Phreatomagmatic activity means that erupting magma reacts with external water, e. In contrast, if only magma is erupted and driven only by gasses originally contained in the magma, it is called magmatic activity.
If no magma itself erupts, but heated ground water drives explosions and eruptions of older material, the activity is called phreatic. Try our free app! Android iOS version. Vanuatu Volcano Tours One of the most exciting volcano travel destinations in the world! Hazards associated with these types of eruptions include ejection of crater lakes and generation of lahars e. Ballistics and ash emissions typically have less impact compared to type 1 eruptions due to the very wet nature of type 2 eruptions, which tends to restrict the aerial extent of the finer eruptive products.
Increasing magmatic gas input into the lake raises the vapor—liquid boundary, resulting in vaporization of confined liquid water, generating volume change, pressurization, and eruption. Compared to type 1 phreatic systems, here the conduit is more open with a shallower magma system. Type 2 phreatic eruptions are also very common at Rincon de la Vieja and other hyperacid crater lake systems. The eruptive style is more akin to surtseyan eruptions than vulcanian eruptions which are more related to type 1 phreatic activity.
To our knowledge, no phreatic eruption has been formally and accurately forecast as such or in terms of its size and timing. However, the technical tools and understanding of phreatic eruptions driven by magmatic gas should allow quantitative assessment that can inform hazard assessment. We envision volcanoes prone to phreatic eruptions being monitored using multiple parameters geophysical and geochemical feeding into automated probabilistic calculations forecasting the likelihood of eruption within a given time period.
Here, we examine parameters by which to forecast phreatic eruptive activity. Because these eruptions are commonly small and sudden events, precursory signals may be subtle or absent Maeda et al. In our opinion, the application of broadband seismicity and gas ratios offer the most useful and cost-effective means for forecasting. If these two types of data can be integrated on a real-time or near real-time basis, new insight may be gained in terms of our ability to forecast phreatic eruptions.
Other techniques such as deformation may also be helpful, although many phreatic eruptions appear to occur with no significant precursory inflation, which would suggest no pressure buildup and a type 2 eruption mechanism. However, the lack of recognized deformation may be a result of the shallow deformation source in the hydrothermal system or upper conduit.
Monitoring of systems prone to phreatic eruptions, which do not necessarily involve significant magma movement, will be more likely to detect subtle changes that do occur prior to explosions if instruments are deployed closer to the conduit than would normally be the case for volcanoes prone to magmatic eruptions.
The very long period signals discussed above appear to have common behavior in terms of an inflation—deflation mechanism. The inflationary phase is ascribed to pressurization, commonly from injection of hot magmatic gas into a cooler hydrothermal reservoir system causing boiling. The deflation results from evacuation of the reservoir as the seal ruptures from excessive overpressure, allowing fluid discharge upward through a propagating crack system.
At Aso VLP signals were of long duration — s and shallowly sourced 1—1. The VLP events were thought to originate at deep levels 3—7 km Jolly et al. Mayon volcano in the Philippines periodically experiences phreatic eruptions without clear precursors Catane and Mirabueno ; Maeda et al. These authors suggest that the lack of clear precursors could be due to progressive sealing of the shallow hydrothermal system.
While these data indicate that VLP events can originate at different depths, their occurrence also appears to be an indication of pressurization—depressurization sequences leading to phreatic eruptions. Their recognition is thus an important tool in assessing the probability of a future eruption, although predicting such eruptive events cannot clearly be done using the VLP data alone.
Banded tremor is an unusual seismic signal characterized by periods of tremor interspersed with periods of quiescence. The duration of both the tremor and the quiescent periods are sometimes constant, producing a striking pattern on a seismogram, as can be seen in Fig. The occurrence of banded tremor commonly precedes explosive eruptive activity Martinelli ; Gresta et al.
Hence, it is an important precursory signal. The banded tremor described at Nevado del Ruiz was clearly significant in terms of short-term forecasting. For the 11 September phreatic eruption, banded tremor was first recorded on 5 September 1 week beforehand Martinelli For the devastating 13 November magmatic eruption 2 months later, Voight reported that 3 days of continuous tremor commenced on 10 November before the eruption, although it is unclear if this tremor was banded.
Very similar banded seismic signals were observed at Mt. Banded tremor was first recorded on 1 April, 1 week before the first eruption. Individual tremor bands lasted 25—35 min and quiescent spacings between bands — min, for total cycle durations of — min. During this 1-week interval, tremor amplitudes increased progressively to the time of eruption, then disappeared temporarily before resuming on 13 April, 4 days before the second eruption.
In contrast to these short-term eruption indicators, banded tremor at Karkar volcano, Papua New Guinea, was significantly longer-lived during unrest in — McKee et al. Banded tremor began in July , 6 months prior to the initiation of phreatic explosive activity in January The tremor strengthened appreciably in late August and was associated with increased gas emissions from the craters.
Tremor amplitudes peaked in late October and declined thereafter. In late January , the color of the gas emissions changed from white and blue to dense white vapor.
The most significant eruption, which was phreatic in nature containing no juvenile material, occurred on 8 March The Nevado del Ruiz and Mt. Etna examples illustrate that banded tremor can serve as a short-term precursor to phreatic explosive activity. The Karkar activity demonstrates that occurrence of banded tremor can also extend over an appreciable time period. Nevertheless, the tremor at Karkar was clearly associated with explosive activity which itself extended over 8 months January—August For this type of phreatic activity observed at these and other volcanoes, the presence of magma at shallow levels appears to exert a significant influence on the overlying hydrothermal or groundwater system.
Gas emission monitoring has high potential as an eruption forecasting tool for phreatic eruptions. The delivery of heat from magmatic systems to overlying hydrothermal systems essentially occurs through the upward migration of high-temperature magmatic fluids.
Thus, the fundamental process responsible for driving phreatic eruptions should also be quantifiable and measurable through the gases emitted. However, the lack of gas emission can be equally important, as this could signify the formation of a hydrothermal seal, resulting in accumulation of gas and pressure in the hydrothermal system and ultimately leading to phreatic eruptions e.
Distinguishing between a transition to quiescence and a transition to sealing with ongoing gas input is a key issue requiring integrated assessment of multiple monitoring parameters such as deformation and seismicity.
Additionally, sealing could conceivably result in pressurization without eruption, potentially shutting off magmatic input from below. The field of volcanic gas monitoring is experiencing rapid technological advances. New methods for in situ and remote measurements of gas flux and gas composition have recently been developed e. Hydrothermal—magmatic systems that typically produce phreatic eruptions are rather challenging for monitoring when exclusively using gas fluxes e.
This is primarily due to the fact that systems with high gas and heat fluxes cannot easily establish and maintain hydrothermal systems because meteoric water is rapidly boiled off Pasternack and Varekamp By contrast, volcanoes with inherently lower magmatic gas output can form extensive hydrothermal systems, which interact with magmatic volatiles introduced from below.
These reactions remove reactive volatiles from the magmatic gas phase Symonds et al. Two fundamental dissociation reactions dominate this process e. Reaction 1 produces H 2 S, another gas useful for monitoring, and sulfuric acid. Reaction 2 does not produce a gas species, but instead native sulfur and sulfuric acid. Deeper hydrothermal systems associated with more explosive phreatic eruptions tend to be associated with H 2 S e. The sulfur chemistry at these latter systems is probably dominated by reaction 2, but both reactions can be active at the same time, or may vary in relative significance with time, space, and volcanic activity.
Carbon dioxide is another very important gas species in these systems because it is abundant and readily measurable, and behaves very differently to sulfur gases. Under acidic conditions found in high enthalpy hydrothermal systems, CO 2 is essentially inert and is not removed from the gas phase by interactions with hydrothermal liquids e.
Carbon dioxide is not readily measurable using remote techniques e. Diffuse CO 2 degassing using accumulation chamber methods can be useful to monitor e. These calculations can be done in real-time. Typically, permanent Multi-GAS stations are programmed to analyze 4 times every 24 h, for a period of 30 min during each analytical session. Figure 6 shows an interpretive triangular CO 2 —SO 2 —H 2 S diagram of hydrothermal—magmatic gases, with fields useful for assessing the state of activity in volcanic systems prone to phreatic eruptions.
Low temperature hydrothermal gases typical of volcanoes in a dormant state lack SO 2 and fall along the CO 2 —H 2 S axis. As a volcano reactivates, SO 2 becomes a significant component of gas emissions, characterized by detectable magmatic input e. Within the magmatic and hydrothermal-magmatic fields, we make a distinction between deep CO 2 -rich gases and shallow SO 2 -rich gases.
CO 2 is less soluble in melt than sulfur and thus exsolves at higher pressure. As a magma rises through the crust, the first gases to reach the surface are therefore rich in CO 2 e. As magma continues to rise and then pond at shallow levels, the gases will become richer in SO 2 as magma reaches lower pressure conditions allowing S to exsolve from the melt.
Monitoring of gas compositions can provide crucial insight into the influence of the magmatic system. Generally, as a volcanic system reactivates, the gas emissions will evolve from CO 2 -rich to SO 2 -rich. This plot shows proposed generalized fields for the characterization of hydrothermal-magmatic gas emissions, based on recent studies of Central American volcanoes Aiuppa et al. The exact boundary lines would vary with tectonic setting and other factors Aiuppa et al.
It is important to recognize that the classification in Fig. In dynamic hydrothermal-magmatic systems, distinguishing between gases derived from deep and shallow magmatic degassing that have also been affected by hydrothermal processes such as scrubbing producing H 2 S at the expense of SO 2 , oxidation or remobilization of sulfur species, and mixing between gas sources, can be extremely difficult without additional information e.
Magma intruded at shallow levels releases large amounts of gas due to decompression first boiling. Once emplaced in this comparatively shallow and cold environment, the magma solidifies by crystallization and further gas release second boiling.
For type 2 phreatic systems, we propose that the combined effects of shallow magmatic gas input and vaporization of the liquid-dominated hydrothermal system typically below a crater lake drive phreatic activity. Both banded tremor and VLP seismic signals appear to be reliable indicators of pressurization of the shallow hydrothermal system, although the timescales of pressurization may be variable.
Banded tremor pressurization timescales vary from days e. VLP timescales appear to be short, on the order of minutes before an eruption occurs, although they can also occur without eruption, e. The additional presence of long period seismic signals is also clearly significant, indicating increased or continuing pressurization and most importantly an increased probability of eruption, as was seen at Karkar in February—March and at Ontake in September We suggest that gas ratios may be able to play a key role in forecasting magmatically driven phreatic eruptions.
Recent high-frequency Multi-GAS time series data show that individual phreatic eruptions are associated with pulses of H 2 S-poor magmatic gas Battaglia et al. However, if these changes are accompanied by anomalous low-frequency seismic signals indicating continued magmatic gas input to the hydrothermal system, the system is likely undergoing pressurization.
In some cases, gas ratios could provide the only information that a system is sealing, e. Characterization of the extent and depth of hydrothermal systems at volcanoes prone to phreatic eruptions e. The process of hydrothermal sealing plays a direct role in determining the explosivity of phreatic eruptions.
Understanding and recognizing this sealing process is a key direction for research and monitoring efforts. If a lot of gas is trapped within magma, pressure will build and build until eventually the magma erupts explosively out of the volcano. This builds pressure inside the bottle and when you release the pressure by opening the bottle, the gas rushes out of the top carrying some of the liquid with it. Phreatomagmatic eruptions are a type of explosive eruption that results from magma erupting through water.
Some submarine volcanoes are phreatomagmatic if the magma is gas-rich, for example Surtsey in Iceland. This eruption formed a new island. Explosive eruptions can form pyroclastic flows that sweep down valleys, destroying everything in their path. They also send ash high into the atmosphere, forming plumes.
If a magma has low viscosity it is runny , gas can escape easily, so when the magma erupts at the surface it forms lava flows. These eruptions are relatively! If a magma rises very slowly within the conduit. Phreatomagmatic fragmentation implies that external water, with resulting rapid melt quenching and water vaporization, played a role in fragmenting the magma Table 1.
This varies in detail, but commonly is based on particle shape, degree of quenching, and whether there is a glassy fluidal exterior film, and for a bulk sample, proportions of differently classified clasts. It is widely known that no single one of these clast-specific criteria is entirely diagnostic and other criteria at the local deposit level are often considered, such as welding, particle aggregation, and lithic fragment abundance.
For whole-deposit grain populations the proportion of fines is often considered, and is sometimes applied even at local deposit level. The range of products produced by magmatic fragmentation is broad and well known, and includes dense to highly vesicular fragments with many different shapes. A test is run against criteria considered proof of phreatomagmatic origin, and if the test fails a magmatic origin is considered proven. In other words, the absence of evidence for phreatomagmatic fragmentation is taken as evidence of its absence.
Positive arguments are needed, and an acceptance that fragmentation process es of an eruption cannot always be determined from its deposits. Vesicles are among the most obvious and varied features of particles in primary volcaniclastic rocks White and Houghton, Dense, glassy, almost vesicle-free volcanic particles from deep-sea eruptions are one possible magma-water end member, opposite basaltic pumice or reticulite from Hawaiian fountains as the magmatic end member Table 2.
This simple distinction fails for rhyolitic compositions, where both pumice and obsidian are formed from dry magmatic eruptions. A more rigorous way of using vesicles to distinguish between magmatic and phreatomagmatic used in this paper broadly for all magmatism involving external water particles was clearly expressed in an empirical study by Houghton and Wilson , in which eruptions inferred to have involved magma-water interaction produced particle populations with a greater range of vesicularity, and a lower median value, than did eruptions or eruptive phases without water.
The explanation for the broader vesicularity range of phreatomagmatic particles is that those with lower vesicularity were formed and quenched either before their volatiles could exsolve or before small bubbles could grow under decompression to reach high volume proportions. In contrast, higher vesicularity particles encountered water higher in the conduit, or not at all, allowing further exsolution of volatiles and growth of bubbles during depressurization.
This distinction has been complicated by recent studies of deposits from observed Hawaiian and Strombolian eruptions, which were clearly driven by magmatic volatiles but contain clasts with a wide range of vesicularity, even within individual clasts, due to entrainment of degassed melt in conduits by erupting more gas-rich melt e. Vesicle-population analysis has become an important tool for assessing conditions and rates of magma ascent e.
Despite the detail available from such studies, no unique fingerprint of magma-water interaction has emerged e. Walker and Croasdale recognized a relationship that is now presented in textbooks and many papers; in the basaltic deposits they studied inferred phreatomagmatic particles were typically equant and fracture bound, while many from inferred dry magmatic eruptions were achnelithic, having aerodynamic forms Table 2. Heiken , followed by many others, termed the equant fracture-bound particles blocky.
A key concept applicable to both magmatic and phreatomagmatic fragments is that not all particles hold information about fragmentation dynamics that relates specifically to the explosion mechanism. Magma surrounding an explosion or expansion site can be broken into fragments without contributing to that explosion or expansion; the internal textures of those fragments carry no information about the explosion mechanism.
Fluid magmas, for example, can be torn apart to form a shower of fluidal achnelithic fragments by bursts of air in experiments, by magmatic gas in hornitos, lava fountains, and Strombolian bursts, or by vaporization of external water trapped below or in the magma Fisher, ; Zimanowski et al. An additional problem in small-scale eruptions such as those of Stromboli Capponi et al. For particles formed in more highly explosive magmatic eruptions there can be a similar distinction; where outgassed magma is shattered by explosive decompression of an underlying magma body, the particle population reflects the history of the shattered magma.
Blocky dense particles are formed by magmatic eruptions of this sort Heiken and Wohletz, , including blast e. Basalt magma quenched by water commonly forms sideromelane granules e. Such quenching is rare for dry basaltic eruptions, but is sometimes achieved in vigorous lava fountains e. Silicic magma, in contrast, can form glass even when cooling is relatively slow, such as in obsidian domes Fink and Manley, , so silicic glass is of little help in inferring cooling rates.
To stick primary volcanic particles back together after they are formed, heat must be retained or applied, along with some force. Basaltic bombs or coarse lapilli formed as fluidal, spatter clasts can become agglutinated upon landing or with subsequent loading when their interiors remain hot and ductile during emplacement and their accumulation rate is sufficiently high Head and Wilson, Retaining this characteristic requires that the clasts not be fully quenched, but this is not a difficult threshold to meet because loss of heat from the interior of a clast is limited to diffusive rates.
Agglutinate is common in hornitos small lava-flow—fed spatter cones , around lava fountains, and in layers within scoria cones. It is also known from submarine eruptions Kaneko et al. Agglutination of original pyroclasts also takes place within vents, and perhaps especially in phreatomagmatic ones, to form composite bombs e.
Particle aggregates, such as accretionary and armored lapilli, commonly form by adhesion of damp ash to a nucleus of ice or rock. They are a well-known product of phreatomagmatic eruptions Moore et al.
Different types can be distinguished, and a range of inferences drawn Moore and Peck, ; Schumacher and Schmincke, ; Brown et al. They can provide compelling evidence of water droplets in the eruption column, plume, or density currents, but do not indicate the nature of fragmentation that produced the ash being aggregated. Fragments of country rock are important constituents of maar-forming eruptions, which are defined by their crater cut below the preeruptive surface that remains open at the end of the eruption e.
It is widely accepted that basaltic maars result from phreatomagmatic eruptions White and Ross, , and references therein , and lithic-rich horizons in tuff cones and rings, and in scoria cones, are commonly attributed to episodes of phreatomagmatic activity e. In other contexts, such as eruptions produced during caldera subsidence, increased abundance of lithics need not involve water or phreatomagmatic processes Druitt and Bacon, ; Bear et al.
An important feature often used to diagnose effects of water on an eruption is the proportion of fine particles. This is embedded in the well-known Walker eruption classification that plots fall-deposit dispersal against grain size Walker, , and finds support also in thermodynamic analyses of magma-water fuel-coolant interactions Sheridan and Wohletz, ; Wohletz and McQueen, ; Wohletz, Strong fragmentation of magma requires considerable energy Zimanowski et al.
Diagnosing phreatomagmatic interaction on the basis of high proportions of fines in an examined deposit remains, despite this sensible background, problematic Table 2. First, only a whole-deposit grain-size distribution can be considered representative. It is a common misperception that all juvenile particles erupted by a phreatomagmatic explosion should have the signature of magma-water interaction.
Zimanowski et al. Environmental factors can introduce a bias in grain size; for example, local water flushing Talbot et al. Specific to the evolution of maar-diatreme eruptions, there is also good evidence that the tephra ring deposits, for which one might obtain a whole-deposit grain-size inventory, do not represent well the overall fragment population produced by the volcano because of systematic retention of coarser fragments within the maar-diatreme subsurface structure e.
The abundant fines in many inferred phreatomagmatic deposits have conspicuously high proportions of country rock or recycled fragments that were not formed by fragmentation during the explosion that deposited them Houghton and Nairn, ; White, Fine-grained lithic debris can be produced by repeated explosions before being finally ejected onto a tephra ring, for example, but also depends upon the original characteristics of the host material.
A mudstone or shale, or poorly indurated fine sands and silts, will eject as fine-grained material that is not reflective of any particular degree of volcanic fragmentation.
Magmatic eruptions can also produce very high proportions of juvenile fines in some instances Palladino and Taddeucci, ; Dellino et al. In contrast, there is no published evidence that submarine or other subaqueous eruptions generate particularly large proportions of fines.
This may be because fines in such settings are very efficiently separated by hydraulic sorting, but until some whole-deposit grain-size data become available, this must be considered no more than a reasonable hypothesis.
Surtsey is the classic example for near-vent enrichment in fine particles produced from a shallow-submarine phreatomagmatic vent, but even there separating the signature of water in the jets and plumes, which drive near-source deposition and leave their imprint on depositional features, from a fragmentation signature is difficult. It is compounded by strong evidence for particle recycling through the vent Thorarinsson et al. In deposits of both Surtseyan eruptions and deeper submarine ones, interactive particles have been identified Schipper et al.
Such particles are known from MFCI experiments, and imply phreatomagmatic fragmentation Zimanowski et al. There are other simple features that are less commonly available but that can be helpful.
Normal jointing on large particle surfaces is well known from water-chilled fragments e. Cauliflower bombs Lorenz, have deeply quenched rough surfaces; these differ in surface-fracture geometry and internal vesicularity from the breadcrust bombs common in some dry eruptions e.
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