A Study of Some Aspects of the Volcanic History of the Lake Taupo Area, North Island, New Zealand

<p>Rhyolitic pyroclastic eruptives from the Taupo area, New Zealand have been mapped as nine tephra formations of Holocene (0-10 kyr B.P.), and six of late Pleistocene age (20-c.50 kyr B.P.). Only the 10 younger tephras are dated by radiocarbon. All formations contain PLINIAN type airfall units but three, KAWAKAWA, WAIMIHIA and TAUPO also contain a major pyroclastic flow deposit (IGNIMBRIIE) unit. Dome extrusion can only be demonstrated for KARAPITI eruptive episode, but is inferred for the other Holocene episodes. TAUPO IGNIMBRITE is the product of the most recent eruption and is a particularly well preserved and extensive, unwelded pyroclastic flow deposit, up to 50m thick. Its variety of appearance is described in terms of three lithofacies; valley facies, fines depleted facies and veneer facies, each being formed by particular mechanisms within a pyroclastic flow. Abundant charred logs, lying prone within Taupo Ignimbrite, are radial about the source and attest to a radially outward moving mass dominated by laminar flow. Lake Taupo today covers most of the volcanic source area, preventing close examination and the identification of individual source vents. A vent for each Holocene tephra is inferred from isopachs, grainsize and lake bathymetry, but the vents so inferred show no spatial distribution with time. Nevertheless they are evenly spaced along a northeast trending line and lie on intersections with a northwest trending set of lineations, indicating deep, crustal, structural control on volcanism. Cumulative volume of airfall and ignimbrite material erupted in the Taupo area in the last 50 kyr has amounted to about 175 km3 of magma. Eruptions have proceeded in a step-wise manner, indicating the period to the next eruption is about 8 kyr. By the same approach, the next eruption from the Okataina area, 50 km to the north of Taupo is expected in less than 400 years. Whole rock and mineral chemistry clearly distinguishes between the Holocene and the late Pleistocene tephras, but within each group variations are subtle and no trends with time are apparent. None of the formations exhibit evidence for a chemically zoned magma body, but some data, especially pyroxene phenocryst chemistry, suggests magma inhomogeneities of mafic elements. The Holocene tephra were probably all erupted from the same magma chamber in which crystallisation was the dominant process but convection, crystal element diffusion and chamber replenishment were all probably operative. Results obtained by electron microprobe analysis of glass shards are critically dependent on the beam diameter and current used. By standardising these at 10 microns and 8 nanoamps respectively, comparable major element analyses on glass shards from numerous tephras ranging in age from 20 kyr to 600 kyr were obtained. The stratigraphic relationships between sets of samples (located mainly distal from source) and the close chemical similarity of some samples enabled a comprehensive tephrostratigraphy to be established. In particular, MT. CURL TEPHRA has a glass chemistry quite different from other stratigraphically separate tephras, establishing correlation of Mt. Curl Tephra to Whakamaru Ignimbrite. Likewise, other ignimbrite formations can be correlated to widespread airfall tephras, so establishing an absolute ignimbrite stratigraphy. Microprobe analysis of glass shards provides a method for indirectly determining the amount of hydration. For dated samples from a known weathering environment, the parameters controlling hydration can be quantified. For glass of uniform chemistry, shard size and porosity, ground temperature and groundwater movements are the most important parameters. No shards in the 63-250 micron size range have been found with more than 9% water, suggesting once this maximum is reached, glass rapidly alters to secondary products. Detailed knowledge of the volcanic history of the Taupo area, particularly since 50 kyrs B.P. allows the volcanic hazards of the region to be assessed. Fifteen major eruptions in 50 kyr gives a frequency of 1 in 3300 years, but the timing of individual events is not evenly spread throughout that time. Monitoring for volcanic Precursory events (not being undertaken at present) is essential to gauge the present and short-term future volcanic activity of the Taupo Volcanic Zone.</p>

A Study of Some Aspects of the Volcanic History of the Lake Taupo Area, North Island, New Zealand

<p>Rhyolitic pyroclastic eruptives from the Taupo area, New Zealand have been mapped as nine tephra formations of Holocene (0-10 kyr B.P.), and six of late Pleistocene age (20-c.50 kyr B.P.). Only the 10 younger tephras are dated by radiocarbon. All formations contain PLINIAN type airfall units but three, KAWAKAWA, WAIMIHIA and TAUPO also contain a major pyroclastic flow deposit (IGNIMBRIIE) unit. Dome extrusion can only be demonstrated for KARAPITI eruptive episode, but is inferred for the other Holocene episodes. TAUPO IGNIMBRITE is the product of the most recent eruption and is a particularly well preserved and extensive, unwelded pyroclastic flow deposit, up to 50m thick. Its variety of appearance is described in terms of three lithofacies; valley facies, fines depleted facies and veneer facies, each being formed by particular mechanisms within a pyroclastic flow. Abundant charred logs, lying prone within Taupo Ignimbrite, are radial about the source and attest to a radially outward moving mass dominated by laminar flow. Lake Taupo today covers most of the volcanic source area, preventing close examination and the identification of individual source vents. A vent for each Holocene tephra is inferred from isopachs, grainsize and lake bathymetry, but the vents so inferred show no spatial distribution with time. Nevertheless they are evenly spaced along a northeast trending line and lie on intersections with a northwest trending set of lineations, indicating deep, crustal, structural control on volcanism. Cumulative volume of airfall and ignimbrite material erupted in the Taupo area in the last 50 kyr has amounted to about 175 km3 of magma. Eruptions have proceeded in a step-wise manner, indicating the period to the next eruption is about 8 kyr. By the same approach, the next eruption from the Okataina area, 50 km to the north of Taupo is expected in less than 400 years. Whole rock and mineral chemistry clearly distinguishes between the Holocene and the late Pleistocene tephras, but within each group variations are subtle and no trends with time are apparent. None of the formations exhibit evidence for a chemically zoned magma body, but some data, especially pyroxene phenocryst chemistry, suggests magma inhomogeneities of mafic elements. The Holocene tephra were probably all erupted from the same magma chamber in which crystallisation was the dominant process but convection, crystal element diffusion and chamber replenishment were all probably operative. Results obtained by electron microprobe analysis of glass shards are critically dependent on the beam diameter and current used. By standardising these at 10 microns and 8 nanoamps respectively, comparable major element analyses on glass shards from numerous tephras ranging in age from 20 kyr to 600 kyr were obtained. The stratigraphic relationships between sets of samples (located mainly distal from source) and the close chemical similarity of some samples enabled a comprehensive tephrostratigraphy to be established. In particular, MT. CURL TEPHRA has a glass chemistry quite different from other stratigraphically separate tephras, establishing correlation of Mt. Curl Tephra to Whakamaru Ignimbrite. Likewise, other ignimbrite formations can be correlated to widespread airfall tephras, so establishing an absolute ignimbrite stratigraphy. Microprobe analysis of glass shards provides a method for indirectly determining the amount of hydration. For dated samples from a known weathering environment, the parameters controlling hydration can be quantified. For glass of uniform chemistry, shard size and porosity, ground temperature and groundwater movements are the most important parameters. No shards in the 63-250 micron size range have been found with more than 9% water, suggesting once this maximum is reached, glass rapidly alters to secondary products. Detailed knowledge of the volcanic history of the Taupo area, particularly since 50 kyrs B.P. allows the volcanic hazards of the region to be assessed. Fifteen major eruptions in 50 kyr gives a frequency of 1 in 3300 years, but the timing of individual events is not evenly spread throughout that time. Monitoring for volcanic Precursory events (not being undertaken at present) is essential to gauge the present and short-term future volcanic activity of the Taupo Volcanic Zone.</p>

Some Studies of the Geology, Volcanic History, and Geothermal Resources of the Okataina Volcanic Centre, Taupo Volcanic Zone, New Zealand

<p>Okataina Volcanic Centre is the most recently active of the four major rhyolite eruptive centres in the Taupo Volcanic Zone of New Zealand. Within the Centre lies Haroharo Caldera, a complex of overlapping collapse structures resulting from successive voluminous pyroclastic eruptions from the same general source area. At least four main and possibly two minor caldera-forming eruptions have occurred during the last 250,000 years, although poor exposure means that attempts to interpret the early structural history are highly speculative. Although there is no compelling evidence of structural updoming within Haroharo Caldera, magma resurgence has followed the last major caldera-forming eruption of the Rotoiti Breccia at [greater than or equal to] 42,000 years B.P. Eruption of this magma within the caldera has formed the two large rhyolite lava and pyroclastic piles of the Haroharo Volcanic Complex and Tarawera Volcanic Complex, plus two subsidiary adjacent complexes at Okareka and Rotoma. All these intracaldera eruptives are younger than 20,000 years B.P., with the most recent eruptions from Tarawera; of rhyolite at c. 700 years B.P., and of basalt in 1886 A.D. A considerable amount of earlier work carried out at Okataina was directed mainly at petrology and chemistry of the rhyolites forming the Tarawera and Haroharo Volcanic Complexes. The present study has arisen from a 1:50,000 mapping programme at Okataina and has sought to examine structures and volcanic history in greater detail, and to consider the resulting geological implications for geothermal resources. Caldera boundaries have been mapped, and two major vent lineations are defined, apparently related to fundamental basement fractures which have controlled location of the Tarawera and Haroharo Volcanic Complexes. An intracaldera ring fault is also suggested by the sub-circular arrangement of some young volcanic vents. The Haroharo and Tarawera Complexes are mapped, with locations of source vents, and dating of the major lavas and pyroclastic deposits. All the post-20,000 year eruptives are placed in four main emptive episodes at Haroharo, and five at Tarawera. The near-source pyroclastic surge and flow deposits are 14C dated, and with their associated widespread plinian fall deposits they provide time planes for dating the associated lavas. The emptive episodes generally appear to have been of much shorter duration than the intervening quiescent periods which lasted for thousands of years. All the eruptive episodes at Haroharo involved multiple eruptions from vents spread out over several kilometres along the vent lineations. Similar multiple vent eruptions can be demonstrated for some of the Tarawera eruptive episodes. More than 500 km3 of magma has been erupted from Haroharo Caldera during the last 250,000 years, 80 km3 of which was erupted in the Last 20,000 years. This history suggests that a large magmatic heat source should continue to underlie the Okataina Volcanic Centre. However, very little surface hydrothermal activity occurs within Haroharo Caldera. It is suggested that the large external hydrothermal fields at Tikitere, Waimangu-Waiotapu-Waikite, and possibly Kawerau, are related to Haroharo Caldera heat sources. Presently available data are summarized for hydrothermal fields in and adjacent to Haroharo Caldera, and new analyses are presented for some warm springs discovered within the caldera. Estimates and measurements of chloride fluxes in lakes and rivers are reported. The chloride flux values suggest the occurrence of larger hydrothermal heat flows into lakes and rivers than are apparent at the surface. Measurements of chloride flux in the Tarawera River showed that 280 g s-1 of chloride is added to the river within Haroharo Caldera below the Lake Tarawera outlet. Only 80 g s-1 of this chloride comes from known geothermal sources. A total chloride flux of 760 g s-1 in the Tarawera River passing out of the Okataina Volcanic Centre indicates a minimum geothermal heat flow of 600 MW. Estimates of heat flows in other drainage paths from Haroharo Caldera suggest that minimum total heat flow from the caldera may exceed 1500 MW. A large heat flow from the caldera would appear consistent with the volcanic history. Some suggestions are made for further investigation of the geothermal resources</p>

Some Studies of the Geology, Volcanic History, and Geothermal Resources of the Okataina Volcanic Centre, Taupo Volcanic Zone, New Zealand

<p>Okataina Volcanic Centre is the most recently active of the four major rhyolite eruptive centres in the Taupo Volcanic Zone of New Zealand. Within the Centre lies Haroharo Caldera, a complex of overlapping collapse structures resulting from successive voluminous pyroclastic eruptions from the same general source area. At least four main and possibly two minor caldera-forming eruptions have occurred during the last 250,000 years, although poor exposure means that attempts to interpret the early structural history are highly speculative. Although there is no compelling evidence of structural updoming within Haroharo Caldera, magma resurgence has followed the last major caldera-forming eruption of the Rotoiti Breccia at [greater than or equal to] 42,000 years B.P. Eruption of this magma within the caldera has formed the two large rhyolite lava and pyroclastic piles of the Haroharo Volcanic Complex and Tarawera Volcanic Complex, plus two subsidiary adjacent complexes at Okareka and Rotoma. All these intracaldera eruptives are younger than 20,000 years B.P., with the most recent eruptions from Tarawera; of rhyolite at c. 700 years B.P., and of basalt in 1886 A.D. A considerable amount of earlier work carried out at Okataina was directed mainly at petrology and chemistry of the rhyolites forming the Tarawera and Haroharo Volcanic Complexes. The present study has arisen from a 1:50,000 mapping programme at Okataina and has sought to examine structures and volcanic history in greater detail, and to consider the resulting geological implications for geothermal resources. Caldera boundaries have been mapped, and two major vent lineations are defined, apparently related to fundamental basement fractures which have controlled location of the Tarawera and Haroharo Volcanic Complexes. An intracaldera ring fault is also suggested by the sub-circular arrangement of some young volcanic vents. The Haroharo and Tarawera Complexes are mapped, with locations of source vents, and dating of the major lavas and pyroclastic deposits. All the post-20,000 year eruptives are placed in four main emptive episodes at Haroharo, and five at Tarawera. The near-source pyroclastic surge and flow deposits are 14C dated, and with their associated widespread plinian fall deposits they provide time planes for dating the associated lavas. The emptive episodes generally appear to have been of much shorter duration than the intervening quiescent periods which lasted for thousands of years. All the eruptive episodes at Haroharo involved multiple eruptions from vents spread out over several kilometres along the vent lineations. Similar multiple vent eruptions can be demonstrated for some of the Tarawera eruptive episodes. More than 500 km3 of magma has been erupted from Haroharo Caldera during the last 250,000 years, 80 km3 of which was erupted in the Last 20,000 years. This history suggests that a large magmatic heat source should continue to underlie the Okataina Volcanic Centre. However, very little surface hydrothermal activity occurs within Haroharo Caldera. It is suggested that the large external hydrothermal fields at Tikitere, Waimangu-Waiotapu-Waikite, and possibly Kawerau, are related to Haroharo Caldera heat sources. Presently available data are summarized for hydrothermal fields in and adjacent to Haroharo Caldera, and new analyses are presented for some warm springs discovered within the caldera. Estimates and measurements of chloride fluxes in lakes and rivers are reported. The chloride flux values suggest the occurrence of larger hydrothermal heat flows into lakes and rivers than are apparent at the surface. Measurements of chloride flux in the Tarawera River showed that 280 g s-1 of chloride is added to the river within Haroharo Caldera below the Lake Tarawera outlet. Only 80 g s-1 of this chloride comes from known geothermal sources. A total chloride flux of 760 g s-1 in the Tarawera River passing out of the Okataina Volcanic Centre indicates a minimum geothermal heat flow of 600 MW. Estimates of heat flows in other drainage paths from Haroharo Caldera suggest that minimum total heat flow from the caldera may exceed 1500 MW. A large heat flow from the caldera would appear consistent with the volcanic history. Some suggestions are made for further investigation of the geothermal resources</p>

Old Volcanic Stories—Bringing Ancient Volcanoes to Life in Ireland’s Geological Heritage Sites

Active or recently active volcanic areas present very visible and easy to understand phenomena for the broad population to appreciate as geological heritage. However, in a geologically stable country such as Ireland, with no volcanism evident for tens of millions of years and few clearly visible traces of volcanoes of a ‘school textbook’ nature, the significance of ancient volcanic remains is much harder to explain or to present to visitors to geological heritage sites. This paper explores the wide range of evidence of ancient volcanic activity within recognised geological heritage sites across Ireland, both in County Geological Sites and in the UNESCO Global Geoparks. Some of the stories that can be told using the available evidence are documented, including some of the current efforts to present Ireland’s volcanic geological heritage. The stories are told within the context of the geological and volcanic history of Ireland over the past 500 million years. As such, the promotion of geological heritage is at an early stage, and this contribution may provide inspiration or ideas for approaches to this problem for other countries or terrains with similar ancient volcanic rocks.