Egmont Volcano, New Zealand: three-dimensional structure and its implications for evolution

Egmont Volcano (Mt. Taranaki), a large active andesite stratovolcano, is characterised by a well-defined large positive residual gravity anomaly of 350 g.u. Detailed three-dimensional modelling of extensive gravity data delineates a large subedifice intrusion of andesite density extending to at least basement depths (6 km) and a core of similar density within the edifice. These andesite bodies are attributed mainly to repeated magma injection from deeper magma chambers into both the underlying sediments and the volcanic edifice. The dense edifice core of Egmont Volcano probably represents significant dyke intrusion; such dykes may have played a major role not only in edifice construction but also in edifice collapse. Egmont Volcano appears to share a common evolution with the three older relict centres in the Taranaki succession since all four Taranaki volcanoes are shown to have large subedifice intrusions and similar dense edifice cores. However, intrusive volumes are somewhat smaller below the older centres which, given their ages, suggests that magma production rates may have increased with time; the total volume of magma involved in the formation of the Taranaki volcanoes is estimated to be at least 1500 km3.