Mayor Island / Tà «hua is a dormant shield volcano located off the Bay of Plenty coast of New Zealand's North Island. It covers .
The island is quite steep along its coast and rises to above sea level. The closest major port entrance is at Tauranga, away to the southeast and it is from the Bay of Plenty coast. A saddle about deep separates it from the North Island, while the other side of the volcano rises from the seafloor some beneath the waves. Approximately 18,000 years ago during the Last Glacial Maximum when sea levels were over lower than present day levels, Mayor Island / Tà «hua was connected to the rest of New Zealand. Sea levels began to rise 7,000 years ago, separating Mayor Island / Tà «hua from the mainland. Hot springs may be found on the island's northern side, and there are two small crater lakes, Lake Aroarotamahine (Green Lake) and Lake Te Paritu (Black Lake). These lie within the calderas formed in explosive eruptions.
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. It is the largest and most complex volcano located in the southwest margin of the 128 edifice Tà «hua volcanic field which covers at least of the ocean floor. This field is a polygenetic volcanic field, although up to 55 of the volcanoes are likely monogenetic, having only erupted once.
Three caldera forming events have occurred named, C1 at 36 thousand years BP, C2, perhaps a gradual collapse, at about 9,200 years BP and C3 as a VEI 5 event at 7,600 ñ 100 years BP.
The most recent lava flows were historically dated by appearance at between 500 and 1,000 years old, however radiocarbon dating of lake sediments is consistent with volcanic activity destroying pre-existing vegetation about a thousand years earlier than this.
The Tà «hua Caldera formed in the > eruptive volume C3 event that occurred at ñ100 and is also a partial collapse crater. It is by 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 thick, at Rotorua up to thick and Lake Waikaremoana where it is 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:
The tephra can contain a distinctive purple crystal mineral tuhualite (Na,K)Fe<sub>2</sub>Si<sub>6</sub>O<sub>15</sub> which the island's volcano was only known source of, until 2016.
The island would likely be sterilised in a major pyroclastic eruption as last occurred about 7,000 years ago, 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. Large ash fall over land would devastate agriculture and ecosystems for some time due to fluorosis and chlorine poisoning as happened after the 1783 eruption at Laki, in Iceland. Usually however the ash fall will be away from land. However it has been estimated that a worse case tsunami could be 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 above sea level so is at risk of complete destruction.
Tà «hua is the ancestral home of Te Urungawera, hapà « of Te WhÃÂnau a Tauwhao within the iwi of NgÃÂi Te Rangi. The island is considered special by MÃÂori (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 Tà «hua by MÃÂori, who called the island by the same name. Over 80% of obsidian excavated from Matakawau on Great Mercury Island had been sourced from Tà «hua. Several pa sites are known on the island, the final of which was inhabited until 1901.
Captain James Cook called it the Mayor 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 NgÃÂti Whakaue led a military expedition to the island in 1842, after a tribesman was killed by the Whanau a Tauwhao, a hapu of NgÃÂi Te Rangi
The island, since 2014 has been administered by the Tà «hua Trust Board, and access is restricted. Since 1953 it has been a wildlife refuge. 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. Since 1990 the island itself, has no longer been the base for big game fishing. Some of the waters close to the island, since 1993, are a small marine reserve. There are several tramping tracks around the island, and it is also popular with divers. 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). There are no permanent residents presently but the island is usually occupied during the summer months.