Amplified hazard of small-volume monogenetic eruptions due to environmental controls, Orakei Basin, Auckland Volcanic Field, New Zealand
- 558 Downloads
Orakei maar and tuff ring in the Auckland Volcanic Field is an example of a basaltic volcano in which the style and impacts of the eruption of a small volume of magma were modulated by a fine balance between magma flux and groundwater availability. These conditions were optimised by the pre-85 ka eruption being hosted in a zone of fractured and variably permeable Plio-Pleistocene mudstones and sandstones. Orakei maar represents an end-member in the spectrum of short-lived basaltic volcanoes, where substrate conditions rather than the magmatic volatile content was the dominant factor controlling explosivity and eruption styles. The eruption excavated a crater ≫80 m deep that was subsequently filled by slumped crater wall material, followed by lacustrine and marine sediments. The explosion crater may have been less than 800 m in diameter, but wall collapse and wave erosion has left a 1,000-m-diameter roughly circular basin. A tuff ring around part of the maar comprises dominantly base surge deposits, along with subordinate fall units. Grain size, texture and shape characteristics indicate a strong influence of magma–water and magma–mud interactions that controlled explosivity throughout the eruption, but also an ongoing secondary role of magmatic gas-driven expansion and fragmentation. The tuff contains >70 % of material recycled from the underlying Plio-Pliestocene sediments, which is strongly predominant in the >2 ϕ fraction. The magmatic clasts are evolved alkali basalt, consistent with the eruption of a very small batch of magma. The environmental impact of this eruption was disproportionally large, when considering the low volume of magma involved (DRE < 0.003 km3). Hence, this eruption exemplifies one of the worst-case scenarios for an eruption within the densely populated Auckland City, destroying an area of ~3 km2 by crater formation and base surge impact. An equivalent scenario for the same magma conditions without groundwater interaction would yield a scoria/spatter cone with a diameter of 400–550 m, destroying less than a tenth of the area affected by the Orakei event.
KeywordsBasaltic monogenetic volcanoes Orakei Basin Phreatomagmatic eruptions
The authors acknowledge support from the Foundation for Research, Science and Technology International Investment Opportunities Fund, Project MAUX0808 “Facing the challenge of Auckland volcanism” and KN was assisted by the Massey University Research Fund (RM13444). Prof. Claus Siebe and an Anonymous Reviewer provided helpful reviews and we also thank Editors Jeremy Phillips and James White for their contributions toward improving the manuscript.
- Allen SR, Smith IEM (1994) Eruption styles and volcanic hazard in Auckland Volcanic Field, New Zealand. Geosci Rep Shizuoka Uni 20:5–14Google Scholar
- Auer A, Martin U, Németh K (2007) The Fekete-hegy (Balaton Highland Hungary) “soft-substrate” and “hard-substrate” maar volcanoes in an aligned volcanic complex—implications for vent geometry, subsurface stratigraphy and the palaeoenvironmental setting. J Volcanol Geotherm Res 159:225–245CrossRefGoogle Scholar
- Bultitude RJ, Green DH (1971) Experimental study of crystal–liquid relationships at high pressures in olivine nephelinite and basanite compositions. J Petrol 12:121–147Google Scholar
- Büttner R, Dellino P, La Volpe L, Lorenz V, Zimanowski B (2002) Thermohydraulic explosions in phreatomagmatic eruptions as evidenced by the comparison between pyroclasts and products from Molten Fuel Coolant Interaction experiments. J Geophys Res-Solid Earth 107(B11): art. no.-2277Google Scholar
- Gebhardt AC, De Batist M, Niessen F, Anselmetti FS, Ariztegui D, Haberzettl T, Kopsch C, Ohlendorf C, Zolitschka B (2011) Deciphering lake and maar geometries from seismic refraction and reflection surveys in Laguna Potrok Aike (southern Patagonia, Argentina). J Volcanol Geotherm Res 201(1–4):357–363CrossRefGoogle Scholar
- Lesti C, Giordano G, Salvini F, Cas RAF (2008) Volcano tectonic setting of the intraplate, pliocene–holocene, newer volcanic province (southeast Australia): Role of crustal fracture zones. J Geophys Res—Solid Earth 113(B7): Article #: B07407Google Scholar
- Lorenz V (2003) Maar-diatreme volcanoes, their formation, and their setting in hard-rock or soft-rock environments. Geolines 15:72–83Google Scholar
- Németh K (2010) Monogenetic volcanic fields: Origin, sedimentary record, and relationship with polygenetic volcanism. In: E. Cañón-Tapia and A. Szakács (Editors), What Is a Volcano? GSA Special Papers [Geological Society of America, Boulder, Colorado] 470: 43–67Google Scholar
- Ort MH, Wohletz K, Hooten JA, Neal CA, McConnel VS (2000) The Ukinrek maars eruption, Alaska, 1977: a natural laboratory for the study of phreatomagmatic processes at maars. Terra Nostra 2000(6):396–400Google Scholar
- Sohn YK, Cronin SJ, Brenna M, Smith IEM, Németh K, White JDL, Murtagh RM, Jeon YM, Kwon CW (2012) Ilchulbong tuff cone, Jeju Island, Korea, revisited: a compound monogenetic volcano involving multiple magma pulses, shifting vents, and discrete eruptive phases. Geol Soc Am Bull 124(3–4):259–274CrossRefGoogle Scholar