A vesicularity index for pyroclastic deposits

Keywords:extrusive eruption explosive eruption pyroclastic edifice collapse volcanic hazard Taiwan Taipei City, with a population of around 8 million, as well as two nuclear power plants is located in close proximity to the Quaternary, dominantly andesitic Tatun Volcanic Group (TVG) of Northern Taiwan. We have investigated the stratigraphy of the youngest volcaniclastic deposits, as well as the morphology of lava flows and domes of the TVG in order to reconstruct the character and timing of the most recent eruptions and related hazardous events in the area. Our data indicate that recent eruptions of the group were dominated by long-term, voluminous extrusions of crystal-rich, very viscous lavas. These eruptions formed closely spaced monogenetic domes and lava flows. Based on morphological parameters of the lava flows (thicknesses 80-150 m, lengths up to 5.6 km, and volumes up to 0.6 km 3 ), average rates of magma effusion ranged from 1 to 10 m 3 /s, eruption durations from 500 to 1800 days, and lava front speeds from 0.5 to 6 m/h. Explosive activity of TVG was diverse, ranging from weak phreatic to highly explosive (VEI 4) Plinian eruptions; vulcanian activity with deposition of lithic ashes was most common. Interaction of rising magma with ground water frequently occurred during the eruptions. This study presents the first radiocarbon dates of various volcaniclastic deposits of the TVG, which indicate that Cising, Siaoguanyin, and possibly Huangzuei volcanoes had magmatic eruptions in the period 13,000-23,000 years ago. In addition, Mt. Cising had a phreatic eruption 6000 years ago, and possibly an effusive eruption just before that. Gravitational collapses of volcanic edifices with volumes 0.01-0.1 km 3 and H/L 0.16-0.25 were also common. They occurred on intersections with tectonic faults and may have been triggered by seismic activity. The youngest collapses occurred at Mt. Siaoguanyin (23,000 BP) and Mt. Cising (6000 BP). It is concluded that the TVG should be considered volcanically active. The results of this study provide a basis for volcanic hazard assessment and mitigation in the area.

Section: Deposits Of Pumice Fallout Of Plinian Eruptionsmentioning

confidence: 62%

Deposits, character and timing of recent eruptions and gravitational collapses in Tatun Volcanic Group, Northern Taiwan: Hazard-related issues

Belousov

Belousova

Chen

et al. 2010

“…The overall abundance of this facies in the field area is estimated to <1%. 7-hosts steeply dipping rafts (fragments >2 m across), described in Table 3 -observations 4-5 indicate that country rock was extensively fragmented; that magma cooled rapidly & was fractured in the brittle regime; and that it was disrupted at different points of its degassing history (generally early), under a range of strain/decompression rates -together these features are the hallmark of phreatomagmatic fragmentation (e.g., Wohletz, 1983;Fisher & Schmincke, 1984;Barberi et al, 1989;Houghton & Wilson, 1989;Heiken & Wohletz, 1991;Houghton et al, 1996Houghton et al, , 1999White, 1996;Morrissey et al, 2000) -an origin as one or several lahars (Hanson & Elliot, 2001) -"rags" = relatively vesicular, glassy basaltic fragments, elongate, up to several dm long, w/ bended shapes, delicate ends forming spiral shapes & displaying accommodation of surrounding clasts -"rags" can be aligned in any orientation or be 'randomly' dispersed; alignments are inconsistent between outcrops, e.g., the outcrop next to one displaying sub-horizontally aligned "rags" may show nearly random "rag" orientation, although these outcrops might be at the same topographic level & relatively close laterally -"rags" transported while still plastic (high temperature)…”

Section: Acknowledgementsmentioning

confidence: 99%

“…7, lower left). Most ash-grade basaltic clasts are non-vesicular to incipiently vesicular (vesicularity index of Houghton and Wilson, 1989), although some have more vesicles, in particular in the basalt-rich facies TB hr and TB j (Figs. 6b and 7).…”

Section: Grain Size and Componentry Of Non-bedded Volcaniclastic Rocksmentioning

confidence: 99%

Debris jets in continental phreatomagmatic volcanoes: A field study of their subterranean deposits in the Coombs Hills vent complex, Antarctica

Ross

White

2006

115

The Ferrar large igneous province of Antarctica contains significant mafic volcaniclastic deposits, some of which are interpreted to fill large vent complexes. Such a complex was re-examined at Coombs Hills to map individual steepsided cross-cutting bodies in detail, and we found several contrasting types, two of which are interpreted to have filled subterranean passageways forcefully opened from below into existing, non-consolidated debris. These transient conduits were opened because of the propagation of debris jets -upward-moving streams of volcaniclastic debris, steam, magmatic gases +/-liquid water droplets -following explosive magma-aquifer interaction. Some debris jets probably remained wholly subterranean, whereas other made it to the surface, but the studied outcrops do not allow us to differentiate between these cases. The pipes filled with country rock-rich lapilli-tuff or tuff-breccia are interpreted to have formed following phreatomagmatic explosions occurring near the walls or floor of the vent complex, causing fragmentation of both magma and abundant country rock material. In contrast, some of the cross-cutting zones filled with basalt-rich tuff-breccia or lapilli-tuff could have been generated following explosions taking place within preexisting basalt-bearing debris, well away from the complex walls or floor. We infer that once focused jets were formed, they did not incorporate significant amounts of existing Mawson debris while travelling through them; instead, incorporation of fragments from the granular host took place near explosion sites. Other basalt-rich tuff-breccia zones, accompanied by domains of in situ peperite and coherent basalt pods, are inferred to have originated by less violent processes.

“…9 * Vesicularity index of Houghton and Wilson (1989) throughout the paper. consists of black, dense, finely crystalline 'basalt'; middle 2 m displays well-developed columnar joints, whereas rest has more irregular jointing patterns; petrographic texture typical of rapidly quenched tholeiitic lava with abundant glass, randomly oriented plagioclase laths (~55%) & very small opaque minerals (OU74530, Fig.…”

Section: Numerous Very Small Outcrops (Locality 2 Onmentioning

confidence: 99%

Geological evolution of the Coombs–Allan Hills area, Ferrar large igneous province, Antarctica: Debris avalanches, mafic pyroclastic density currents, phreatocauldrons

Ross

White

McClintock

2008

The Jurassic Ferrar large igneous province of Antarctica comprises igneous intrusions, flood lavas, and mafic volcaniclastic deposits (now lithified). The latter rocks are particularly diverse and well-exposed in the Coombs-Allan Hills area of South Victoria Land, where they are assigned to the Mawson Formation. In this paper we use these rocks in conjunction with the pre-Ferrar sedimentary rocks (Beacon Supergroup) and the lavas themselves (Kirkpatrick Basalt) to reconstruct the geomorphological and geological evolution of the landscape. In the Early Jurassic, the surface of the region was an alluvial plain, with perhaps 1 km of mostly continental siliciclastic sediments underlying it. After the fall of silicic ash from an unknown but probably distal source, mafic magmatism of the Ferrar province began. The oldest record of this event at Allan Hills is a ≤180 m-thick debris avalanche deposit (member m 1 of the Mawson Formation) which contains globular domains of mafic igneous rock. These domains are inferred to represent dismembered Ferrar intrusions emplaced in the source area of the debris avalanche; shallow emplacement of Ferrar magmas caused a slope failure that mobilized the uppermost Beacon Supergroup, and the silicic ash deposits, into a pre-existing valley or basin.The period which followed ('Mawson time') was the main stage for explosive eruptions in the Ferrar province, and several cubic kilometres of both new magma and sedimentary rock were fragmented over many years. Phreatomagmatic explosions were the dominant fragmentation mechanism, with magma-water interaction taking place in both sedimentary aquifers and existing vents filled by volcaniclastic debris. At Coombs Hills, a vent complex or 'phreatocauldron' was formed by coalescence of diatreme-like structures; at Allan Hills, member m 2 of the Mawson Formation consists mostly of thick, coarse-grained, poorly sorted layers inferred to represent the lithified deposits of pyroclastic density currents. Meanwhile at Carapace Nunatak, mafic clasts were mixed with detrital material to form the Carapace Sandstone in alluvial and eventually lacustrine environments.Eruptions then became largely effusive, producing hundreds of metres of flood lavas that covered the landscape ('Kirkpatrick time'). In places, lava flowed into ephemeral lakes to form pillow-palagonite breccias (base of sequence, Carapace Nunatak) or pillow lavas (top of sequence, Coombs Hills). Several generations of Ferrrar intrusions were emplaced during the course of these events; at least three can be distinguished based on field relations. New geochemical data indicates that for the Ferrar province, magma involved in the explosive eruptions had the same major element composition as that which produced shallow intrusions and lavas. We also note the possibility that flood lavas were fed by plugs cross-cutting the Mawson Formation at Coombs Hills, rather than by major dikes extending to the surface. Finally, we infer that eruption plumes were limited to the troposphere and that direct...