or continued deep evolution of the final eruptible melt leading to increased overpressures or a combination of the two.

Evolution and variability of vesicle textures

For lapilli with little or no post-fragmentation textural alteration, crystallinity, bulk vesicularity and vesicle shape and size are all controlled by magma properties and ascent dynamics (Cashman and Mangan 1994). Viscosity, volatile content and ascent rate of the melt all influence vesicle and crystal nucleation and growth (Cashman and Mangan 1994). The textural fingerprint of a lapilli population is thus the outcome of spatial and temporal variations in these parameters. Hence, even qualitative comparison of these features can identify key step changes in ascent dynamics that lead to transitions in eruptive style.

By considering how vesicle textures develop prior to fragmentation, the relative influence of bubble growth, coalescence and shear on the melt for each stage of an eruption can be discerned. This principle is used to interpret characteristic vesicle textures of type 1–5 lapilli clasts and their relative abundance throughout the eruption to understand changes in eruptive style. Figure 10 shows how polydisperse vesicle textures evolve under sheared and non-sheared conditions and the dominant processes acting at each stage, as recorded in type 1–5 lapilli clasts. Immature vesicle populations forming in the absence of shear are recorded by type 1 clasts: maturation of vesicle textures through continued growth promotes bubble interference and coalescence (Sparks 1978; Klug and Cashman 1996)—type 2 clasts. However, ascending melt is subject to transverse velocity gradients: drag at conduit margins reduces ascent velocity relative to the centre of the flow, creating zones of intense shear associated with rapidly changing velocity (Polacci et al. 2003; Shea et al. 2012; Cassidy et al. 2018). If an immature vesicle population is sheared, small elongate vesicles will result (type 3). If a mature population is sheared, coarse elongate vesicles result (e.g. type 4), and shear-induced coalescence hastens development of permeable networks. When a permeability threshold is reached, outgassing occurs: if gas flux is insufficient, bubble collapse ensues (Shea et al. 2012). Resultant textures are heterogeneous: clasts have irregular vesicles, thick glass walls and lower bulk vesicularities (type 5, Fig. 10) (Polacci et al. 2003; Shea et al. 2012). Shea et al. (2012) showed that shear-induced vesicle collapse can increase the density of the eruption column sufficiently to induce column collapse and PDC formation.

Fig. 10
figure 10

Diagram showing bubble nucleation, growth and coalescence processes in the absence and presence of shear to form the five textural types observed in the Echo Canyon (EC) lapilli population. Types 1 and 2 record multiple nucleation events and progressive vesicle growth without shear. Types 3 and 4 show how shear impacts vesicle shape and coalescence. Type 5 occurs after coalescence leading to outgassing through permeable networks and bubble collapse. Black arrows show movement of volatiles through permeable networks. Shaded rectangles show initiation of main processes during textural evolution. Greyed-out type 5 represents this process in the absence of shear—a lesser contributor to EC clast textures.

Whether a juvenile clast population has immature or mature vesicle textures depends on when fragmentation occurs relative to growth, coalescence and relaxation (τr) timescales (Moitra et al. 2013). As such, when magma properties of the rising melt column vary laterally, interaction with the fragmentation surface/threshold produces texturally heterogeneous pumice populations (Polacci et al. 2003). In the EC pumices, fragmentation captures vesicle populations: (i) before extensive bubble growth (type 1 and 3 if sheared); (ii) after coalescence is initiated but within τr (type 2); (iii) after shearing and some coalescence, but before outgassing and vesicle collapse (types 3 and 4); and (iv) after outgassing and vesicle collapse (type 5). Changing proportions of sheared and dense clasts may then be associated with development of low velocity shear zones and the extent to which outgassing could progress before fragmentation. Conduit modification and changes in fragmentation depth and magma flux may have influenced the proportion of melt and maturity of vesicle populations within shear zones and hence the density profile at the point of fragmentation (cf., Klug and Cashman 1996; Shea et al. 2012; Cassidy et al. 2018).

We interpret the influx of sheared clasts in Unit 3 (Fig. 8) to reflect a widening of marginal shear zones within the conduit. The lack of dense clasts indicates that outgassing and vesicle collapse were not extensive prior to fragmentation. In contrast, dense clasts make up 25% of Unit 4, reflecting onset of bubble collapse and densification of the eruption column. Further densification of the column occurred during eruption of Unit 9 (middle cone) with an even higher proportion of dense and sheared clasts marking a tipping point, beyond which column collapse and PDC formation dominated. Obsidian clasts in PDC deposits may reflect continued densification of melt in marginal shear zones—the source region for type 5 clasts—explaining the low abundance of type 5 clasts in Unit 11 (cf., Shea et al. 2017).

Bulk vesicularity, fragmentation and explosivity

When reconstructing explosive volcanic eruptions, it is important to characterise the explosivity/intensity/magnitude of individual phases, as this informs our understanding of the associated hazards, transitioning eruption styles and timescales for change. The volcanic explosivity index (VEI) as defined by Newhall and Self (1982) has been widely used to semi-quantitatively categorise and compare eruptions based on relationships between magnitude, intensity and rate of energy release. Continuous scales of magnitude and intensity relate closely to VEI and better describe observed extremes in eruptive style (Pyle 2015). However, each of these require accurate calculation of eruption mass, mass eruption rate or plume height (Pyle 2015), which is often precluded in small-volume eruptions by poor preservation or minimal exposure. Houghton et al. (2013) extended the VEI to better capture explosivity of small-volume basaltic eruptions on Hawaii, but this also requires accurate determination of eruption volumes. Standard scales of magnitude and intensity also fail to correctly capture the explosive potential, transient nature and associated hazards of individual phases of an eruption, or the timescales over which they may occur. Despite the coarseness of the VEI, its widespread use in volcanology and the media make it valuable when identifying and communicating potential hazards from future eruptions as they are often quoted in hazard assessments (Mastrolorenzo et al. 2017, Campi Flegrei and Sieron et al. 2019, Ceboruco volcano), and so careful comparisons—with caveats—can be worthwhile. In order to make interpretations of changing bulk processes in the ascending magma and link these to changing eruptive style for unobserved and poorly preserved eruptions, Mueller et al. (2011) compared clast bulk vesicularities of several eruptions with well-constrained eruption styles, dynamics and intensities. By doing so, they showed how such studies allow reconstruction of fragmentation mechanisms and eruption style. This is a useful approach, as the physical processes controlling a vesicle population also influence the explosivity and nature of fragmentation which are preserved in easily sampled juvenile clasts. By comparing vesicularity data of individual phases from the EC eruption to eruption data collated by Mueller et al. (2011), we identify step-changes in fragmentation mechanism and explosivity that would otherwise be masked or inappropriately categorised using bulk properties of deposits. Mueller et al. (2011) preferentially sampled clasts from phases that represented the peak intensity for the VEI assigned to the eruption, e.g. only the Plinian fallout phases of the Novarupta 1912 eruption. This means that direct comparisons can be made between these eruptions and the EC units as both datasets represent specific eruption phases rather than an integral of bulk deposit vesicularity for the eruption as a whole.

Figure 11 is adapted from Mueller et al. (2011) and shows the BVDs (lines) and variance (bracketed numbers) of each EC pyroclastic unit alongside BVDs for several other eruptions. Shaded fields on Fig. 11 are the BVDs of several explosive eruptions of well-constrained eruption style and intensity. Explosive felsic eruptions represent sub-Plinian to ultra-Plinian peak activity. Mafic eruptions represent Hawaiian-Strombolian activity.

Fig. 11
figure 11

Comparison of Echo Canyon (EC) vesicularity distributions with eight eruptions of well-constrained eruption style and intensity. Non-EC bulk vesicularity curves (shaded regions) are adapted from Mueller et al. (2011) Fig. 2. Bulk vesicularity distributions of EC units are plotted as solid and dashed lines. Variance of clast vesicularity for each EC unit is displayed in brackets.

Variance of BVDs can be used to infer fragmentation mechanism. Low variance BVDs reflect fragmentation by bubble interference and rupture (Sparks 1978). In contrast, high variance reflects transient fragmentation controlled by decompression processes where bubble overpressure exceeds the strength of the melt (Fowler et al. 2010).

For the EC eruption, variance decreases between Units 1 and 3, then increases by a factor of 3 in Unit 4 and peaks in Unit 9. Variance in the PDC deposits and overlying OBP are lower than that of Unit 9 but remain high. We suggest, therefore, that the dominant fragmentation mechanism shifted during the eruption. Fragmentation controlled by bubble-interference became increasingly important as early activity progressed. From Unit 4 onwards, transient fragmentation controlled by decompression processes became increasingly dominant. This coincides with the onset of vesicle collapse, PDC formation and ultimately the explosive-effusive transition. Bulk vesicularity distributions of pumice fall Units 1–3 progressively narrow, closely matching the dacitic-rhyodacitic, Plinian eruption of Krakatau in 1883. Units 4, 9 and 11 have broader distributions and lower modal bulk vesicularities than Units 1–3, but neither their shape nor modal vesicularities match closely with those of the mafic explosive eruptions. Bulk vesicularity distributions are shifted to higher vesicularities in systems with polydisperse and deformed vesicles that have not yet undergone collapse (Mueller et al. 2011). The position of the porosity distribution thus relates to magmatic volatile content and degree of outgassing at the point of fragmentation (Mueller et al. 2011): BVDs of Units 1–3 suggest higher magmatic volatile content for early phases. Later stages, e.g. Units 4 and 9, were more outgassed, displaying lower bulk vesicularities and supporting our interpretation of clast densification through progressive development of connectivity and outgassing.

The trachytic Campanian Ignimbrite is classified as Plinian (Polacci et al. 2003), as is Krakatau 1883 (Mueller et al. 2011), showing that low viscosity trachytic melts are capable of highly explosive activity producing stable plumes. As the BVDs for Unit 3 and the Krakatau 1883 eruption are similar, we infer that Unit 3 may record a comparable transient peak intensity during the EC eruption. The low lithic content, thickness and massive, clast-supported nature of this unit support the interpretation of a stable plume. Units 1 and 2 have very similar BVDs to Unit 3 but contain more lithic clasts and are interbedded with ash beds, indicating the lack of a well-developed plume in contrast to Unit 3. Unit 4 is unusual as it has very similar deposit characteristics to Unit 3, but the BVD (Fig. 11) and juvenile clast population for Unit 4 (Fig. 8) indicate significant changes to fragmentation mechanism, vesicle connectivity and outgassing. The mismatch between deposit characteristics, BVD and the lapilli population indicates some lag between the onset of densification processes and changing eruption style. The eruption does not shift to unsteady, sub-plinian activity until the middle and upper cone, Units 9 and 11. For Unit 9, the BVD (Fig. 11), lapilli population (Fig. 8) and lithologic characteristics (Figs. 3, 7D and 8 and Appendix 1) are all consistent with a transition to a destabilising plume and ultimately PDC formation producing Unit 11.

Bulk vesicularity distributions, when combined with careful interpretation of deposit characteristics can provide insights into the bulk behaviour of ascending magmas, changing conduit dynamics and resulting eruptive style (Mueller et al. 2011). This is vital for reconstruction of unobserved eruptions. However, there are limitations to this approach. While important interpretations of volcanological processes and the associated hazards can be made, quantification of degassing, water exsolution and discharge rates and hence column heights require further analyses. Quantification of textures using vesicle and crystal size distributions (VSD and CSD) and microlite number densities allow modelling of these parameters (Toramaru 2006 and Toramaru et al. 2008). However, such textural analyses, particularly VSDs, require careful assessment of coalescence, bubble relaxation and elongation prior to analysis (e.g. Mitchell et al. 2019; Houghton et al. 2010; Shea et al. 2010). In addition, intensive parameters, e.g. initial and eruptive melt volatile concentrations and temperatures, must be first established to produce reliable model results (Toramaru 2006). While quantification of textures in this manner is beyond the scope of this paper, interrogation of the deposit characteristics, pervasive clast textures and BVDs provide a volcanological framework onto which quantitative modelling of eruption dynamics using micro-scale textures can later be applied.

The geometry, extent and proximal nature of the preserved EC fall and PDC deposits, lava and dome, suggest an eruptive volume of approximately 0.01–0.3 km3 comparable to Mayor Island and Aluto (Table 1). In isolation, the volume would yield an approximate VEI of 2 for this eruption, suggesting low intensity, fountaining eruptive styles. The deposit componentry, clast textures and BVDs, however, reveal a transient Plinian-style eruption peak followed by unsteady, sub-Plinian activity. While a VEI 6 classification is precluded by the small eruptive volumes, it is clear that explosion intensities and stable eruption columns, equivalent to those observed during VEI 6 eruptions, can be achieved, if only transiently, in the course of events like the Echo Canyon eruption. Rapid evolution of vesicle textures and outgassing combined with almost complete degassing of the melt promoted column collapse, later progressed to effusive activity, indicating the stable column could not be sustained for long. Similar complex eruption dynamics are preserved in other alkaline pumice cones, e.g. Pu’u Wa’awa’a, Hawaii (Shea et al. 2017), Aluto (Clarke 2020) and Mayor Island (Houghton et al. 1985; Houghton et al. 1992) indicating rapid transitions in eruptive style and intensity are common for eruptions of this composition and volume. The role of obsidian and tuffisite formation in accelerating these transitions is discussed in the following section.

Obsidian breccia, tuffisite formation and degassing pathways

Fragments of homogenous, banded and brecciated obsidian are first observed in PDC deposits, Unit 11, of the upper cone stratigraphy, coincidental with destabilisation of the plume and initiation of transition to effusive activity. Eruption of obsidian may evidence processes underpinning transitioning eruptive style. Obsidian pyroclasts can form via densification of melt during bubble collapse due to permeable outgassing (Shea et al. 2017) or through sintering and relaxation of ash particles (Gardner et al. 2017): both can occur at varying depths within the conduit. Crystallinity of the EC obsidian is varied, some clasts contain <<1% microlites, while others have >40%. Cracks and small rounded-elongate vesicles in crystal-poor obsidian may suggest formation by sintering (Gardner et al. 2017). However, irregular vesicles aligned parallel to microlites visible in thin sections of crystal-rich obsidian, and vesicular banding seen in hand specimen, indicate formation by vesicle collapse (Shea et al. 2017). Similarities in macro-scale textures between the EC clasts and the Shea et al. (2017) obsidian lead us to favour vesicle collapse as the dominant formation process and further evidence of extensive degassing prior to effusive activity.

Observations made during the eruptions of Chaitén (2008–2009) and Cordón Caulle in Chile (2011–2012) and subsequent work on tuffisites (Alfano et al. 2012; Castro et al. 2014; Pistolesi et al. 2015; Heap et al. 2019) have highlighted the complexity of degassing processes during eruptions of silica-rich melt. Brecciated obsidian tuffisites have been shown to record processes of magmatic autobrecciation and explosive degassing via the influx of hot gas, ash and rock mixtures at the conduit margins (Rust et al. 2004; Saubin et al. 2016; Heap et al. 2019). The occurrence of tuffisite fragments in the PDC deposits suggests accumulation of outgassed melt on the conduit walls and brittle development of permeable pathways prior to the explosive-effusive transition. Castro et al. (2014) showed that tuffisites extending 10’s–100’s of metres into the conduit can act as a transient degassing system shifting activity from explosive to effusive. Strongly bimodal porosity/bulk vesicularity distributions are inferred to represent sampling of material with multiple degassing histories, for example, dome growth or plug formation (Mueller et al. 2011). Units 9 and 15 have BVDs that very weakly tend towards bimodality which indicates material with a different degassing history may have been sampled. This second degassing history does not, however, exert as strong a control on the shape of the BVD as one might expect for a period of dome/plug formation. Tuffisites, however, are a record of transient, explosive degassing within the volcanic system (Castro et al. 2014). A secondary degassing pathway which is transient may exert a weaker control on the BVD, resulting in the weakly bi-modal BVDs of Units 9 and 15 of the EC eruption. If tuffisites intersected with deeper magmatic foams, they may have also contributed to densification of ascending melt prior to effusive activity (Saubin et al. 2016). Obsidian and tuffisite formation, therefore, may have had an important role in facilitating rapid transitions from explosive to effusive activity during the EC eruption.

Melt composition, viscosity and eruption dynamics

There is now a wealth of evidence that trachytic eruptions can exhibit a similar range in eruptive style as larger calc-alkaline eruptions (Table 1). However, the question of how the physical properties of trachytic melts influence eruption dynamics, particularly in the case of small-volume events, remains. Bubble growth, coalescence, melt relaxation and collapse have a key influence on eruption dynamics and are all strongly controlled by the viscosity and volatile content of the melt (Cashman and Mangan 1994). At reasonable estimates of 1–4 wt % water (e.g. those seen for other eruption on Ascension cf., Kar et al. 1998; Chamberlain et al. 2016, 2019, 2020)), the EC eruption products have an estimated viscosity of 105.5–103.5 Pas, two orders of magnitude lower than a calc-alkaline rhyolite with the same water content at a reasonable eruptive temperature (Table 2). For the EC trachytes, low viscosity and high H2O diffusivity mean high bubble overpressures are unlikely to be maintained (Shea et al. 2017). Under such conditions, brittle fragmentation and high intensity explosive activity is more difficult to achieve with reasonable ascent rates and conduit geometries (Hughes et al. 2017). Nevertheless, localisation of strain within the melt column changes melt rheology at multiple length and timescales during ascent and can facilitate brittle fragmentation in peralkaline melts (Hughes et al. 2017). Inter- and intra-clast textural heterogeneity suggests strain localisation was prevalent in the melt column during the EC eruption. Furthermore, Hughes et al. (2017) suggested that high initial magmatic H2O concentrations and low viscosity may promote rapid decompression within peralkaline melts. With an estimated 1–4 wt% initial H2O content in the EC melt, the tendency of trachytes to vesiculate rapidly during decompression (Shea et al. 2017) and evidence of strain localisation may explain the high intensity of early explosive phases. In addition, the water content at the point of fragmentation may be lower than the peak pre-eruptive value, increasing the viscosity and further easing fragmentation to some degree. Low viscosities also reduce timescales of melt re-organisation, allowing vesicle networks to respond rapidly to shear strain and outgassing, pushing eruptions towards effusive activity—which may explain why these high intensity explosive phases seem to be so short-lived. Although beyond the scope of this study, quantification of pre-eruptive and late-stage H2O concentrations would provide further constraints on the evolution of trachytic melt viscosity during the eruption and will be the focus of future work.

Degassing-induced crystallisation is another contributing factor in the transition from explosive to effusive activity, as increasing viscosity induces shear stress in the conduit promoting construction of spines and lobes (Deniel et al. 2020). In calc-alkaline dome-forming eruptions, groundmass crystallinities rarely exceed 50% locally, and PDCs are regularly triggered by Vulcanian blasts due to gas overpressure in the dome (Deniel et al. 2020). In contrast, the trachytic Puy de Dôme in France is glass-poor, and only a few syn-dome explosions are recorded. The Puy de Dôme eruption closed with an explosive magmatic phase with a volume subordinate to the main edifice—similar to the final, OBP, phase of the EC eruption (Deniel et al. 2020). Deniel et al. (2020) argued that high crystallinities reflect efficient and near-complete degassing of the magma prior to and during dome construction. The intrinsic properties of trachyte melts and high dome permeability due to extensive fracturing facilitate this process (Deniel et al. 2020). For the effusive products of the EC eruption, high crystallinities may also record extensive degassing. As argued by Deniel et al. (2020), the physical properties of trachytic melts help to facilitate this process. In this instance, however, the additional high permeability may have been achieved via the presence of transient open fracture networks, evidence for which is the tuffisite fragments, transiently increasing conduit permeability at the explosive-effusive transition (e.g. Castro et al. 2014). The explosive post-dome eruption of the OBP mirrors that of the Puy de Dome eruption: so, significantly, dome formation does not preclude further explosive magmatic activity during short-lived, small-volume eruptions.

The physical behaviour of trachytic melts during ascent can produce high-intensity explosive phases and swift explosive-effusive transitions. High-intensity phases are short-lived as bubble overpressures are not maintained, rapidly evolving vesicle textures promote outgassing and extensive degassing-induced crystallisation occurs. The “kinetic reactivity” (Shea et al. 2017) of trachytic melts explains the varied eruption dynamics of these small-volume eruptions, as recorded in their diverse juvenile clast populations.

Eruption sequence and dynamics summary

Interpretations of stratigraphic observations, juvenile clast populations and textures and BVDs are integrated and evolution of EC eruption dynamics reconstructed and summarised below and in Fig. 12. Where possible, we draw comparisons between the EC eruption and other small-volume trachytic eruptions from Table 1 to show how the EC eruption enhances our understanding of these events worldwide.

Fig. 12
figure 12

Boxes A–I represent key phases in the Echo Canyon eruption, as discerned from eruption stratigraphy and analyses of juvenile clasts. An idealised cone is presented with stratigraphic units as described in main text accumulating after each phase. Underlying topography and wind direction are not reflected here. Numbered inset boxes show schematics of processes of vesicle evolution as interpreted from juvenile clast texture, from 1 to 7 show increasing importance of marginal shear zones on bubble structure, connectivity and outgassing. Box 8 shows a detailed view of hybrid explosive effusive activity that formed Unit 12 at the projected vent region. Box 9 shows a possible mechanism for production of clasts with crystalline regions in the orange-brown pumice.

Early Strombolian-Vulcanian fountaining is recorded by Units 1 and 2 (Fig. 12A–C), common for these small-volume trachytic events, e.g. the non-welded pumice fall facies at Payún Matrú, Argentina (Hernando et al. 2019) (Table 1). A volumetrically subordinate cycle of accumulation and disruption of dense melt in the conduit then produced the VVLP clasts and ash beds of Unit 2 (Fig. 12B and C). Bulk vesicularity distributions (Fig. 11) and lapilli populations (Fig. 8) for Units 1–3 indicate during early phases, volatile-rich melt vesiculated rapidly during ascent and decompression, fragmenting by bubble interference and rupture (Fig. 12A–D). Lithologic characteristics of Units 3 and 4 indicate the subsequent formation of a stable plume with a transient Plinian peak in activity, supported by the BVD of Unit 3 (Fig. 11). Initiation of marginal shear zones in the conduit introduced sheared clasts to the juvenile population in Unit 3. Unit 4 juvenile clasts record propagation of marginal shear zones and enhanced connectivity of vesicle networks and initiation of melt densification and accumulation: the BVD of Unit 4 shows transient fragmentation became more dominant (Fig. 12D and E5). The contradiction between the juvenile clast population and the BVD (Figs. 8 and 11) of Unit 4, and its lithologic characteristics, indicates a lag between the initiation of melt densification and destabilisation of the plume. Heterogeneous clast textures and obsidian formation are common in trachytic eruptions (Houghton et al. (1985); Shea et al. 2017) suggesting trachytic melts support multiple complex degassing histories in the conduit even for small eruptive volumes (Shea et al. 2017). Tuffisite formation on the conduit walls provided an additional outgassing pathway (Fig. 12E6) as continued melt densification initiated partial column collapse and PDC formation (Fig. 12E). Subsequent conduit destabilisation sampled accumulated obsidian and tuffisites during deposition of Unit 11 (Fig. 12E and F). In late stages, near-complete degassing resulted in high crystallinities and aided a transition to effusive activity forming the dome and brecciated lava, with hybrid activity likely at the initial transition (Fig. 12G and H). Pressurisation beneath the dome produced a final magmatic explosive eruption, the OBP, incorporating accumulations of degassed melt from the conduit walls producing heterogeneously microcrystalline clasts (Fig. 12I). The excellent exposure of the EC eruption deposits provides a near-complete record of the eruption and shows that many of the processes identified across the eruptions in Table 1 can occur within a single eruption period and may even be the norm for events of this type.


We use unusually well-preserved and exposed deposits on Ascension Island, South Atlantic, to reconstruct a small-volume explosive-effusive trachytic eruption. By combining the detailed eruption stratigraphy with analysis of individual and bulk clast densities, we demonstrate that eruptions of this type are capable of short-lived periods of high intensity explosions and rapid transitions in eruptive style. In particular we show that:

  1. (1)

    The Echo Canyon eruption had multiple transitions in eruptive style controlled by bulk regimes which transitioned from that controlled by rapidly vesiculating magma to one controlled by the development of shear zones and networks that allowed transient fragmentation to dominate, initiating column collapse.

  2. (2)

    Magmas of trachytic composition are capable of producing Plinian explosive phases equivalent to those seen at the peak of VEI 6 eruptions, but our evidence and the global record suggest that these are unlikely to be sustained for long periods, as a consequence of relatively more efficient outgassing. Ultimately, this and degassing controlled crystallisation in these ‘kinetically reactive’ magmas encourages the development of effusive activity.

  3. (3)

    Our detailed analysis reveals that these abrupt transitions could be characteristic of these low-volume silicic magmas which are found worldwide. The varied eruption styles and swift transitions exhibited by small-volume trachytic eruptions have significant implications for our understanding of volcanic hazard, particularly in remote ocean island settings.

  4. (4)

    Combined analysis of BVDs, macro-scale clast textures and deposit characteristics allows reconstruction of the bulk characteristics of ascending melts revealing changes to conduit and eruption dynamics. Quantification of micro-scale vesicle and crystal textures and changing volatile content and viscosity would enable modelling of useful parameters such as degassing and discharge rates and will be the focus of future work.

Textural studies of other preserved deposits are vital to identify patterns in eruption style, and further investigation of fragmentation in trachytes is required to understand how high-intensity eruptive periods initiate. Recognising signals of these processes in monitoring data will be critical for ongoing hazard assessment in regions that produce small-volume trachytic eruptions.