Fact Sheet Rangitoto Volcano

Description ••Rangitoto is a volcanic island in the Hauraki Gulf visible from most parts of City. ••It is the most recent, largest and least altered volcano in the Auckland which is made up of around 50 small volcanoes. ••It was formed by at least 2 eruptions 600-700 years ago. ••The highest part is 260m, and it is 5.5km wide. ••The island is part of the Hauraki Gulf Maritime Park and is administered by the Department of Conservation. ••Past activities include quarries, military installations and at one time it had a small permanent  Rangitoto can be seen from most parts of population. .

Scoria cone  Rangitoto is in the Auckland volcanic field - an area that has a concentration of flows, from which bubbles sporadically surface, creating scoria cones. Concentrated magma reservoir

Maori Name Eruptive history Last eruptive activity •• Rangitoto means ‘Bloody sky’ and is •• The Auckland Volcanic Field is •• Rangitoto erupted 600-700 years ago thought to refer to the serious injury of a monogenetic, meaning each volcano over an unknown time span. Maori chief during a battle on the island. usually only erupts once. Approximately •• The sequence of events was likely to Features 50 volcanoes have formed over the last have been: •• Roads and tracks allow visitors to walk 250.000 years. The field is still active and there is no way to predict where or when - A violent pyroclastic eruption as cold over lava fields and through lava caves sea water met molten rock, creating an (tubes left behind by the passage of liquid the next ‘bubble’ of magma will rise to the surface and create a new volcano. explosion crater and a tuff ring. lava). - Ongoing fire fountaining built scoria •• Vegetation varies from ‘raw’ lava fields •• The size and length of each eruption cones once water could no longer reach to scrub and sparse forests, including the depends on how big the ‘bubble’ the magma largest pohutukawa forest in NZ. of magma was, so Rangitoto was a - Lava flows from the base of the scoria comparatively large ‘bubble’ of magma. •• A moat like ring around the summit is cone due to subsidence of the mountaintop as Eruptive material Monitoring underlying lava flows cooled and shrank. •• If the magma mixes with water •• 8 seismographs are operated jointly (seawater or groundwater) super heated by GeoNet and the Auckland Regional Type steam blows it apart. This causes a Council. •• It is an intra-plate or hot spot volcano. pyroclastic eruption that produces fall These occur away from plate boundaries and flow deposits and has created the and are not related to subduction. low rings of pyroclastic rock (called tuff) •• The volcano consists of scoria cones on around the craters of many Auckland top of a broad ring of lava flows. volcanoes, eg . Cause •• When the magma has no contact •• A mantle hot spot exists about 100 km with water, lava can fountain out less below Auckland. When rock is melted explosively and build a cone of tephra. by this extra heat it will separate from Basalt tephra is called scoria so the cones the surrounding solid rock and rise to the are commonly called scoria cones eg, One surface. The melted rock is basalt magma Tree Hill. which has a low viscosity (flows easily) •• Rangitoto makes up nearly 60% of the and may rise to the surface at speeds of total volume of material erupted by all up to 5 km/hour. volcanoes in the Auckland Field. www.gns.cri.nz