An assessment of volcanic risk on and from Mayor Island, New Zealand

Abstract

Mayor Island is the emergent part of a silicic volcano in the Bay of Plenty, New Zealand, which was last active about 1000 years ago. The volcano is considered to be dormant and in a period of quiescence. Several lengthy periods of quiet have occurred in its past and each has ended abruptly with initially Vulcanian-style, small magnitude, vent-clearing eruptions which then proceeded to Plinian events of larger magnitude. It is expected that this cycle of activity will occur again in the future; should it do so, then there will be great risk both to the island itself and to the neighbouring mainland.

An assessment of the factors contributing to the risk of any area shows that even the most sheltered parts of the island would not be completely safe for the survival of man during even the smallest eruptions. The entire island is at risk from tephra falls, although the risk is greater on the eastern side of the island where falls would be thickest. Similarly, all parts of the island are at risk from pyroclastic flows, especially the valleys that penetrate the caldera rim. The low-lying caldera floor is the area of greatest risk on the island, as it is here that future vents are likely to form, and it is an area that would be overwhelmed by both pyroclastic and lava flows. The low-lying coastal flats around the island could be easily inundated by tsunamis.

The sea floor linearly shallows from Mayor Island onto the east coast of the North Island, and tsunamis from Mayor Island could inundate much of the low-lying ground along the Bay of Plenty coast. The nearest seaside communities of Waihi and Whangamata would be hit by them within about 30 minutes of their being generated.

A tephra dispersal model that is based on the average wind patterns in the atmosphere above the island, indicates that tephra from an eruption on Mayor Island could be deposited on the mainland. However, it would be most likely only in the summer months when an eruption column could reach above 20 km altitude.

Keywords:

• Bay of Plenty
• Mayor Island
• pyroclastics
• flows
• ash falls
• tsunamis
• tephra dispersion
• volcanic risk