Rangitoto Island - Geology

Geology

Rangitoto was formed by a series of eruptions between 550 and 600 years ago. The eruptions occurred in two episodes, 10-50 yrs apart, and are thought to have lasted for several years during the later shield forming episode The first episode erupted most of the volcanic ash that mantles Motutapu Island next door, and also produced the lower, northern, scoria cone. The second episode built most of Rangitoto erupting all the lava flows and main scoria cone at the apex.The 2.3 cubic kilometres of material that erupted from the volcano was about equal to the combined mass produced by all the previous eruptions in the Auckland Volcanic Field, which were spread over more than 250,000 years.

The volcano is not expected to become active again, although future eruptions are likely within the volcanic field. Subsidence back down the throat during the cooling process has left a moat-like ring around the crater summit, which may be viewed from a path which goes right round the rim and up to the highest point.

In some parts of the island, fields of lightweight, clinker-like black lava stones called scoria are still exposed, appearing very recent to a casual eye. About 200 metres from the top of the mountain on the eastern side visitors can walk through some of about seven known lava tubes — tubes left behind after the passage of liquid lava. The more accessible of the caves are signposted. Lava tubes are formed when low-viscosity molten lava known as pahoehoe flows and cools on the outside due to contact with the ground and air, to form a hard crust allowing the still-liquid molten lava to continue to flow through inside. At Rangitoto the large tubes are cave-like. A torch is needed to explore the caves. The longest known cave is about 50m long.

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