Mayor Island / Tūhua

Mayor Island / Tūhua is a dormant shield volcano located off the Bay of Plenty coast of New Zealand's North Island. It lies 35 km north of Tauranga and covers 13 km².

Geography
The island is quite steep along its coast and rises to 355 m above sea level. A saddle about 75 m deep separates it from the North Island, while the other side of the volcano rises from the seafloor some 400 – beneath the waves. Approximately 18,000 years ago during the Last Glacial Maximum when sea levels were over 100 metres lower than present day levels, Mayor Island / Tūhua was connected to the rest of New Zealand. Sea levels began to rise 7,000 years ago, separating Mayor Island / Tūhua from the mainland. Hot springs may be found on the island's northern side, and there are two small crater lakes, Green Lake and Black Lake. These lie within two overlapping calderas formed in explosive eruptions 36,000 and 6,340 years ago.

Geology
Mayor Island is characterised as a peralkaline volcano and has exhibited a wide range of eruptive styles, including lava fountains, Strombolian explosions, extrusion of lava domes, phreatomagmatic explosions, Plinian falls and ignimbrite. The most recent lava flows have been dated at between 500 and 1,000 years old.

The Tūhua Caldera formed in the >1 km3 eruptive volume event of 6,340 years ago and also is a partial collapse crater. It is 2.2 km by 2.5 km in size. The Tuhua Tephra is quite distinctive and due to it having two dispersion patterns has been found in the Auckland area where it is up to 7 cm thick, at Rotorua up to 10 cm thick and Lake Waikaremoana where it is 4 cm thick, making it useful as a mid-Holocene marker horizon. It has also been characterised from multiple sea bed cores off the East coast from the Havre trough off the Bay of Plenty to the southern North Island off Cape Turnagain.

Te Paritu Tephra is assigned to a Mayor Island eruption but the full recent significant eruptive sequence required sea cores as prevailing winds are off land. In summary recent eruptions are:

Risks
The island would likely be sterilised in a typical major pyroclastic eruption at their current frequency of about every 7,000 years although a small eruption could be confined within the caldera. Tsunami activity and ash affecting the Bay of Plenty are possible with initial vent clearing and ash falls toward the major cities of the northern half of the North island being most likely in summer, with economic disruption to especially ports and airports. Usually however the ash fall will be away from land. However it has been estimated that a worse case tsunami could be 35 m high when it reached Bay of Plenty coastal resorts and the city of Tauranga with possibly only 30 minutes warning. Most of the infrastructure of these coastal towns is less than 15 m above sea level so is at risk of complete destruction.

History
Captain James Cook called it Mayor Island when he sighted it on 3 November 1769, in recognition of the Lord Mayor's Day to be held in London a few days later.

The Ngāti Whakaue led a military expedition to the island in 1842, after a tribesman was killed by the Whanau a Tauwhao, a hapu of Ngāi Te Rangi

The island is considered special by Māori (the indigenous people of New Zealand) partly because of the presence of black obsidian, a volcanic glass created by the rapid cooling of silica-rich lava, prized as a cutting tool. The obsidian was called Tūhua by Māori, who called the island by the same name. Over 80% of obsidian excavated from Matakawau on Great Mercury Island had been sourced from Tūhua. Several pa sites are known on the island, the final of which was inhabited until 1901.

Uses and recreation
The area of the Bay of Plenty around the island is renowned for game fishing, with marlin, mako sharks, and swordfish all inhabiting the surrounding waters. Some of the island and the waters close to its shores, however, are now a small marine reserve. There are several tramping tracks around the island, and it is also popular with divers. Mayor Island today is a wildlife refuge. A small number of holiday houses are located in Opo Bay on the south coast of the island, along with a camp ground and a few rental cabins. The 2001 census showed a population of three, after zero in 1996 and 12 in 1991 (all figures randomised for privacy on a Base-3 system).