Thursday, 9 August 2012

Volcanic activity on White Island, New Zealand.

White Island, or Te Puia o Whakaari, is a volcanic island located 48 km offshore in the Bay of Plenty, New Zealand. The island is the tip of a submerged stratovolcano (cone-shaped volcano made up of successive lavers of ash and lava), reaching 321 m above sea-level and measuring 2 × 2.4 km. The volcano is highly active, having erupted numerous times since written records began in 1826, and with more eruptions mentioned in Maori oral traditions (the name Te Puia o Whakaari means 'The Dramatic Volcano'. The island is uninhabited, due to its small size and the presence of an active volcano (an attempt to mine sulphur on the island ended in disaster in 1914, when an eruption triggered a lahar - sudden flow of water, mud and ash - that killed all ten miners), but it is observed closely by GeoNet, the New Zealand body that monitors geological hazards.

A fumarole over White Island. GeoNet.

The crater on White Island has had an intermittent lake since 2000, when an eruption altered much of the islands surface (prior to this it had been forested with Pōhutukawa trees). This lake had slowly evaporated away in 2011-2, but abruptly refilled to a depth of 3-5 m on 27-28 July 2012. Gas and steam emissions were seen emerging from this new lake (via webcam), and raised levels of sulphur dioxide (SO₂) detected by monitors on the island. There was also an increase in earthquake activity on the island in July-August (often a sign of magma movements beneath a volcano, which can be a sign of a forthcoming eruption). Early on the morning of 5 August a plume of steam and ash emerged from the lake, which has persisted. On 7 August this turned from white-to-brown in colour, implying an increase in ash content. An ash-cone appears to be forming around this column within the lake.

Webcam image from White Island Crater. Otago Daily Times/Institute of Geological and Nuclear Sciences.

The volcanoes of New Zealand are fed by the subduction of the Pacific Plate beneath the Islands, which sit on the eastern margins of the Australian Plate. As the Pacific Plate sinks into the Earth, a combination of heat from the friction and from the planet's interior partially melts the plate, and some of the melted material rises through the overlying Australian Plate, supplying the volcanoes of New Zealand with liquid magma.


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