Research Article| January 01, 2007 Double trouble: Paired ignimbrite eruptions and collateral subsidence in the Taupo Volcanic Zone, New Zealand D.M. Gravley; D.M. Gravley 1Geology Department, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand Search for other works by this author on: GSW Google Scholar C.J.N. Wilson; C.J.N. Wilson 1Geology Department, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand Search for other works by this author on: GSW Google Scholar G.S. Leonard; G.S. Leonard 2GNS Science, P.O. Box 30368, Lower Hutt 5040, New Zealand Search for other works by this author on: GSW Google Scholar J.W. Cole J.W. Cole 3Department of Geological Sciences, University of Canterbury, Private Bag 4800, Christchurch, New Zealand Search for other works by this author on: GSW Google Scholar Author and Article Information D.M. Gravley 1Geology Department, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand C.J.N. Wilson 1Geology Department, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand G.S. Leonard 2GNS Science, P.O. Box 30368, Lower Hutt 5040, New Zealand J.W. Cole 3Department of Geological Sciences, University of Canterbury, Private Bag 4800, Christchurch, New Zealand Publisher: Geological Society of America Received: 01 Nov 2005 Revision Received: 24 May 2006 Accepted: 17 Jul 2006 First Online: 08 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 GEOLOGICAL SOCIETY OF AMERICA GSA Bulletin (2007) 119 (1-2): 18–30. https://doi.org/10.1130/B25924.1 Article history Received: 01 Nov 2005 Revision Received: 24 May 2006 Accepted: 17 Jul 2006 First Online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation D.M. Gravley, C.J.N. Wilson, G.S. Leonard, J.W. Cole; Double trouble: Paired ignimbrite eruptions and collateral subsidence in the Taupo Volcanic Zone, New Zealand. GSA Bulletin 2007;; 119 (1-2): 18–30. doi: https://doi.org/10.1130/B25924.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Large explosive eruptions are generally rare, random events in the history of any particular volcano, volcanic area, or worldwide. In the Taupo Volcanic Zone, New Zealand, temporal clustering of eruptions occurs on a <1 yr to ∼300 k.y. basis, which implies that some controls lead to nonrandom eruption timing. We describe two closely paired large Taupo Volcanic Zone eruptions dated at ca. 240 ka that terminated a large-scale cluster of 7 caldera-forming and >15 smaller eruptions over a total ∼100 k.y. period. After a precursor eruption from a nearby source (and a break of years to decades), these paired eruptions in turn generated a wet ash-fall deposit and a dry pumice-fall deposit; the Mamaku ignimbrite (>145 km3 magma); a fine-grained vitric ash-fall deposit; then the Ohakuri ignimbrite (>100 km3 magma). Rotorua and Ohakuri, spaced ∼30 km apart, are the inferred collapse calderas associated with the Mamaku and Ohakuri ignimbrites, respectively. The early wet and dry fall deposits came from southerly sources, close to or within the subsequent Ohakuri caldera, while the fine-grained vitric ash is inferred to represent a co-ignimbrite ash from the Mamaku ignimbrite. At its southwest margin, the Mamaku ignimbrite overlies, but is also intercalated within and then overlain by, the pumice fall deposit, demonstrating that at least two widely spaced vents were active simultaneously for part of the eruption sequence. The post-Mamaku vitric ash-fall deposit underwent only trivial reworking prior to emplacement of the Ohakuri ignimbrite. This and other field evidence imply continuity, or time gaps of only days to months, in the whole paired sequence. Syneruptive volcanotectonic faulting may have permitted accumulation of >400 m of nonwelded Ohakuri ignimbrite through graben subsidence. Posteruptive faulting within years to decades of the eruption produced an ∼300 m extra-caldera offset of the Mamaku ignimbrite and collateral subsidence of a >40 km2 area immediately south of Rotorua caldera. Temporal linkages between ignimbrite eruptions and graben subsidence, the NNE-SSW alignment of associated faulting between the Rotorua and Ohakuri calderas, and the eruption-related subsidence indicate a tectonic control on volcanism associated with Taupo Volcanic Zone rifting processes. Statistical forecasts of the frequency of large-volume explosive events based on averages may be inaccurate because of tectonic triggering effects. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Abstract We have carried out paleomagnetic analyses of tephras from the Taupō eruption, one of the most violent eruptions on Earth in the past 5000 years. Pyroclastic deposits were collected with 7 cm 3 cubes pushed into each unit of the Taupō eruption sequence, consisting of airfall units and overlying ignimbrite. Where possible, we targeted fine-ash layers and matrix, as lapilli sized material can significantly affect the quality of the analysis. The samples were oriented using a collection device specially designed to maximize accuracy. All samples were subjected to alternating field demagnetization, while samples from Taupō ignimbrite (Y7)—the only unit deposited hot were also subjected to thermal demagnetization. The characteristic remanent magnetizations (ChRMs) for specimens from unit Y1, the lower and upper parts of unit Y4, and unit Y7 are well determined ( α 95 < 3.3°). The declinations and inclinations of site-mean ChRMs range from 3.0° to 7.1° and − 53.4° to − 56.2°, respectively, in close agreement with published results from lithic fragments of the Taupō ignimbrite (Y7). The mean ChRM of unit Y3 does not fit within 95% confidence limits of the ChRM of other units. We presume this is a consequence of unit Y3 samples containing relatively coarse grains and of probable secondary process of the deposit. This outlier aside, our results show consistency between thermoremanent magnetizations of ignimbrite and detrital remanences of co-eval ashfalls, thus validating our method for further tephra research. Both geological observations and paleomagnetic estimation using angular difference suggest that the duration of the Taupō eruption sequence was less than a few tens of years. Furthermore, matching the overall mean ChRM direction (Dec = 4.3°, Inc = − 55.3°, α 95 = 1.3°, N = 38 specimens) to the New Zealand paleosecular variation record using the MATLAB dating tool, most likely supports a younger age (ca. 310 CE) than the reported wiggle match eruption age of 232 ± 10 CE. Graphical Abstract
Pyroclastic density currents (PDCs) are a destructive volcanic hazard. Quantifying the types, frequency and magnitudes of PDC events is essential for effective risk management, but since historical records at best extend a few hundred years this usually relies on identifying deposits in the geological record. However, small volume unconsolidated PDC deposits have low preservation potential and can be difficult to distinguish from other volcaniclastic units, especially in proximal locations. Consequently many small or poorly exposed deposits can be overlooked. Here, we introduce a structured field method for assessing volcaniclastic deposits of unknown origin with a particular focus on identifying deposits from concentrated PDCs (pyroclastic flows). The method differs from traditional identification schemes in that it does not uniquely attribute a deposit to a single depositional process, but instead assesses how confidently different volcaniclastic processes could explain the observed deposit features. Therefore, the underlying uncertainties in the assessment are explicitly addressed. The method allows consistent, rapid assessment of candidate pyroclastic flow deposits in the field, and the concept could easily be adapted for assessing other types of volcaniclastic deposit. The introduction of confidence levels in deposit interpretations should be useful for carrying though uncertainties into probabilistic assessments of volcanic hazards.
Adularia and sanidine are polymorphs of potassium feldspar commonly present in felsic, hydrothermally altered volcanic deposits. Sanidine is a high-temperature volcanic mineral, whereas adularia forms post deposition by hydrothermal processes. Petrographically differentiating between these polymorphs in hydrothermally altered volcanic rocks may be utilised to distinguish geological units as well as provide insights into fluid–rock interactions. However, petrographic identification may be difficult or not possible in fine-grained drill cuttings. Here, polymorphic-sensitive, Raman spectroscopy and electron microprobe analyses are utilised to characterise adularia and potential sanidine in drill cuttings from the Ngatamariki Geothermal Field, Taupo Volcanic Zone, New Zealand. Differences in Raman spectra are capable of distinguishing between adularia and sanidine whether using peak positions or principal component analysis. All the Ngatamariki Geothermal Field potassium feldspars analysed by Raman spectroscopy were found to be adularia, as expected, with typical high K, low Ca compositions between Or94 and Or99 confirmed with electron microprobe analyses. This applied approach demonstrates that Raman spectroscopy is a fast and effective method for lending confidence to adularia and sanidine identification, which can be utilised in geothermal fields worldwide.
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