If there was a Book of Records for volcanoes, Taupo would certainly be in it.

 

Taupo is the most frequently active and productive rhyolite volcano in the world.
The Taupo caldera occupies about the same area as metropolitan Auckland.
The Oruanui eruption of 26,500 years ago produced enough material to build three Ruapehu-sized cones.

The vast and scenic Lake Taupo is not always recognised as a volcano, yet it has had a fiery and violent history. It has erupted 28 times in the past 27,000 years. Although most of these eruptions were small, the most recent - the Taupo eruption of 181AD - was extremely large and violent.

Lake Taupo's shape and size (600 sq km) was largely created by the Oruanui eruption 26,500 yeras ago. This eruption formed a 500 metre deep caldera (large collapsed crater) that was enlarged by the 181AD eruption.

The 26,500 year-old and 181AD eruptions were extraordinarily complex and violent, and they have attracted interest from scientists internationally. The other 26 eruptions in between were small, many not much larger than a typical Mt Ruapehu eruption.

WHAT IS A CALDERA VOLCANO?…

There are two very different types of volcano - cones and calderas. Cone volcanoes generate many small eruptions from the same site. An example is Mt Ruapehu which has been erupting almost continuously for about 260,000 years. The amount of ash and lava they erupt is usually between 0.001 and 0.2 cubic km. The many frequent eruptions from cone volcanoes result in the accumulation of large volumes of volcanic debris close to the vent producing steep-sided cones like Ruapehu, Egmont/Taranaki, and Ngauruhoe.

Caldera volcanoes such as Taupo produce larger and less frequent eruptions. Sometimes their eruptions are as large as 50 cubic km or even bigger, and form new caldera structures. Other eruptions are smaller and contained within the existing caldera like many of Taupo's eruptions over the past 20,000 years. These small eruptions are typically between 0.5 and 10 cubic km. Caldera-forming eruptions drain the magma reservoir beneath the volcano, causing the ground to collapse, so that the eruption forms a depression in the earth's surface. New Zealand's two most active caldera volcanos are Taupo and Okataina, which last erupted from Mt Tarawera in 1886, killing 108 people.

All large caldera volcanoes have associated geothermal systems, where large bodies of underground water are heated by the volcano. An example is the Wairakei geothermal area, which produces about 8 percent of New Zealand's electricity. New Zealand-developed technology associated with Wairakei has been used to assist with geothermal energy developments in many countries over the past four decades.

THE ORUANUI ERUPTION…

The 26,500 year-old Oruanui eruption produced huge volumes of ash and other volcanic material that buried parts of the central North Island. Close to the vent the ash reached depths of about 100m. The size of the eruption is difficult to grasp. Roughly 800 cubic km of pumice and ash were ejected in this one event. The ash blanketed a huge area of ocean floor to the east of New Zealand with a layer that varied in thickness from 20cm to 1cm. Even the Chatham Islands, 800km to the east of New Zealand, received an 11cm coating. The rapid eruption of so much material caused several hundred square km of the area surrounding the vent to collapse to form the Lake Taupo basin, now partly filled by the lake.

THE 181AD TAUPO ERUPTION…

This eruption took place from a vent or vents near the Horomatangi Reefs, now submerged on the eastern side of Lake Taupo. The eruption lasted between several days and several weeks and produced a sequence of pumice deposits that blanketed the landscape east of Taupo. In total about 50 cubic km was erupted.

The Taupo eruption was extremely complex, partly due to the influence of the lake water. At some stages the eruption material was dry and fall deposits consisted of blocks of pumice and rock fragments with no fine ash. During other stages, the eruption was 'wet' as abundant lake water mixed into the eruption column, producing fine ash.

At the climax of this eruption, about 30 cubic km of pumice, ash and rock fragments was erupted in only a few minutes and travelled horizontally as a liquid flow, moving at speeds estimated at between 600-900 kmh. It crossed every obstacle in its path except the top of Mt Ruapehu.

The 181AD Taupo eruption is unusual in several ways:

• it was the most violent eruption in the world in the last 5000 years
• the effects of the eruption were seen in the sky as far away as Europe and China
• the eruption devasted an area now populated by over 200,000 people
• if the same eruption occurred today, ashfall and other debris would cause chaos from Hamilton to Palmerston North, and buildings in Rotorua and Gisborne could be damaged or destroyed.

THE CHALLENGE OF THE TAUPO VOLCANO…

Taupo volcano represents a major scientific challenge in that its activity is so variable. In the last 50,000 years it has had eruptions that vary in volume from 0.05 cubic km (slightly larger than a typical Ruapehu eruption) to over 800 cubic km. Some eruptions have been 'dry' and formed pumice deposits, while others have been 'wet' where lake water was mixed into the eruption column and formed fine ash deposits.

It is impossible to forecast when the next eruption will occur at Taupo, or what size it will be. The best available information shows that there is no relation between the size of the eruption and the time break between eruptions. The next eruption might be next year, or not for hundreds of years. It might produce a small lava dome, or it might destroy the central North Island as the volcano has done in the past. All that is certain, is that Taupo will erupt again.

Background to the proposal to dive with the submersible JAGO on Lake Taupo.…

 Lake Taupo is New Zealand's largest freshwater lake and is situated in the centre of the North Island. The lake is roughly circular in shape and measures about 30km in diameter with maximum depths of between 150 and 165m. Lake Taupo marks the location of one of the world's most active rhyolitic volcanoes as part of the Taupo volcanic centre, located at the southern end of the Taupo Volcanic Zone (Wilson and Walker 1985). However, as a result of various eruptions much of the volcanic centre has collapsed and now lies beneath the lake.

 Lake Taupo thus represents the focal point of some of New Zealand's largest rhyolitic eruptions, including one 26,500 years ago that involved 800 cubic km of erupted material; approximately 28 separate eruptions have been identified since that time, the most recent being the 35 cubic km of material associated with the 181AD Taupo eruption (Wilson 1993). This was a violent rhyolitic eruption which ejected rock, pumice and ash with the latter covering most of the central parts of the North Island.

An area approximately 15 x 10 km (as determined by gravity data) in the northwest part of the lake marks what is considered to be the collapse structure of the 181AD eruption crater, which has since bee infilled by 3 km of low density, uncompacted, volcaniclastic material (Bibby et al., 1995; Davy, 1993; Davy and Caldwell, 1998). The 'vent' for the 181AD eruption is located in an area known locally as the Horomatangi Reefs. These reefs include two NE trending ridges that are considered juvenile features, almost certainly rhyolite lavas extruded in the dying stages of the Taupo eruption (Wilson and Walker, 1985; Davy, 1993).

There is significant evidence for continued hydrothermal activity located at the 181Ad eruption vent site. For example, lake bottom temperature gradient measurements indicate conductive heat flow in the general Horomatangi Reefs area (e.g., Calhaem, 1973; Whiteford, 1992; Whiteford et al., 1994). An area of apparent low resistivity forms a ring-like structure, open to the west, around a central, more resistive area coinciding with the Horomatangi reefs (Caldwell and Bibby, 1992). In other areas of the Taupo Volcanic Zone such low values of resistivity are associated with geothermal fields. The higher resistivities associated with the reefs suggest these features contain less geothermal fluid and are less hydrothermally altered than the surrounding material. This is consistent with the interpretation that the reefs are juvenile features. The resistivity data is interpreted as showing the inferred hydrothermal system to lie within, and is confined to, the collapse structure associated with the 181AD eruption. The reefs thus appear to form a 'plug' within a more permeabe zone formed by the disrupted material of the collapse structure and are not covered by sediment. Hot fluids are therefore thought to enter the lake in a ring-loke structure surrounding the reefs resulting in the observed pattern of resistivity (caldwell and Bibby, 1992).

Occasional observations of gas streaming near the Horomatangi Reefs (e.g., Northey, 1983) is further evidence of an active hydrothermal system in the area.

References cited

Bibby, H.M., Caldwell, T.G., Davey, F.J. and Webb, T.H., 1995, Geophysical evidence on the structure of the Taupo Volcanic Zone and its hydrothermal circulation. J.Volcan. Geothermal Res. 68, 29-58.

Caldwell, T.G. and Bibby, H.M., 1992, Geothermal implications of resistivity mapping in Lake Taupo. Proc. 14th New Zealand Geothermal Workshop, 1992, University of Auckland, 207-212.

Calhaem, I.M., 1973, Heat flow measurements under some lakes in the North Island of New Zealand. Ph.D. thesis, Victoria University, Wellington, New Zealand.

Davey, B.W., 1993, Seismic reflection profiling of the Taupo caldera, New Zealand. Exploration Geophysics 24, 443-454.

Davey, B.W. and Caldwell, T.G., 1998, Gravity, magnetic and seismic surveys of the caldera complex, Lake Taupo, North Island, New Zealand. J.Volcan. Geothermal Res., in press.

Northey, I.M., 1983, Seismic studies of the structure beneath Lake Taupo. Ph.D. thesis, Victoria University, Wellington, New Zealand.

Wilson, C.J.N., 1993, Stratigraphy, chronology, styles and dynamics of late Quaternary eruptions from Taupo volcano New Zealand. Phil. Trans. Roy. Soc. London A343, 205-306.

Wilson, C.J.N. and Walker, G.P.L., 1986, The Taupo eruption, New Zealand I. General aspects. Phil.Trans. Roy. Soc. London A314, 199-228.

Whiteford,P.C., 1994, Heat flow measurements in the sediments of Lake Taupo, New Zealand. Tectonophysics 257, 81-92.

Whiteford, P.C., Caldwell, T.G. and Bibby, H.M., 1994, Examination of heat flow and resistivity values at the boundaries of the geothermal systems beneath Lake Taupo, North Island, New Zealand. Proc. 16th New Zealand Geothermal Workshop, 1994, University of Auckland, 157-162.…

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