Third Day: 11 November
White Island lies at the north tip of the TVZ and is among the most active and accessible andesite-dacite volcanoes in the world. It is also perhaps one of the best understood, having been intensively studied in terms of both its physical and chemical characteristics (e.g. Hamiliton and Baumgart, 1959; Giggenbach, 1987; Houghton and Nairn, 1989; Houghton and Nairn, 1991).
According to legend, White Island is the source of volcanic-hydrothermal activity in the North Island. One story describes the arrival of Chief Ngatoro-i-rangi who brought fire from Hawaiiki. He left his sisters at Whakaari and travelled south to Tongariro. He found it very cold there and he called his sisters to send fire. The subterranean passage taken by the spirits carrying fire marked a trail of thermal activity, which is the subject of this tour. The first European to record sighting White Island is Captain James Cook, who named it as it appeared to him during his frrst visit to New Zealand in 1769.
The island is made up of two overlapping cones, an older western cone and a younger central cone. The main crater occupies the eastern and central portions and is breached to the southeast (Fig. 25). The western part of the main crater includes several deep coalescing subcraters, formed about the most active vents. Their deepest point is about 150m below sea level. Fumarolic activity is concentrated in this area, but acid brines periodically discharge from springs further to the east. Baseline studies are in progress for
Figures & Tables
A simplified geologic map of the North Island is shown in Figure 1. The oldest rocks which form the basement are of late Paleozoic to Mesozoic age. The New Zealand lithosphere only began to develop as a separate crustal entity in the late Cretaceous-early Tertiary when it broke away from the Gondwana supercontinent as the Tasman Sea opened (Sporli, 1987). Much of the geology of this continental fragment, which extends from New Caledonia to the Campbell Plateau, is obscured by the fact that about 70% of it is submerged (Fig. 2).
New Zealand's Cenozoic history relates to its proximity to a major active boundary between the Indian and Pacific plates (Figs. 3 and 4). Accurate reconstruction of the plate boundaries for most of this era is difficult, although several versions are published (e.g. Cole and Lewis, 1981; Ballance et al., 1982; Brothers, 1984; Walcott, 1987). In the following summary, we focus on the products of arc magmatism and closely related hydrothermal activity (Fig. 5). We start with the present situation.
The TVZ is a complex volcano-tectonic depression, filled with pyroclastic deposits and lavas, that is related to the westward dipping subduction zone of the Hik:urangi Trough (Fig. 6). It extends offshore into the Tonga-Kermadec arc and marks the start of the Pacific 'horseshoe of fire'. Convergence along the Hikurangi Trough is increasingly oblique southward to form a transform plate boundary as delineated by the Alpine Fault in the South Island (Figs. 2 and 3).
The Benioff zone dips at a very shallow angle west of the Hikurangi Trough but becomes steeper westward to where it lies at about 80 km depth beneath the TVZ (Fig. 6). An accretionary prism, comprising Tertiary and younger sediments, lies above the shallower Benioff zone. Bounding the accretionary prism to the west are the Axial Ranges which are made up of Mesozoic greywackes and argillites of the Torlesse terrane; the North Island Shear Belt comprises a set of dextral north-trending faults that cuts across them (Fig. 7).
The TVZ lies adjacent to the Axial Ranges, about 250 km west of the Hikurangi Trough, and extends from White Island to Tongariro. Its margins are defined by steep gravity gradients and distribution of volcanic vents, except to the northwest where it merges with the Coromandel Volcanic Zone (Rogan, 1982; Wilson et al., 1984). Northeast trending normal faults dominate the structural fabric, forming a series