Terrestrial basaltic volcanic fields consist of tens to hundreds of single-eruptive-episode (monogenetic) volcanoes. These fields are among the most common volcanic features on Earth’s surface and can cover areas up to thousands of square kilometers. Although such fields commonly are active for hundreds of thousands of years, individual eruptions are rare (perhaps one every thousand or tens of thousands of years). The eruptions are small and short-lived and last several days to decades, producing a small cone or crater. Despite the low eruption frequency of each volcano within the field, the spread of human infrastructure into these fields in many parts of the world necessitates a better understanding of future hazards.
Mexico City, Mexico, is an example of a metropolitan area built close to or on basaltic fields. Long-term storage of nuclear waste within a basaltic field in Nevada has been previously proposed. These are just a few examples of where future volcanic activity would significantly impact society.
Therefore, several major questions need to be addressed: (1) How long do such eruptions last? (2) How does the magma rise to the surface, and does its ascent provide measurable phenomena that could act as precursor signals? (3) How does the style of eruption change during the event, and thus, what spectrum of hazards would be expected?
Rangitoto Island Volcano
Auckland City, New Zealand, is built on the Auckland Volcanic Field (AVF) (Figure 1) [Kermode, 1992]. The AVF has been the intense focus of hazard and risk studies because of the city’s large population (~1.4 million people) and its economic significance to New Zealand [Houghton et al., 2006]. The field comprises at least 50 volcanic centers and has been active for the past 250,000 years.
The youngest volcano, Rangitoto Island, which is only 8 kilometers from the central business district (Figure 1), erupted about 550 years ago. It is a symmetrical, approximately 6-kilometer-wide shield volcano rising about 260 meters above sea level with an estimated dense-rock volume of 1.78 cubic kilometers. This is about half of the estimated erupted magma volume of the field.
Rangitoto comprises a gently dipping lava field and several small scoria cones at the summit, although the absence of deep dissection of the volcano has prevented the development of a stratigraphic framework of volcanic events. Volcanic ash sourced from Rangitoto and preserved in nearby lake sediments suggests that the volcano may have been active intermittently, starting about 1500 years ago and persisting for about 1000 years [Shane et al., 2013]. This finding challenges the fundamental model for the mechanisms and hazards associated with these small volcanoes, typically considered monogenetic.
Drilling Investigation
The new finding prompted a new scientific drilling initiative by a consortium of geologists from the University of Auckland and Massey University, funded by New Zealand’s Earthquake Commission, to investigate Rangitoto’s eruption history. The aim of the project was to drill through the entire volcano edifice to recover a continuous record of deposits. Hence, there is the potential to develop an unparalleled insight into the birth, life, and death of a small monogenetic volcano that will have wider applications to understanding these systems globally.
The team selected a drill site at an elevation of about 120 meters above sea level on the western flank of the volcano (Figure 1) to optimize the thickness (and thus completeness) of the stratigraphic record obtained while avoiding the potentially chaotic deposits of the main vent region near the summit. Drilling was completed in approximately 3 weeks in February 2014.
The approximately 150-meter-deep drill hole resulted in excellent core recovery (>95%). The upper 128 meters of core comprise at least 27 lava flows with thicknesses in the range 0.3–15 meters, representing the main shield-building phase. The lavas overlie marine sediments interbedded with lava and pyroclastic (explosive) deposits. The pyroclastic sequence comprises about 8 meters of phreatomagmatic (water-magma interaction) ash and lapilli, representing the subaqueous birth of the volcano. Miocene (about 20 million years ago) sediments were encountered at about 150 meters.
Preliminary Results
A preliminary geochemical investigation of the core revealed a suite of relatively uniform transitional basalts (magnesium oxide between 8.1 and 9.7 weight percent). However, distinct compositional trends are evident in the sequence (Figure 1), suggesting that multiple magma batches were erupted. In particular, the youngest lavas (0–25 meters) are distinguished by their low magnesium oxide levels and high abundance of some trace elements (e.g., strontium; Figure 1).
The core places the lava compositions in a time series, which can be correlated to the surface lava field. This will allow a geometrical reconstruction of the shield growth. Additional petrologic investigations are under way to provide insight into magma ascent processes. This will ultimately lead to better models for magma production and eruption duration. Chronological investigations, including radiocarbon dating and paleomagnetic secular variation studies, are in progress, in an attempt to constrain the duration of volcanism.
The drilling attracted significant attention from the media, including TV, radio, and newspaper items in New Zealand and Australia. Eventually, the research will contribute to public and scientific awareness of the history of Rangitoto volcano and volcanic hazards associated with such volcanoes globally.
Acknowledgments
The drilling investigation team comprised Paul Augustinus, Tamzin Linnell, Jan Lindsay, and Ian Smith (University of Auckland) and Shane Cronin (Massey University).
References
Houghton, B. F., C. Bonadonna, C. E. Gregg, D. M. Johnston, W. J. Cousins, J. W. Cole, and P. Del Carlo (2006), Proximal tephra hazards: Recent eruption studies applied to volcanic risk in the Auckland volcanic field, New Zealand, J. Volcanol. Geotherm. Res., 155, 138−149.
Kermode, L. O. (1992), Geology of the Auckland urban area, Geol. Map 2, scale 1:50,000, Inst. of Geol. Nucl. Sci., Lower Hutt, New Zealand.
Shane, P., M. Gehrels, A. Zawalna-Geer, P. Augustinus, J. Lindsay, and I. Chaillou (2013), Longevity of a small shield volcano revealed by crypto-tephra studies (Rangitoto volcano, New Zealand): Change in eruptive behavior of a basaltic field, J. Volcanol. Geotherm. Res., 257, 174−183.
—Phil Shane, University of Auckland, New Zealand; email: pa.shane@auckland.ac.nz
© 2014. American Geophysical Union. All rights reserved.
© 2014. American Geophysical Union. All rights reserved.