Rangitoto Island is a volcanic island in the Hauraki Gulf near Auckland, New Zealand. The 5.5 km (3.4 mi) wide island is a symmetrical shield volcano cone capped by central scoria cones, reaching a height of 260 m (850 ft). Rangitoto is the youngest and largest of the approximately 50 volcanoes of the Auckland volcanic field, having erupted in two phases about 1450 CE and 1500 CE and covering an area of 2,311 ha (5,710 acres). It is separated from the mainland of Auckland's North Shore by the Rangitoto Channel. Since World War II, it has been linked by a causeway to the much older, non-volcanic Motutapu Island. Rangitoto is Māori for 'Bloody Sky', with the name coming from the full phrase Ngā Rangi-i-totongia-a Tama-te-kapua ("The days of the bleeding of Tama-te-kapua"). Tama-te-kapua was the captain of the Arawa waka (canoe) and was badly wounded on the island, after having lost a battle with the Tainui iwi (tribe) at Islington Bay.
Geology
Rangitoto formed during two phases of eruptions that may have lasted only 5–10 years, about 600 years ago. The first part of the eruption sequence, dated by radiocarbon methods at about 629 years ago (= 553 ± 7 yrs BP) was initially wet and produced surges of volcanic ash that mantled neighbouring Motutapu Island from alkaline olivine basalt eruptives sourced from the north cone area. The later part of the eruption dated at about 580 years ago (= 504 ± 5 yrs BP) was dry and built most of Rangitoto, erupting all the sub-alkaline basaltic lava flows of the shield and the southern scoria cone at the apex that mostly buried the north cone. The 2.3 km3 (0.55 mi3) of material that erupted from the volcano was nearly as much as the combined mass produced by all the previous eruptions in the Auckland volcanic field, which were spread over nearly 200,000 years.
Eruptive history In 2013, scientists from Auckland University reported that Rangitoto had been much more active in the past than previously thought, suggesting it had been active on and off for around 1000 years before the final eruptions around 600 years ago. This was based on recognition of a number of horizons of tiny volcanic glass shards, seemingly erupted from Rangitoto, in sediment cores from Lake Pupuke. To test this hypothesis, a 150 m (490 ft) deep hole was drilled through the western flank of Rangitoto in February 2014. No supporting evidence for these hypothesised small eruptions up to 1000 yrs before the main eruption of Rangitoto was found. The university scientists did, however, core a thin basalt lava flow within marine sediment dated at 6000 years old, and hypothesised that this was an even earlier eruption from Rangitoto. Civil Defence officials said the discovery did not make living in Auckland any more dangerous, but did change their view of how an eruption might proceed. These headline-grabbing results were controversial and not accepted by all geologists. In 2018 many of the original group of Auckland University geologists reported on their latest research and reinterpretation of the evidence and concluded that Rangitoto only erupted once, about 600 yrs ago, possibly in two phases. The thin lava flow within 6000 yr-old sediment in the Rangitoto drillcore has exactly the same chemistry as the lowest/oldest flows of the cored shield volcano above and has recently been shown to also have the same U-Th-Ra signature and both must have been erupted and solidified at virtually the same time. The thin "flow" is therefore likely to be basalt lava that was intruded into the sediment as the shield volcano was starting to be formed. This is similar to the situation seen on the coast at Queen Victoria Rock, near the Rangitoto Lighthouse, where lava flows have ploughed into the soft seafloor sediment and intruded into it, bringing at least one sediment slab up to high tide level. Thus no evidence has been found so far to support the hypotheses of earlier eruptions of Rangitoto between 620 and 8000 yrs ago. It is possible that Rangitoto buried a smaller and much older and unrelated volcano, a view possibly supported, but certainly not proven, by recent microfossil research from the marine sediments obtained from beneath Rangitoto in the stratigraphic drillhole.
Structure
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