Subaqueous basaltic magmatic explosions trigger phreatomagmatism: A case study from Askja, Iceland
Introduction
The question of what controls the transition from basaltic effusive to explosive activity in water-rich environments has been a matter of debate over the last 20 years (Wohletz, 1986, Houghton and Schmincke, 1989, Houghton and Nairn, 1991, White, 1996a, Zimanowski et al., 1997, Wohletz, 2002, Wohletz, 2003, Zimanowski et al., 2003, Mastin et al., 2004). The answer to this question has important implications for modeling volcanic hazards, such as the potential for explosions or the grain size distribution, height and duration of ash plumes in wet environments. Submarine exploration technology has advanced our ability to describe subaqueous explosive volcanic deposits from the submarine environment (Clague and Davis, 2003, Clague et al., 2003, Eissen et al., 2003, Clague et al., 2009). However, the study of explosively generated deposits formed in an ice-confined (glaciovolcanic) environment offers much more accessible deposits that are commonly well-preserved in three-dimensions. We argue that such centers preserve sequences that record in situ transitions from effusive to explosive activity at a single vent. The detailed textural and stratigraphic study of such sequences offers the best opportunity for understanding the onset of explosive activity and the interplay of fragmentation mechanisms in natural subaqueous settings.
The focus of this study is three ca. 30 m thick, glass-rich, incipiently palagonitized basaltic pyroclastic deposits that directly overlie pillow lavas from Askja volcano, Iceland (Fig. 1). All three clastic sequences are massive but display a systematic and continuous fining upward from pillow-fragment breccia to fluidly-shaped bomb-bearing breccia and then into vitric lapilli tuff. Ostensibly similar sequences of pillow lavas overlain by breccias, containing pillow fragments, and capped by vitric lapilli tuffs, have been described from many areas, including ophiolite sequences (Carlisle, 1963), Archean basalt provinces (Dimroth et al., 1978), ocean island settings (Fujibayashi and Sakai, 2003) and several glaciovolcanic sequences (Jones, 1970, Werner and Schmincke, 1999, Skilling, 2009). Such sequences could clearly be derived through many processes. Common interpretations for these sequences can be divided into those where the clastic deposits were (1) erupted from a different vent from that which produced the lavas, (2) produced by the same vent that produced the lavas, but not as part of a temporally continuous eruption, or (3) erupted from the same vent as lavas and were part of a continuous eruption. Mechanisms that produce unrelated sequences of pillow lavas and clastic deposits include deposition of density currents from nearby vents or post-emplacement collapse of deposits on top of the pillows. Similar sequences produced from the same vent without continuous eruption include flow related collapse (autoclastic breccias), intrusion of pillowed dikes into clastic deposits, or explosive activity instigated under already solidified lava. Based on a detailed textural analysis of the sequences from the Austurfjöll massif of Askja, we argue that the deposits were produced from the same vent over a brief eruption that transitioned from effusive activity to magmatic explosive, and then to phreatomagmatic explosive eruptive activity.
The interpretation of such clastic deposits relies heavily on the distinction of the influence of different fragmentation mechanisms in the formation of subaqueous basaltic pyroclasts along the contact between facies within the transitional sequences. Evidence for the mechanisms of fragmentation, transportation and deposition is preserved in the textures of subaqueous pyroclasts over the full range of grain sizes, including bombs, lapilli and fine ash, though most previous research has focused on fine ash textures (Heiken, 1971, Mattox and Mangan, 1997, Büttner et al., 1999, De Rosa, 1999, Dellino et al., 2001, Dellino and Liotino, 2002, Ersoy et al., 2006, Durig et al., 2012b). These textural data can then be used to make inferences about the controls of the onset of subaqueous explosive activity and specifically phreatomagmatic activity. In this study we present data on textural characteristics of fine ash to block-sized clasts that could be used to help distinguish phreatomagmatic from magmatic fragmentation, and argue that the phreatomagmatic eruptions in our study area were generated following an initial mingling (premixing) of magma and water driven by magmatic fragmentation. It is not clear how important or common initial mingling by magmatic fragmentation might be in basaltic phreatomagmatic sequences elsewhere, and this is probably only one mechanism for instigating such eruptions. The uniquely dynamic nature of the ice-confined lakes may play a role in the initiation of magmatic gas expansion through depressurization caused by lake drainage. Nevertheless, similar textural investigations may be used to identify the mechanisms during the onset of explosivity in other basaltic phreatomagmatic systems.
Basaltic glaciovolcanic systems under thick ice (> 400 m) typically evolve into ice-confined lacustrine centers (Allen et al., 1982, Gudmundsson et al., 1997, Werner and Schmincke, 1999, Gudmundsson, 2003). Within the ice-confined lake the water level may change rapidly and repeatedly through time (Gudmundsson et al., 1997, Bjornsson, 2002, Höskuldsson et al., 2006, Smellie et al., 2008). Simplified models of such centers include initial subaqueously emplaced effusive products, dominated by pillowed lavas, followed by a shift towards more explosive activity with deposition of glassy fragmental deposits (Jones, 1970, Allen, 1980, Moore et al., 1995, Werner et al., 1996). Within this model it is assumed that there is a decreasing fragmentation and dispersal of subaqueous eruptions as confining pressure or water depth increases, particularly with depths greater than 400 m (Allen, 1980, Clague et al., 2003, Zimanowski and Büttner, 2003). However, investigations of submarine basaltic deposits are revealing the presence of explosively derived deposits at depths up to 3 km (Clague and Davis, 2003, Fujibayashi and Sakai, 2003, Wohletz, 2003, Sohn et al., 2008, Portner et al., 2010, Schipper and White, 2010, Schipper et al., 2010a, Helo et al., 2011, Schipper et al., 2011a). This uncertainty about the importance of controls other than confining pressure (White, 1996a, Mastin et al., 2009, Schipper et al., 2011b) on the triggering of subaqueous explosions emphasizes the importance for detailed studies of natural deposits that may record the onset of basaltic explosions in water.
Askja is one of the largest and best-exposed formerly ice-confined volcanoes on Earth. Most research to date has been on its Holocene (ice-free) evolution. It comprises a complex of basaltic glaciovolcanic massifs that are dominated by pillow lavas and subaqueously emplaced vitric lapilli tuff deposits. These massifs are cut by at least three calderas and surrounded by Holocene subaerial lava flows (Fig. 1). The greatest volume of glaciovolcanic deposits at Askja is the eastern mountain massif, Austurfjöll, which is truncated by the two youngest calderas. Austurfjöll has been described briefly by Brown et al., 1994, Sigvaldason, 1968, Sigvaldason, 2002 and more recently in detail by Graettinger et al. (2012).
Austurfjöll is incised on its eastern side by large gullies that extend up to 3 km into the massif. The vertical exposure within the gullies is between 10 and 100 m. These exposures are dominated by pillow lava sheets, lava breccias and vitric lapilli tuffs. Three of these gullies contain well-exposed sequences that display gradual transitions up-section between effusive pillow lavas at their base, to an upward-fining pillow fragment and bomb-bearing breccia and vitric lapilli tuff sequence. The sequences have lateral continuities of tens of meters and can be traced in multiple directions. The three gullies from north to south are named Drekagil, Nautagil and Rosagil (Fig. 1).
Section snippets
Methods
This study is based on field work conducted over two seasons at Austurfjöll. The three sequences of basaltic pillow lava, pillow-fragment breccia, fluidal bomb-bearing breccia and vitric lapilli tuff were identified in 2010 and revisited in 2011 for more detailed sampling and study. Samples were collected from the top of the basal pillow units, the lowermost breccia at the contact with the pillow lavas and then progressively up through the section of overlying breccias and vitric lapilli tuffs,
Overview of lithofacies
All three sequences are exposed in steep-walled gullies incised into the base of the eastern margin of the 750 m high Austurfjöll massif of Askja volcano (Fig. 1). The sequences have basal pillow lavas overlain by pillow-fragment and fluidal bomb-bearing breccias and capped by vitric lapilli tuffs. Fluidal bombs also occur within the lapilli tuff sequences, but decrease in abundance up-section. The transition from breccia to lapilli tuff is defined as the point where the fluidal bomb presence is
Interpretation of deposits
Sequences of pillow lava, to pillow fragment and fluidal bomb breccias, to lapilli tuff sequences are not uncommon in subaqueous basaltic sequences (Carlisle, 1963, Jones, 1970, Dimroth et al., 1978, Werner and Schmincke, 1999, Fujibayashi and Sakai, 2003, Skilling, 2009). The gross similarities between these pillow lava, breccia and tuff sequences belie the range of mechanisms that can produce outwardly similar deposits. The following discussion will use detailed stratigraphic and textural
Magma fragmentation at the onset of basaltic phreatomagmatic explosions
The kinetic disruption of magma is accomplished in two main ways: ductile deformation of the melt and brittle deformation of the cooling glass/lava. Ductile, or inertial fragmentation, is the breakup of magma during decompression in the form of inertial stretching and breakup of the liquid magma (Houghton and Gonnermann, 2008; and references within). Basaltic magmatic explosions predominantly involve the ductile disruption of the melt through the nucleation, growth and buoyant rise of vesicles (
Conclusion
Pyroclast textures in three subaqueous basaltic sequences from Askja reveal the interaction of magmatic and FCI fragmentation mechanisms as the eruption transitioned from effusive to explosive behavior. The identification of the signature of different fragmentation mechanisms in natural subaqueous phreatomagmatic eruption deposits remains a challenge. However, the comparison of textures in multiple grain sizes across important facies transitions, as in this study, can reveal subtle changes in
Acknowledgments
This work was made possible by a National Science Foundation grant to IPS, DG and AH. Our gratitude goes to Háskóli Íslands, NORVOLK and the Vatnajökull National Park, for field logistics and permits. Dickinson College is acknowledged for the use of the SEM and Robert Dean should be thanked for his assistance using the equipment. Field assistance from R. Wham, R. Lee, A. Lema and M. Ellis was invaluable. This manuscript was improved greatly by comments by two anonymous reviewers and editor L.
References (116)
- et al.
The complex facies architecture and emplacement sequence of a Miocene submarine mega-pillow lava flow system, Muriwai, North Island, New Zealand
Journal of Volcanology and Geothermal Research
(2007) - et al.
Diamict fans in subglacial water-filled cavities — a new glacial environment
Quaternary Science Reviews
(2006) - et al.
The architecture, eruptive history, and evolution of the Table Rock Complex, Oregon: from a Surtseyan to an energetic maar eruption
Journal of Volcanology and Geothermal Research
(2009) - et al.
Geomorphological evidence for jokulhlaups from Kverkfjoll volcano, Iceland
Geomorphology
(2004) - et al.
Widespread strombolian eruptions of mid-ocean ridge basalt
Journal of Volcanology and Geothermal Research
(2009) - et al.
Observations of eruptive plume dynamics and pyroclastic deposits from submarine explosive eruptions at NW Rota-1, Mariana arc
Journal of Volcanology and Geothermal Research
(2011) - et al.
Image processing analysis in reconstructing fragmentation and transportation mechanisms of pyroclastic deposits
Journal of Volcanology and Geothermal Research
(1996) - et al.
Evolution of an englacial volcanic ridge: Pillow Ridge tindar, Mount Edziza volcanic complex, NCVP, British Columbia, Canada
Journal of Volcanology and Geothermal Research
(2009) - et al.
Texture discrimination of volcanic ashes from different fragmentation mechanisms: a case study, Mount Nemrut stratovolcano, eastern Turkey
Computers and Geosciences
(2006) - et al.
Supercooled rocks: development and significance of varioles, spherulites, dendrites and spinifex in Archean volcanic rocks, Abitibi Greenstone belt, Canada
Precambrain Research
(2002)
Intrusion of basalt into frozen sediments and generation of Coherent-Margined Volcaniclastic Dikes (CMVDs)
Journal of Volcanology and Geothermal Research
Volcanic investigations of the Puna Ridge, Hawai'i: relations of lava flow morphologies and underlying slopes
Journal of Volcanology and Geothermal Research
MFCI experiments on the influence of NaCl-saturated water on phreatomagmatic explosions
Journal of Volcanology and Geothermal Research
Englacial tephrostratigraphy of Erebus volcano, Antarctica
Journal of Volcanology and Geothermal Research
Deep submarine pyroclastic eruptions: theory and predicted landforms and deposits
Journal of Volcanology and Geothermal Research
Basaltic explosive volcanism: constraints from deposits and models
Chemie der Erde
SEM-based methods for the analysis of basaltic ash from weak explosive activity at Etna in 2006 and the 2007 eruptive crisis at Stromboli
Physics and Chemistry of the Earth
Sheet hyaloclastite: density-current deposits of quench and bubble-burst fragments from thin, glassy sheet lava fows, Seamount Six, Eastern Pacific Ocean
Marine Geology
What makes hydromagmatic eruptions violent? Some insights from the Keanakako'i Ash, Kilauea Volcano, Hawai'i
Journal of Volcanology and Geothermal Research
An experimental study of hydromagmatic fragmentation through energetic, non-explosive magma–water mixing
Journal of Volcanology and Geothermal Research
Littoral hydrovolcanic explosions: a case study of lava–seawater interaction at Kilauea volcano
Journal of Volcanology and Geothermal Research
Textural variation in juvenile pyroclasts from an emergent, Surtseyan-type, volcanic eruption: the Capela tuff cone, Sao Miguel (Azores)
Journal of Volcanology and Geothermal Research
Depositional characteristics and volcanic landforms in the Lake Natron-Engaruka monogenetic field, northern Tanzania
Journal of Volcanology and Geothermal Research
Degassing and differentiation in subglacial volcanoes, Iceland
Journal of Volcanology and Geothermal Research
Pyroclast textures of the Ilchulbong ‘wet’ tuff cone, Jeju Island
Journal of Volcanology and Geothermal Research
Drivers of explosivity and elevated hazard in basaltic fissure eruptions: the 1913 eruption of Ambrym Volcano, Vanuatu (SW-Pacific)
Journal of Volcanology and Geothermal Research
Contrasting patterns of vesiculation in low, intermediate, and high Hawaiian fountains: a case study of the 1969 Mauna Ulu eruption
Journal of Volcanology and Geothermal Research
Influence of the substrate on maar-diatreme volcanoes — an example of a mixed setting from the Pali Aike volcanic field, Argentina
Journal of Volcanology and Geothermal Research
Changing conditions of magma ascent and fragmentation during the Etna 122 BC basaltic Plinian eruption: Evidence from clast microtextures
JVGR
Syn- and post-fragmentation textures in submarine pyroclasts from Lo'ihi Seamount, Hawai'i
Journal of Volcanology and Geothermal Research
Explosive submarine eruptions driven by volatile-coupled degassing at Lo'ihi Seamount, Hawai'i
Earth and Planetary Science Letters
Textural, geochemical, and volatile evidence for a Strombolian-like eruption sequence at Lo'ihi Seamount Hawai'i
Journal of Volcanology and Geothermal Research
Experimental interaction of magma and “dirty” coolants
Earth and Planetary Science Letters
Textural studies of vesicles in volcanic rocks: an integrated methodology
Journal of Volcanology and Geothermal Research
Fluidal-clast breccia generated by submarine fire fountaining, Trooper Creek Formation, Queensland, Australia
Journal of Volcanology and Geothermal Research
Subglacial to emergent basaltic volcanism at Hlöðufell, south-west Iceland: a history of ice-confinement
Journal of Volcanology and Geothermal Research
Six million years of glacial history recorded in volcanic lithofacies of the James Ross Island Volcanic Group, Antarctic Peninsula
Palaeogeography, Palaeoclimatology, Palaeoecology
Growth of an emergent tuff cone: fragmentation and depositional processes recorded in the Capelas tuff cone, Sao Miguel, Azores
Journal of Volcanology and Geothermal Research
Evolution and facies architecture of Paleogene Surtseyan volcanoes on Chatham Islands, New Zealand, Southwest Pacific Ocean
Journal of Volcanology and Geothermal Research
Icelandic subglacial volcanism: thermal and physical studies
Journal of Geology
Subglacial volcanism in North-Central British Columbia and Iceland
Journal of Geology
Pillow lava and isolated-pillow breccia of rhyodacite composition from the Fishguard Volcanic Group, Lower Ordovician, S.W. Wales, United Kingdom
Journal of Geology
Dynamics of crustal rifting in NE Iceland
Journal of Geophysical Research
Subglacial lakes and jokulhlaups in Iceland
Global and Planetary Change
Compositional and textural characteristics of the Strombolian and Surtseyan K-Trig basalts, Taupo Volcanic Centre, New Zealand: implications for eruption dynamics
New Zealand Journal of Geology and Geophysics
Identifying magma–water interaction from the surface features of ash particles
Nature
Thermohydraulic explosions in phreatomagmatic eruptions as evidenced by the comparison between pyroclasts and products from Molten Fuel Coolant Interaction experiments
Journal of Geophysical Research
Physics of thermohydraulic explosions
Physical Review E
Pillow breccias and their aquagene tuffs, Quadra Island, British Columbia
Journal of Geology
Miocene submarine fire fountain deposits, Ryugazaki Headland, Oshoro Peninsula, Hokkaido, Japan: implications for submarine fountain dynamics and fragmentation processes
Cited by (25)
Hydrovolcanic eruptions of the Paraná Igneous Province: Insights from mafic volcaniclastic deposits in Sertanópolis, Paraná, Brazil
2023, Journal of Volcanology and Geothermal ResearchMagmatic and phreatomagmatic contributions on the ash-dominated basaltic eruptions: Insights from the April and November–December 2005 paroxysmal events at Karthala volcano, Comoros
2022, Journal of Volcanology and Geothermal ResearchCitation Excerpt :All the detailed data described in this section, as well as crystal compositions and REE concentrations can be found on Supplementary Table S5. The distinction between magmatic and phreatomagmatic fragmentation regimes, especially concerning mafic magmas, is widely discussed in volcanology (e.g., Houghton et al., 1996; Ort and Carrasco-Núñez, 2009; Graettinger et al., 2013; Colombier et al., 2019a, Latutrie and Ross, 2020; Thivet et al., 2020c), but in some cases, is unattainable looking only at deposit features of a given eruptive event (White and Valentine, 2016). In our study, pre-, syn and post-eruptive visual observations of the active crater are available, providing a reliable support for interpreting on the eruption dynamics (Fig. 2).
The role of external water on rapid cooling and fragmentation of magma
2020, Earth and Planetary Science LettersCitation Excerpt :Heat loss from magma to external water (such as groundwater, lake water or ice) is considered as one of the key driving forces for explosive magma fragmentation during phreatomagmatic (sometimes referred to as hydromagmatic or hydrovolcanic) eruptions (e.g., Németh et al., 2001; Head and Wilson, 2007; Geshi et al., 2011; White and Ross, 2011; Graettinger et al., 2013; Wohletz et al., 2013; Houghton et al., 2015; Liu et al., 2015; van Otterloo et al., 2015; Fitch et al., 2017).
Cambrian shallow-marine to emergent alkaline volcanism near Ouinguigui (Ougnat inlier, eastern Anti-Atlas, Morocco): Volcanic facies, geochemistry and geodynamic setting
2020, Journal of African Earth SciencesCitation Excerpt :A relative poor sorting and the chaotic nature of the diatreme deposits may be caused by high fallout rates and direct deposition from subaqueous convective plumes and tephra jets (Maicher et al., 2000; Graettinger et al., 2013). For example, the Askja deposits in Iceland described by Graettinger et al. (2013) show also a normal grading of outsized clasts (bombs), which is equivalent to the observation of our study. Though, the origin of the bombs is different.
Insights into the dynamics of mafic magmatic-hydromagmatic eruptions from volatile degassing behaviour: The Hverfjall Fires, Iceland
2018, Journal of Volcanology and Geothermal ResearchLarge explosive basaltic eruptions at Katla volcano, Iceland: Fragmentation, grain size and eruption dynamics
2018, Journal of Volcanology and Geothermal ResearchCitation Excerpt :Recent studies have shown that fragmentation in subglacial eruptions is not solely driven by magma/water-interaction (Graettinger et al., 2013; Jude-Eton et al., 2012; Cioni et al., 2014; Liu et al., 2015). Graettinger et al. (2013) reported feedback loops between decoupled gas magma eruption and phreatomagmatic explosions, which are in line with our observations. Heap et al. (2014) showed that large vesicles in low porosity magma could decrease the brittle strength of the magma more than small vesicles at the same porosities.