Tarawera Volcanic Complex Research Articles

Understanding Degassing Pathways Along the 1886 Tarawera (New Zealand) Volcanic Fissure by Combining Soil and Lake CO2 Fluxes

The 1886 eruption of Tarawera, New Zealand, produced a ~17 km long fissure across the Tarawera Volcanic Complex (TVC), Lake Rotomahana (LR), and the Waimangu Geothermal System (WGS). We combine new soil CO2 flux and isotope measurements of TVC with previous data from LR and WGS to fingerprint the CO2 source, understand the current pathways for degassing, and quantify the CO2 released along the entire fissure. The total CO2 emissions from the fissure are 1227 t·d-1 (742–3398 t·d-1). The CO2 flux from WGS and LR is far higher than from TVC (>549 vs. ~4 t·d-1 CO2), likely influenced by a shallow silicic body at depth and caldera rim faults increasing permeability at the southern end of the fissure. Highly localised regions of high CO2 flux occur along the fissure and are likely caused by cross-cutting faults that focus the flow. One of these areas occurs in the TVC, which is emitting ~1 t·d-1 CO2 with a δ13CO2 of -5.5 ± 0.5 ‰, and comparison with previous observations shows that activity is declining over time. Future CO2 flux surveys could be compared to this baseline survey to understand changes in volcanic activity.

Silicic recharge of multiple rhyolite magmas by basaltic intrusion during the 22.6 ka Okareka Eruption Episode, New Zealand

Deposits of the 22.6 ka Okareka Eruption Episode from Tarawera Volcanic Complex record the sequential and simultaneous eruption of three discrete rhyolite magmas following a silicic recharge event related to basaltic intrusion. The episode started with basaltic eruption (∼ 0.01 km 3 magma), and rapidly changed to a plinian eruption involving a moderate temperature (750 °C), cummingtonite-bearing rhyolite magma (T1) with a volume of ∼ 0.3 km 3. Hybrid basalt/rhyolite clasts demonstrate direct basaltic intrusion that helped trigger the eruption. Crystals, shards and lapilli of two other rhyolite magmas then joined the eruption sequence. They comprise a cooler (720 °C) crystal-rich biotite–hornblende rhyolite magma (T2) (∼ 0.3 km 3), and a hotter (780 °C), crystal-poor, pyroxene–hornblende rhyolite magma (T3) (∼ 4.5 km 3). All mid to late-stage ash units contain various mixtures of T1, T2 and T3 components with a general increase in abundance of T3 and rapid decline of T1 with time. About 4 km 3 of T3 magma was extruded as lavas at the end of the episode. Contrasts in melt composition, crystal and volatile contents, and temperatures influenced viscosity and miscibility, and thus limited pre-eruption mixing of the rhyolite magmas. The eruption sequence and the restricted direct basaltic intrusion into only one magma (T1) is consistent with the rhyolites occupying separate melt pods within a large crystal-mush zone. Melt–crystal equilibria and volatile contents in melt inclusions indicate temporary magma storage depths of < 8 km. Each of the magmas display quartz crystals containing melt inclusions that are compositionally highly evolved relative to the accompanying matrix glass, and thus point to a stage of more complete crystallisation. The matrix glass, enriched in Sr and Ti, represents a re-melting event of underlying the crystal pile induced by basaltic intrusion, presumably part of the same event that erupted scoria at the start of the eruption. This recharge rhyolite melt percolated upward and hybridised with the resident melts in each of the three magma pods. The Okareka episode rhyolites contrast with other well-documented rhyolites that are either continuously or discontinuously zoned, or have been homogenised during re-activation to a uniform composition. Rapid basalt dike intrusion to shallow levels appears to have (prematurely?) triggered the Okareka rhyolites into eruption, so that their early ponding in separate melt pods has been recorded before it could be masked by mixing or stratification had amalgamation into a larger body occurred.

Multiple rhyolite magmas and basalt injection in the 17.7 ka Rerewhakaaitu eruption episode from Tarawera volcanic complex, New Zealand

The 17.7 ka Rerewhakaaitu eruption episode (volume ∼ 5 km 3 DRE rhyolite magma) was the second of five major episodes that have built the Tarawera volcanic complex in the Okataina Volcanic Centre over the past 22 kyr. The Rerewhakaaitu episode produced a widespread tephra fall deposit, associated proximal pyroclastic flow deposits, and voluminous rhyolite lava extrusions. Two different rhyolite magmas (T1 and T2) were simultaneously erupted from the main vent area throughout much of the eruption episode. T1 magma was a crystal-poor orthopyroxene-hornblende rhyolite that is highly evolved (whole rock SiO 2 = 77 wt.%), with a moderate temperature (∼ 760 °C, based on Fe–Ti oxides). T2 is a crystal-rich biotite-hornblende rhyolite that is less evolved (SiO 2 = 75 wt.%), with a Fe–Ti oxide temperature of ∼ 700 °C. Ejecta from the simultaneous and sequential eruption of these two magmas include some pumice clasts with mixed (hybrid) and mingled glass compositions and crystal populations. A third rhyolite magma (T3) was extruded from another vent 3 km distant to form an apparently contemporaneous lava dome. T3 was the least evolved (SiO 2 = 74 wt.%) and hottest (∼ 820 °C) of the three magmas. Saturation pressures calculated using dissolved H 2O and CO 2 contents of melt inclusions in quartz crystals indicate that T2 magma stagnated and crystallised at about 12 km depth, while small quartz crystals in T1 magma grew during ascent through ∼ 8 km depths. Some T1 and T2 rhyolite clasts contain vesicular brown blebs with widely variable (andesite to rhyolite) glass compositions, accompanied by olivine, clinopyroxene and calcic plagioclase crystals that are interpreted as xenocrysts derived from injected basalt. Temperatures over 1000 °C estimated from pyroxene phase equilibria in these clasts reflect intrusion of the more mafic magma, which is now identified as the priming and triggering mechanism for three of the four post-22 ka Tarawera rhyolite eruption episodes. However, the rhyolite magma bodies and conduits modelled for each episode have considerable differences in characteristics and geometry. Our preferred model for the Rerewhakaaitu episode is that eruptions occurred from three laterally and vertically isolated rhyolite magma bodies that were initially primed and triggered by basalt intrusion during a regional rifting event. The ascending hotter and less viscous T1 rhyolite magma intersected and further invigorated a stagnant pond of cooler, denser and more viscous T2 magma, and lubricated its transport to the surface.

Probabilistic modeling of tephra dispersal: Hazard assessment of a multiphase rhyolitic eruption at Tarawera, New Zealand

The Tarawera Volcanic Complex comprises 11 rhyolite domes formed during five major eruptions between 17,000 B.C. and A.D. 1886, the first four of which were predominantly rhyolitic. The only historical event erupted about 2 km3 of basaltic tephra fall (A.D. 1886). The youngest rhyolitic event erupted a tephra fall volume more than 2 times larger and covered a wider area northwest and southeast of the volcano (∼A.D. 1315 Kaharoa eruption). We have used the Kaharoa scenario to assess the tephra fall hazard from a future rhyolitic eruption at Tarawera of a similar scale. The Plinian phase of this eruption consisted of 11 discrete episodes of VEI 4. We have developed an advection‐diffusion model (TEPHRA) that allows for grain size‐dependent diffusion and particle density, a stratified atmosphere, particle diffusion time within the rising plume, and settling velocities that include Reynolds number variations along the particle fall. Simulations are run in parallel on multiple processors to allow a significant implementation of the physical model and a fully probabilistic analysis of inputs and outputs. TEPHRA is an example of a class of numerical models that take advantage of new computational tools to forecast hazards as conditional probabilities far in advance of future eruptions. Three different scenarios were investigated for a comprehensive tephra fall hazard assessment: upper limit scenario, eruption range scenario, and multiple eruption scenario. Hazard curves and probability maps show that the area east and northeast of Tarawera would be the most affected by a Kaharoa‐type eruption.

Volcanic stratigraphy and phase chemistry of the 11 900 yr BP Waiohau eruptive episode, Tarawera Volcanic Complex, New Zealand

The 11 900 yr BP Waiohau eruptive episode is a moderately sized rhyolitic event of the Tarawera Volcanic Complex. Subdivision of the pyroclastic fall deposits previously mapped as Waiohau Tephra has allowed the chronology of the eruption to be deciphered. At least 15 events (units A‐O) with volumes in the range <0.01–1.7 km3 can be recognised in proximal to medial settings. The eruption commenced with a series of minor pyroclastic falls and surges, which rapidly changed to a major phase of pyroclastic surges (unit D) that reached up to 14 km from the vent with ash clouds recognisable up to 30 km. Eastern Dome is considered to have been extruded early in the episode. Several prominent plinian phases (unit E, L, M) followed, two of which are estimated to have had column heights of c. 30 km. One event was associated with the destruction of an obsidian dome and dispersal of obsidian lapilli (unit F). These plinian events were accompanied by thin pyroclastic flows punctuated by minor magmatic and phreatomagmatic fall eruptions. There is no evidence for prolonged quiescence during these eruptions. The climactic pyroclastic fall phase (unit M) was followed by a major failure of the northeast flanks of the Tarawera massif, either by a series of block and ash flows or debris avalanche. Effusive activity resulted in the emplacement of Waikakareao lava flows, and voluminous and widespread pyroclastic flows (unit N) to the north and northeast. The Waiohau episode concluded with effusive activity producing the Pokuhu flow and Kanakana Dome. A new volume estimate for the combined units A‐O is 4.5 km3. This is a minimum because it does not properly incorporate distal ash (>50 km) poorly preserved in paleosols. Previous volume calculations are considered overestimates because they included tephric loess. Effusive activity represents 3.9 km3, and pyroclastic debris in the northern sector adds another 2.4 km3, but much of the latter is xenolithic. Deposits from the entire Waiohau eruptive episode were derived from a hypersthene (+ minor hornblende) rhyolite magma that lacked compositional (SiO2 = 78 ± 0.2 wt%) and mineralogic gradients. Electron microprobe analyses reveal that phenocrystic phases lack compositional variation. The bulk of the magma also lacked physical gradients (T= 780–790°C; logfO2 = c. ‐15) as determined by Fe‐Ti oxide geothermometry. Eastern Dome and the Pokohu lava display lower temperature (<740°C) and log fO2 (c. ‐16). There is no evidence for mafic triggering of the Waiohau eruption, unlike other eruptive episodes of Tarawera, and the magma represents a separate batch, unrelated to earlier and later biotite‐bearing magmas. The Waiohau eruptive products appear to have been derived from a rapidly convecting, low aspect ratio magma body that did not reside in the crust for a prolonged time.

Basalt triggering of the c. AD 1305 Kaharoa rhyolite eruption, Tarawera Volcanic Complex, New Zealand

The ∼AD 1305 Kaharoa eruption episode of Tarawera Volcanic Complex was the largest (∼4 km3 magma) eruption in New Zealand in the last 1000 years. High-silica (∼76% SiO2) rhyolite was erupted from seven vents along an 8-km linear zone to form pumiceous pyroclastic deposits and lava domes. Initial vent-clearing explosions were followed by a series of plinian pumice eruptions that spread tephra downwind, and pyroclastic density current deposits over the volcano slopes. The early pyroclastic eruptions were followed by extrusion of a dome in the summit vent and the migration of further plinian activity to adjacent vents. The episode finished with the extrusion of three summit lava domes, accompanied by block-and-ash flows. Two main types of rhyolite were erupted: (1) a low-Zr, low-Sr, high-Rb type comprising the early plinian pyroclastics and (2) a high-Zr, high-Sr, low-Rb type comprising the late eruptives including the summit domes. Basaltic material (basalt and basaltic andesite) is commonly present as free clasts within the Kaharoa pyroclastic deposits, along with rare clasts of granodiorite, diorite, gabbro and olivine clinopyroxenite. The basaltic clasts are often veneered by Kaharoa rhyolite. Basaltic material is also common as inclusions within rhyolite pumices, along with olivine and augite xenocrysts derived from the basalt. The basaltic clasts and inclusions contain complexly zoned plagioclase and corroded olivine (commonly with a reaction rim) consistent with disequilibrium caused by magma mixing. Some basaltic clasts and inclusions are hornblende-free; others contain groundmass hornblende. ‘Plumes’ of brown glass often extend from hornblende-bearing basaltic inclusions into the surrounding rhyolite. These plumes demonstrate magma mixing, as does the presence of rhyolitic glass and quartz xenocrysts in some basaltic inclusions. Most basaltic inclusions have crenulate boundaries, suggesting they were fluidal when coming into contact with the rhyolite magma. Diorite, gabbro and olivine clinopyroxenite clasts have mineralogy similar to the basalts, commonly contain patches of fine-grained basaltic material, and are considered co-magmatic with the basalts. Intrusion of basalt magma into the base of the rhyolite magma chamber appears to have occurred for some time before the Kaharoa eruptions began, allowing considerable mixing with the rhyolite magma, transfer of water to some basalt inclusions and formation of hornblende in their groundmass. Conversely, the hornblende-free basalt inclusions did not have time to react with the rhyolite magma and must have been intruded shortly before the start of the eruption. Intrusion of this hornblende-free basalt appears to have triggered the Kaharoa eruption, after the magma chamber had been primed by the earlier intrusions.

Basalt dikes in the 1886 Tarawera Rift

Abstract Basalt intruded into Tarawera Rift during the 1886 eruption occupies an en echelon pattern of dike-filled eruptive fissures. The leftstepping pattern of dilation fractures is consistent with dextral/normal movement on an underlying, older, master fault which appears to have influenced location of the earlier rhyolite eruptive vents of Tarawera Volcanic Complex, and controlled the orientation of deep basaltic intrusions. Although the Tarawera Rift was previously regarded as a pure normal fault, the 1886 dike pattern provides further geological evidence for the occurrence of dextral shear in the Taupo Volcanic Zone.

Rotomahana—Waimangu eruption, 1886: base surge and basalt magma

Abstract Steam explosions ejecting country rock and lake-floor sediments from Rotomahana-Waimangu accompanied eruption of basaltic scoria from Tarawera Volcanic Complex during the early morning of 1886 June 10. The near-source Rotomahana ejecta displays wave-like internal bed forms typical of base surge eruption mechanisms. Deposition occurred from high velocity, steam-fluidised density currents, which carried ejecta laterally outwards from the base of a large vertical eruption column. The surges extended at least 6 km west of the source and passed over steep hills that rise to 350 m above the eruptive vents. Controversy has existed as to whether basalt magma was erupted from craters in the previously highly active Rotomahana hydrothermal area. New exposures clearly demonstrate that basalt was erupted from Waimangu in 1886; pre-1886 basic scorias outcrop in the Rotomahana area; much near-source Rotomahana ejecta consists of little or non-altered rock. These data are consistent with the existence of a cont...

Structure and eruptive history of the Tarawera Volcanic Complex

Abstract The Tarawera Volcanic Complex comprises 11 rhyolite domes and associated flows and one plug. These extrusions, together with pyroclastic debris, were formed during the Rerewhakaaitu eruption (14,700 ± 400 yr before 1960), the Waiohau eruption (11,250 ± 250 yr before 1960), and the Kaharoa eruption (930 ± 70 yr before 1960). The Tarawera eruption of 10 June 1886 was basaltic, forming the Tarawera Basalt, and terminating in a violently explosive phase which left a linear series of craters cutting three of the rhyolite domes, and providing good internal sections.

Petrography of the rhyolite lavas of Tarawera Volcanic Complex

Abstract The rhyolite domes and associated lava flows of the Tarawera Volcanic Complex, North Island, New Zealand, can be divided into four rock types: hypersthene-hornblende rhyolite; hornblende-biotite-rhyolite; hypersthene rhyolite, and biotite rhyolite. Two types of biotite are found within the lavas; “normal” biotite with a γ refractive index of 1.65–1.67 and “ferrian” biotite with a γ refractive index of 1.72–1.74. Four types of hornblende occur: a pale one (similar to cummingtonite), and green, brown, and basaltic ones. The concentration of K2O in the lava was probably the controlling factor which determined whether biotite or hornblende (or both) crystallised. Plagioclase occurs in all lavas, and quartz in most. Where they both occur initial crystallisation appears almost synchronous and subsequent resorption was probably due to variations in water pressure within the magma reservoir.

Petrology of the basic rocks of the Tarawera Volcanic Complex

Abstract Basic rocks were included as blocks in the Kaharoa eruption of 1020 A.D. and erupted as a tephra deposit on 10 June 1886. The 1886 lava is primarily aphyric although it does contain a few xenocrysts. The earlier basic rocks, which presumably formed an intrusion before the Kaharoa eruption, contain olivine and augite phenocrysts and many xenocrysts. Sinking of the olivine and augite phenocrysts within this intrusion has formed a pyroxene dolerite, and assimilation of rhyolitic material by the basic magma of the intrusion has formed a series of bytownite dolerites.

Basalt Dikes in the Tarawera Volcanic Complex, New Zealand

Abstract Magnetic and geological measurements indicate that a basalt dike occupies the fissure formed during the 1886 eruption in the Tarawera Volcanic Complex, and is confined to the Complex. Parallel to this dike, but 300 yd to the south-east, lies a second dike which is not visible at the surface.