Thursday 26 July 2012

Movement beneath the Tongariro Volcanic Complex.

The Tongariro Volcanic Complex is an andesitic volcanic massif hosting at least 12 distinct volcanic cones, situated in the central part of North Island, New Zealand, northeast of Mount Ruapehu and southwest of Lake Taupo. The complex is thought to have been active for slightly over a quarter of a million years, with over 70 eruptions between 1839 and 1977, since when the complex has produced fumaroles (gas emissions) and occasional bouts of Earthquake activity, but no eruptions. The complex forms part of the Tongariro National Park and UNESCO World Heritage Site.

Aerial photograph of the Tongariro Volcanic Complex seen from the northeast. PhotoVolcanica.

Between 13 and 23 July 2012 the GetNet Data Center recorded a series of small tremors (less than magnitude 2.5) beneath the Tongariro Volcanic Complex, clustered between the Emerald Lakes and Te Mari Craters. Such tremors beneath volcanoes are monitored closely by volcanologists as they may be indicative of magma movements, which in turn may be a precursor of new eruptions; though this is not always the case, Tongariro suffered periods of Earthquake activity in 1983, 2006 and 2008-9, none of which led to an eruption. 

Map of the Tongariro National Park. Green triangles within circles represent volcanic craters. TripFinding.

New Zealand is located on the boundary beneath the Australian and Pacific Plates. Beneath the islands the Pacific Plate is being subducted beneath the Australian Plate. This causes a great deal of friction which causes Earthquakes where the boundary between the two plates is close to the surface; this is to the east of North Island, but onshore on South Island, where it can lead to strong Earthquakes such as the ones felt in Christchurch recently. Technically such quakes also occur where the plate margin is deeper, but these are felt less strongly as the rocks between the boundary and the surface absorb much of the energy, making strong tremors much less frequent on North Island. As the Pacific Plate sinks deeper into the Earth it is partially melted by the friction and the heat of the planet's interior. Some of the melted material then rises through the overlying Australian Plate, fueling the volcanoes of New Zealand.

Diagrammatic representation of the passage of the Pacific Plate beneath North Island, New Zealand, and how this fuels volcanoes there. Natural Hazards Research Platform

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