Geology topics

Graham S. Leonard

Publications and source records attributed to Graham S. Leonard.

7 recordsLinked to original sources

USGS2025

High-precision 40Ar/39Ar dating of Quaternary basalts from Auckland Volcanic Field, New Zealand, with implications for eruption rates and paleomagnetic correlations

The Auckland Volcanic Field (AVF), which last erupted ca. 550 years ago, is a late Quaternary monogenetic basaltic volcanic field (ca. 500 km 2 ) in the northern North Island of New Zealand. Prior to this study only 12 out of the 53 identified eruptive centres of the AVF had been reliably dated. Careful sample preparation and 40 Ar/ 39 Ar analysis has increased the number of well-dated centres in the AVF to 35. The high precision of the results is attributed to selection of fresh, non-vesicular, non-glassy samples from lava flow interiors. Sample selection was coupled with separation techniques that targeted only the groundmass of samples with < 5% glass and with groundmass feldspars > 10 μm wide, coupled with ten-increment furnace step-heating of large quantities (up to 200 mg) of material. The overall AVF age data indicate an onset at 193.2 ± 2.8 ka, an apparent six-eruption flare-up from 30 to 34 ka, and a ≤ 10 kyr hiatus between the latest and second-to-latest eruptions. Such non-uniformity shows that averaging the number of eruptions over the life-span of the AVF to yield a mean eruption rate is overly simplistic. Together with large variations in eruption volumes, and the large sizes and unusual chemistry within the latest eruptions (Rangitoto 1 and Rangitoto 2), our results illuminate a complex episodic eruption history. In particular, the rate of volcanism in AVF has increased since 60 ka, suggesting that the field is still in its infancy. Multiple centres with unusual paleomagnetic inclination and declination orientations are confirmed to fit into a number of geomagnetic excursions, with five identified in the Mono Lake, two within the Laschamp, one within the post-Blake or Blake, and two possibly within the Hilina Pali.

Auckland Volcanic Field↗

USGS2022

Earthquakes and tsunami

Earthquakes occur as a burst of sudden ground shaking created by the release of accumulated stress along a fault, often influenced by movement of the world’s tectonic plates. Ground shaking from an earthquake can generate additional hazards, including landslides, liquefaction, and tsunami. According to the 2019 “Global Assessment Report on Disaster Risk Reduction”, earthquakes combined with tsunami are the most damaging environmental hazards globally. Impacts of earthquakes and tsunami on people have increased around the world as human development of built infrastructure continues to expand. Adverse earthquake and tsunami impacts can be reduced through strategies including land-use planning, engineering, mitigation and preparedness, emergency planning, warnings, and exercises. The specific disaster risk reduction approaches taken will depend on the country, considering the geography, built environment, and social and cultural contexts. Wherever the location, it is important that such measures are considered and implemented holistically, as singular approaches may not be effective in addressing earthquake and tsunami challenges.

Book chapter↗

USGS2021

From anecdotes to quantification: Advances in characterizing volcanic eruption impacts on the built environment

Over the past 20 years, our understanding of volcanic eruption impacts on the built environment has transformed from being primarily observational with small datasets to one grounded in field investigations, laboratory experiments, and quantitative modeling, with an emphasis on stakeholder collaboration and co-creation. Here, we summarize key advances and knowledge gaps of impacts across volcanic hazards and built environment types from the past 20 + years. Studies have concentrated on impacts from tephra fall (ash) and to buildings, with less examination of other hazards’ impacts to critical infrastructure. As we look to the next decade, we speculate on likely research directions, including the increasing role of new technologies, higher resolution modeling, transdisciplinary collaborations, and evidence-based mitigative solutions.

Bulletin of Volcanology↗

USGS2020

Geologic map of the Paeroa Fault block and surrounding area, Taupo Volcanic Zone, New Zealand

The Taupo Volcanic Zone (TVZ), New Zealand, is the most productive area of explosive silicic volcanism in the world. Faulted early and middle Pleistocene volcanic products are generally concealed beneath voluminous, generally unfaulted, younger volcanic products. An exception is the southeast margin of the TVZ where the two parallel, northeast-trending Paeroa and Te Weta Fault blocks expose Quaternary volcanic products consisting predominantly of caldera-related, rhyolitic ignimbrites and lacustrine sediments. The Taupo-Reporoa Basin is situated along the eastern part of the map area, and its northernmost part underwent collapse to form Reporoa Caldera. The Paeroa Fault block is the largest exposed fault block within the TVZ, and it encompasses early and middle Pleistocene ignimbrites and sedimentary deposits that are buried throughout the Taupo-Reporoa Basin to the east. This map displays the volcanic and sedimentary geology of ~430 km 2 of the Paeroa Fault block and the adjacent Te Weta Fault block at a scale of 1:50,000. Volcanic and sedimentary rocks are divided into the Reporoa Group, Whakamaru Group, and Huka Group (from oldest to youngest), which are overlain by relatively unfaulted late Pleistocene and Holocene surficial volcanic and sedimentary deposits.

Open-File Report↗

USGS2019

Ar-Ar age constraints on the timing of Havre Trough opening and magmatism

The age and style of opening of the Havre Trough back-arc system is uncertain due to a lack of geochronologic constraints for the region. 40 Ar/ 39 Ar dating of 19 volcanic rocks from across the southern Havre Trough and Kermadec Arc was conducted in three laboratories to provide age constraints on the system. The results are integrated and interpreted as suggesting that this subduction system is young (<2 Ma) and coeval with opening of the continental Taupo Volcanic Zone of New Zealand. Arc magmatism was broadly concurrent across the breadth of the Havre Trough.

Kermadec Arc–Havre Trough↗

USGS2018

New petrological, geochemical, and geochronological perspectives on andesite-dacite magma genesis at Ruapehu volcano, New Zealand

Time–composition relationships in eruptive sequences at composite volcanoes can show how the ongoing intrusion of magmas progressively affects the lithosphere at continental convergent margins. Here, new whole-rock and microanalytical major and trace element data from andesite-dacite lava flows are integrated with previous studies and existing isotopic data, and placed within the framework of a high-resolution chronostratigraphy for Ruapehu volcano (southern Taupo Volcanic Zone, New Zealand). The geochemical evolution of lavas erupted over the ∼200 kyr lifetime of the exposed edifice reflects variable degrees of fractionation and systematic changes in the type of crustal assimilation in the Ruapehu magma system. Lavas erupted from ∼200–150 ka have previously been distinguished from those erupted <150 ka based on Sr-Nd isotopic characteristics, which indicate that the oldest lavas were sourced from magmas that assimilated oceanic crust. Such source rocks underlie the regionally widespread Mesozoic meta-sedimentary greywacke-argillite basement, which was conversely assimilated by <150 ka magmas. New results from this work reveal that since 150 ka, an upper limit of magma differentiation occurred from ∼50–35 ka. High K 2 O (∼6 wt%) and Rb contents (∼270 ppm) in melt inclusions, interstitial glass, and glass from in situ quenched melts of partially fused crustal xenoliths are reported for andesite-dacite lavas erupted during this period. In addition to crystal fractionation, selective partial melting and assimilation of K- and Rb-rich mineral phases (e.g., biotite, K-feldspar) that are significant components of the meta-sedimentary basement rocks is inferred to explain these geochemical characteristics. These processes coincided also with the effusion of high-MgO andesitedacite lavas that display petrological evidence for mixing between andesite-dacite and more mafic magmas. An influx of hotter mafic magma into the system explains why the extent of crustal assimilation recorded by Ruapehu lavas peaked during the ∼50–35 ka eruptive period. From 26 ka to the present, andesite lavas have reverted to more mafic compositions with less potassic melt inclusion and whole-rock compositions when compared to the ∼50–35 ka lavas. We suggest that the younger lavas assimilated less-enriched melts because fertile phases had been preferentially extracted from the crustal column during earlier magmatism. This scenario of bottom-up heating of the lithosphere and exhaustion of fertile phases due to the progressive intrusion of magma explains the geochemical evolution of Ruapehu lavas. This model may be applicable to other long-lived composite volcanoes of the circum-Pacific continental arcs.

Taupo Volcanic Zone↗

USGS2014

Variable population exposure and distributed travel speeds in least-cost tsunami evacuation modelling

Evacuation of the population from a tsunami hazard zone is vital to reduce life-loss due to inundation. Geospatial least-cost distance modelling provides one approach to assessing tsunami evacuation potential. Previous models have generally used two static exposure scenarios and fixed travel speeds to represent population movement. Some analyses have assumed immediate departure or a common evacuation departure time for all exposed population. Here, a method is proposed to incorporate time-variable exposure, distributed travel speeds, and uncertain evacuation departure time into an existing anisotropic least-cost path distance framework. The method is demonstrated for hypothetical local-source tsunami evacuation in Napier City, Hawke's Bay, New Zealand. There is significant diurnal variation in pedestrian evacuation potential at the suburb level, although the total number of people unable to evacuate is stable across all scenarios. Whilst some fixed travel speeds approximate a distributed speed approach, others may overestimate evacuation potential. The impact of evacuation departure time is a significant contributor to total evacuation time. This method improves least-cost modelling of evacuation dynamics for evacuation planning, casualty modelling, and development of emergency response training scenarios. However, it requires detailed exposure data, which may preclude its use in many situations.

Natural Hazards and Earth System Sciences↗