Karoly Nemeth - Profile on Academia.edu (original) (raw)

Papers by Karoly Nemeth

Earth, Planets and Space

Accurate dating of young eruptions from explosive volcanoes is essential for forecasting future e... more Accurate dating of young eruptions from explosive volcanoes is essential for forecasting future eruptions and for defining the hazardscape of volcanic fields. However, precise dating of Quaternary eruptions is often challenging due to limited number of applicable dating methods or lack of datable eruptive phases. Moreover, small volume eruptions (e.g., monogenetic type), despite their significance on regional scale, have traditionally deserved less attention than their large volume counterparts. Puketerata is a maar-lava dome complex in the central Taupō Volcanic Zone (New Zealand), encompassing mafic and silicic phreatomagmatic eruptions with well-preserved pyroclastic deposits sourced from closely spaced vents. Its most recent activity is estimated to ca. 16 ka based on medial and distal stratigraphic surveys. Here, we carried out two independent age determinations and an additional paleomagnetic analysis on the volcanic succession of the Puketerata maar-lava dome complex with an ...

Microlites Size Distributions and P-T-t-X(H 2 O) Constraints of Central Plateau Tephras, New Zealand: Implications for Magma Ascent Processes of Explosive Eruptions

AGUFM, Dec 1, 2018

Morphological characteristics of glassy pyroclasts from the AD 1913 rift-edge phreatomagmatic eruption on Ambrym, Vanuatu

AGU Fall Meeting Abstracts, Dec 1, 2009

ABSTRACT

Journal of Petrology, Jul 18, 2020

To assess whether magma ascent rates control the style of volcanic eruption, we have studied the ... more To assess whether magma ascent rates control the style of volcanic eruption, we have studied the petrography, geochemistry and size distribution of microlites of plagioclase and pyroxene from historical eruptions from Tongariro, Ruapehu and Ngauruhoe volcanoes located in the southern Taupo Volcanic Zone, New Zealand. The studied deposits represent glassy andesitic and dacitic tephra shards from the Mangamate, Mangatawai, Tufa Trig and Ngauruhoe tephra formations, ranging in age from 11 000 years BP to AD 1996. Covering a range in eruption styles and sizes from Strombolian to Plinian, these samples provide an excellent opportunity to explore fundamental volcanic processes such as pre-eruptive magma ascent processes. Our quantitative petrographic analysis shows that larger microlites (>30 mm) display complex growth zoning, and only the smallest crystals (<30 mm) have formed during magma ascent in the conduit. Using a combination of orthopyroxene geothermometry, plagioclase hygrometry, and MELTS modelling, we show that these microlites nucleated at maximum pressures of 550 MPa (c. 16Á5 km) from hot andesitic magmas (1010-1130 C) with low H 2 O content (0-1Á5 wt%). Size distributions of a total of >60 000 microlites, involving 22 tephras and 99 glass shards, yield concave-up curves, and the slopes of the pyroxene microlite size distributions, in combination with well-constrained orthopyroxene crystal growth rates from one studied tephra, indicate microlite population growth times of $3 6 1 days, irrespective of eruption style. These data imply that microlites form in response to cooling of melts ascending at velocities of <5 cm s -1 prior to H 2 O exsolution, which occurs only at <33 MPa. Maximum magma ascent rates in the upper conduit, calculated using the exsolution of water during final decompression, range between 3 and 12 m s -1 ; that is, at least an order of magnitude lower than the hypersonic vent velocities typical of Vulcanian or sub-Plinian eruptions (up to 400 m s -1 ). This implies that magma ascent from depths of an average of 4 km occurs in dykes, and that vent velocities at the surface are controlled by a reduction of conduit cross-section towards the surface (e.g. dyke changing to cylindrical conduit).

Geological Society, London, Special Publications, 2017

Small-scale volcanic systems are the most widespread type of volcanism on Earth and occur in all ... more Small-scale volcanic systems are the most widespread type of volcanism on Earth and occur in all of the main tectonic settings. Most commonly, these systems erupt basaltic magmas within a wide compositional range from strongly silica undersaturated to saturated and oversaturated; less commonly, the spectrum includes more siliceous compositions. Small-scale volcanic systems are commonly monogenetic in the sense that they are represented at the Earth's surface by fields of small volcanoes, each the product of a temporally restricted eruption of a compositionally distinct batch of magma, and this is in contrast to polygenetic systems characterized by relatively large edifices built by multiple eruptions over longer periods of time involving magmas with diverse origins. Eruption styles of small-scale volcanoes range from pyroclastic to effusive, and are strongly controlled by the relative influence of the characteristics of the magmatic system and the surface environment.

Shallow-seated explosions in the construction of the Motukorea tuff ring (Auckland, New Zealand): Evidence from lithic and sedimentary characteristics

Journal of Volcanology and Geothermal Research, Oct 1, 2015

At least 52 eruption centres are scattered within the ~ 360 km2 Auckland Volcanic Field (AVF). Mo... more At least 52 eruption centres are scattered within the ~ 360 km2 Auckland Volcanic Field (AVF). Motukorea, now an island in the Waitemata Harbour, is one of 39 AVF volcanoes that experienced a phreatomagmatic explosive phase, before a magmatic phase. The volcano erupted through a ~ 200–300 m-thick, consolidated, mudstone/sandstone sequence of the Miocene Waitemata Group, which overlies the Waipapa Terrane greywacke basement. Detailed field descriptions of the sedimentary characteristics of the early phreatomagmatic deposits were carried out, along with examination of lithics. The ejecta ring deposit comprises 55 to 60 vol.% lithics, of which Waitemata Group fragments constitute approximately 90 vol.%, whereas < 10 vol.% are Waipapa fragments, suggesting a dominance of shallow fragmentation. The sedimentary characteristics of the stratigraphic sequence at Motukorea suggest a dominance of wet surges at the beginning of the eruption with progression into drier sequences upwards. This is reflected in increasing inter-bedded juvenile-pyroclast-dominated fall deposits up-sequence. These characteristics are attributed to the changing hydrogeological conditions within the diatreme and the host rocks. These findings shed light on the eruption dynamics of phreatomagmatic eruptions through consolidated rocks in the AVF and enable the depiction of a scenario of future eruptions within the field in similar substrates.

Journal of Volcanology and Geothermal Research, Apr 1, 2011

Fissure-eruptions along linear structures can extend for several tens of kilometres with distinct... more Fissure-eruptions along linear structures can extend for several tens of kilometres with distinct separate manifestations of volcanism along their length. They typically involve low-viscosity mafic magmas forming long lava flows and cinder cones. Eruptions in 1894 and 1913 on Ambrym volcano, Vanuatu, showed how these mildly explosive eruptions can rapidly transform into violent explosive events, producing significant hazard and widespread volcanic ash clouds. During the 1913 episode, a fissure began in the central caldera and basaltic magmas broke out in a series of locations down the island's western flank. In all sites over 100 m in elevation, fissure outbreaks produced vigorous lava fountains and highly fluid lava flows that travelled rapidly to the shoreline. When the outbreaks propagated along the island's axis into coastal plain areas, a climactic series of explosive eruptions occurred, producing a 1.2 km long by 600 m wide maar and tephra ring. A further small tuff ring was formed later, creating a temporary island 400 m offshore. The onshore tephra ring destroyed a hospital and associated buildings. Its last evacuating occupants were close witnesses to the eruption processes. Deposits exposed in the lower portion of the tephra ring show that this part of the eruption began with a mild phreatomagmatic explosive eruption from a narrow vent, followed by a magmatic scoria-producing phase. Subsequently a complex sequence of highly explosive phreatomagmatic eruptions occurred, producing pyroclastic surges, along with repeated distinctive breccia-horizons, rich in coral and lava country rock. These features tally with eye-witness accounts to indicate that the main eruption phase was produced by a periodically shifting locus of phreatomagmatic fragmentation and eruption along a single E-W fissure. The glassy and vesicle-poor pyroclasts produced during this eruption phase were dominantly fragmented in a brittle manner by magma water interaction. Low volatile content of the magma upon fragmentation is confirmed by FTIR analysis showing b 0.5% H 2 O in chilled glass. These findings highlight that a degassed, mafic, fissure-fed eruption can under certain circumstances pose a major volcanic hazard if dykes intersect substrates with abundant available water.

Goldschmidt Abstracts, 2020

Journal of Volcanology and Geothermal Research, Mar 1, 2009

tuff ring tuff cone maar volcanic island basalt rift scoria sideromelane accretionary lapilli bas... more tuff ring tuff cone maar volcanic island basalt rift scoria sideromelane accretionary lapilli base surge Ambae Island is a mafic stratovolcano located in the northern Vanuatu volcanic arc and has a NE-^SW rift-^controlled elongated shape. Several hundred scoria cones and fissure-^fed lava fields occur along its long axis. After many decades of quiescence, Ambae Island erupted on the 28th of November 2005, disrupting the lives of its 10, ^000 inhabitants. Its activity remained focused at the central (crater-^lake filled) vent and this is where hazard-^assessments were focused. These assessments initially neglected that maars, tephra cones and rings occur at each tip of the island where the eruptive activity occurred b500 and b ^300 yr ^B.P. The products of this explosive phreatomagmatic activity are located where the rift axis meets the sea. At the NE edge of the island five tephra rings occur, each comparable in size to those on the summit of Ambae. Along the NE coastline, a near-^continuous cliff section exposes an up to 25 m thick succession of near-^vent phreatomagmatic tephra units derived from closely spaced vents. This can be subdivided into two major lithofacies associations. The first association represents when the locus of explosions was below sea level and comprises matrix-^supported, massive to weakly stratified beds of coarse ash and lapilli. These are dominant in the lowermost part of the sequence and commonly contain coral fragments, indicating that the loci of explosion were located within a reef or coral sediment near the syn-^eruptive shoreline. The second type indicate more stable vent conditions and rapidly repeating explosions of high intensity, producing fine-^grained tephra with undulatory bedding and cross-^lamination as well as megaripple bedforms. These surge and fall beds are more common in the uppermost part of the succession and form a few-^m-^thick pile. An older tephra succession of similar character occurs below, and buried trees in growth position, as well as those flattened within base surge beds. This implies that the centre of this eruption was very near the coastline. The processes implied by these deposits are amongst the most violent forms of volcanism on this island. In addition, the lowland and coastal areas affected by these events are the most heavily populated. This circumstance is mirrored on many similar volcanic islands, including the nearby SW Pacific examples of Taveuni (Fiji), Upolu and Savai'i (Samoa), and Ambrym (Vanuatu). These locations are paradoxically often considered safe areas during summit/central-^vent eruptions, simply because they are farthest from the central sources of ash-^fall and lahar hazard. The observations presented here necessitate a revision of this view.

Volcanic craters, pit craters and high-level magma-feeding systems of a mafic island-arc volcano: Ambrym, Vanuatu, South Pacific

Geological Society, London, Special Publications, 2008

Ambrym is one of the largest volcanic islands of the Vanuatu arc. It has been the focus of exclus... more Ambrym is one of the largest volcanic islands of the Vanuatu arc. It has been the focus of exclusively mafic volcanism and has a structure dominated by a central 13 km-diameter caldera. Contained within the caldera are two major cone complexes, Marum and Benbow, which have been the locus of most historic eruptions. Vents within these are constantly in a state of strong degassing, with visible lava lakes periodically being observed in several subcraters. Vulcanian and strombolian explosive eruptions occur at least yearly, along with larger subplinian events every 20–30 years. The active vent systems are enclosed by several 100 m-deep vertical-walled pit craters that expose cross-sectional views through the transition zone between the conduit and the crater. Units include coherent magma bodies with interbedded pyroclastic successions. One of the Marum craters, Niri Taten, exposes portions of solidified lava lakes, magma pods that fed spatter cones, small shallow-level intrusions and larger sills that connect through a complex network of dykes to the surface and/or into the pyroclastic edifice. These features show that shallow-level infiltration of degassed and low-viscosity melts into pyroclastic-deposit-dominated volcanic sequences of Marum plays an important role in the growth of scoria and spatter cones. Once solidified, the large intrusive bodies apparently provide important buttressing of pyroclastic cones, but during emplacement they may also cause cone collapse and lateral escape of magma to form lava flows.

Weka Trainable Segmentation Plugin in ImageJ: A Semi-Automatic Tool Applied to Crystal Size Distributions of Microlites in Volcanic Rocks

Microscopy and Microanalysis, Dec 1, 2018

Crystals within volcanic rocks record geochemical and textural signatures during magmatic evoluti... more Crystals within volcanic rocks record geochemical and textural signatures during magmatic evolution before eruption. Clues to this magmatic history can be examined using crystal size distribution (CSD) studies. The analysis of CSDs is a standard petrological tool, but laborious due to manual hand-drawing of crystal margins. The trainable Weka segmentation (TWS) plugin in ImageJ is a promising alternative. It uses machine learning and image segmentation to classify an image. We recorded back-scattered electron (BSE) images of three volcanic samples with different crystallinity (35, 50 and ≥85 vol. %), using scanning electron microscopes (SEM) of variable image resolutions, which we then tested using TWS. Crystal measurements obtained from the automatically segmented images are compared with those of the manual segmentation. Samples up to 50 vol. % crystallinity are successfully segmented using TWS. Segmentation at significantly higher crystallinities fails, as crystal boundaries cannot be distinguished. Accuracy performance tests for the TWS classifiers yield high F-scores (>0.930), hence, TWS is a successful and fast computing tool for outlining crystals from BSE images of glassy rocks. Finally, reliable CSD’s can be derived using a low-cost desktop SEM, paving the way for a wide range of research to take advantage of this new petrological method.

Geochemical dissection of a monogenetic eruption: Motukorea Volcano, the Auckland Volcanic Field, New Zealand

AGU Fall Meeting Abstracts, Dec 1, 2012

depending on many factors including flow volumes, distance from source, type of source, and flow ... more depending on many factors including flow volumes, distance from source, type of source, and flow composition. The largest silicic volcanic events are comparable in size to the largest recorded mafic events; however, they are potentially more catastrophic if erupted as ignimbrite flows. Facies and facies associations identified in CFBs include simple-classic flows, compound flows, ponded flows, truncation-onlap volcanic disconformities, burial-onlap volcanic disconformities, prograding hyaloclastite facies, preserved shield volcanic features, and sill facies. Many of these features occur on an intermediate to large basin wide scale and may only be revealed by detailed fieldwork, photogrammetry, and 3D geological models (e.g., Jerram and Robbe 2001; Single and Jerram 2004). Using examples of well constrained 3D geological earth models from the Skye Main Lave Series it is possible to work out lava stacking patterns and flow directions (Fig. ), volumes of different lava packages and how they have been influenced by faults (Fig. ). The 3D models can also be of use in terms of rock property information such as density and velocity of key horizons in lavas/intrusions, known as intra-facies (Single and Jerram 2004). Such intra-facies maps allow geophysical interpretation of the behavior of the igneous rocks which is of importance when exploring areas in offshore settings with significant flood basalt cover such as offshore North Atlantic Margin. Dougal A. Jerram, Mike Widdowson (2005) The anatomy of continental flood basalt provinces: geological constraints on the processes and products of flood volcanism. Lithos 79:385-405 Richard T. Single, Dougal A. Jerram (2004) The 3-D facies architecture of flood basalt provinces and their internal heterogeneity: examples from the palaeogene skye lava field.

scoria cone in Tongoa show much small-scale complexity. Where this has occurred, the cone consist... more scoria cone in Tongoa show much small-scale complexity. Where this has occurred, the cone consists of a sheeted network of slopeparallel and steeply crosscutting coherent igneous rocks, autobreccia, and locally developed peperite, all intercalated with the host pyroclastic material. These outcrops also provide information of potential use in hazard evaluation. The dykes and sills developed from the lava suggest that the studied cone must have accumulated at least 10 m of scoria on top of a still-active buried lava flow. During the eruption this buried lava may have been invisible to onlookers, but it still had a fluid core capable of feeding lava to the surface beyond the zone of rapid burial, or, perhaps more importantly of causing collapse of a growing cone by deforming within it and injecting sheets of magma upward or laterally.

Lepusztult maar/diatrema szerkezetek a Bakony-Balaton Felvidék Vulkáni Területröl (Eroded maar/diatrema structures from the Bakony-Balaton Highland Volcanic Field)

... 1998, NÉMETH, MARTIN 2001). Bármely kőzettöredék, amelyet a vulkáni robbanás repít ki egy vul... more ... 1998, NÉMETH, MARTIN 2001). Bármely kőzettöredék, amelyet a vulkáni robbanás repít ki egy vulkán kürtőjéből, piro-klasztnak tekintendő (FISHER, SCHMINCKE 1984). ... M icro vesicu la r, micro lite -rich sidero m elane. Főként triász karbonát, alárendelt en pannóniai hom ok. ...

Contributions to Mineralogy and Petrology, 2021

Arc magmas typically contain phenocrysts with complex zoning and diverse growth histories. Microl... more Arc magmas typically contain phenocrysts with complex zoning and diverse growth histories. Microlites highlight the same level of intracrystalline variations but require nanoscale resolution which is globally less available. The southern Taupo Volcanic Zone (TVZ), New Zealand, has produced a wide range of explosive eruptions yielding glassy microlite-bearing tephras. Major oxide analyses and textural information reveal that microlite rims are commonly out of equilibrium with the surrounding glass. We mapped microlites and microcrysts at submicron resolution for major and trace element distributions and observed three plagioclase textural patterns: (1) resorption and overgrowth, (2) oscillatory zoning, and (3) normal (sharp) zoning. Pyroxene textures are diverse: (1) resorption and overgrowth, (2) calcium-rich bands, (3) hollow textures, (4) oscillatory zoning, (5) sector zoning, (6) normal zoning and (7) reverse zoning. Microlite chemistry and textures inform processes operating dur...

Elucidating stratovolcano construction from volcaniclastic mass‐flow deposits: The medial ring‐plain of Taranaki Volcano, New Zealand

Sedimentology, 2021

Long‐lived stratovolcanoes display a thick volcanic apron surrounding the edifice. This sedimenta... more Long‐lived stratovolcanoes display a thick volcanic apron surrounding the edifice. This sedimentary succession incorporates the majority of the deposits from both growth and destruction phases of a volcanic massif. The ring plain of Taranaki Volcano (>200 ka) is composed of volcaniclastic mass‐flow deposits that are exceptionally well‐exposed along its coastal‐cliff shoreline, at 20 to 30 km distance from the edifice. Overall, the volcaniclastic deposits in the southern and south‐western sector record three growth phases (65 to 34 ka) which can be investigated due to access and stratigraphic control of the ring‐plain section. Each cyclic growth phase is represented by a sequence of mass‐flow deposits. Lithostratigraphic units or repeated packages with similar properties were identified in order to understand the depositional sequences. The mass‐flow units within these growth phases can be described by three criteria subdivided into nine distinct sedimentological textural types (f...

Shallow subaqueous to emergent intra-caldera silicic volcanism of the Motuoapa Peninsula, Taupo Volcanic Zone, New Zealand – New constraints from geologic mapping, sedimentology and zircon geochronology

Journal of Volcanology and Geothermal Research, 2021

Abstract Motuoapa Peninsula, located in the southeast of the Taupo Volcanic Centre, New Zealand, ... more Abstract Motuoapa Peninsula, located in the southeast of the Taupo Volcanic Centre, New Zealand, is dominated by a silicic pyroclastic cone and overlaying lavas. The pyroclastic succession has not been recognised and studied before, and its thickness and sedimentological characteristics indicate completely different eruption mechanisms than proposed for the other pyroclastic successions within the central Taupo Volcanic Zone. Here, we present the results of field mapping and sedimentological characterisation of accessible pyroclastic deposits, and complement these data with combined U-Th and (U-Th)/He zircon geochronology providing first constraints on the succession's crystallization and eruption history. (U-Th)/He zircon eruption ages of 77.2 ± 6.3, 81.3 ± 9.2 and 34.5 ± 3.1 ka indicate that volcanic activity in the Motuoapa Peninsula occurred in two distinct eruptive episodes that were separated by ca. 45 kyrs. The earlier rhyolitic eruption at ca. 80 ka is inferred to have commenced in a shallow subaqueous environment. Its lowermost succession includes breccias and tuff breccias sourced from an extruding lava dome by autobrecciation, quench-fragmentation and localised debris flows. With gradual emergence of the growing volcanic pile, explosive hydrovolcanic activity became dominant, constructing an emergent cone by pyroclastic density currents and fall-out. The lack of exotic/accidental clasts, along with an abundance of low-vesicularity rhyolitic juvenile fragments, suggests fragmentation driven by magma-water interaction, which predominantly occurred at shallow depths within the outgassed part of the ascending magma. The frequency and thickness of ash-dominated units increases upwards, suggesting a gradual increase in explosive energy of tephra jets. The final phase of the rhyolitic activity was dominated by emplacement of viscous lava that breached the crater rim and flowed onto the SE sector of the pyroclastic cone. The remnant of the Motuoapa pyroclastic cone, along with the bedded structure of deposits that comprise fallout and surge-dominated units, appears very similar to Surtseyan tuff cones and silicic tuff/pumice cones described elsewhere. A dacitic eruption that produced a nearby lava dome at ca. 35 ka, represents a significantly younger event that occurred after substantial erosion of the earlier pyroclastic cone. The Motuoapa Peninsula deposits most likely record the evolution of a subaqueous silicic eruption, where hydrovolcanism played a fundamental role on subaerial pyroclastic cone formation in a terrestrial environment with abundant surface water availability. The similarities between the environment of the Taupo area today and the area during the Motuoapa activity at ca. 80 ka may provide an analogue model for future subaqueous eruptions in the region.