Mayor Island / Tūhua is a dormant shield volcano located off the Bay of Plenty coast of New Zealand's North Island. It covers .
Geography
<!-- although better code and would allow labelling is pushing Lua processing time so might fail on updates to page
had to use this code as more efficient by 4 secs-->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 / 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, Lake Aroarotamahine (Green Lake) and Lake Te Paritu (Black Lake). These lie within the calderas formed in explosive eruptions.
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. It is the largest and most complex volcano located in the southwest margin of the 128 edifice Tūhua 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.
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. 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.
In summary recent eruptions are:
{| class="wikitable sortable"
|+ Mayor Island eruptions
|-
!class="unsortable"|Phases
!class="unsortable"|Tephra/eruption name(s)
!data-sort-type="number"|Date before present (ka BP)
!class="unsortable"| Comment
|-
|rowspan="8" | <b>Early cone building phase</b> ||
| align="center"|<span style="display:none">150</span>from ~150 ||Consistently rhyolitic peralkaline eruptives, with two large and complex lava shields at north-east and south-west of island
Risks
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. 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,
Uses and recreation
The island, since 2014 has been administered by the Tūhua Trust Board, and access is restricted. 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.