Unveiling the Hidden Gas Legacy of Auckland's Volcanoes

A groundbreaking study led by researchers from the University of Auckland has quantified the massive volcanic gas emissions from the Auckland Volcanic Field over its 200,000-year history. The research reveals that this urban volcanic system beneath New Zealand's largest city has released approximately 26,000 kilotonnes of carbon dioxide (CO2), 9,000 kilotonnes of sulphur dioxide (SO2), 470 kilotonnes of hydrogen chloride (HCl), and 2,220 kilotonnes of hydrogen fluoride (HF). These figures, derived from petrologic analysis of ancient magma samples, mark the first comprehensive estimate for the field and highlight the potent, yet previously unquantified, gas hazards lurking beneath Auckland's 1.8 million residents.

The Auckland Volcanic Field (AVF), a monogenetic basaltic system spanning about 400 square kilometres, features 53 recognised eruptive centres, from small maars to larger scoria cones like Rangitoto Island. Eruptions have been sporadic, with the most recent around 600 years ago, but the field remains active, with a roughly 10% probability of another event in the next 50 years. This new research, part of the long-standing DEVORA (DEtermining VOlcanic Risk in Auckland) programme, underscores the University of Auckland's pivotal role in advancing volcanic hazard science in New Zealand.

The DEVORA Programme: University-Led Collaborative Research

DEVORA, launched in 2008, is a multi-agency initiative spearheaded by volcanologists at the University of Auckland in partnership with GNS Science, Earth Sciences New Zealand, Auckland Council, and the Natural Hazards Commission Toka Tū Ake. Its mission is to reduce uncertainties in volcanic risk assessment for Auckland through transdisciplinary research, providing data for emergency planning and public resilience. The gas emissions study exemplifies this effort, funded by Toka Tū Ake and Auckland Council, and involving experts from multiple New Zealand universities.

Lead author Dr. Elaine R. Smid, a volcanologist at the University of Auckland's School of Environment, brings extensive experience from geothermal and volcanic projects, including her PhD on AVF mechanisms. Co-authors include Michael C. Rowe and Professor Jan M. Lindsay from Auckland, and Dr. Carol Stewart from Massey University's College of Health. Their combined expertise bridges petrology, geochemistry, and hazard modelling, positioning New Zealand universities at the forefront of monogenetic volcanism research.

Petrologic Method: Reconstructing Ancient Gas Releases Step-by-Step

The petrologic degassing method, a cornerstone of modern volcanology, estimates pre-eruptive gas contents by analysing melt inclusions—tiny pockets of pristine magma trapped in crystals during eruption—and comparing them to degassed groundmass glass in tephra. First, researchers collect samples from five compositionally diverse AVF eruptions: Te Kopua Kai-a-Hiku/Panmure Basin (small maar), Ōrākei Basin (small maar), Motukorea (tuff cone with flows), and two phases of Rangitoto (alkali and subalkali).

Volatile concentrations (CO2 via FTIR on 67 inclusions, S/Cl/F via EPMA on hundreds of glasses and inclusions) are measured. Initial magmatic values are corrected for post-entrapment crystallisation using models like Petrolog3. Gas loss is calculated as δv (ppm difference), then emissions M (kg) = V × φ × δv × 10^{-6} / ρ, where V is dense-rock equivalent volume, φ melt fraction (~0.8), ρ density. Sulphur converts to SO2 (×1.99), etc. These are minima, excluding diffuse degassing.

• Emissions scale with volume and composition: Small maars emit modestly (e.g., Panmure Basin: 14 kt CO2), while Rangitoto's Central Scoria Cone dominates (6,430 kt CO2, 3,947 kt SO2).
• Extrapolation uses geochemical-volume trends to assign values to unsampled centres.
• Open-system fluxes model ascent-induced degassing with EVo software; closed-system uses log-normal discharge curves.

Emissions from Key Eruptions: Rangitoto Takes the Lead

Rangitoto, the field's largest and youngest volcano (~0.65 km³ DRE for Central Scoria Cone), accounts for 25-48% of total AVF emissions. Its subalkali phase alone rivals Bárðarbunga 2014-15 in CO2/HCl and exceeds La Palma 2021's SO2. Smaller centres like Motukorea punch above weight relative to volume, emitting gas akin to Parícutin 1943.

These minima highlight how monogenetic bursts concentrate emissions, unlike steady polygenetic output.

Photo by Matthew Stephenson on Unsplash

Total Field Emissions: Negligible Globally, Critical Locally

Extrapolated to all 53 centres, AVF totals average 0.135 kt/yr CO2 and 0.045 kt/yr SO2—insignificant versus global subaerial volcanoes (~39,000 kt/yr CO2). Yet ~65-72% erupted in the last 60,000 years, suggesting clustered activity. In urban context, daily peaks in scenarios reach 70 kt/d SO2 (closed-system), matching hazardous events and dwarfing NZ's ~0.14 kt/d anthropogenic SO2.

This underscores University of Auckland's contribution to local resilience, informing DEVORA scenarios where gases were previously qualitative.

Health and Infrastructure Hazards in Urban Auckland

SO2 irritates lungs, exacerbating asthma (200% risk rise at >1,000 µg/m³); HF causes fluorosis in livestock; HCl corrodes electronics; CO2 asphyxiates in low spots. In Auckland's CBD, plumes could trigger alerts, evacuations. Past soil CO2 baselines (Smid & Mazot 2013) show low fluxes, but eruption fluxes demand upgraded networks. Natural Hazards Commission's Jo Horrocks notes concentrations far exceed Rotorua's, stressing respiratory risks.

Fluxes and Scenarios: Daily Peaks Rival Global Events

DEVORA scenarios (A-H, varying vent size/location) yield max daily SO2 30-70 kt/d, comparable to Bárðarbunga or Miyakejima. Duration matters: Short bursts acute; prolonged corrosive. Open-system pre-eruptive CO2/SO2 (up to 60 days) offers warning potential, vital since vents are unpredictable.

Monitoring Advances: From Soil Gas to Real-Time Networks

Building on 2012 soil CO2 surveys (no anomalies), recommendations include dense SO2/CO2 stations. GeoNet watches for changes signaling magma ascent. University research drives this, enhancing early warning for Auckland's unique distributed field.

Photo by James Lee on Unsplash

University of Auckland's Volcanology Excellence

The School of Environment leads NZ volcanology, with DEVORA fostering PhDs, grants, and international ties. Smid's work exemplifies how uni research translates to policy, protecting communities while advancing global monogenetic knowledge. Collaborations with Massey bolster health/volcanic interfaces.

For aspiring researchers, opportunities abound in AVF studies, from petrology to modelling. DEVORA resources offer public insights.

Future Outlook: Refining Risks for a Resilient Auckland

Updated scenarios will integrate these data, revising volumes/durations for realism. As climate change amplifies vulnerabilities, uni-led monitoring ensures preparedness. This study cements New Zealand universities' role in hazard science, safeguarding the city atop volcanoes.

Auckland Council eyes incorporation into plans, affirming academic impact. Read the full study for technical depth.