This text was written by Elaine R Smid a, in collaboration with Dr Bruce W Hayward b, Thomas Stolberger c, and Ewart Barnsley d
a PhD candidate & DEtermining VOlcanic Risk in Auckland (DEVORA) Research Assistant, University of Auckland
b Author, Retired geologist from University of Auckland and Geomarine Research
c DEVORA Research Assistant, University of Auckland
d Media Manager, City Rail Link
In some recent news articles, Maungawhau/Mt Eden volcano was confirmed to be 28,000 years old. This is the story of how that came about.
Maungawhau and Auckland City. Image credit: Bruce W Hayward. Used with permission.
In February 2019, the City Rail Link (CRL) micro-tunnel boring machine “Jeffie” became entangled in a large tree as it was digging a tunnel to move the stormwater line out of the way of the new underground rail tunnels. Jeffie was dubbed “a little legend” for her excellent digging work.
City Rail Link’s micro-tunnel boring machine “Jeffie” encountered a tree that was used to find the age of Maungawhau’s eruption. Photo by City Rail Link.
The tree was found at a depth of about 15 m, approximately 50 m north of the rail line between Mt Eden Rd and Shaddock St in Eden Terrace, Auckland. The tree was a very large obstruction for the tunneler–over 1 m in diameter, so large that it steered the drilling off course. See below for the approximate location of the tree, viewed from the surface.
The CRL tree was found at 15 m depth, approximately 50 m north of the rail line between Mt Eden Rd. and Shaddock St. in Eden Terrace, Auckland. Image from Google Earth Pro. Location data from CRL.
Before the tunneling started, CRL dug some vertical boreholes along the surface trace of the tunnel so that they knew what to expect while Jeffie was digging horizontally. This showed that the tree was found 1 – 1.5 m below lava flows from Maungawhau, as extrapolated from the boreholes (see below). This key piece of information–the tree’s location beneath lava flows–allows us to get a date for Maungawhau’s eruption. Whatever the age of the tree, the eruption followed soon after.
a) Cross-section excerpt of the area where the tree was found by CRL (blue star); b) Legend for stratigraphy shown in cross-section. Cross-section and legend courtesy of CRL.
CRL pulled the tree fragments to the surface (see below). Select samples of tree fragments were chosen by DEtermining VOlcanic Risk in Auckland (DEVORA) researchers for radiocarbon analysis during a visit to CRL offices in April 2019. The tree fragments are well-preserved, with light charring or carbonisation evident on parts of some fragments.
a) Tree in mud or tuff, entangled in the City Rail Link micro-tunnel boring machine near Maungawhau; b & c) Fragments of tree brought to the surface; d) Tree fragments sampled for radiocarbon dating. Fragments are well-preserved and/or carbonised in some spots. Photos a – c courtesy of CRL; Photo d by Elaine Smid.
TREE ENVIRONMENT AND ITS DEMISE
The tree was found approximately 1.2 km away Maungawhau’s main vent (Fig. 4), in what looks to be soft mud or volcanic tuff (see a, above). From the borehole information and geologist accounts during drilling, the tree was likely growing in a forested valley that was later filled in by Maungawhau’s lava flows. The lava flows baked the sediment underneath. We do not know if the tree was standing in place or had been knocked over when the volcano started erupting.
The uncertainty about what material surrounds the tree (mud or tuff) leads to several potential scenarios for the tree’s demise, as described in my explanation, below (video created by City Rail Link & used with permission).
Scenario 1: If the tree was found in soft mud, it may have fallen into a pond-like environment up to several hundred years prior to the eruption. The apparent lack of black, organic-rich mud at the tunneling site indicates it did not fall into a swamp or peat bog. Pond sediment then buried the tree, followed by the lava flow some time later.
Scenario 2: Alternately, the tree may have been left in-place or felled by the eruption’s initial shock waves or explosive base surges, partly or wholly buried in the resulting tuff, and then subsequently covered over by lava flows. The black appearance of some of the samples may be due to charring during the surge and/or the lava flow stages of the eruption, or simply could be due to carbonization during natural decay processes.
The presence of tuff from Maungawhau, indicating an explosive start to the eruption, has been inferred in Auckland maar (explosion crater) cores (Hopkins et al., 2017) and reported within the site description of a previously-dated totara log near Lauder Rd., found during the construction of two Mt Eden Corrections Facility buildings (see below and Table 1; East and George, 2003).
Locations of radiocarbon samples used to date Maungawhau’s eruption. These include: 1. a totara log buried in tuff found during construction of buildings within the Mt Eden Corrections Facility; and 2. the CRL tree (see text for details). The trees were discovered ~1.1 and ~1.2 km from Maungawhau’s main vent, respectively, and are roughly 0.5 km apart. Image from Google Earth Pro.
Edited to Add New Information! Scenario 3: A third scenario has to be added to the mix. Geologists that examined the drilled tunnel have said that the mud in the photos could have been introduced into the drilling apparatus for lubrication, and may not be representative of the sediment around the tree at the time it died. Lava flows have been known to sink into soft mud or bulldoze material in front of it (see here for a lava flow ‘bulldozing’ a shed in Hawai’i). In this scenario, sediment may have built up in front of the advancing lava flow. This sediment insulated and buried the tree before the lava flow covered it all
Tree fragments will be provided to a University of Auckland anthropologist, who may be able to positively identify the species, likely growth environment, and the cause of the black veneer or “charring.”
TREE AGE & IMPLICATIONS FOR MAUNGAWHAU ERUPTION AGE
The GNS National Isotope Centre’s Rafter Radiocarbon Laboratory (sample ID CRL-1; NZA 69409) returned a 2-sigma (error) age range of 27,666 to 28,274 calibrated years before present (before present = before 1950). This new result places the eruption age within error of previously obtained ages.
There are two previous reliable ages for the volcano. One age was from a totara log found within presumed tuff from Maungawhau. This tree was discovered 1.1 km north of Maungawhau’s main vent and about a half-kilometer from where the CRL tree was found (see image above). Another comes from the age of ash layers within five maar cores. First, the ash layers were geochemically matched to Maungawhau lava. Researchers then used sedimentation rates (how quickly sediment accumulates at the bottom of lakes) to figure out the age of the Maungawhau ash layers within the cores (Table 1).
Table 1. Known Reliable Ages for Maungawhau Samples
||Tree in soft mud or tuff
15 m depth; 50 m N of Rail Line between Shaddock St & Mt Eden Rd, Eden Terrace; 1.2 km from Maungawhau
||Totara log buried in tuff
||36°52’3.58″S, 174°45’56.90″E; under the Administration Building at Mt Eden Corrections Facility; 1.1 km from Maungawhau
||East and George, 2003; Lindsay et al., 2011
||geochemically correlated ash layers within maar cores
||Orakei Basin; Hopua; Onepoto; Pupuke; Pukaki
||Hopkins et al., 2017
a Coordinates in WGS 84 Web Mercator; b radiometric age in calibrated years before present; c estimated average age in years before present, calculated from sedimentation rates in five cores.
Ages obtained using other techniques (e.g. thermoluminescence; K-Ar) were deemed unreliable (McDougall et al. 1969; Lindsay et al., 2011). This new result confirms Maungawhau’s ~28 kyr eruption age, and means that Maungawhau was potentially the 37th volcano to erupt and form in the AVF, with potentially 15 other volcanoes forming and erupting in Auckland after it did (some ages are still unknown).
East, G. R. W., & George, A. K. (2003). The construction of the Auckland Central Remand Prison on the Mt Eden basalt flow. In Geotechnics on the Volcanic Edge: Tauranga, March 2003, New Zealand Geotechnical Society Symposium (p. 387). Institution of Professional Engineers New Zealand.
Hopkins, J. L., Wilson, C. J., Millet, M. A., Leonard, G. S., Timm, C., McGee, L. E., Smith, I. E. M. & Smith, E. G. (2017). Multi-criteria correlation of tephra deposits to source centres applied in the Auckland Volcanic Field, New Zealand. Bulletin of Volcanology, 79(7), 55.
Lindsay, J. M., Leonard, G. S., Smid, E. R., & Hayward, B. W. (2011). Age of the Auckland Volcanic Field: a review of existing data. New Zealand Journal of Geology and Geophysics, 54(4), 379-401.
McDougall, I., Polach, H. A., & Stipp, J. J. (1969). Excess radiogenic argon in young subaerial basalts from the Auckland volcanic field, New Zealand. Geochimica et Cosmochimica Acta, 33(12), 1485-1520.