Km3 Of Lava Research Articles

Development of Shalbatana Vallis (Mars) by dry volcanic processes

Shalbatana Vallis is a large Martian outflow system that extends into Chryse Planitia from areas of topographic disturbance located north of Ganges Chasma. Development of this system has previously been attributed to processes including: 1) the expulsion of groundwater generated by interactions between magmatic intrusions and ground ice; and 2) the northward conveyance of water along subsurface conduits connected to a deep lake impounded within Ganges Chasma. However, aqueous interpretations of Shalbatana Vallis suffer from weaknesses such as the implausible nature of hypothesized outburst processes, discrepancies between the magnitudes of subsidence that characterize parts of the system and realistic volumes of excess ice that might have once existed in the subsurface, and contradictions between the nature of hypothesized aqueous processes and the mineralogy of associated geological materials. The attributes of Shalbatana Vallis are instead consistent with dry development as a result of the rapid and voluminous effusion of low-viscosity lavas of mafic or ultramafic composition. Volcanic development of the main channel of Shalbatana Vallis is estimated to have required a minimum effusion of ∼360,000 ​km3 of lava, though a total of several times this volume is more likely to have been necessary since channel development cannot be assumed to have taken place with full efficiency. Involved lavas are hypothesized to have been fed by a deeply-rooted igneous plumbing system, and terrain collapse and disturbance located at the head of the system is expected to be partly related to the collapse of intrusions during the latter stages of igneous events. A volcanic origin for Shalbatana Vallis further highlights the critical role played by igneous processes in channel development on rocky bodies of the inner solar system.

A new 1D velocity model and absolute locations image the Mayotte seismo-volcanic region

In May 2018, a seismically quiet region of the Indian Ocean awoke. More than 130 magnitude 4+ earthquakes were recorded in the first month, including a MW 5.9 event on May 15th, 2018. This seismic activity was later identified as being related to an exceptional underwater volcanic eruption offshore Mayotte island, which had emitted more than 6.5 km3 of lava by the time of writing. To better constrain the geodynamic processes responsible for the seismic and volcanic activity, a new network of ocean-bottom seismometers and land stations has been deployed around the seismically active region since February 2019. We present here an improved 1D velocity model for the active area and relocations of manually-picked earthquakes using this new model. The best-constrained events image detailed structures within two clusters of seismic activity east of Mayotte. The westernmost, proximal cluster, close to Mayotte's Petite-Terre island, has a “donut” shape horizontally and an “hourglass” shape in depth. The events distribution suggests the presence of a magma reservoir at around 27 km depth, with earthquakes focused along its sides, and a collapsing system underneath, related to the drainage of another, deeper magma storage zone. The distal cluster, focused 30–50 km offshore of Petite-Terre island, highlights the propagation of a dike between 45 and 25 km depth, aligned towards the new volcanic activity on the seafloor. We interpret this cluster as the fluid pathway towards the new volcano and nearby active seafloor lava fields. The improved velocity model also permits more robust daily monitoring of the seismicity using land stations, allowing local authorities to better assess seismic and volcanic hazards and to communicate them to the island's population.

Onset and evolution of Kīlauea's 2018 flank eruption and summit collapse from continuous gravity

Prior to the 2018 lower East Rift Zone (ERZ) eruption and summit collapse of Kīlauea Volcano, Hawai‘i, continuous gravimeters operated on the vent rims of ongoing eruptions at both the summit and Pu‘u ‘Ō‘ō. These instruments captured the onset of the 2018 lower ERZ eruption and the effects of lava withdrawal from both locales, providing constraints on the timing and style of activity and the physical properties of the lava lakes at both locations. At the summit, combining gravity, lava level, and a three-dimensional model of the vent indicates that the upper ∼200 m of the lava lake had a density of about 1700 kgm−3, slightly greater than estimates from 2011–2015 and possibly indicating a gradual densification over time. At Pu‘u ‘Ō‘ō, gravity and vent geometry were used to model both the density and the rate of crater collapse, which was unknown owing to a lack of visual observations. Results suggest the withdrawal of at least 11×106 m3 of lava over the course of two hours, and a material density of 1800–1900 kgm−3. In addition, gravity data at Pu‘u ‘Ō‘ō captured a transient decrease and increase about an hour prior to crater collapse and that was probably related to a small, short-lived fissure eruption on the west flank of the cone and possibly to dike intrusion beneath Pu‘u ‘Ō‘ō. The fissure was the first event in the subsequent cascade that ultimately led to the extrusion of over 1 km3 of lava from lower ERZ vents, collapse of the summit caldera floor by more than 500 m, and the destruction of over 700 homes and other structures. These results emphasize the importance of continuous gravity in operational monitoring of active volcanoes.

Geomorphological characterization of the 2014–2015 Holuhraun lava flow-field in Iceland

The 2014–2015 Holuhraun eruption extruded >1 km3 of lava in a barren region of the Icelandic highlands. Due to its large volume and the abundance of data for this eruption, Holuhraun is an ideal site to investigate fissure-fed eruption products for comparison with other large lava flow-fields on Earth and other planetary bodies. To characterize lava morphologies associated with this event, we used 0.01–0.5 m/pixel image data, acquired from aerial surveys and small Unoccupied Aircraft Systems (sUAS) to create a 1:800-scale facies map that was ground-truthed using field observations during the summers of 2015–2019. Each facies region exhibits similar attributes in the remote sensing data, including albedo, surface texture, and geomorphology. However, at our mapping scale of 1:800, the facies typically include mixtures of lava types. Results show that transitional lava types (e.g., rubbly pāhoehoe, slabby pāhoehoe, and spiny pāhoehoe) dominate the 2014–2015 Holuhraun lava flow-field (83.82 km2), rather than the traditional end-members of ʻaʻā and classical pāhoehoe. At 1:800-scale, we distinguish the following eight facies (with the percentage of total flow-field area shown in parentheses): rubbly (57.35%), spiny (25.96%), undifferentiated rubbly–spiny (9.59%), shelly (5.58%), pāhoehoe (1.24%), flat-lying–knobby (0.58%), vent-proximal edifice (0.19%), and channel interior (0.16%). Field observations show that initial coherent pāhoehoe surfaces were episodically disrupted to produce slabby and rubbly textures that resemble ʻaʻā in remote sensing data. Our observations also show that continued solidification of the lava beneath brecciated surfaces can cause the surfaces of disrupted lobes to stabilize, or restabilize and undergo inflation. These factors complicate the use of surface texture as a direct indicator of emplacement style, which can change over the course of an eruption. This complexity has important implications for reconstructing the emplacement history of flow-fields on Earth and other planetary bodies.

The Deccan Volcanic Province (DVP), India: A Review

The Deccan Volcanic Province (DVP), India: A Review

Constraints on the nature of the effusive volcanic eruptions that incised Ravi Vallis, Mars

Ravi Vallis is a Martian outflow channel that is widely interpreted as a product of one or more catastrophic outbursts of groundwater from adjacent Aromatum Chaos. However, solar system analogs are unknown for formation of large channels by outbursts from aquifers, and the Ravi Vallis region lacks the obvious sedimentological and mineralogical signatures expected of aqueous origins. Instead, the basic nature of this channel system is consistent with volcanic origins involving voluminous effusions of low-viscosity lavas. On bedrock slopes of only 0.2°, 50-m-deep lava flows with viscosities of 1 Pa s are predicted to have been characterized by discharge rates of ∼3 × 107 m3/s for 25-km-wide flows, with associated mechanical incision rates of ∼4 m/day and thermal incision rates of ∼2 m/day. Formation of Aromatum Chaos and the preserved 215-km-long extent of Ravi Vallis is conservatively estimated on the basis of thermal considerations to have required eruption of a minimum of ∼64,000 km3 of lava, but greater volumes would have been necessary to form the original (unknown) length of this channel system. A volcanic origin for Ravi Vallis is consistent with development of all Martian outflow channels by igneous processes, and with the broader premise that early development of immense volcanic channel systems is typical of all large rocky bodies.

Insights Into the Stress Field Around Bárarbunga Volcano From the 2014/2015 Holuhraun Rifting Event

AbstractThe two weeklong rifting event at Bárðarbunga volcano in 2014 led to the Holuhraun eruption, which produced 1.5 km3 of lava and was the largest in Iceland in over 200 years. Predicting when and where an intrusion will lead to eruption requires detailed knowledge of the underlying stress field. Previous studies have explained the dike propagation path with a model that includes a tectonically induced stress field set up by a uniform amount of plate spreading across a straight rift axis. Here we test this hypothesis by modeling the tractions acting on the dike walls, constrained by data from Global Navigation Satellite System and Interferometric Synthetic Aperture Radar. Our results show that the majority of the opening and shearing in the final two dike segments is due to stresses built up by plate spreading since the last eruption at Holuhraun, as expected, but that the tectonically induced stress magnitude must be much lower to explain the movement of the dike walls further south. This result implies that most of the tectonically induced stress beneath the ice cap has been released, presumably due to intrusions associated with the Bárðarbunga volcanic system and the nearby Grímsvötn volcanic system, which have not been detected due to their subglacial nature. Modeling of the 2014 Bárðarbunga rifting event therefore not only yields insights into the event but also provides a window into undetected volcanic activity in the past.

Extended SO2 outgassing from the 2014\u20132015 Holuhraun lava flow field, Iceland

The 2014–2015 Holuhraun eruption was the largest fissure eruption in Iceland in the last 200 years. This flood basalt eruption produced ~ 1.6 km3 of lava, forming a lava flow field covering an area of ~ 84 km2. Over the 6-month course of the eruption, ~ 11 Mt of SO2 were released from the eruptive vents as well as from the cooling lava flow field. This work examines the post-eruption SO2 flux emitted by the Holuhraun lava flow field, providing the first study of the extent and relative importance of the outgassing of a lava flow field after emplacement. We use data from a scanning differential optical absorption spectroscopy (DOAS) instrument installed at the eruption site to monitor the flux of SO2. In this study, we propose a new method to estimate the SO2 emissions from the lava flow field, based on the characteristic shape of the scanned column density distribution of a homogenous source close to the ground. Post-eruption outgassing of the lava flow field continued for at least 3 months after the end of the eruption, with SO2 flux between < 1 and 9 kg/s. The lava flow field post-eruption emissions were not a significant contributor to the total SO2 released during the eruption; however, the lava flow field was still an important polluter and caused high concentrations of SO2 at ground level after lava effusion ceased.

Tremor-rich shallow dyke formation followed by silent magma flow at Bárðarbunga in Iceland

The Barðarbunga eruption in Iceland in 2014 and 2015 produced about 1.6 km3 of lava. Magma propagated away from Barðarbunga to a distance of 48 km in the subsurface beneath Vatnajokull glacier, emerging a few kilometres beyond the glacier’s northern rim. A puzzling observation is the lack of shallow (<3 km deep), high-frequency earthquakes associated with shallow dyke formation near the subaerial and subglacial eruptive sites, suggesting that near-surface dyke formation is seismically quiet. However, seismic array observations and seismic full wavefield simulations reveal the presence and nature of shallow, pre-eruptive, long-duration seismic tremor activity. Here we use analyses of seismic data to constrain the relationships between seismicity, tremor, dyke propagation and magma flow during the Barðarbunga eruption. We show that although tremor is usually associated with magma flow in volcanic settings, pre-eruptive tremor at Barðarbunga was probably caused by swarms of microseismic events during dyke formation, and hence is directly associated with fracturing of the upper 2–3 km of the crust. Subsequent magma flow in the newly formed shallow dyke was seismically silent, with almost a complete absence of seismicity or tremor. Hence, we suggest that the transition from temporarily isolated, large, deep earthquakes to many smaller, shallower, temporally overlapping earthquakes (<magnitude 2) that appear as continuous tremor announces the arrival of a dyke opening in the shallow crust, forming a pathway for silent magma flow to the Earth’s surface. Magma movement is thought to trigger volcanic tremor. However, analysis of seismic data suggests that tremor prior to the Barðarbunga eruption in Iceland instead marked the crust cracking open, whereas subsequent magma flow was seismically silent.

Satellite detection, long‐range transport, and air quality impacts of volcanic sulfur dioxide from the 2014–2015 flood lava eruption at Bárðarbunga (Iceland)

AbstractThe 2014–2015 Bárðarbunga‐Veiðivötn fissure eruption at Holuhraun produced about 1.5 km3 of lava, making it the largest eruption in Iceland in more than 200 years. Over the course of the eruption, daily volcanic sulfur dioxide (SO2) emissions exceeded daily SO2 emissions from all anthropogenic sources in Europe in 2010 by at least a factor of 3. We present surface air quality observations from across Northern Europe together with satellite remote sensing data and model simulations of volcanic SO2 for September 2014. We show that volcanic SO2 was transported in the lowermost troposphere over long distances and detected by air quality monitoring stations up to 2750 km away from the source. Using retrievals from the Ozone Monitoring Instrument (OMI) and the Infrared Atmospheric Sounding Interferometer (IASI), we calculate an average daily SO2 mass burden of 99 ± 49 kilotons (kt) of SO2 from OMI and 61 ± 18 kt of SO2 from IASI for September 2014. This volcanic burden is at least a factor of 2 greater than the average SO2 mass burden between 2007 and 2009 due to anthropogenic emissions from the whole of Europe. Combining the observational data with model simulations using the United Kingdom Met Office's Numerical Atmospheric‐dispersion Modelling Environment model, we are able to constrain SO2 emission rates to up to 120 kilotons per day (kt/d) during early September 2014, followed by a decrease to 20–60 kt/d between 6 and 22 September 2014, followed by a renewed increase to 60–120 kt/d until the end of September 2014. Based on these fluxes, we estimate that the eruption emitted a total of 2.0 ± 0.6 Tg of SO2 during September 2014, in good agreement with ground‐based remote sensing and petrological estimates. Although satellite‐derived and model‐simulated vertical column densities of SO2 agree well, the model simulations are biased low by up to a factor of 8 when compared to surface observations of volcanic SO2 on 6–7 September 2014 in Ireland. These biases are mainly due to relatively small horizontal and vertical positional errors in the simulations of the volcanic plume occurring over transport distances of thousands of kilometers. Although the volcanic air pollution episodes were transient and lava‐dominated volcanic eruptions are sporadic events, the observations suggest that (i) during an eruption, volcanic SO2 measurements should be assimilated for near real‐time air quality forecasting and (ii) existing air quality monitoring networks should be retained or extended to monitor SO2 and other volcanic pollutants.

Next article &gt;&gt; &lt;&lt; Previous article Environmental pressure from the 2014–15 eruption of Bárðarbunga volcano, Iceland

The effusive six months long 2014-2015 Barðarbunga eruption (31 August-27 February) was the largest in Iceland for more than 200 years, producing 1.6 ± 0.3 km3 of lava. The total SO2 emission was 11 ± 5 Mt, more than the amount emitted from Europe in 2011. The ground level concentration of SO2 exceeded the 350 μg m−3 hourly average health limit over much of Iceland for days to weeks. Anomalously high SO2 concentrations were also measured at several locations in Europe in September. The lowest pH of fresh snowmelt at the eruption site was 3.3, and 3.2 in precipitation 105 km away from the source. Elevated dissolved H2SO4, HCl, HF, and metal concentrations were measured in snow and precipitation. Environmental pressures from the eruption and impacts on populated areas were reduced by its remoteness, timing, and the weather. The anticipated primary environmental pressure is on the surface waters, soils, and vegetation of Iceland.

SAR mapping of Burfellshraun: A terrestrial analog for recent volcanism on Mars

Images of the Northern plain on Mars have revealed up to 1500 km long lava flows. The low density of impact craters on the lava flows implies that these lava flows were formed during the most recent volcanic activity on Mars. Estimates of the ages of the flows are controversial but generally between 10 and 100 Myr. Parts of the lava flows show smooth kilometer‐sized plates that appear to have rafted on the moving lava flow. We have found a terrestrial analogue with similar features, the Burfellshraun lava flow in Northern Iceland. This is the only known lava flow on Earth where kilometer‐sized rafting plates are observed and constitute the dominating feature. Using a combination of airborne remote sensing data and field observations we have studied the emplacement and physical properties of the flow. On the basis of our observations we have reconstructed the sequence of events that led to the disruption of the lava surface. Our observations show that a ∼45 km2 lava lake, holding about 0.7 km3 of lava, catastrophically collapsed releasing a flood of lava. During the collapse the crust of the lava lake was broken into plates up to a kilometer in width and a few meters thick that floated downstream for almost 10 km.

Records of the influence of Deccan volcanism on contemporary sedimentary environments in Central India

Deccan volcanism, with a duration of ∼5 Myr, represents a major continental flood basalt province in which ∼2 million km3 of lavas were formed. Such voluminous and rapid volcanism in Phanerozoic earth history has commonly been advanced as a cause for mass extinctions. Despite this significance, few studies have attempted to understand the influence of continental flood basalt volcanism on contemporary sedimentary environments.

Low-magnitude earthquakes in Rome: structural interpretation and implications for the local stress field

On 1995 June 12, a ML = 3.6 earthquake occurred in the Cecchignola area of southern Rome. A detailed structural investigation of this area and an analysis of macroseismic and instrumental data indicate a seismogenic area for the local low-magnitude seismicity that affects the city of Rome. A peripheral volcanic field in Cecchignola shows that the local seismicity could be related to the same volcanotectonics that affects the Colli Albani (Alban Hills) region. Fracture-field emission is proposed for the emplacement of about 2.5 km3 of lava, which developed a small plateau that was subsequently intruded by the feeder dyke of a large ignimbrite. The geometry of this dyke as well as the faults disrupting the lava field indicates NE–SW extension. Younger tectonic features show a NW-striking σ3 in the studied area, suggesting that dextral N–S strike-slip displacement generated structures that are not compatible with the local present-day stress field, characterized by a NE-striking σ3. An analysis of focal solutions and the spatial distribution of the 1995 June 12 microearthquake sequence suggests an E–W seismogenic structure whose trend is atypical of the structural setting of the area. The present study identifies surface faulting with the same trend as the focal mechanisms in the epicentral area and suggests that these faults represent an inactive surface expression of the deeper structures. It is also suggested that these structures originated during a different tectonic regime and are presently reactivated at depth by the NE–SW extension. The general NE orientation of the T axes from focal mechanisms for the Colli Albani earthquakes and for the 1995 June 12 Cecchignola event strongly supports a regional interpretation for the present-day stress field previously observed only in the Colli Albani area. It is likely that a combination of different tectonic styles during the last 600 kyr is necessary to explain the structural features of the Cecchignola area and of the Central Tyrrhenian Sea margin of Italy in general.

Post‐11,000‐year volcanism at Medicine Lake Volcano, Cascade Range, northern California

Eruptive activity during the past 11,000 years at Medicine Lake volcano has been episodic. Eight eruptions produced about 5.3 km3 of basaltic lava during an interval of a few hundred years about 10,500 years B.P. After a hiatus of about 6000 years, eruptive activity resumed with a small andesite eruption at about 4300 years B.P. Approximately 2.5 km3 of lava with compositions ranging from basalt to rhyolite vented in nine eruptions during an interval of about 3400 years in late Holocene time. The most recent eruption occurred about 900 years B.P. A compositional gap in SiO2 values of erupted lavas occurs between 58 and 63%. The gap is spanned by chilled magmatic inclusions in late Holocene silicic lavas. Late Holocene andesitic to rhyolitic lavas were probably derived by fractionation, assimilation, and mixing from high‐alumina basalt parental magma, possibly from basalt intruded into the volcano during the early mafic episode. Many basaltic to andesitic lavas contain iron‐rich crystals and have high FeO*/MgO (characteristics caused by mixing of high‐alumina basalt with ferrobasalt liquid produced by fractionation of parental high‐alumina basalt). When ferrobasalt and high‐alumina basalt are contaminated with a granitic crustal component, a calc‐alkaline trend is produced. Some eruptions have produced both tholeiitic and calc‐alkaline compositions. The eruptive activity is probably driven by intrusions of basalt that occur during east‐west stretching of the crust in an extensional tectonic environment. Vents are typically aligned parallel or subparallel to major structural features, most commonly within 30° of north. Intruded magma should provide adequate heat for commercial geothermal development if sufficient fluids can be found. The nature and timing of future volcanic activity cannot be predicted from the observed pattern, but eruptions high on the edifice could produce high‐silica products that might be accompanied by explosive activity, whereas eruptions lower on the flanks are likely to vent more fluid mafic lavas.

Changes in magma composition at Arenal volcano, Costa Rica, 1968?1985: Real-time monitoring of open-system differentiation

Arenal volcano in Costa Rica has been erupting nearly continuously, but at a diminishing rate, since 1968, producing approximately 0.35 km3 of lavas and tephras that have shown consistent variations in chemistry and mineralogy. From the beginning of the eruption in July 1968 to early 1970 (stage 1, vol.=0.12 km3) tephras and lavas became richer in Ca, Mg, Ni, Cr, Fe, Ti, V, and Sc and poorer in Al2O3 and SiO2. Concentrations of incompatible trace elements (including Sr) decreased by 5%–20%. Phenocryst contents increased 20–50 vol%. During stage 2 (1970–1973, vol. = 0.13 km3) concentrations of compatible trace elements rose, and concentrations of incompatible trace elements either remained constant or also rose. Al2O3 contents decreased by 1 wt%. Phenocryst content increased slightly, principally due to increased orthopyroxene. During stage 3 (mid-1974 to the present, vol.= 0.10 km3) concentrations of SiO2 increased by 1 wt%, compatible trace elements decreased slightly, and incompatible trace element concentrations increased by 5% to 10%. Although crystals increased in size during stage 3, their overall abundance stayed roughly constant. Our modeling suggests that early stage-1 magmas were produced by boundary layer fractionation under high-p H2O conditions of an unseen basaltic andesitic magma that intruded into the Arenal system after approximately 500 B.P. Changes in composition during stage 2 resulted from mixing of this more mafic original magma with new magma that had a similar SiO2 content, but higher compatible and incompatible element concentrations. The changes during stage 3 resulted from continued influx of the same magma plus crystal removal. We conclude that the eruption proceeded in the following way. Before 1968 zoned stage-1 magma resided in the deep crust below Arenal. A new magma intruded into this chamber in July 1968 causing ejection of the stage-1 magmas. The intruding magma mixed with mafic portions of the original chamber producing the mixed lavas of stage 2. Continued mixing plus crystal fractionation along the chamber and conduit walls produced stage-3 lavas. The time scales of crustal level magmatic processes at Arenal range 100–103 years, which are 3–6 orders of magnitude shorter than those of larger, more silicic systems.

A geophysical study of the Taupo Volcanic Zone New Zealand

The Taupo Volcanic Zone is a region of intense volcanic activity, from which over 16,000 km3 of lavas and pyroclastics have been erupted since the Late Pliocene. Analysis of gravity and magnetic anomalies over the zone has revealed the presence of five major centers, each associated with an approximately circular negative gravity anomaly and a positive pseudogravity anomaly. These centers are interpreted as collapse features resulting from large‐scale withdrawal of magma from the upper crust. Four of the depressions occur beneath recognized volcanic centers or calderas; the fifth (Kapenga) is inferred to underlie an extinct hidden volcanic center. Three‐dimensional models of the thicknesses of low density and magnetized rocks in the Taupo Volcanic Zone have been determined by inversion of gravity and magnetic anomalies. Beneath three of the centers, the thicknesses of low density and magnetized rocks are approximately equal (2–3 km); that is, the collapse structures are infilled by magnetized volcanics. However, there is a significant discrepancy between the thicknesses of the gravity (5 km) and magnetic models (1.5 km) in the region of the Okataina Volcanic Centre. It is therefore suggested that a significant volume of low density, nonmagnetic rock, possibly acidic magma, underlies Okataina.

Patterns of Quaternary basaltic volcanism in the northern North Island, New Zealand

Abstract North of Taupo Volcanic Zone (TVZ) are 4 major Quaternary volcanic fields; South Auckland, Auckland, Whangarei, and Kaikohe-Bay of Islands. Volcanoes in these fields are monogenetic and include maars and scoria cones atop lava pedestals. Volumes are small (<1 km3) although a few centres erupted approximately 5 km3 of lava and scoria. Early,, often explosive, undersaturated volcanism is followed by eruption of more voluminous saturated basalts. Differences in chemistry and volcanic style between earlier and later volcanism are matched by differences in texture and mineralogy of the lavas. A similar style of basaltic volcanism occurred throughout the Pliocene and Miocene. The basalts are explained as products of progressive melting of mantle diapirs formed by convection in the mantle wedge to the rear of the TVZ. Melting probably occurs between 70 and 120 km, which is a zone of low Q beneath the northern North Island. Ascent rates of mantle diapirs of the order of 102 cm/year are suggested by the r...

Quaternary alkalic and sub-alkalic volcanism in South Auckland, New Zealand

A significant feature of Quaternary volcanism in New Zealand is the presence of several compact fields of monogenetic basaltic volcanism, in many respects comparable to back-arc volcanism of Japan and intra-plate volcanism in S.E. Australia. Volcanism in South Auckland between 1.56 m.y. (B.P.) and 0.51 m.y. has produced some 20 km3 of lava and scoria from at least 70 centres. Older lavas are alkalic (basanite and alkali-olivine basalt), and are associated with maars and small scoria cones and many contain mafic and ultramafic xenoliths and megacrysts. Younger subalkalic lavas (mainly transitional tholeiite) form large scoria cones with extensive lava aprons.