Reconstructing Rangitoto volcano from a 150-m-deep drill core

Abstract

Investigation of a 150-m-deep core drilled into Rangitoto volcano, Auckland, reveals a volcanic history extending farther back in time than previously known. The oldest lava flow has been dated by radiocarbon ages on wood and shell from enclosing estuarine sediments at about 6000 years old.

Most of Rangitoto is made of a pile of lava flows and these post-date 3800 years ago. Traditionally it was thought that Rangitoto volcano had formed in one or two brief episodes about 550 years ago. However, the rocks in the drill core suggests this was a late phase of activity that post-dates most of the volcano’s construction. The prolonged and perhaps episodic activity is consistent with the studies of microscopic ash from Rangitoto preserved in sediments at Lake Pupuke. The composition of magma erupted from Rangitoto has changed with time due to processes of melting in the mantle.

However, there no evidence of interaction between the rising magmas and Earth’s crust. This implies the magmas ascend fast and there would be limited warning times in any future eruptions. The prolonged life of Rangitoto means that future eruptions could potentially occur at Rangitoto, in addition to other sites around Auckland.

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Technical Abstract

A 150-m-deep hole was drilled through the upper western flank of Rangitoto volcano, Auckland, in February 2014. The core comprises about 128m of lavas from the main shield-building phase overlying marine sediments with intercalated lava and pyroclastics. All of the volcanic deposits in the drill core classify as transitional basalts, and display a relatively narrow range in major element composition (SiO2 ~48-49 wt % and Mg# ~0.60 to 0.64). The rocks comprise at least four chemical affinities which relate to stratigraphic sequence. Variations in incompatible trace elements and uniformity of radiogenic isotope compositions are best explained by the magmas representing discrete batches reflecting different degrees of partial melting in the mantle. The uniformity in isotopic composition and lack of crustal signal indicates all magmas shared a common source region and experienced minimal crustal contamination during ascent or storage. An important implication is that magma ascent was sufficiently rapid to preclude assimilation of country rock. Thus, potential future eruptions could display limited precursor activity.

Organic materials dated by the 14C method produce a coherent stratigraphic sequence from ~7,400 years at 139.38 m to 3,800 years at 128 m depths. However, ages of ~1,000 years at 135.39 m and 1,500 years at 133.11 m are stratigraphically inverted, and discordant with ages above and below. This is likely to reflect fluidized extrusion of pyroclastics into the core barrel. Ages of ~5,900 and 6,800 years bracket the oldest lava flow at 137 m depth, thus providing an age for the earliest volcanism. Traditionally it was thought that Rangitoto volcano had formed in one or two brief episodes about 550 years ago, based on ages associated with ash layers on Motutapu Island. However, ages from the drill core suggests the ash layers are a late phase of activity that post-date most of the volcano’s construction. This is further supported by the lack of alkalic rock compositions in the drill core that would match the North Scoria Cone at the summit of the volcano. Thus, implying pyroclastic activity post-dating main edifice construction. The prolonged and perhaps episodic activity is consistent with the studies of microscopic ash from Rangitoto preserved in sediments at Lake Pupuke. Future modeling should consider the possibility of repeat eruptions from volcanic centers in the Auckland Volcanic Field and prolonged activity. The socio-economic impact would differ from that resulting from short-lived phenomena.