Phreatomagmatic and Related Eruption Styles

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Chapter 30

Phreatomagmatic and RelatedEruption Styles

Bruce HoughtonDepartment of Geology and Geophysics, National Disaster Preparedness Training Center, University of Hawai‘i, Honolulu, HI, USA

James D.L. WhiteGeology Department, University of Otago, Dunedin, New Zealand

Alexa R. Van EatonU.S. Geological Survey, WA, USA

Chapter Outline1. Introduction 538

2. Transient Explosions 538

2.1. Drain-Back, Heated Wall Rock, and

Groundwater: Halema’uma’u 1924, Hawaii 538

2.2. Wall-Rock Collapse and Decoupled Magmatic

Volatiles: Halema’uma’u March 19, 2008, Hawaii 539

2.3. Lava and Surface Water: Kalapana 1988e2013,

Hawaii 540

2.4. Lava and Groundwater or Ice: Laki, Iceland 542

2.5. Vesiculating Magma and Surface Water: Surtsey,

Iceland 542

2.6. Vesiculating Magma and Groundwater: Ukinrek

Maars, Alaska 543

3. Sustained Eruptions 544

3.1. Characteristics of Sustained “Wet” Plumes 544

3.2. Outgassed Magma and Surface Water:

Rotongaio, New Zealand 544

3.3. Vesiculating Magma and Surface Water:

Oruanui, New Zealand 545

3.4. Vesiculating Magma and Groundwater: Phase

C Askja 1875 Eruption, Iceland 546

4. Discussion 548

4.1. “Ash Aggregation” in “Wet” Plumes 548

4.2. Models for “Wet” Plumes 549

4.3. Role of the Vent and Shallow Conduit 549

4.4. Role of the Magma in Wet Volcanism 550

4.5. Role of the Preeruptive State of Water 551

4.6. Heterogeneity of Magma-Water Mixing

and Thermal Efficiency 551

4.7. Concluding Thoughts 552

Further Reading 552

GLOSSARY

accretionary lapilli Spherical aggregates of volcanic ash, commonlyhaving a concentric structure and formed by clumping of moistash. Often used as a general term for all ash aggregates includingstructureless “ash lumps/pellets.”

armored lapilli Solid fragments coated with one or more layers ofvolcanic ash; essentially a variant of “accretionary lapilli.”

ash aggregation Formation of clusters of ash particles duringtransport and sedimentation.

ash lump/ash pellet Ash aggregate of irregular shape and with nodiscernible, concentric internal structure.

cock’s tail jets A common type of tephra jet with an arching, mul-tifingered form likened to the shape of a rooster’s tail. Also calledcypressoid jet.

continuous uprush Sustained phreatomagmatic eruption columnthat is fed by ongoing or closely spaced explosions.

external water Any water phase involved in explosive volcanismthat was not originally dissolved in the magma, including surface,ground, and atmospheric water.

mud rain Ash-bearing droplets of water from an eruption plume.phreatoplinian deposits Widespread phreatomagmatic fall deposits,

defined by a dispersal index of !50 km2, and a fragmentationindex generally >80%.

spall dome Subhemispherical, gas-driven shock zone expandingoutward from a subaqueous explosion.

tachylite/sideromelane Varieties of basaltic glass. Sideromelane istranslucent; tachylite is opaque as a result of abundantnanolites and microlites, inferred to result from a slightly slowercooling.

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tephra jets Discrete ejections containing tephra and water vapor,driven by steam expansion and momentum of large clasts.

vesiculated tuff Ash deposit containing spherical to irregularlyshaped cavities, reflecting entrapment of air or water droplets inwet, fine-grained ash.

1. INTRODUCTION

Here, we describe the spectrum of “wet” explosive erup-tions that result from the combination of magmatic heat and“external water,” such as groundwater, lakes, or oceans(Figure 30.1). These eruptions have been grouped in thepast under descriptors such as phreatomagmatic, phreatic,and hydrothermal. Although definitions vary, most authorsuse phreatomagmatic to characterize eruptions wheremagma interacts directly with external water, phreaticwhere magma heats surrounding fluids without actuallyerupting at the surface, and hydrothermal to describeeruptions of geothermal fluids. However, water-influencederuptions in nature occur well outside the boundaries ofthese simplified definitions. The case studies given belowillustrate some of the defining characteristics of “wet”eruptions, and how they can be recognized through directobservation and in the geological record. In revisiting theseexamples, we illustrate how water can modify or, in somecases, govern the dynamics of volcanic explosions, andaddress the following open questions:

1. How does water involvement modulate eruption style,transport processes, and the resulting deposits?

2. How do eruption dynamics reflect the source of water(magma, surface water, groundwater, and atmosphere)and its physical state (ice, liquid, and vapor)?

3. Are phreatomagmatic products necessarily finer grainedthan their dry counterparts?

4. Does water involvement at the volcanic source neces-sarily leave behind “wet” signatures in the deposits?

2. TRANSIENT EXPLOSIONS

2.1. Drain-Back, Heated Wall Rock, andGroundwater: Halema’uma’u 1924,Hawaii

A 20-day sequence of multiple discrete explosions atHalema’uma’u crater on Kilauea volcano occurred whenmagma withdrawal from the lava lake back down theconduit led to multiple collapses of the conduit walls, andsignificant crater widening (Figure 30.2). This event hasbeen called an example of phreatic activity, that is, explo-sions culminating from the indirect transfer of magmaticheat to external water via wall rock, but the presence ofsmall amounts of juvenile ejecta suggests that it wasphreatomagmatic.

Explosions began on May 10, 1924, reaching a peakintensity on May 18. Several “large” explosions andnumerous smaller events occurred each day, sometimes<1 min apart. The last explosions were recorded on May29. The locus of the activity was not fixed, and each ex-plosion produced a discrete apron of ejecta, but the north-eastern wall of the crater was the main source. The activitygenerated transient plumes to heights of 6.5 km(Figure 30.2(D)), depositing ash, lapilli, “mud rain,” andash aggregates, and ejecting ballistic blocks (up to10 tonnes) to 1 km from vent. The blocks ranged greatly intemperature, from ambient to approximately 700 "C, andwere in part breadcrusted. Ash fall was recorded >30 kmdownwind, consisting almost entirely of wall rock, princi-pally preexisting lava flows.

Before 1924, Halema’uma’u had contained a long-lived, active lava lake. The lava lake subsided and dis-appeared in February 1924 as part of a more widespread,1- to 4-m subsidence of the volcano summit, accompanyingmagma removal from the shallow summit reservoir.Geodetic data suggest the draining of approximately0.4 km3 of magma into the East Rift Zone. In February, thefloor of Halema’uma’u sank to a depth of 115 m, leavingthe crater 520 m wide. By the end of the eruption, the depthof the crater was 417 m, and its diameter was915 # 1065 m. Jagger estimated the collapse volume to beapproximately 2 # 108 m3, and the ejecta volume was7 # 105 m3.

A model for these explosions requires there to have beenthermal quasiequilibrium established during the extendedperiod when lava was ponded in the Halema’uma’u crater(Figure 30.2(A)). Adjacent to the shallow, magma-filled

Aquifer

Magma flow

Water flow

Fragmentation

Interaction of magma orpyroclast/gas mixtureswith sea or lake water

Interaction of magmaand groundwater

Tephra jets

‘Wet’ convective plumeUmbrella

cloud

Ash &aggregates

FIGURE 30.1 Conceptual cartoon for subaerial phreatomagmaticeruptions showing two alternative external sources for the water that isflashed into steam. These are either surface water in lakes or streams, orgroundwater held in pores and cracks within the shallow rocks.

538 PART | IV Explosive Volcanism



conduit was extremely hot wall rock (probably at least700 "C), which formed the incandescent blocks seen duringexplosions. Outboard from this region, colder wall rockcontained groundwater in cracks, fractures, joints, and porespace. The water phase was necessarily in a liquid state toenable significant expansion on heating. This state wasmaintained until support was removed from the conduit wallsby withdrawal of magma from the conduit (Figure 30.2(B)).The collapses of the conduitwalls that followed brought thesethermally heterogeneous wall-rock materials into rapid,intimate contact. It is inferred that pockets of water weretrapped during the collapses and heated rapidly, flashing tosteam, expanding perhaps 5000x in volume, and poweringimpulsive, short-lived explosions (Figure 30.2(C) and (D)).The observed thermal heterogeneity of the ejected blocks isexplained by the short time scales of the mixing events. Theexplosions thus involved magmatic heat interacting with anexternal source of liquid water, but only after temporarystorage in conduit wall rock. By this mechanism, explosiveactivity was initiated without direct involvement of magma.

2.2. Wall-Rock Collapse and DecoupledMagmatic Volatiles: Halema’uma’u March19, 2008, Hawaii

The March 19, 2008, eruption at Halema’uma’u, K!ılauea isincluded as an example of an eruption where neither

external water nor magma was involved (Figure 30.3).Instead, a rock fall blocked the release of magmatic vola-tiles from the conduit, leading to a transient buildup ofpressure. The eruption was thus powered by a suddenexpansion of the decoupled, but trapped, magmaticvolatiles.

Enhanced seismic tremor at Halema’uma’u was firstobserved in November 2007, followed by increasing SO2

emissions in December. Microearthquakes increased infrequency on March 12, 2008, when the gas emissionsbecame centered upon a 150-m-wide area at the base of thesouthern wall of Halema’uma’u, which was to become thefuture vent (Figure 30.3(B)). Incandescence (rock temper-atures >500 "C) was first seen on March 14. On March 17,there were at least 16 incandescent gas vents in an area of15 # 30 m. The final measurement of SO2 flux prior to theexplosion was 1630 $ 130 tons/day on March 18, 2008, upfrom a background of approximately 150 tons/day.

The explosion on March 19, 2008, was preceded bythree small, high-frequency earthquakes, inferred to havebeen caused by rock falls (Figure 30.3(C)). It was associatedwith a long-period seismic event and a decompressiveinfrasound signal, attributed to a pressure release at source(Fee et al., 2010). Its total duration, inferred fromgeophysical techniques, was only 53 s. TheMarch 19, 2008,explosion produced a steep-walled 35-m-wide crater at least30 m deep (Figure 30.3(D)), in the center of the degassingarea, and ejected only wall-rock particles (Swanson et al.,

Ground-water

Water Table

Rockfall

Block-age

Lava

lake(A) (B) (C)

(D)

(E)

FIGURE 30.2 (A) Cartoon, modified after Swanson et al. (2011) showing the model for 1924 explosions. Drainage of the long-lived lava lake (A)dropped the free surface below the water table and withdrew support from the steep, hot walls of the conduit. (B) Wall collapse and vent blockagestriggered (C) a series of transient explosions involving hot wall rock and a small proportion of largely outgassed magma. (D) Photograph of the plumeformed by explosion at 11:15 (Hawaii Standard Time) on May 18 at the peak of the 1924 explosive activity. Observers noted fallout of ballistic blocks, hotash, and accretionary lapilli. Photographer K. Maehara, HVO archive. (E) Impact crater and 8- to 10-tonne block thrown 1 km (SE) by the explosion in(D). Photographer H.T. Stearns, Hawaiian Volcano Observatory archives.

539Chapter | 30 Phreatomagmatic and Related Eruption Styles



2009; Patrick et al., 2011). The volume of ejecta was, atmost, 1% of the volume of the collapse crater, producing atotal mass of 4.1 # 105 kg (wall-rock particles only), whichequates to a time-averaged eruption rate of 8 # 103 kg s%1.The major volume discrepancy between ejecta and thecollapse crater indicates that a significant preeruptive cavityexisted before eruption, which was only partially blocked atthe surface by the talus derived from the Halema’uma’uwall. This cavity was a pathway for the free escape ofmagmatic gas, which before March 19, was dischargedthrough a network of pore space (Figure 30.3(B)). Themagma, however, was significantly deeper in the conduitand was not involved in this explosion. Juvenile ejecta firstappeared four days later (Swanson et al., 2009) and has beenfollowed by more than six years of magmatic activity.

It can be reasonably inferred that the gas phase involvedon March 19, 2008, had the same origin as the decoupledmagmatic volatiles that were being discharged freely priorto and after the eruption. There is also no evidence forinvolvement of external water; the water table in theK!ılauea caldera is approximately 600 m below the floor ofthe caldera. The flux of magmatic volatiles immediatelyafter the explosion (1620 tons/d) was essentially the sameas on March 18. The erupted volatiles represent a pocket of

magmatic gas that was temporarily trapped between theprecursory rock falls and the explosive eruption 3 min later.

No external water or magma was involved in thiseruption, yet the products look identical to phreatic de-posits. It cannot be accommodated at all in existingclassifications of eruption styles.

2.3. Lava and Surface Water:Kalapana 1988e2013, Hawaii

When pahoehoe lava enters the ocean, it forms a broadvolcaniclastic delta traversed by lava tubes. Interactionwith seawater becomes most explosive when lava fluxes arehigh and focused in relatively few tubes. Major littoralexplosions are often linked to external mechanical triggers,such as cracking or collapse of lava tubes or bench col-lapses. Activity in these cases takes three dominant forms:episodic “tephra jets” (Figure 30.4 (C)) lithic-rich directedexplosions, and bubble bursts (Figure 30.4 (D)).

Episodic tephra jets: This activity is linked to thesevering of lava tube by bench collapse (Figure 30.4(E) and(F)). Breaking waves then disrupt the stream of melt, pro-ducing closely spaced successions of tephra jets up to tensof meters high (Figure 30.4(B) and (C)). The explosions are

(A)

(C)

(D)

East West

Halema’uma’ucrater floor

Air-filled cavity

Taluspile

Magmafree-surface

(B)

FIGURE 30.3 Three-stage model for the March 19, 2008, at Halema’uma’u crater, Kilauea. The explosion occurred well above the approximately600-m-deep water table and was powered by the confinement and subsequent expansion of decoupled magmatic volatiles exsolved at depths of severalhundred meters below the floor of the crater. (A) Sketch of vent geometry. The new vent formed close to the foot of the wall of the preexisting Hale-ma’uma’u crater, through a loose talus scree of fallen blocks. The subsequent disparity between the volume of the vent and that of the ejecta suggested thatthere was considerable void space beneath the talus, represented schematically here, in an oversimplified way. (B) Rise of basaltic magma from theshallow storage region at depths>1 km was accompanied by the release of magmatic volatiles which, a week prior to the eruption, were being dischargedfrom numerous sources across a 150-m-wide region on the surface of the talus fan. By 5 days prior to the explosion, the discharge was incandescent andconcentrated on perhaps 10e15 points immediately above the future vent. (C) Rock falls accompanied by three small impulsive high-frequency seismicevents at and just after 2:55 a.m. on March 19, 2008, temporarily blocked the system, inhibited gas expansion, and release. (Note for simplicity, theblockage is shown just below at the base of the talus fan but could have been significantly deeper.) (D) Three minutes later, the vent is explosively clearedby the expanding trapped volatiles ejecting only wall-rock particles in a short-lived plume.




exceptionally thermally inefficient, with incandescenttemperatures visible in large clasts for tens of seconds afterdeposition. This activity results in two types of eruptionproducts: (1) proximal littoral cones, which can growrapidly at rates up to meters per day, and (2) a downwindtephra blanket. The cone is constructed mostly from largelyoutgassed but exceptionally fluidal bombs, which spatter onlanding and often form thin sheets. Other ejecta take theform of centimeter wide lava ropes, known informally as“Pele’s locks” (Figure 30.4(A)), Pele’s hairs, fluidal ash,fine lapilli, and glass flakes. The distal blanket contains amix of Pele’s hair, fluidal ash, and glass flakes.

Directed explosions: During collapse of unstable lavadeltas (Figure 30.4(F)), incandescent bench faces areexposed, and brought into contact with seawater, whichtriggers explosions, ejecting blocks from the hot “wallrock” of the lava delta (Figure 30.4(E)) and lesser amounts

of fluid lava. Ballistic clasts are produced and dispersedsignificantly more widely than tephra jets, and the ejectaforms discontinuous and asymmetric block fields that canextend up to 200 m inland (Figure 30.4 (H)). On April 19,1993, a collapse killed one visitor and the resulting blockfall injured several others (Mattox and Mangan, 1997).

Bubble bursts: Any process that traps seawater, forexample, fracturing of the roof of lava tubes close to sealevel, may produce 1- to 10-m diameter steam bubbles.Explosive ruptures of bubbles (Figure 30.4(D)) eject amixture of fluidal pyroclasts (Figure 30.4(G)), limu-o-Peleand Pele’s hair. On rare occasions, tube fracture insteadproduces continuous fountaining lasting tens of minutes.

Mattox and Mangan (1997) summarize the activity asrepresenting either open or confined mixing of seawaterwith largely outgassed lava. Tephra jets and lithic blasts arethe result of “open” or unconfined mingling with an

(A) (B) (C)

(D) (E) (F)

(G) (H)

FIGURE 30.4 Photographs of explosions and products of Kalapana littoral explosions. (A) Pele’s locks and (B) fluidal, outgassed clast enclosing anangular wall-rock block, formed during episodic tephra jets in 2008 of the type shown in (C). (C) Tephra jetting at Kalapana ocean entry of lava.J.D. Griggs, USGS, February 3, 1988. (D) Burst of steam bubble formed under fluid lava, an excellent example of hydrodynamic fragmentation.J.D. Griggs, USGS, October 5, 1988. (E) Angular, blocky fragments torn from the walls of lava tubes during major explosions accompanying collapse ofthe lava delta. (F) Outpouring of a lava stream from a severed tube following one such collapse. J.D. Griggs, USGS, November 27, 1989. (G) Exceptionallyfluidal, meter-long clast (under yellow notebook) formed during events as seen in (D). (H) Scattered blocks from the collapse-triggered explosions in theforeground overlying pahoehoe lava. In the background, the littoral cone formed during more sustained periods of tephra jet activity in 2008.




extended contact area caused by tube or bench collapse.The formation of tephra jets mostly involves hot fluid lava;blasts incorporate very hot glassy material from the head ofthe tube. Bubble bursts (and fountains) are the result ofconfined mingling, whereby seawater trapped in tubes orelsewhere develops small confining pressures, permittingvapor expansion to disrupt overlying melt.

Littoral cones have an inherently small preservationpotential when formed on lava deltas or eroding coasts, dueto the dynamic environment in which they form. Pahoehoe-fed cones are generally only preserved when they arewelded or armored by subsequent lava. Larger cones formwhen ‘a‘!a lava flows into the ocean, generally at muchhigher lava fluxes. Tephra jets are the dominant transportmechanism, and the ejecta consist of ragged fluidal bombsand lapilli.

2.4. Lava and Groundwater or Ice:Laki, Iceland

The processes giving rise to rootless cones are similar inmany respects to those producing littoral cones in coastalareas. Both are driven by explosive entrapment andmingling of lava flows and external water rather than adirect magmatic source. In the case of rootless cones, thewater comes from a saturated or icy substrate, and debrisfrom the substrate is often included in the ejecta. It is clearfrom the accumulation of rootless cones on top of lavaflows fed by the same magma that the cones formed afterinitial advance of the lava (Figure 30.5).

After pahoehoe overrides a wet substrate, the lava inte-rior remains molten. The flow thickens by inflation, withlava tubes feeding the advancing flow front. As the flowthickens, it begins to sink into the underlying deformingsubstrate,most strongly at siteswhere the lava is the thickest,where there is also sustained lava supply. Cracking of thebase of the lava during this subsidence provides direct accessto underlying water-saturated sediment, producing blasts

and tephra jets like those of littoral cones. In some cases, thisalso results in localized density currents. The ejected tephracontains both lava fragments and sediment, andmany showaprogressive coarsening or “drying-out” with time.

A prominent feature of rootless cones is their distribu-tion along lava “pathways” in cone fields. In other words,they tend to be localized along paths of sustained lavapassage, in channels or tubes. During the eruption of Laki(1783e1784), hundreds of rootless cones, grouped intoclusters, formed where lava advanced over swampy low-lands. Channel-fed rootless cones often have a U-shape inplan view, the result of tephra being carried away byflowing lava in the channel. In contrast, rootless conesformed above lava tubes are more equant.

2.5. Vesiculating Magma and SurfaceWater: Surtsey, Iceland

Once Surtsey built above the sea, Thorarinsson (1967)noted “ ... two clearly discernible types of activity. Whenthe sea had easy access to one or more of the vents ... theactivity was the typical one for submarine eruptions... Aftereach explosion a tephra-laden mass rushes up, and out of itshoot numerous lumps of liquid or plastic lava, calledbombs, each with a black tail of tephra.

Within a few seconds these black tails turn grayishwhite and furry as the superheated vapor . cools andcondenses” (pp 17e18).

Such activity from a flooded vent is termed tephrajetting, or cock’s tail plumes (as seen in Figure 30.4(C)). Incontrast, “continuous uprush” activity was linked withtephra accumulation that blocked seawater from pouringinto the vent. “Instead of intermittent explosions the uprushof vapor and tephra was then continuous. at the base ofthe eruption column the speed of the uprush was about 400feet per second. The black columns of tephra frequentlyreached half a mile in height. accompanied by a heavyrumbling noise. The tephra production from such acontinuous uprush was much greater than from the inter-mittent explosive activity. uprush of this kind might lastfor a few hours at a time.” (Thorarinsson, 1967; pp. 18e19).

Tephra jets carried tephra and steam along modifiedballistic paths outward from the vent. Several jets perminute were commonly produced, and jets also occurred inisolation at widely spaced and irregular intervals, or inincreasingly close succession at the onset of continuousuprush episodes. Strong, pulsatory vapor plumes commonlyrose from the vent while jetting took place and were asso-ciated with lightning, whirlwinds, and tephra-hail showers,in which each hailstone was cored by a sand-sized tephragrain. As the inner parts of larger jets at Surtsey descended,they formed steam and tephra density currents that floweddown the cone and out over the sea. Continuous uprush

LavaVent

TephraCone Platform

Sheet

FIGURE 30.5 Conceptual model of a rootless cone at Laki, afterHamilton et al. (2010). The deposit has typically three facies: a distal sheetof material from multiple source vents, an intermediate tephra platform,and a proximal cone centered on a crater above the rootless vent.




activity formed relatively stable, dark-colored eruptioncolumns up to a couple of kilometers in height, carryingwater vapor, and heavily charged with tephra (Figure 30.6).Incandescent clasts were abundant in such columns and fellback onto the cone. Buildup of tephra around the vent(tephra dams) was observed to prevent flooding of the ventduring uprush activity.

Subaerially deposited tephra at Surtsey is poorly sortedand, in many places, includes “vesiculated tuff” rich in“accretionary lapilli” and sideromelane. Deposits containwidely scattered pebbles and local blocks of induratedmarine sediments from the preeruption seafloor, and manyfragments of thin vesicle-layered dikes. Vesiculated tuffbeds on steep cone slopes (>20") are broken by channelsformed by small tephra debris flows. On even steeper slopes(>35"), these tuff beds failed en masse, breaking intoplastic slabs. Quenched bombs, with ubiquitous incorpo-rated wall rock, and a rugged, glassy exterior are present inlate-stage beds; they indicate heterogeneous interaction ofwater and recycled tephra with erupting fluidal magma.

2.6. Vesiculating Magma andGroundwater: Ukinrek Maars, Alaska

A 10-day eruption at Ukinrek, Alaska, resulted from risingand ponded magma interacting with small amounts ofshallow, perched, initially frozen groundwater withinglacial till and underlying silicic pyroclastic deposits

(Kienle et al., 1980). It formed two maar craters (volume4 # 106 m3) and ejected 2 # 107 m3 of olivine basalt andwall-rock lithic clasts. West Maar formed over the firstthree days of eruption and was 170 m wide and 35 m deep.The larger East Maar formed over the following 7 days, andis 300 m wide and 70 m deep (Figure 30.7(A)). Craterformation was gradual over the course of the eruption.Magma formed an incandescent pond in the vent late in theeruption sequence (Kienle et al., 1980).

Eruption plumes occasionally reached 6 km high(Figure 30.7(B)), and distal ash fall was recorded over anarea of 20,000 km2. The eruptions were phreatomagmaticthroughout, but activity at both vents showed a progressivetrend toward drier activity (transitional to Strombolianeruptions), marked by a lower content of wall-rock clastsand an increased abundance of ragged scoria lapilli in thedeposits. This change reflects depletion of the water supply,delivered by flow through and collapse of the shallowaquifers. From day 5 after the start of eruption, contrastingeruptive styles were observed from two vents within EastMaar. One vent within the lava pond experienced relatively“dry explosions” coeval with dark ash-rich plumes from thesecond vent (Kienle et al., 1980).

The deposits, which reach a maximum thickness of26 m on the rim of the East Maar, are of four types:(i) lithic-rich ash and lapilli fall deposits, (ii) scoria-richlapilli fall deposits, (iii) late-stage lithic-block apronsextending to only 800 m from the rims of the maars, and

Tephra fall

Water

Arrows indicatematerial movementdirection

Vesiculatingmagma to pyroclastsSubaqueousvolcanic pile

Mobilized slurryin vent/jet

Bedded tephra(Not necessarily to scale)

FIGURE 30.6 Conceptual model for processes in the Surtseyan vent in which shear-instability entrainment and mixing of a slurry of recycled tephra andwater with rising magma drives discrete or continuous uprush activity. Depending on the setting, the wall rock may be loose to consolidated sediment inaddition to buried volcanics. Modified after Kokelaar (1983).




(iv) minor deposits of dilute pyroclastic density currents(PDCs) (Figure 30.7(B)). The lithic-rich lapilli falldeposits contain up to 75 weight% wall-rock clasts andsubordinate rounded dense, cauliflower juvenile lapilli, yetare well sorted but, on grain size criteria alone, thephreatomagmatic units would not be distinguished fromStrombolian ones.

3. SUSTAINED ERUPTIONS

3.1. Characteristics of Sustained“Wet” Plumes

Large-scale wet plumes are distinctive from their drycounterparts in several ways. Classic Plinian (“dry”)eruption plumes produce well-sorted lapilli beds thatbecome progressively finer grained with distance. Coarser,lapilli-sized particles settle more or less individually,allowing them to fractionate by size and density duringatmospheric transport. In contrast, wet plumes form poorlysorted and fine-grained deposits even close to source. Thedisparity arises from two key processes: (1) additionalfragmentation during magma-water interaction leading toincreased ash production, and (2) wet aggregation of ash,which rapidly scavenges fine-grained particles out of theatmosphere. However, a number of complexities existwithin this simplified framework for “wet” versus “dry”volcanism. Even the type examples of wet volcanism on thelargest scale, known as phreatoplinian eruptions, exhibit aspectrum of eruptive styles and behaviors. Here, we usethese type examples of phreatoplinian volcanism to explorethe characteristics of sustained, water-rich plumes.

3.2. Outgassed Magma and Surface Water:Rotongaio, New Zealand

The 1.8-ka Rotongaio Ash, Taupo, New Zealand, has asimilar dispersal and similar whole-deposit grain sizecharacteristics to the preceding Hatepe Ash, formed during

the same eruption, but is different in three key ways. Itsdistinguishing characteristics are exceedingly fine grainsize with close to 100 wt% of the deposit finer than 1 mm, afinely laminated character even in the most proximalexposures, and a lack of lateral continuity for laminaebetween proximal sites.

There is a marked contrast in the dispersal pattern ofrelatively coarse (lapilli-bearing) and relatively fine-grainedsubunits. Coarse-grained packages were significantly morewind advected, and hence have more restricted crosswinddispersals similar to the Plinian phases of the eruption, ash-rich subunits have much more symmetrical patterns ofdispersal around the vent. Concentrically zoned accretionarylapilli are rare in the Rotongaio Ash, but many fall laminaeconsist of partially flattened, structureless, <1- to 2-mmdiameter ash “pellets.” The products of PDCs are, subordi-nate in volume to the fall deposits, confined to proximalareas and are interpreted as the products of relatively dilutesuspension currents that temporarily interrupted fall depo-sition at any particular locality.

The Rotongaio Ash is distinctive as a superb and rela-tively rare example of syneruptive reworking due to localrainstorms precipitated by the volcanic event itself. Adelicate balance apparently existed between accumulation,reworking, and erosion during deposition. Locallyreworked beds are juxtaposed with, or transitional into,primary beds. Individual beds show a complete range oftextures from primary fall through slope-affected materialto water-reworked ash on meter scales. Plastic deformationand thickness variations along outcrop reflect the localtopography. In some cases, ash has clearly flowed off highground as a viscous liquid to produce flat-topped gully fills(Figure 30.8). Plastic deformation and thickness variationsalong outcrop reflect the local topography. These fieldrelationships clearly show that the water reworking wassyneruptive, and a complex and intimate relationshipexisted between primary pyroclast deposition and second-ary reworking from downbursts generated by the wetvolcanic activity.

(A) (B)

West Maar

East Maar

Dilute PDC

FIGURE 30.7 (A) Easterly view of Ukinrek Maars, Alaska, on April 3, 1977, showing black ejecta surrounding the steaming West Maar, and darker,ash-rich phreatomagmatic plume from East Marr. Photograph by Larry Conyers. (B) Photograph looking north at East Ukinrek Maar in vigorousphreatomagmatic eruption April 5, 1977, 16:30 AST. Note the chevron-shaped, dilute PDC. Photograph by Jim Faro. Modified from Kienle et al. (1980).




This clearly shows that such secondary processes weresyneruptive, and a complex and intimate relationship existedbetween primary pyroclast deposition and secondaryremobilization.

The Rotongaio Ash shows conspicuous evidence forprolonged episodicity. Like the Ikedako ash from Ikedacaldera in Japan, and Unit E of the Terra Blanca Jovan tephrafrom Ilopango caldera in El Salvador, it contains numeroussharply definedmillimeter- to centimeter-thickbeds thatwerethe products of numerous powerful but discreteexplosions that gave rise to individual short-lived plumes andnumerous short-lived episodes of deposition. The model forthe eruption involves abundant lake water coming intocontact with magma well after the peak of degassing,transient powerful explosions, and the development ofunstable, short-lived eruption plumes with wide fluctuationsof the water-pyroclast ratio. The Rotongaio Ash has anextremely low wall-rock lithic content, and a predominanceofdenseglassy juvenile clasts. Fragmentation appears to havetaken place after outgassing, and explosivity driven by rapidvesiculation appears to have been unimportant.

3.3. Vesiculating Magma and SurfaceWater: Oruanui, New Zealand

The Oruanui eruption of Taupo volcano, New Zealand (ca.25.4 14C-ka) is one of the largest phreatomagmatic erup-tions documented worldwide. Over a period of weeks tomonths, the eruption produced approximately 530 km3 ofhigh-silica rhyolite (dense-rock equivalent) in 10 majorphases of activity (Wilson, 2001). Each phase producedinterlayered fall and PDC deposits, and is inferred to haveundergone some level of interaction with water. The

following observations are consistent with eruption througha large, long-lived lake:

l Extremely fine-grained volcanic deposits, rich in ashaggregates;

l Entirely nonwelded ignimbrite even where >200 mthick, suggesting relatively low-temperature emplace-ment; and

l Widespread lacustrine sedimentation leading up to theeruption, with ubiquitous incorporation of freshwaterdiatoms (algae skeletons) in the volcanic deposits.

Ejecta are dominated by vesicular pumice and bubble wallshards, consistent with an actively vesiculating magmaticfoam meeting lake water during rapid ascent to the surface.However, the eruption appears to have alternated between“wet” and “dry” phases of deposition throughout, as seen inchanges in the abundance of fine ash and aggregates be-tween units. For example, deposits with a clear phreato-magmatic signature (e.g., units 3, 6, and 8) are dominatedby PDC deposits and associated co-PDC fall layers. Theash aggregates in these units span a wide range of internalmicrostructures, from mud rain to complexly layeredaccretionary lapilli.

An intriguing feature of the wettest phase, unit 3, is areversal of the usual fining trend; overall mean grain sizeis finest near the vent, becoming progressively coarseroutward to approximately 110 km from the source.Similar outward coarsening has been recognized in otherdeposits, including the 1.8-ka Hatepe ash, Taupo, and1991 co-PDC ash from Unzen, Japan, and is attributed toearly fallout of fine ash particles (<250 mm) by vigorous,wet aggregation. Despite their water-rich nature, the wetOruanui units lack the kind of syn-depositional remobi-lization observed in deposits from the Rotongaio Ash,

1

2

3

4

5

6

7Unit

3

4

(A) (B)

(C)

FIGURE 30.8 Stratigraphic log (A) and photographs (B,C) of the products of the 1.8-ka eruption of Taupo volcano. Unit 4, the Rotongaio Ash, isnoticeably fine grained in even the most proximal outcrops (C) and distinctively dark in color (B) due to the abundance of poorly to moderately vesicularpyroclasts derived from outgassed magma.




suggesting somewhat more modest liquid water contentsoverall (less than w30 wt.%, the slurrying threshold forfine ash).

In contrast, units 2 and 5 are similar to “dry” Pliniandeposits, comprising relatively well-sorted lapilli falllayers that become progressively finer away from source.The subordinate amounts of ash aggregates are mainlyweakly bound particle clusters and small, massive ashpellets, reflecting lower water contents in the eruptedmixture overall. These drier units also contain smallerproportions of PDC deposits, indicating a dominantlystable, vent-derived plume (Figure 30.9(A)).

It is interesting that the most pronounced eruptioncolumn instability appears to have occurred during thewettest phases of eruption. The complexly interlayered falland PDC deposits point to a hybrid plume system, arisingfrom both vent-derived and co-PDC clouds for sustainedperiods (Figure 30.9(B)). Given the difficulty of dis-tinguishing emplacement mechanisms on the basis ofgrain size alone (due to their fine-grained, poorly sortednature overall), the microstructures of ash aggregatesbecome a particularly useful tool for these kinds of de-posits. In this regard, the wettest Oruanui units show adistinct lateral transition. Where high-level plumes

interacted with vigorous updrafts from PDCs in theproximal region, deposits contain accretionary lapilli withultrafine outer rims (ash dominantly <10 mm, seeFigure 30.10(B)). Beyond the runout distances of PDCs(>40e80 km from source, depending on the unit), ashaggregates become progressively smaller, less internallycomplex, and land purely as fall deposits (Figure 30.9(B)).Generation of voluminous PDCs is thus inferred to be animportant factor in the growth of large and complex ag-gregates during this style of volcanism.

3.4. Vesiculating Magma andGroundwater: Phase C Askja 1875Eruption, Iceland

The 17-h-long rhyolitic eruption of Askja volcano, Icelandin 1875 was a complex but well-documented silicicexplosive eruption characterized by abrupt and reversibleshifts in eruption style, for example, from “wet” to “dry”eruption conditions, and transitions between fall to PDCtransport and sedimentation (Figure 30.11(A)). We includeit here because the phreatoplinian eruption phase C inter-acted with a very small pond of water fed by groundwater

Minor,dilute PDCs

Fluctuatingheight

Spreading umbrella

Spreading umbrella

(B)(A)

Concentrated& dilute PDCs

Co-PDCash plumes

Lake water

Ash pellets& mud rain

Tropopause

Vent-derivedplume

Accretionarylapilli

Ashpellets

FIGURE 30.9 Cartoon showing two styles of sustained, wet plume development and associated deposits during phreatoplinian volcanism. (A) During anevent like the 1.8-ka Hatepe Ash at Taupo, New Zealand, vent-derived plumes are characterized by fluctuating heights, and dilute PDCs play a relativelyminor role. Ash aggregates fall mainly as structureless pellets or coalesce into vesiculated tuff. In an Oruanui-like scenario (B), voluminous PDCs (diluteor concentrated) significantly affect transport processes in the proximal area, leading to a hybrid eruption column made up of vent-derived and co-PDCplumes. Weakly structured ash pellets that are reentrained (especially into co-PDC plumes) develop a distinctively finer-grained outer rim. This leads to ageneral decrease in aggregate size and structure with distance away from proximal PDCs. Note that wet plumes can significantly propagate upwind due totheir high density.




(A)

ignimbrite

54

6

7

8

21

unit 3

85 km paleosol

(B)

(C)

ignimbrite facies

fall facies

FIGURE 30.10 (A) Deposits of the25.4 ka Oruanui eruption, Taupo, NewZealand, 85 km from the source, showingignimbrite scouring the top of the fall layers.Units 1e8 are labeled on image. Note thescalloped base of units 7 and 8 at this site,caused by ash aggregates impacting intounderlying layers. Dry hand samples showthe matrix-supported ash aggregates char-acteristic of Oruanui ignimbrite (B) andclast-supported ash aggregate structure offall deposits (C).

1 m

C2

C1

D

B

UC1

LC2

MC2

UC2

LC1

(A) (B)

(C)

FIGURE 30.11 Proximal deposits of the1875 Askja eruption, Iceland. (A) Strati-graphic log showing the contrast offine-grained phreatoplinian unit C andcoarse-grained subplinian unit B and Pli-nian unit D, after Carey et al. (2009). (B)Image showing a proximal section for UnitC. The lower fall ash has discontinuouspartings of fine-medium lapilli. The over-lying PDC deposits show indications of aprogressive decrease in the water/magmaratio. (C) Image showing the close-up viewof finely bedded nature of the C1 fall de-posit at a proximal site.




and ice melt runoff rather than a large caldera lake. Thephreatoplinian C phase reflects the interaction of rapidlyvesiculating magma and shallow groundwater, which wascontained within joints and fractures in basaltic lavasinfilling an older Holocene caldera.

The main eruption began with a subplinian dry phase(B), followed by a shift in vent location and the onset of thephreatoplinian fall phase (unit C1). Dilute density currents(unit C2) were emplaced after the phreatoplinian fall (Figure11(B)) but were largely confined to the larger Askja calderaregion. A shift in vent position was then associated with anabrupt change in eruptive style producing Plinian Unit D.The phreatoplinian phase lasted for approximately 1 h,depositing 0.45 km3 of “wet, sticky gray ash.” This C1 falldeposit consists of very fine, pale gray, uniformly massiveash up to 2 m thick (Figure 11(C)), which contains vesiclesin the matrix, but only rare ash aggregates. Lack of finebedding suggests steady, sustained eruption conditions andthe time-averaged mass discharge rate was 7 # 107 kg s%1,and is very similar to those inferred for phreatoplinianphases at Taupo volcano. Grain size analyses conducted bySparks et al. (1981) suggest that 99 wt.% of this deposit isfiner than 1 mm. The fall tephra fall deposits are dominatedby vesicle wall fragments, suggesting that the magma wasactively vesiculating and a foam at fragmentation. Vesiclesize and volume distributions suggest that the processes ofnucleation at depth, decompression, and ascent rates weresimilar between wet (C) and dry (B,D) phases of the 1875eruption, and therefore, water source and availability wereprimary factors influencing the eruption style and plumebehavior. Unit C is widely dispersed to the eastenortheast,extending to Scandinavia where it cannot be distinguishedfrom the Plinian fall. In the proximal area, unit C1 isoverlain by C2 dilute density current deposits. Three majorintervals of PDC deposition occurred, of successively“drier” character.

Transitions between “dry” and “wet” styles were drivenby shifts in vent position, permitting movement of the ventin and out of groundwater. The shift between C1 and C2 isinferred to be a function of the reduced availability ofwater, and vent widening, decreasing the exit velocity ofthe jet.

4. DISCUSSION

4.1. “Ash Aggregation” in “Wet” Plumes

One of the key ways in which external water affects theairborne transport of volcanic products is by promotingaggregation of ash. Aggregation presents a challenge totraditional deposit characterization technique of grain sizeand componentry. Grain size analysis typically ignores anyaggregated clasts that were originally present on deposi-tion, due to unintentional breakup during sievingda

practice that obscures the sizes and types of the particles asthey originally landed.

Investigating how and when aggregation occurs iscrucial to our understanding of plume dynamics, particu-larly from “wet” eruptions. In the simplest sense, aggre-gation takes place whenever attractive forces betweenparticles overcome the dispersive forces breaking themapart. However, there are factors at a range of length scalesthat influence aggregate growth and structure: particlebinding and cementation processes (at the scale of mi-crons), volcanic plume dynamics (meters to kilometers),and interaction with large-scale weather patterns (hundredsof kilometers).

On the particle scale, binding forces operate in twodistinct categories: hydrostatic and electrostatic. Weak, butlong-range electrostatic attraction is thought to be omni-present in volcanic plumes, forming fragile, porous clus-ters akin to dry, snowflakes. This mechanism mainlyaffects particles <63 mm in the absence of a liquid phase.In contrast, wet aggregation dominates when liquid isabundant (more thanw5% by weight in the case of water).The particles adhere by hydrostatic forces due to cohesionof water molecules, forming liquid bridges between grains.In general, increasing the amount of liquid waterinvolvement leads to larger, denser aggregates that traplarger grains, although fine ash <250 mm tends to be moststrongly affected. Once individual particles have beenbrought into close range by these binding forces, morepermanent cementation occurs by precipitation of salts outof solution, ice formation, and perhaps by the products ofglass alteration.

On the scale of dynamic plume processes (meters tomany kilometers), the evolving style of aggregationresponds to two factors: (1) availability of liquid waterand (2) presence of turbulent updrafts. There is someindication that in water-rich eruptions, wet aggregation inthe eruption column and near-source umbrella cloud canoccur extremely rapidly, within minutes to tens ofminutes. In these cases, cyclic collection, freezing andremelting of ashy water droplets contribute to aggregategrowth during moist convection. There is also a complexinterplay between rapid, wet growth in warm regions ofthe plume, and weaker, electrostatic aggregation else-where. This is reflected in the detail of aggregate struc-tures; the most complexly layered types representmultiple stages of growth by recycling through warm,moist updraftsdespecially those rising from ground-hugging PDCs (Figure 30.9(B)). Growth of the outer-most ultrafine rim (rich in <10-mm ash) seen on manyaccretionary lapilli is thought to occur during final pas-sage through a low-level cloud of elutriated, co-PDC ash,which may or may not still be connected to a laterallymoving density current (Brown et al., 2010; Van Eatonand Wilson, 2013).




On the largest scale of continental or global transport,aggregation becomes primarily a meteorological phenom-enondin other words, controlled by background weatherpatterns. In these distal regions, well downwind of theeruption column, volcanic particles serve as nucleationsites for precipitation processes, and are eventually scav-enged out of the atmosphere by rain, snow, or as delicate,electrostatically bound clusters. These processes generatedistal fall deposits with a low-density, porous texture.

4.2. Models for “Wet” Plumes

Volcanic plumes are remarkably sensitive to the addition ofexternal water. Glacial melting or water flashing to steamfundamentally redistributes the energy in a rising column ofparticles and gases. Although an influx of external watercools the erupting mixture, it does not necessarily producea sluggish, low-level plume. In fact, “phreatoplinian de-posits” are among the most powerfully dispersed deposits(Self and Sparks, 1978), leading to the apparent paradox ofwidespread dispersal despite cool, water-rich, and oftenunstable plume behavior.

The key factor here is that water vaporization transfersan enormous amount of energy to the gas phase, where it isstored as latent heat. As soon as the erupted mixture ex-pands outward and entrains enough cool, ambient air forcondensation (and eventually freezing) to occur, that storedenergy is released, leading to rapid expansion of the cloud(Koyaguchi and Woods, 1996). Other phase changes ofwater provide sources and sinks of energy and buoyancyduring the lifetime of a volcanic plume. This process,known as moist convection, is characteristic of conven-tional thunderstorms, with the key difference being thatvolcanic clouds usually start with more thermal energy andmomentum, allowing them to puncture the tropopause andascend into the stratosphere (Van Eaton et al., 2012).

Early one-dimensional (1D) models of wet plumesconcluded that the most intense water-rich eruptions(e.g., Oruanui) were likely to collapse before condensa-tion could occur, along with its boost in buoyancy, therebyproducing low-temperature PDCs confined to lower levelsof the atmosphere (Koyaguchi and Woods, 1996). How-ever, more recent 3D multiphase models accounting formicrophysical processes show that ash plumes can risevigorously from these ground-hugging flows (Van Eatonet al., 2012). Convective columns of fine ash feed into, andmerge with, the main jet issuing from vent, producing ahybrid system that expands outward as a powerfullyspreading umbrella cloud (Figure 30.9(B)). Such “dirtythunderstorm” dynamics, powered by moist convection,provide an overall picture that is more consistent with theobservational evidence for powerful dispersal in the mostintense wet eruptions. Interestingly, this redistribution ofenergy helps delineate wet and dry behavior in another

way. While dry, plinian eruption columns tend to have thehighest vertical velocities near the vent, where energy andmomentum are the greatest, unstable, water-rich plumesexperience a delayed onset of the maximum verticalvelocities, once rapid growth during moist convectionoccurs some distance vertically (and laterally) away fromthe vent.

Althoughwet plumes containingwater contents of>15%aremore unstable and prone to collapse than are dry ones, thereality is that water involvement does not have a simple,predictable effect on eruption style. Three-dimensionalnumerical models suggest that changing the external watercontent of the erupted mixture by even a few weight percentproduces nonlinear effects in the dynamic system that cannotbe categorized in a simpleway (Koyaguchi andWoods, 1996;VanEaton et al., 2012). However, in a broad sense, increasingwater involvement does seem to increase the likelihood ofrapid oscillations between Plinian-style (buoyant) ascentand partial to wholesale column collapse (Figure 30.9(B)).This leads to overprinting of multiple transport levels;coarse ash and lapilli are transported farther duringstable phases of eruption, yet deposited closer to sourceduring periods of fluctuating or lower plume heights(Figure 30.10(A)). Further, in cases where moist convec-tion dominates the plume behavior, a substantial percent-age of erupted mass stays in the troposphere (like athunderstorm), where a temperature inversion inhibits therise of weaker portions of the eruption column. Here, theplume spreads outward radially as a gravity current. Thiscombination of fluctuating, multilevel transport andupwind/crosswind expansion at the tropopause (Figure30.10(A) and (B)) may contribute to the more radiallysymmetric distribution of wet eruption deposits aroundtheir volcanic source areas.

4.3. Role of the Vent and Shallow Conduit

Vent/conduit wall properties: The walls of the vent allow,in one or more ways, external water to encounter magmaand contribute to initial and/or secondary fragmentationand eruption. The simplest way in which this happens,exemplified by activity at Surtsey, Iceland, or Taupo,New Zealand, is for surface water to occupy or flowdirectly into the vent during eruption. In such a situation,the first phreatomagmatic explosions take place byinteraction of magma with some small part of an effec-tively unlimited supply of clear ocean, river, or lakewater, and the ejecta consist of juvenile pyroclasts andwater. Under these circumstances, the form of the ventand shallow conduit strongly influences the geometry ofthe interaction.

Many subaerial phreatomagmatic eruptions begin underdifferent circumstances than those of subaqueous vents. Inparticular, there is no body or sizeable domain of water




with which magma can interact. Instead, magma encoun-ters interstitial water in fractures of the surrounding wallrocks, or contained within a porous and permeable frame-work in these deposits. Two end-member modes of inter-action can be described for this situation: (1) water flowsthrough pores or fractures in the wall rock to interact withmagma; (2) the wall rock and contained interstitial waterare disrupted, and together interact with magma as wetblocks or an impure fluid.

In many explosions, the yield strength of the shallowwall rock is negligible, and the primary grain size ofvent-wall material has a strong effect on the grain size ofthe wall-rock lithic clast population of the resultingpyroclastic deposit. In particular, if there is abundantash-sized wall-rock material, its inclusion (often withoutany modification to its size distribution by the explosions)can lead to a misleadingly high estimate of the efficiencyof fragmentation. Surtsey exemplifies another type ofwall instability, in that the fragmental deposits formedearly in the eruption form the walls of the vent later in theeruption. In this situation, the vent is occupied by a slurryof seawater and tephra that sloughed and slid back intothe vent (Figure 30.6), which strongly affects eruptionstyle.

Vent geometry: The geometry of the vent will change asan eruption proceeds, and this may feed back to modify thestyle of the eruption. Cone growth will influence the surfacehydrology of the vent, and in general act to restrict theingress of water. Growth of the cone or tuff ring may alsopermit magma to “pond” in the growing vent at levelsabove the water table, bringing an end to “wet” phreato-magmatic fragmentation. Vent-wall collapse is moreimportant in phreatomagmatic than in dry magmaticeruptions. Changes of vent geometry, such as collapse ofthe inner walls of the growing tuff cone or ring, may pro-mote ongoing magma-water interaction or may temporarilyblock the vent leading to episodic vent-clearing explosions.Alternatively, vent widening as a result of shallow collapsemay promote a change to wetter explosions if surface watercan pond in the crater.

Recycling of clasts is a process strongly influenced byvent geometry. Most pyroclastic deposits contain bothfirst-cycle juvenile clasts, derived from magma at theinstant of eruption, and recycled juvenile clasts that werefragmented in earlier explosions but then fell or collapsedback into the vent. The recycled clasts are similar to wall-rock lithic clasts in that they contribute no heat to furthermagma/water interaction, but they can be difficult toseparate from “first-cycle” juvenile clasts. Texturalfeatures, such as a combination of uneven mud coatingand a mixture of fluidal and angular faces on many clasts,indicate that they were fragmented at least twicedoncewhile the magma was fluid and subsequently in a brittlestate.

4.4. Role of the Magma in Wet Volcanism

As in all volcanic eruptions, magma is the source of energyfor phreatomagmatic explosions. Many phreatomagmaticevents are modifications of explosive eruptions that wouldhave occurred even in the absence of external water,essentially the phreatomagmatic equivalents of Hawaiian,Strombolian, subplinian, and plinian eruptions. Exceptionsare the equivalent of extrusive events, such as the RotongaioAsh, which in the absence of water would have erupted onlylava, or Kilauea 1924, which prior to the explosive eruptionwas an actively receding lava lake. In interpreting the styleand products of phreatomagmatic eruptions, it is importantto consider the influences of physical properties of themagma and the “background” magmatic processes (cooling,vesiculation, crystallization, etc.).

Intrinsic properties of the magma include temperature,viscosity, dissolved volatile content, vesicularity, andcrystallinity. During fragmentation, thermal energy istransferred to the external water phase (latent and sensibleheat) and/or is converted to seismic and acoustic energy,fragmentation energy, and kinetic energy. This energypartitioning has major influences of the form of thevolcanism. Preeruptive magma (and water) temperaturesinfluence the heat transfer rates and the temperatures in theeruptive jet and/or plume. The absolute amount of heatavailable is clearly also a factor.

Magma viscosity is strongly dependent on chemicalcomposition, volatile content, temperature, and crystalcontent. An increase in viscosity will retard the mixing ofmagma and water, yet, by slowing the ascent of magma, itmay prolong the opportunity for contact between the twophases. The magmas described in this chapter show a rangeof viscosity at liquidus temperatures from few tens ofPascal seconds, in the case of basalts, to 107 to 109 Pa-s forsilicic magmas. A 200 "C decrease in temperature willproduce a 10e100 times decrease in viscosity. Viscosity ofthe magmatic phase at the time of fragmentation mayreflect both lowered temperatures as a result of coolingduring ascent and early interaction with the conduit wallsand groundwater, and the effects of degassing and microlitecrystallization that accompanied ascent. Vesiculation alsoincreases melt surface area while reducing the density ofthe bulk melt phase and may therefore promote better ge-ometry of mixing.

Dynamic properties of the magma include ascent rate,discharge velocity and steadiness, and extent of vesicu-lation and outgassing. Processes of magma ascent closeto the fragmentation zone have a major influence on theensuing style of phreatomagmatism. The extent ofdegassing and microlite crystallization is strongly influ-enced by the rates of ascent in the deeper conduit, anddegassing and crystallization and, in turn, influencesmagma rheology and rise rate at shallower levels.




A magma that rises rapidly and continuously to the sur-face is likely to undergo most of its vesiculation atshallow levels and be relatively fluid, hot, and homoge-neous at the point of contact with external water. Rela-tively simple phreatomagmatic eruption styles result, astypified by most phreatoplinian eruptions. If magma as-cends in a staged fashion and/or subsequently stagnatesin the shallow conduit, then at least some portion of thismelt will have undergone quenching, and outgassing,often becoming physically and thermally heterogeneous.Complex magma-water interaction and phreatomagma-tism often result.

4.5. Role of the Preeruptive State of Water

The availability of water or other external fluid has a first-order influence on the style of phreatomagmatism. Relativeproportions of water and magma, and the efficiency withwhich they mix, determine the thermodynamic states ofwater phases in phreatomagmatic eruptions. If very littlewater interacts, it can be heated to high temperatures, andhigh pressures if confinement is available, but its expansionto vapor can fragment only a relatively small volume ofmagma (Sheridan and Wohletz, 1981). Most of the work offragmentation is done during the volume increase fromliquid water to superheated fluid, rather than by subsequentexpansion in the vapor phase. Alternatively, if the externalphase is already steam (see below), then its ability toexpand on further heating, and thus drive fragmentation, ishighly limited. At the other extreme, water is presentbeyond the mixing domains, and is ejected or slowly boiledduring post-fragmentation expansion; this water contrib-utes nothing to magma fragmentation. At intermediate ra-tios of interacting magma and water, the available heat iscapable of vaporizing all the water, thus resulting in effi-cient magma fragmentation.

Avariety of relatively shallow influences can disturb thesupply of external water or the effectiveness of mixing andinteraction, so most wet eruptions are characterized byabrupt changes in the proportions of water and magma andhence eruptive behavior. The simplest example is the for-mation of a deposit that excludes influx of surface water;thus, many phreatomagmatic eruptions end with “dry”phases. In models and some field situations, the fluidinvolved in magma-water interactions is a liquid initially atambient temperatures. However, phreatomagmatic erup-tions commonly also include examples of magma con-tacting one or more of

1. vapor-dominant geothermal fluid at temperatures up to800 "C,

2. superheated geothermal liquids at temperatures of200e250 "C,

3. snow or ice,

4. impure fluids in which sediment particles are suspendedin water,

5. vent slurries with significant yield strengths.

Where magma intrudes a preexisting hydrothermal system,the water phase prior to interaction may be vapor, two-phase vaporewater mixtures, or hot water. Vapor (drysteam) is a minor contributor to explosivity because themajor volume change is that of vaporization, and subse-quent (super)heating causes only a minor volume increase.The state of water most effective in forming phreato-magmatic explosions is superheated liquid water; it willundergo large volume changes upon vaporization, whichcan be accomplished by small additions of heat frominteraction with magma and by subsequent disruption ofthe hydrothermal system.

Water involved in the interaction typically containssuspended particles (sediment or tephra) or dissolved spe-cies (e.g., salt in seawater, or silica and chlorine in hydro-thermal fluids). These impurities change the physicalproperties of the water phase and introduce nonvaporizablecomponents. Depending on the concentration of impuritiesin the water, its viscosity may increase up to an order ofmagnitude, the density may double and the heat capacitymay decrease by 25%. Precipitation of dissolved ionsduring vaporization absorbs energy.

There is a subtle distinction that could be made between“wet” and phreatomagmatic eruptions. In the latter, it isassumed that fragmentation is induced or enhanced by theflashing of water to steam during the magma’s ascent to thesurface. However, water can also become involved afterfragmentation. A challenge therefore exists in relating clearevidence of wet deposition to a case for magma-waterinteraction in the vent. Larger eruptions have an opportunityto interact with external water well beyond the vent area,including large-scale weather systems. While there areplenty of wet eruptions for which the source of externalwater is known or reasonably inferred, others are not soclear cut, including the preclimactic eruptions of Pinatubo1991. The source of external water remains cryptic in thesecases, despite the noted role of moist convection and pre-cipitation processes. Might some unconventional pathways,such as the incorporation of wall rock derived from a water-saturated volcanic edifice, or vaporization of rivers, streams,and moist vegetation by hot PDCs, deliver sufficient waterto induce particle aggregation? Further work is required tolearn how and when these mechanisms play a role in blur-ring the line between wet and dry plume behavior.

4.6. Heterogeneity of Magma-WaterMixing and Thermal Efficiency

The extent/efficiency with which magmatic heat can beused during fragmentation and transport is important; this




is strongly influenced by a number of processes describedabove. The unifying characteristics of phreatomagmaticeruptions are the involvement of external water andheterogeneity of eruption processes and products. Inparticular, many of the case studies above illustrate howheterogeneous heat transfer can be.

Dry magmatic eruptions (Strombolian, Hawaiian, etc.,see Chapter 27) involve two phases, the silicate liquid and agas/vapor phase, which we can often treat as thermally,compositionally, and physically homogeneous. In wetsystems, the starting point is magma at high temperature incontact with a source of external water and any materialthat contains or confines the water, both of which are atmuch lower, commonly ambient temperatures. The endproduct is a mixture of juvenile particles, magmatic gas,steam, wall-rock particles, and often liquid water(Figure 30.1). Both the starting ingredients and the endproducts are heterogeneous (Figure 30.11).

Total mixing is rarely achieved, and a wide temperaturerange often characterizes the products of a single smallexplosion (i.e., it is not uncommon for both liquid waterdroplets and steam to be present in the eruption and for coldand hot clasts to be ejected simultaneously). The inescap-able conclusion from this observation is that, in many ex-plosions, some water is ejected that is never in significantcontact with magmatic heat and some magma never in-teracts with external water. For larger sustained eruptions,thermal and physical mixing is often more complete. butrapid and reversible temporal shifts in the magma-waterratio are recorded, and usually ascribed to changing ventlocations, construction of tephra barriers, or grounddeformation (inflation/deflation).

4.7. Concluding Thoughts

We have a clear need for a more comprehensive set ofcriteria to define phreatomagmatic eruption styles. This isone of two great challenges remaining; the other iscomprehensive modeling of the thermodynamics of erup-tions of the types described in the case studies above.

FURTHER READING

Brown, R.J., Branney, M.J., Maher, C., Davila-Harris, P., 2010. Origin of

accretionary lapilli within ground-hugging density currents: evidence

from pyroclastic couplets on Tenerife. Geol. Soc. Am. Bull. 122,

305e320.

Carey, R.J., Houghton, B.F., Thordarson, T., 2009. Abrupt shifts between

wet and dry phases of the 1875 eruption of Askja Volcano: micro-

scopic evidence for macroscopic dynamics. J. Volcanol. Geotherm.

Res. 184, 256e270.

Fee, D., Garces, M., Patrick, M., Chouet, B., Dawson, P., Swanson, D.,

2010. Infrasonic harmonic tremor and degassing bursts from

Halema’uma’u Crater, Kilauea Volcano, Hawaii. J. Geophys. Res.

115, B11316,. http://dx.doi.org/10.1029/2010JB007642.

Hamilton, C.W., Thordarson, T., Fagents, S.A., 2010. Explosive lavad

water interactions I: architecture and emplacement chronology of

volcanic rootless cone groups in the 1783e1784 Laki lava flow,

Iceland. Bull. Volcanol. 72, 449e467.

Houghton, B.F., Smith, R.T., 1993. Recycling of magmatic clasts during

explosive eruptions: estimating the true juvenile content of phreato-

magmatic volcanic deposits. Bull. Volcanol. 55, 414e420.

Kienle, J., Kyle, P.R., Self, S., Motyka, R.J., Lorenz, V., 1980. Ukinrek

Maars, Alaska, I. April 1977 eruption sequency, petrology and tec-

tonic setting. J. Volcanol. Geotherm. Res. 7, 11e37.

Kokelaar, P., 1983. The mechanism of Surtseyan volcanism. J. Geolo.

Soc. London 140, 939e944.

Kokelaar, B.P., 1986. Magma-water interactions in subaqueous and

emergent basaltic volcanism. Bull. Volcanol. 48, 275e289.

Koyaguchi, T., Woods, A.W., 1996. On the formation of eruption columns

following explosive mixing of magma and surface water. J. Geophys.

Res. 101, 5561e5574.

Mattox, T.N., Mangan, M.T., 1997. Littoral hydrovolcanic explosions: a case

study of lavadseawater interaction at Kilauea Volcano. J. Volcanol.

Geotherm. Res. 75, 1e17.

Patrick, M., Wilson, D., Fee, D., Orr, T., Swanson, D., 2011. Shallow

degassing events as a trigger for very-long-period seismicity at

K!ılauea Volcano, Hawai’i. Bull. Volcanol. 73, 1179e1186.

Self, S., 1983. Large-scale silicic phreatomagmatic volcanism: a case

study from New Zealand. J. Volcanol. Geotherm. Res. 17, 433e469.

Self, S., Sparks, R.S.J., 1978. Characteristics of widespread pyroclastic

deposits formed by the interaction of silicic magma and water. Bull.

Volcanol. 41, 196e212.

Sheridan, M.F., Wohletz, K.H., 1981. Hydrovolcanic explosions: the

systematics of waterdpyroclast equilibration. Science 212,

1387e1389.

Sparks, R.S.J., Wilson, L., Sigurdsson, H., 1981. The pyroclastic deposits

of the 1875 eruption of Askja, Iceland. Phil. Trans. R. Soc. London

Ser. A 299, 241e273.

Swanson, D., Wooten, K., Orr, T., 2009. Buckets of ash track tephra flux

from Halema’uma’u Crater, Hawai’i. Eos Trans. Am. Geophys.

Union. 46, 427e428.

Thorarinsson, S., 1967. Surtsey. The New Island in the North Atlantic.

The Viking Press, New York, pp. 47.

Van Eaton, A.R., Wilson, C.J.N., 2013. The nature, origins and distribu-

tion of ash aggregates in a large-scale wet eruption deposit: Oruanui,

New Zealand. J. Volcanol. Geotherm. Res. 250, 129e154.

Van Eaton, A.R., Herzog, M., Wilson, C.J.N., McGregor, J., 2012. Ascent

dynamics of large phreatomagmatic eruption clouds: the role of

microphysics. J. Geophys. Res. 117, B03203. http://dx.doi.org/

10.1029/2011JB008892.

Walker, G.P.L., 1981. Characteristics of two phreatomagmatic ashes and

their water-flushed origins. J. Volcanol. Geotherm. Res. 9, 395e407.

White, J.D.L., 1991. Maar-diatreme phreatomagmatism at Hopi Buttes,

Navajo Nation (Arizona), USA. Bull. Volcanol. 53, 239e258.

Wilson, C.J.N., 2001. The 26.5 ka Oruanui eruption, New Zealand: an

introduction and overview. J. Volcanol. Geotherm. Res. 112,

133e174.




http://dx.doi.org/10.1029/2010JB007642

http://dx.doi.org/10.1029/2011JB008892

http://dx.doi.org/10.1029/2011JB008892

Phreatomagmatic and Related Eruption Styles

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