Geophysical signatures of the Taranaki and Tongariro
volcanoes reflect similarities in their age, composition and hydrothermal
histories, but also reflect differences in host rock settings and edifice
evolution. The Taranaki volcanoes have strong positive gravity anomalies showing
that their edifices largely consist of dense andesite which reflects substantial
lava production or dyke injection during construction and that large sub-edifice
dyke/stock complexes occur.
In contrast, the Tongariro
volcanoes have strong negative gravity anomalies indicating low density edifices
of volcaniclastic material; no sub-edifice intrusions can be identified,
probably because of the lack of density contrast with host basement rocks.
Strong positive magnetic anomalies over the younger cones can mainly be
attributed to near-surface andesite lavas or dykes whereas subdued magnetic
effects are associated with hydrothermally altered zones within both edifices
and sub-edifice intrusions.