Australia: The Land Where Time Began
Tangaroan Eruptions - Submarine Volcanic Eruption of Highly Vesicular Pumice - Foam
Highly vesicular pumice deposits are found on many submarine caldera volcanoes that has typically been attributed to explosive activity that is especially vigorous (Allen & McPhie, 2009; Fiske et al., 2001; Fiske et al., 1998; Kano, 2003). The authors1 suggest the relating of specific styles of eruption to volcanic products is difficult (Arculus, 2011; Sigurdsson, 2000). In this article they document the vesicularity and textural characteristics of pumice clasts that they dredged from the surface of Macauley Volcano off the coast of New Zealand, a volcano in the Kermadec Arc, southwest Pacific Ocean. The resulting clasts display a modal distribution, with corresponding differences in abundances and shapes of vesicles, specifically with a sharp mode at a vesicularity of 91 %, and at 65-80 %, a broad mode. The authors1 attribute bimodality to an eruptive style, that has previously been undocumented, that is neither effusive nor explosive. The rate of eruption is not high enough to cause the magma to explosively fragment, though too high to feed a lava dome, instead, the magma foam detaches buoyantly at the vent, rises as discrete parcels of magma, or blebs, while internally it continues to vesiculate. These blebs either disintegrate or become waterlogged only after being distributed widely by ocean currents. The individual clasts, from rim and interior fragments, that result from disintegration, yield the bimodal vesicularity characteristics. The authors1 concluded that highly explosive activity is not required to generate and disperse widely in the marine environment pumice that is highly vesicular.
There is a first-order distinction between explosive and effusive volcanic activity in volcanism that is subaerial, with the explosive type being subdivided into specific styles of eruption, such as Plinian and Vulcanian, etc., that are linked to characteristics of clast and deposit, that can be quantified for eruptions of the past (Sigurdsson, 2000). Though the first-order distinction also applies to the subaqueous eruptions, a factor that is unique to subaqueous eruptions is that, whether by effusive or explosive activity, any material generated may have enough buoyancy to float in or on the water column as a result of its vesicularity, before it becomes waterlogged and sinks, to be deposited on the sea floor. Studies of uplifted deposits (Allen & McPhie, 2009), in situ dredge and remotely operated vehicles are used for investigations (Fiske et al., 2001), as well as reports of floating pumice rafts (Fiske et al., 1998), are used in studies of the fundamental volcanism distinction between volcanic activity that is effusive and activity that is explosive, the fundamental distinction between the 2 types of eruption being applied widely to the products of subaqueous volcanism. According to the authors1 the endmember regimes, for silicic magmas in particular, are: driven by exsolution of magmatic volatiles (Allen & McPhie, 2009; Fiske et al., 2001; Fiske et al., 1998; Kano, 2003), which cause them to be vigorously explosive, and dome-building, that varies from wholly effusive to partly explosive ( Kano, 2003; Allen, Fiske & Tamura, 2010). Insights into the processes (Houghton & Wilson, 1989; Klug, Cashman & Bacon, 2002; Houghton et al., 2010; Spieler, 2004) of vesiculation and fragmentation in subaerial erupted clasts are provided by the use of density spectra and vesicularity texture, though these methods have yet to be widely applied to subaqueous deposits.
Vesicular pumice (about 70-85 % vesicles) (Houghton & Wilson, 1989; Sparks, 1978), with vesicular textures that are evenly developed, similar bubble size distribution (BSD) and a narrow range in bubble number densities (Klug, Cashman & Bacon, 2002) (BDN), is characteristically produced by exsolution of magmatic volatiles that drive explosive subaerial silicic eruptions. Comparably vesicular pumice in deposits on the seafloor are similarly interpreted in terms of their style of eruption (Allen & McPhie, 2009; Fiske et al., 2001) In both subaerial and subaqueous silicic lava domes, material forming the pumiceous carapace, though sometimes vesicular to moderate to high degrees, inevitably displays evidence of outgassing and accompanying crystallisation (Blundy & Cashman, 2005). When such domes are disrupted by explosive activity (Vulcanian) in a subaerial environment fragments are produced that have a high crystal content, extensive degassed-bubble textures, vesicles with thick walls, and many microlites (Houghton et al., 2010) The authors1 applied the tools such as clast density (Houghton & Wilson, 1989) and analysis of the texture of vesicles (Shea et al., 2010) to the pyroclasts that were dredged from Macaulay Volcano, Kermadec Arc. Their results suggest the distinctive features of these pyroclasts are inconsistent with an origin by either endmember eruptive style.
Macauley Island, with an area of 3 km2, is the uppermost section of Macauley Volcano, most of which is below the surface. The caldera of the volcano, about 8x11 km has been interpreted (Lloyd et al., 1996) to have been formed by the eruption of more than 1-5 km3 of magma at 6.1 14Ckyr, with the deposition of 100 m of ignimbrite, Sandy Bay Tephra (SBT), on the island. The authors1 collected material from the subaerial SBT, and dredged clasts from 8 sites on the submerged flanks of the volcano, that were originally believed to be from the same eruption that produced the SBT, It was subsequently shown by biochemical data from these clasts (Barker et al., 2012) that in the past there had been other felsic eruptions, not only at Sandy Bay17-19,(Barker et al., 2012; Smith et al., 2003; Barker et al., 2012) that had contributed to the layer of seafloor surficial pumice that had been sampled.
The results from this study have shown the pyroclasts from the SBT to have a density spectrum that is narrow, with a dominant mode, 0.42 gm/cm3, that is consistent with explosive magmatic fragmentation (Cashman et al., 2000) as seen at many subaerial sites around the world (Houghton & Wilson, 1989; Mueller et al., 2011) A contrasting density distribution has been found for the pyroclasts that were dredged, with a narrow mode at 0.20 g/cm3, and a broad mode and tail that is more than 0.5 g/cm3. Low density mode clasts are rounded, are highly vesicular, with an evenly developed foamy appearance (Barker et al., 2012) Clasts from the tail, that are of higher density, are dominantly sub-angular and blocky, with prismatic joint faces and vesicles that are stretched to oblate ellipsoids that are perpendicular to jointing. Continuous gradients, from pumice that is denser blocky to foamy vesicular, are shown by about 5-10 % of clasts ('gradient clasts'), over distances that are as short as 2-3 cm, though with glass chemistry that is homogeneous. The dredged pumices show features unusual in, or absent from, subaerial silicic pumices, but in their distribution of density and appearance.
Vesicles at the denser, blocky end are elongated parallel to the clast margin that are thick, ≤ 50 μm, glass that are microlite-free, are disclosed by back-scattered electron images of gradient clasts. Corresponding BSD modes and BND values are similar to those in clasts of similar bulk composition and density that had been erupted at Raoul volcano subaerially. Vesicularities increase by up to 94 % towards the foamy ends of gradient clasts, and bubbles become almost spherical and their walls thin to about 1 μm. Also, the BSD mode shifts to larger sizes, greater than the typical of subaerially erupted pumices (Klug, Cashman & Bacon, 2002; Houghton et al., 2010; Shea et al., 2010), including vesicular clasts from Raoul that are comparable. From the blocky end to foamy regions, that has been interpreted to reflect the continuing coalescence and growth that occurs under isotropic conditions of stress in the absence of shearing and bubble distortion. In individual gradient clasts regions of higher and lower density correspond to textures in clasts that are homogeneous from the high- and low-density modes, with similar BND values and BSD modes. The authors1 thus infer the homogeneous clasts to be derived from the fragmentation of bigger clasts which had gradients that developed on a scale larger than the sampled pieces - decimeter to metre scale.
The implication of the density and texture ranges in the dredged Macauley pumices is that they were produced, and consequently dispersed, by different mechanisms than the mechanisms involved in endmember effusive or explosive styles that have been prosed for subaqueous eruptions. According to the authors1 2 different eruptive styles cannot be reflected in the bimodality of density spectra and textures of vascularity, as these contrasts occur within individual gradient clasts. The clasts of lower density have homogeneous textures and small ranges in BND and BSD values, which differs from pyroclasts from deposits at Macauley and Raoul that are subaerial that fragmented explosively. The higher-density blocky clasts are precluded from representing magma carapace14 that was disrupted explosively (Vulcanian), that shows degassed bubble textures, both rinds and interiors have lower vesicularities and higher microlite contents, which are indicative of magma stalling and loss of gas before fragmentation (Blundy & Cashman. 2005) The microlites and the distorted and deflated bubble shapes that are observed in blocks floated from dome carapaces that are extruded from beneath lakes at both Taupo (Houghton et al., 2010)and Raoul (Barker et al., 2010) and contain low phenocryst contents, less than 5 %.
A proposed new type of eruption
The authore1 have proposed a new, distinctive style of eruption, that is distinct in several key features. They infer that in this style the magma doesn't fragment within the conduit, as is the case in submarine neptunian eruptions (Allen & McPhie, 2009) instead reaching the seafloor as a coherent foam22 that is vesiculating. In this type of eruption the lava is sufficiently vesicular to be positively buoyant at the vent instead of forming a lava extrusion. Blebs of bubbly magma form and rise, as a result of buoyancy forces, by either viscous detachment or by the formation of cooling joints, as magma with a vesicularity of 60 %, that is represented by the high-density blocky clasts, has a density of 0.95 g/cm3. Subsequently, the buoyant blebs disintegrate to form the sampled clast population, though this process may be delayed by a steam sheath23 that is insulating and the impermeability of the high-density material, that has isolated bubbles and thick glass walls. The interiors, that are still molten, continue to vesiculate as a result of the decreasing hydrostatic pressure, as the blebs rise towards the ocean surface. Just inside the quenched margin the bubbles are stretched parallel to the outer margin of the bleb and glass is still plastic, yielding the oblate vesicle texture, though the bubbles in the interior grow and coalesce to more than 90 % vesicularity under conditions of isotropic stress in the absence of shearing, to form homogeneous foamy textures. When water enters the bleb through cooling joints and/or inflation cracks, these processes are halted that causes it to disintegrate, or if the bleb cools slowly and the water in drawn in more gradually until it becomes waterlogged and sinks. Fragments that are widely dispersed, tens of kilometres or more, of the interiors that are foamy and exteriors that are quenched yield the bimodal clast density distribution, that have the corresponding textures and broad range of chemistry at single dredged sites (Barker et al., 2012; Barker et al., 2012 (2)).
According to the authors1 it is inferred that this novel eruptive style reflects intermediate rates of eruption in which magma vesiculates to a state where its buoyancy in water is the primary control on its behaviour as it emerges from the vent. Fragmentation of magma within the conduit will dominate at higher rates of eruption as bubble expansion tears the magma apart, neptunian eruption style (Allen & McPhie, 2009), though the result of this is the inevitable distortion of the vesicle textures over a broad size range of bubbles, which is a reflection of shear strain that is imparted in the conduit. The magma will generate domes or lava flows at lower rates of eruption, that are similar to those on land, accompanied by open-system degassing to generate microlites and vesicle textures that are distinctive in their pumiceous carapaces, from which fragments also detach and float7,10. The authors1 suggest that this eruption style that is unique to subaqueous volcanism, not fitting into either of the pre-existing styles of explosive or effusive regimes, requires the combination of intermediate rates of eruption and water as an overlying substrate.
The authors1 suggest this volcanism style be named Tangaroan, after the research vessel that was used to collect the samples, which was itself named for the Maori god of the sea, given the distinctive characteristics of the pyroclasts from the Macauley Volcano and their inferred origin. Tangaroan clasts can be dispersed widely, as detached blebs will be transported in the heated, buoyant water plume above the vent and at the surface of the sea. When the blebs disintegrate at or below the surface the fragments can drift further as they settle through the water column. The characteristics of the deposit from an eruption may therefore vary widely as a result of the diversity of possible events, possibly including the timing of when/if the blebs quench and disintegrate, the speed and direction of plumes and currents that transport the fragments, and the amount of seafloor reworking, syn- and post-depositional, of the near-neutrally buoyant clasts that are waterlogged. Texturally distinctive clasts may be contributed to a wide variety of subaqueous volcaniclastic deposits, with no single facies geometry that is unique, volume of distribution or range of thickness that corresponds to the style, by the Tangaroan style of eruption.
The authors (Allen & McPhie, 2009) infer that the Tangaroan style of eruption has been recorded or observed in modern aqueous eruptions, though its general significance as a source of submarine pyroclasts has not been recognised, partly as a result of clast-specific data being lacking. There are several silicic examples that have been found, such as West Rota Volcano, Marianas Arc, where rhyolite 'pumice balloons', that are near-neutrally buoyant, on a metre-scale, that have interior cooling joints that are radial, have been observed (Stern et. 2008). In the San Augustin deposits of Ilopango Caldera, El Salvador (Mann et al., 2004), dacite-rhyolite pumices also display evidence that cooling was rapid (joint and quenched exteriors, wide ranges of vesicularities and textures, intermediate rates of eruption and passive clast flotation away from the vent. In the Shin-Iwojima eruption, Japan, of 1934-1935, swarms of rhyolite pumice blocks of metre size rose to the surface of the sea in swarms. Overall this eruption was not violent, which allowed ships to move through the pumice raft (Fiske et al., 1998) that was near the vent. As proposed here by the authors1 for silicic composition, they suggest Tangaroan-style processes could apply to other compositions of magma. The authors1 suggest examples of basaltic eruptions may have occurred near Socorro Island, Mexico (Siebe et al., 1996), and Terceira, Azores (Gaspar et al., 2003; Kueppers et al., 2012), when ellipsoidal 'basalt balloons', that were highly vesicular to hollow, and up to 3 m in diameter, rose to the surfaces in pulses that were at irregular intervals.
The transport and deposition of pyroclasts, both blebs and their disintegration produced fragments, over significant distances is enabled by the distinctive Tangaroan style of eruption under subaqueous conditions. The authors1 suggest the fragments may contribute to the formation of sheets, not necessarily or exclusively forming cones, volcaniclastic deposits in a style of eruption that doesn't require the eruption to be of high volume or highly energetic. Clasts that have endmember characteristics that are blocky and foamy, plus gradient clasts, were found during this study in all dredges as far as 25 km from the rim of the Macauley Caldera, and there may have been other clasts that floated further from the caldera. Exceedingly vesicular pumice, up to more than 90 %, with no explosive activity. The authors1 therefore suggest the presence on submarine caldera volcanoes of widespread blankets of pumice, for example, cannot automatically be interpreted as indicating the existence of large scale explosive volcanism.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|