The Reporoa caldera is a 10 km by 15 km caldera

Caldera

A caldera is a cauldron-like volcanic feature usually formed by the collapse of land following a volcanic eruption, such as the one at Yellowstone National Park in the US. They are sometimes confused with volcanic craters...

 in New Zealand

New Zealand

New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...

's Taupo Volcanic Zone

Taupo Volcanic Zone

The Taupo Volcanic Zone is a highly active volcanic V shaped area in the North Island of New Zealand that is spreading east -west at the rate of about 8mm per year...

. It formed some 230,000 years ago, in a large eruption that deposited the approximately 100 km³ Kaingaroa Ignimbrite

Ignimbrite

An ignimbrite is the deposit of a pyroclastic density current, or pyroclastic flow, a hot suspension of particles and gases that flows rapidly from a volcano, driven by a greater density than the surrounding atmosphere....

s. It contains three rhyolitic

Rhyolite

This page is about a volcanic rock. For the ghost town see Rhyolite, Nevada, and for the satellite system, see Rhyolite/Aquacade.Rhyolite is an igneous, volcanic rock, of felsic composition . It may have any texture from glassy to aphanitic to porphyritic...

 lava dome

Lava dome

|250px|thumb|right|Image of the [[rhyolitic]] lava dome of [[Chaitén Volcano]] during its 2008–2009 eruption.In volcanology, a lava dome is a roughly circular mound-shaped protrusion resulting from the slow extrusion of viscous lava from a volcano...

s (Deer Hill, Kairuru and Pukekahu) and the active Reporoa geothermal field. The Waiotapu

Wai-O-Tapu

Wai-O-Tapu is an active geothermal area at the southern end of the Okataina Volcanic Centre, just north of the Reporoa caldera, in New Zealand's Taupo Volcanic Zone. The area has many hot springs noted for their colourful appearance, in addition to the Lady Knox Geyser...

 geothermal area lies just north of the caldera rim, while the Broadlands thermal area lies to the south. The Waikato River

Waikato River

The Waikato River is the longest river in New Zealand. In the North Island, it runs for 425 kilometres from the eastern slopes of Mount Ruapehu, joining the Tongariro River system and emptying into Lake Taupo, New Zealand's largest lake. It drains Taupo at the lake's northeastern edge, creates the...

 runs through the southern half of the caldera.

In April 2005, a large hydrothermal explosion

Hydrothermal explosion

Hydrothermal explosions occur when superheated water trapped below the surface of the earth rapidly converts from liquid to steam, violently disrupting the confining rock. Boiling water, steam, mud, and rock fragments called breccia are ejected over an area of a few meters up to several kilometers...

 occurred in a cow

Cattle

Cattle are the most common type of large domesticated ungulates. They are a prominent modern member of the subfamily Bovinae, are the most widespread species of the genus Bos, and are most commonly classified collectively as Bos primigenius...

 paddock

Field (agriculture)

In agriculture, the word field refers generally to an area of land enclosed or otherwise and used for agricultural purposes such as:* Cultivating crops* Usage as a paddock or, generally, an enclosure of livestock...

 within the caldera, creating a 50-metre crater. A similar explosion took place in the area in 1948, and smaller explosions have occurred in the intervening years.