Why volcanoes erupt explosively

When volcanic breccias and even ash-size deposits are found in subaqueous settings, the first and most likely hypothesis to test is: are these hyaloclastite, and if not, what is the evidence based on textural characteristics, context, analysis or physical processes, rather than assuming that they are pyroclastic? The high heat capacity, thermal conductivity, and hydrostatic pressure constraints of water indicate that explosive origins become less likely with increasing water depths.

For lavas that have vesiculated to high degrees, but have not been fragmented explosively, the pumice carapaces can also be quench fragmented, producing pumice hyaloclastite Figures 5c,d , 7c ; de Rosen-Spence et al. In addition, spatter ejected from submarine fire fountains will be subject to quench fragmentation in the water column and plume above the vent Figure 8A , producing blocky, ash size glassy fragments and fractured spatter fragments Figure 7f , that then become entrained and dispersed in thermal plumes of seawater above the fountain Cas et al.

Subaqueous eruption column characteristics. A Deep-water fire fountain column, showing quench fragmentation of spatter clasts in the column and the water mass above the vent. After Cas et al. B Complex processes contributing to the dynamics of a relatively deep-water, subaqueous explosive eruption column as a result of initial bubble decompression and expansion, then cooling, condensation and water ingress into a subaqueous eruption column upward.

C Schematic diagram of a relatively shallow water tens to hundreds of meters explosive eruption column collapsing around the vent. The rising buoyant column is subject to condensation of gasses and mixing in of ambient water. Large pumice clasts become water-logged, contributing to collapse of the column, forming a hot mass-flow of pyroclastic debris, steam and water on the seafloor.

After Kano et al. From Chadwick et al. Hot thermal plumes of water that rise above the vent and above propagating seafloor lavas cf. Barreyre et al. They are more likely to be hyaloclastic fallout deposits dispersed by thermal seawater plumes rising above the newly erupting lavas.

The heat capacity and thermal conductivity of water also plays a critical role in determining the dynamics of subaqueous eruption columns, through heat exchange between the eruption column and the ambient water body e. The change in plume dynamics results from a change in the physical state of the plume, particularly temperature, density, and the proportions of water and gas. These factors each influence the dispersal capacity of submarine plumes, which will be discussed below.

Whitham and Sparks first demonstrated that if hot pumice with gas filled vesicles comes in contact with cold water, a large proportion of them quickly become water-logged, denser and sink.

This process can affect both pyroclastic and autoclastic hyaloclastite pumice. It is due to the cooling and condensation of hot gasses in vesicles, resulting in a pressure gradient from high in the host, dense water mass to low in vesicles in pumice clasts, which forces external water into the vesicles if permeability pathways exist. This then increases the bulk density of the pumices, in some cases to higher values than water, which causes them to sink cf.

Figure 5d. Water-logging and densification of pumice by this process can occur syn-eruptively in subaqueous eruption columns Whitham and Sparks, ; Allen et al. Some may, however, eventually become water-logged through slow infiltration of water, even when cold after drifting around for weeks to years on the sea surface Fauria et al.

Subaerial eruption columns and plumes result from the buoyant rise or uplift of a mixture of mostly hot gas, with dispersed water droplets, and solids, driven initially by explosive gas thrust, and the low density and buoyancy of hot gas relative to the density of the atmosphere. In subaerial settings, in addition to hot volcanic gas from the vent, the heating of cold air entrained from the atmosphere through turbulent mixing into the eruption column at its margins helps to maintain a low bulk density and contributes to turbulent plume rise.

Theoretically, the plumes can rise through the entirety of the water column and breach the sea surface if their upward momentum is high, after which they behave more or less like subaerial eruption columns. However, this is more likely from shallow water vents where the eruption column is gas-charged, or seawater has been explosively superheated e. However, cold seawater can be mixed into the margins of the column, particularly if there is a significant density difference between the column and the ambient water, creating a pressure difference between the two, which will drive cold seawater laterally into the column.

As a result, a hot plume of seawater above a vent will cool, become denser and lose buoyancy upward Whitham and Sparks, ; Allen et al. If volatile bubbles are released from a subaqueous vent, they will further reduce the density of the water column or eruption plume rising above the vent. Bubbles will be densest at the vent because the hydrostatic pressure is greatest at the seafloor. As bubbles rise buoyantly in the eruption column to shallower water depths and lower hydrostatic pressures, they decompress and theoretically should expand and cause the eruption column to become less dense and become more buoyant upward.

However, subaqueous eruption columns with a significant gas bubble fraction are also affected by cooling and condensation erosion of gas bubbles, due to the high heat capacity and thermal conductivity of the water column Kano et al. Gas bubbles under high ambient pressure can also collapse or implode as they cool and adjust to ambient pressure.

This could be due to cooling condensation effects and implosions of under-pressured volatile bubbles in magma or in the subaqueous eruption column. They also suggest that such reductions in density and therefore hydraulic pressure in the eruption column at the depth of the vent have the capacity to change eruption styles e.

However, their argument is based on speculative gas bubble abundances in the eruption column, not based on quantitative modeling of likely gas release rates from the erupting vesiculating lava domes. This could only happen if all bubble walls in the erupting pumiceous magma were bursting at the time of the eruption or in the column, which should then produce more ash glass shards than pumice clasts, for which there is little evidence.

The concept could perhaps work where vents lie at shallow water depths of a few hundred meters or less, at much lower hydrostatic pressure, and higher gas overpressure, almost simulating subaerial conditions with high gas over-pressures.

In addition, syn-eruptive cooling and water logging of pumice clasts in rising subaqueous eruption columns will cause the columns to cool and become denser and may even lead to gravitational collapses of parts of eruption columns, producing subaqueous volcaniclastic density currents and deposits around the vent, which are water-supported mass flows of volcanic debris Figure 8C ; Kano et al.

This is analogous to subaerial pyroclastic density currents, which are gas supported. If eruption mass flux is high enough, column collapse-generated subaqueous mass flows that are briefly gas-supported could travel limited distances from vent, producing limited true subaqueous pyroclastic density current deposits e. In summary, subaqueous eruption columns and plumes are therefore likely to have very different buoyancy properties, experience greater ephemeral changes to buoyancy properties, and therefore different dynamics, compared with subaerial eruptions columns.

In particular, every subaqueous eruption, including both effusive and explosive eruptions, will produce a subaqueous eruption column or hot water plume. Subaqueous eruption columns resulting from effusive eruptions can disperse non-explosively generated autoclastic pumice and ash size hyaloclasts to produce autoclast fallout deposits.

This is very different from subaerial effusive eruptions, which can generate gas plumes capable of carrying very fine glassy ash popped off the surfaces of cooling lavas, but not coarse pumice clasts. Furthermore, deep-water subaqueous eruption columns are much more height limited than subaerial counterparts, which has implications for dispersal of pyroclasts, compared with subaerial eruption columns.

However, buoyant eruption columns can entrain pyroclasts if the gas turbulence velocity and column up-rise velocity exceeds the terminal fall velocity of the pyroclasts. Conversely, the rise and settling velocities of clasts in the water column are reduced relative to pyroclasts erupted at subaerial vents, because of the high density, relative viscosity and the viscous drag effects of water compared with air Figure 9 ; Cashman and Fiske, ; Fiske et al.

Water-logged pumice clasts whose bulk density exceeds that of water can sink and be deposited around the eruption vent, although Cashman and Fiske noted that the settling velocity of pumice clasts is markedly slower in water than in air Figure 9.

However, pumice clasts that do not have a permeable network of vesicles can retain their low bulk density and float Fauria et al. For example, pumice from the submarine eruptions along the Tonga arc in Bryan et al. August , Tonga volcanic arc, southwest Pacific. From Jutzeler et al. Surface currents and waves can produce very irregular, changing and circuitous dispersal patterns, reflected by changing pumice raft shapes and drift directions Figure 10b ; Bryan et al.

Different current directions at different depths in the ocean may also affect the distribution and dispersal of clasts. Water settled seafloor deposits of rafted pumice pyroclasts and autoclasts also often coarsen or show no systematic down current variations in grain size Jutzeler et al. In addition, ash sized particles within deep-sea tephra layers could in fact be generated by abrasion between pumice clasts during transit in pumice rafts Jutzeler et al. This contrasts with true subaerial pyroclastic pumice fall deposits, which are dispersed radially around the vent on a windless day, or asymmetrically by strong atmospheric winds following relatively linear paths away from the vent.

The deposit grain size decreases downwind with increasing distance from the vent, and exhibits distribution patterns that reflect plume height, and prevailing wind direction and strength Carey and Sparks, ; Bonadonna and Costa, Few of these controls apply to submarine pumice-forming eruptions and their columns.

Although it is tempting to assign the origin of pumice deposits in submarine settings to a nearby submarine or subaerial explosive eruption, pumice and ash deposits in marine settings can in fact originate from a range of possible processes, many of which are very different from those in subaerial settings. Possible origins Cas and Giordano, , include:. Cas and Giordano have briefly considered this topic and advise against using terms ascribed to subaerial explosive eruption styles and deposits for subaqueous eruptions and deposits.

Subaerial explosive eruption styles and deposits are defined on measurable quantitative parameters, such as eruption column height, dispersal patterns and distances of the deposits, grain size and sorting characteristics relative to dispersal patterns and areas, etc. Walker, ; Cas and Wright, ; Cas et al. These parameters cannot be easily applied to subaqueously erupted deposits because the dispersal processes of vesiculated clasts are influenced by very different processes and conditions in marine environments, and data on dispersal and grain size characteristics are very rarely available or collectable.

We first need to decide on what descriptive and measurable parameters are most useful and what the significance of those parameters is before we can begin to develop a terminological approach. As a result, to further advance understanding of submarine volcanic processes we need continuing documentation of both modern and ancient volcanic settings through detailed mapping, logging of sections and sample collection.

In ancient settings, this will involve traditional mapping methods as well as application of remote sensing methods and data sets such as LiDAR, radiometrics, magnetics, drone based imagery, aerial image interpretation, and in some cases diamond drilling. In addition, further modeling of the physics of deep-water eruptions, eruption columns, magma properties, volatile properties, fragmentation processes including autoclastic and pyroclastic and dispersal processes based on factual data is required to help better understand them and to bring understanding to the same level as for subaerial eruption processes.

High levels of vesicularity are not an indicator of explosivity. Deep-water fountains are jets of fluid magma, but not explosive. Submarine eruption columns behave differently to subaerial columns, and can be dissipated by cooling condensation effects, implosions of gas bubbles, and ingress of cold ambient water.

A new approach to classifying submarine eruption styles and deposits is required, based on careful assessment of deposit characteristics. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

This review has resulted from decades of interest and research on submarine volcanic processes by the senior author and numerous insightful discussions with volcanological colleagues at Monash University, the University of Tasmania, and the international volcanological community at scientific meetings.

Airey, M. Explosive volcanic activity on Venus: the roles of volatile contribution, degassing, and external environment. Space Sci. Allen, R. Reconstruction of the tectonic, volcanic, and sedimentary setting of strongly deformed Zn-Cu massive sulfide deposits at Benambra, Victoria.

Allen, S. Quenching of steam-charged pumice: implications for submarine pyroclastic volcanism. Earth Planet. Effects of water depth on pumice formation in submarine domes at Sumisu, Izu-Bonin arc, western Pacific. Geology 38, — Water-settling and resedimentation of submarine rhyolitic pumice at Yali, eastern Aegean, Greece. Barreyre, T. Dispersal of volcaniclasts during deep-sea eruptions: settling velocities and entrainment in buoyant seawater plumes. Batiza, R. Craters, calderas, and hyaloclastites on young Pacific seamounts.

Res Solid Earth — Sigurdsson, J. Stix, B. Houghton, S. McNutt, and H. Google Scholar. Binns, R. White, J. Smellie, and D. Bischoff, J. Acta 52, — Bonadonna, C. Plume height, volume, and classification of explosive volcanic eruptions based on the Weibull function. Brennen, C. Cavitation and Bubble Dynamics. Oxford: Oxford University Press. Park Passes. Technical Announcements. Employees in the News. Emergency Management. Survey Manual.

Deep within the Earth it is so hot that some rocks slowly melt and become a thick flowing substance called magma. Since it is lighter than the solid rock around it, magma rises and collects in magma chambers.

Eventually, some of the magma pushes through vents and fissures to the Earth's surface. Magma that has erupted is called lava. Some volcanic eruptions are explosive and others are not.

The explosivity of an eruption depends on the composition of the magma. If magma is thin and runny, gases can escape easily from it. When this type of magma erupts, it flows out of the volcano.

Lava flows rarely kill people because they move slowly enough for people to get out of their way. If magma is thick and sticky, gases cannot escape easily. Pressure builds up until the gases escape violently and explode. In this type of eruption, the magma blasts into the air and breaks apart into pieces called tephra.

Tephra can range in size from tiny particles of ash to house-size boulders. Explosive volcanic eruptions can be dangerous and deadly. They can blast out clouds of hot tephra from the side or top of a volcano. These fiery clouds race down mountainsides destroying almost everything in their path.

Ash erupted into the sky falls back to Earth like powdery snow. If thick enough, blankets of ash can suffocate plants, animals, and humans. When hot volcanic materials mix with water from streams or melted snow and ice, mudflows form. Mudflows have buried entire communities located near erupting volcanoes.

Despite their seeming permanence, volcanoes are prone to catastrophic collapse that can affect vast areas in a matter of minutes. Large collapses begin as gigantic landslides that quickly transform to debris avalanches—chaotically tumbling masses of rock debris that can sweep downslope at extremely high velocities, inundating areas far beyond the Ash, mudflows, and lava flows can devastate communities near volcanoes and cause havoc in areas far downwind, downstream, and downslope.

Even when a volcano is quiet, steep volcanic slopes can collapse to become landslides, and large rocks can be hurled by powerful When erupting, all volcanoes pose a degree of risk to people and infrastructure, however, the risks are not equivalent from one volcano to another because of differences in eruptive style and geographic location.

Assessing the relative threats posed by U. At least volcanoes in 12 States and 2 territories have erupted in the past 12, years and have the potential to erupt again. Consequences of eruptions from U. Many aspects of our daily life are vulnerable to volcano hazards, On May 22, , a large explosive eruption at the summit of Lassen Peak, California, the southernmost active volcano in the Cascade Range, devastated nearby areas and rained volcanic ash as far away as miles to the east.

This explosion was the most powerful in a series of eruptions during —17 that were the last to occur in the Viewing an erupting volcano is a memorable experience, one that has inspired fear, superstition, worship, curiosity, and fascination since before the dawn of civilization. In modern times, volcanic phenomena have attracted intense scientific interest, because they provide the key to understanding processes that have created and shaped more than Volcanoes have been erupting in the Cascade Range for over , years.

During the past 4, years eruptions have occurred at an average rate of about 2 per century. This chart shows 13 volcanoes on a map of Washington, Oregon, and northern California and time lines for each showing the ages of their eruptions. Most volcano hazards are associated with eruptions. However, some hazards, such as lahars and debris avalanches, can occur even when a volcano is not erupting.

Our Earth is a dynamic planet, as clearly illustrated on the main map by its topography, over volcanoes, 44, earthquakes, and impact craters. These features largely reflect the movements of Earth's major tectonic plates and many smaller plates or fragments of plates including microplates. Volcanic eruptions and earthquakes are awe Park Passes. Technical Announcements. Employees in the News.

Emergency Management. Survey Manual. Hawaiian volcanoes have produced explosive eruptions ranging in size and vigor from relatively small lava fountains to large eruptions. May 18, - one of many in a series of similar events during May Credit: Maehara, K. Public domain. Plank, T. Kent, A. Melt inclusions in basaltic and related volcanic rocks. Gaetani, G. Rapid reequilibration of H2O and oxygen fugacity in olivine-hosted melt inclusions. Geology 40 , — Wallace, P.

Melt inclusion CO2 contents, pressures of olivine crystallization, and the problem of shrinkage bubbles. Experimental constraints on parameters controlling the difference in the eruptive dynamics of phonolitic magmas: the case of tenerife Canary islands.

Non-explosive silicic volcanism. Gonnermann, H. The fluid mechanics inside a volcano. Fluid Mech. Woods, A. Transitions between explosive and effusive eruptions of silicic magmas. Martel, C. Timescales of bubble coalescence, outgassing, and foam collapse in decompressed rhyolitic melts. Scandone, R. Magma supply, magma ascent and the style of volcanic eruptions. Sparks, R. Nonlinear dynamics of lava dome extrusion. Nature , 37—41 Tarasewicz, J.

Magma mobilization by downward-propagating decompression of the Eyjafjallajkull volcanic plumbing system. Castro, J. Schipper, C. Shallow vent architecture during hybrid explosive-effusive activity at Cordon Caulle Chile, : evidence from direct observations and pyroclast textures. Thomas, M. Understanding which parameters control shallow ascent of silicic effusive magma.

Stevenson, J. Mangan, M. Decompression experiments identify kinetic controls on explosive silicic eruptions. Delayed, disequilibrium degassing in rhyolite magma: decompression experiments and implications for explosive volcanism. Mourtada-Bonnefoi, C.

Experimental study of homogeneous bubble nucleation in rhyolitic magmas. Shea, T. Bubble nucleation in magmas: a dominantly heterogeneous process? La Spina, G. Did magma ascent rate control the explosive-effusive transition at the Inyo volcanic chain, California? Geology 36 , — Endo, E. The Eruptions of Mount St. Helens, Washington 93—, professional paper U. Alidibirov, M. Magma fragmentation by rapid decompression. Dingwell, D. Volcanic dilemma: flow or blow?

Mueller, S. Permeability control on magma fragmentation. Geology 36 , Horwell, C. Kendrick, J. Blowing off steam: tuffisite formation as a regulator for lava dome eruptions. Earth Sci. Boudon, G. What factors control superficial lava dome explosivity?

A model of vulcanian eruptions at Galeras volcano, Colombia. Barmin, A. Periodic behavior in lava dome eruptions. Wright, H. Breadcrust bombs as indicators of Vulcanian eruption dynamics at Guagua Pichincha volcano, Ecuador.

Druitt, T. Rapid and slow: varying magma ascent rates as a mechanism for Vulcanian explosions. Deegan, F. Magma—carbonate interaction processes and associated CO2 release at Merapi volcano, Indonesia: insights from experimental petrology. Troll, V. Crustal CO 2 liberation during the eruption and earthquake events at Merapi volcano, Indonesia.

Zimanowski, B. Analysis of thermohydraulic explosion energetics. The dynamics of bubble formation and growth in magmas: a review and analysis.

Thermal vesiculation during volcanic eruptions. Zhang, Y. A criterion for the fragmentation of bubbly magma based on brittle failure theory. Jaupart, C. Gas content, eruption rate and instabilities of eruption regime in silicic volcanoes. Saar, M. Permeability-porosity relationship in vesicular basalts. Permeability and degassing of dome lavas undergoing rapid decompression: an experimental determination. Takeuchi, S. Permeability measurements of natural and experimental volcanic materials with a simple permeameter: toward an understanding of magmatic degassing processes.

Okumura, S. Shear deformation experiments on vesicular rhyolite: implications for brittle fracturing, degassing, and compaction of magmas in volcanic conduits. Solid Earth , B Farquharson, J. Permeability and porosity relationships of edifice-forming andesites: a combined field and laboratory study. Colombier, M.

The evolution of pore connectivity in volcanic rocks. Rust, A. Permeability of vesicular silicic magma: inertial and hysteresis effects. Magma deformation may induce non-explosive volcanism via degassing through bubble networks. Mechanisms of bubble coalescence in silicic magmas. The effects of outgassing on the transition between effusive and explosive silicic eruptions.

Lindoo, A. An experimental study of permeability development as a function of crystal-free melt viscosity. Crystal controls on permeability development and degassing in basaltic andesite magma.

Geology 45 , — Textural studies of vesicles in volcanic rocks: an integrated methodology. The percolation threshold and permeability evolution of ascending magmas. Permeability controls on expansion and size distributions of pyroclasts. Solid Earth , 1—17 Degassing conditions for permeable silicic magmas: implications from decompression experiments with constant rates. Nguyen, C. Explosive to effusive transition during the largest volcanic eruption of the 20th century Novarupta , Alaska.

Geology 42 , — Kameda, M. Advancement of magma fragmentation by inhomogeneous bubble distribution. Yoshimura, S. Fracture healing in a magma: an experimental approach and implications for volcanic seismicity and degassing.

ADS Google Scholar. Cabrera, A. Melt fracturing and healing: a mechanism for degassing and origin of silicic obsidian. Geology 39 , 67—70 Permeability reduction of fractured rhyolite in volcanic conduits and its control on eruption cyclicity. Kushnir, A. In situ confirmation of permeability development in shearing bubble-bearing melts and implications for volcanic outgassing. Stasiuk, M. Tuffen, H. Repeated fracture and healing of silicic magma generate flow banding and earthquakes?

Geology 31 , — Kolzenburg, S. Strength and permeability recovery of tuffisite-bearing andesite. Solid Earth 3 , — Fault textures in volcanic conduits: evidence for seismic trigger mechanisms during silicic eruptions. Flow banding in obsidian: a record of evolving textural heterogeneity during magma deformation. Evidence for the development of permeability anisotropy in lava domes and volcanic conduits.

Black, B. Ash production and dispersal from sustained low-intensity Mono-Inyo eruptions. Helens, Washington. A Volcano Rekindled Renewed Erupt. Helens, — Professional paper U. Geological Survey, Reston, VA, Merapi eruption-chronology and extrusion rates monitored with satellite radar and used in eruption forecasting.

Explosive volcanism may not be an inevitable consequence of magma fragmentation. Hale, A. The influence of viscous and latent heating on crystal-rich magma flow in a conduit. De Angelis, S. On the feasibility of magma fracture within volcanic conduits: constraints from earthquake data and empirical modelling of magma viscosity. Holland, A. Degassing processes during lava dome growth: Insights from Santiaguito lava dome, Guatemala.

Volcanic drumbeat seismicity caused by stick-slip motion and magmatic frictional melting. Heap, M. From rock to magma and back again: the evolution of temperature and deformation mechanism in conduit margin zones. Multiple origins of obsidian pyroclasts and implications for changes in the dynamics of the B.

Cordonnier, B. The viscous-brittle transition of crystal-bearing silicic melt: direct observation of magma rupture and healing. Gardner, J. Formation of obsidian pyroclasts by sintering of ash particles in the volcanic conduit. Vasseur, J.

Volcanic sintering: timescales of viscous densification and strength recovery. Wadsworth, F. Nonisothermal viscous sintering of volcanic ash. Solid Earth , — Collinson, A. Gas storage, transport and pressure changes in an evolving permeable volcanic edifice. Kennedy, B. Time-and temperature-dependent conduit wall porosity: a key control on degassing and explosivity at Tarawera volcano, New Zealand.

Schneider, A. Conduit degassing and thermal controls on eruption styles at Mount St. Lamb, O. Ball, J. The hydrothermal alteration of cooling lava domes. Effects of conduit geometry on magma ascent dynamics in dome-forming eruptions. Transient effects of magma ascent dynamics along a geometrically variable dome-feeding conduit. Costa, A. Effects of wall-rock elasticity on magma flow in dykes during explosive eruptions. Stability of volcanic conduits during explosive eruptions.

Bachmann, O. Voluminous lava-like precursor to a major ash-flow tuff: low-column pyroclastic eruption of the Pagosa Peak Dacite, San Juan volcanic field, Colorado. Michel, J. Incremental growth of the Patagonian Torres del Paine laccolith over 90 k. Strain localization in vesicular magma: Implications for rheology and fragmentation.

Geology 37 , — Rheological transitions in high-temperature volcanic fault zones. Pore-scale mass and reactant transport in multiphase porous media flows. Oppenheimer, J. Gas migration regimes and outgassing in particle-rich suspensions.

Pistone, M. In situ X-ray tomographic microscopy observations of vesiculation of bubble-free and bubble-bearing magmas. Crystal-rich lava dome extrusion during vesiculation: an experimental study.

The mechanics of shallow magma reservoir outgassing.