Misplaced Pages

#633366

51-554: The Ohakuri Caldera ( also Ōhakuri Caldera ) was formed in a paired single event eruption of Ohakuri ignimbrite and is located in the Taupō Volcanic Zone on the North Island of New Zealand . Its significance was first recognised in 2004, as the geology of the area had been misunderstood until then. The paired eruption resulted in a very large eruption sequence in the Taupō Volcanic Zone about 240,000 years ago that included

102-535: A complete reinterpretation of events in the Taupō Volcanic Zone in the last 250,000 years. There has been interest in the mineral potential close to the western rim of the caldera. Ignimbrite Ignimbrites are made of a very poorly sorted mixture of volcanic ash (or tuff when lithified ) and pumice lapilli , commonly with scattered lithic fragments. The ash is composed of glass shards and crystal fragments. Ignimbrites may be loose and unconsolidated, or lithified (solidified) rock called lapilli-tuff. Near

153-529: A large part of post-erosional rocks in Tenerife and Gran Canaria islands. Yucca Mountain Repository, a U.S. Department of Energy terminal storage facility for spent nuclear reactor and other radioactive waste, is in a deposit of ignimbrite and tuff. The layering of ignimbrites is used when the stone is worked, as it sometimes splits into convenient slabs, useful for flagstones and in garden edge landscaping. In

204-414: A plug flow, with an essentially non-deforming mass travelling on a thin shear zone, and the en masse freezing occurs when the driving stress falls below a certain level. This would produce a massive unit with an inversely graded base. There are several problems with the en masse model. Since ignimbrite is a deposit, its characteristics cannot completely represent the flow, and the deposit may only record

255-601: A similar chemistry and so must have undergone the same compaction process to have the same foliation. The Green Tuff in Pantelleria contains rheomorphic structures which are held to be a result of post-depositional re-mobilization because at that time the Green Tuff was believed to be a fall deposit which has no lateral transport. Similarities between the structures in the Green Tuff and ignimbrites on Gran Canaria suggest post-depositional re-mobilization. This interpretation of

306-444: Is a common form of ignimbrite alteration. There are two types of welding, primary and secondary. If the density current is sufficiently hot the particles will agglutinate and weld at the surface of sedimentation to form a viscous fluid; this is primary welding. If during transport and deposition the temperature is low, then the particles will not agglutinate and weld, although welding may occur later if compaction or other factors reduce

357-690: Is also theorized that transformation occurs at a boundary layer at the base of the flow and that all the materials pass through this layer during deposition. Another model proposed is that the density current became stationary before the rheomorphic structures form. Structures such as pervasive foliation are a result of load compaction, and other structures are the result of remobilization by load and deposition on inclined topography. The tuff deposited at Wagontire Mountain in Oregon and Bishop Tuff in California show evidence of late stage viscous flow. These tuffs have

408-553: Is believed to be defined by the valley of the Mangaharakeke stream that the main highway uses and towards the north west of Ātiamuri the caldera floor extends at just below the 300 metres (980 ft) level above sea level. Ngautuku is a dome at the south western aspect of the caldera. The much larger Maroa Caldera complex is to the south with its northern border on the Waikato River so the two caldera borders are adjacent. However,

459-452: Is currently debate in the field of the relative importance of either mechanism, there is agreement that both mechanisms have an effect. A vertical variation in orientation of the structures is compelling evidence against post-depositional re-mobilization being responsible for the majority of the structures, but more work needs to be carried out to discover if the majority of ignimbrites have these vertical variations in order to say which process

510-524: Is the most common. A model based on observations at the Wall Mountain Tuff at Florissant Fossil Beds National Monument in Colorado suggests that the rheomorphic structures such as foliation and pyroclasts were formed during laminar viscous flow as the density current comes to a halt. A change from particulate flow to a viscous fluid could cause the rapid en masse cooling in the last few meters. It

561-590: Is widening much faster that other continental intraarc rifts, which might drive this evolution during a relatively short geological timeframe. In the Bay of Plenty region the current active faults of the old Taupō Rift can align with those of the modern Taupō Rift. This was illustrated by the Edgecumbe Fault and the off sea White Island Fault in the Whakatāne Graben of the rift. The parallel Tauranga Fault Zone to

SECTION 10

#1732869300634

612-464: The Bay of Plenty as much as 19 mm (0.75 in)/yr. To the north east it is related tectonically to the Havre Trough off the continental shelf which is also an active rift structure. The spread of the rift is associated with the basement graywacke rocks subsiding between the rift walls, so creating grabens infilled with volcanic deposits, sometimes from much higher volcanic mountains than

663-519: The Hikurangi subduction zone . The present young, modern Taupō Rift is defined by events between 25,000 and 350,000 years and the old Taupō Rift system, which can be defined by a gravity anomaly, is now located more to the north being created between 350,000 and 2 million years and is about 70 kilometres (43 mi) wide. Consensus does not yet exist with regard to the cause of the Taupō Rift's extension or

714-480: The Rotorua Caldera eruption by this amount, presumably shortly after the eruption. This fault, in the present day, while active has a much lower displacement rate of the order of 0.14 millimetres (0.0055 in)/year and has been assigned by some as the outer western fault of the modern Taupō Rift although most think this is further to the east. Understanding that there is volcanotectonic interrelationship lead to

765-749: The Sierra Madre Occidental in western Mexico. In the western United States , massive ignimbrite deposits up to several hundred metres thick occur in the Basin and Range Province , largely in Nevada , western Utah , southern Arizona , and north-central and southern New Mexico , and the Snake River Plain . The magmatism in the Basin and Range Province included a massive flare-up of ignimbrite which began about 40 million years ago and largely ended 25 million years ago:

816-494: The Hunter region of New South Wales, ignimbrite serves as an excellent aggregate or "blue metal " for road surfacing and construction purposes. Taup%C5%8D Rift The Taupō Rift is the seismically active rift valley containing the Taupō Volcanic Zone , central North Island of New Zealand . The Taupō Rift ( Taupo Rift ) is a 300 km (190 mi) intra-arc continental rift resulting from an oblique convergence in

867-466: The Taupo Rift exhibits the entire spectrum of behaviour ranging from large, ground rupturing events to swarm activity comprising thousands of small events. In the time since Māori settlement these larger earthquakes can be speculated to have resulted in more indirect loss of life than volcanic activity, although as this is driven by oral tradition reports of hundreds dying in a relatively recent landslip on

918-670: The Tongariro graben which considerably widens at the Ruapehu graben. South of Ruapehu the rift, and its normal faulting, terminates with east to west faulting in the Taupō Rift termination faults . At the scale of the tectonic plate boundary, the rift trends NE-SW (41 ± 2°) but within New Zealand this trend is presently at 30° south of Lake Taupō and is 55° at the Bay of Plenty coast. A significant change in

969-462: The adjacent Maroa Caldera . Possibly Pokai ignimbrite which is found to the east on the faultline of the Paeroa Fault , actually came from a caldera eruption that may have been co-located with the present Ohakuri Caldera about 275,000 years ago, but this is speculation. Ohakuri ignimbrite, which has been characterised as a deposit radiating in decreasing thickness from the Ātiamuri area arises from

1020-497: The base and top, called lower and upper 'vitrophyres', but central parts are microcrystalline ('lithoidal'). The mineralogy of an ignimbrite is controlled primarily by the chemistry of the source magma. The typical range of phenocrysts in ignimbrites are biotite, quartz, sanidine or other alkali feldspar , occasionally hornblende , rarely pyroxene and in the case of phonolite tuffs, the feldspathoid minerals such as nepheline and leucite . Commonly in most felsic ignimbrites

1071-410: The base of the flow cannot be turbulent . The instantaneous deposition of an entire body of material is not possible because displacement of the fluid is not possible instantaneously. Any displacement of the fluid would mobilize the upper part of the flow, and en masse deposition would not occur. Instantaneously cessation of the flow would cause local compression and extension, which would be evident in

SECTION 20

#1732869300634

1122-411: The density current passed over the forming deposit. Vertical variations in the orientations of sheathfolds are evidence that rheomorphism and welding can occur syn-depositionally. It has been disputed that the shear between the density current and the forming deposit is significant enough to cause all of the rheomorphic structures observed in ignimbrites, although the shear could be responsible for some of

1173-475: The deposition of ignimbrites from a pyroclastic density current: the en masse deposition and the progressive aggradation models. The en masse model was proposed by volcanologist Stephen Sparks in 1976. Sparks attributed the poor sorting in ignimbrites to laminar flows of very high particle concentration. Pyroclastic flows were envisioned as being similar to debris flows, with a body undergoing laminar flow and then stopping en masse . The flow would travel as

1224-424: The deposition of the Green Tuff has been disputed, suggesting that it is an ignimbrite, and structures such as imbricate fiamme , observed in the Green Tuff, were the result of late stage primary viscous flow. Similar structures observed on Gran Canaria had been interpreted as syn-depositional flow. Sheathfolds and other rheomorphic structures may be the result of a single stage of shear. Shear possibly occurred as

1275-409: The depositional process. Vertical chemical zonation in ignimbrites is interpreted as recording incremental changes in the deposition, and the zonation rarely correlates with flow unit boundaries and may occur within flow units. It has been posited that the chemical changes are recording progressive aggradation at the base of the flow from an eruption whose composition changes with time. For this to be so,

1326-627: The drainage of the linked deep magma mush body between Rotorua and Ohakuri resulted in more than 250 metres (820 ft) of vertical displacement on the Horohoro Fault scarp and formed the Paeroa Graben, coincident to the north with the Kapenga Caldera between it and the Paeroa Fault to the east. This is an area known as the Horohoro Cliffs escarpment and displaced Mamaku ignimbrite from

1377-449: The edges of the ancient Waikato River course which flowed through the valley before the last major Taupō eruption 1,800 years ago (the Hatepe eruption ). The west cliffs are quarried to get blocks of Hinuera Stone, the name given to welded ignimbrite used for building cladding. The stone is light grey with traces of green and is slightly porous. Huge deposits of ignimbrite form large parts of

1428-591: The exceptional volcanic productivity of the associated Taupō Volcanic Zone . Its geology and landforms are of worldwide interest, and it contains multiple significant faults and volcanoes, with some of the volcanoes having potential for worldwide impact. The recent volcanism of the Taupō Volcanic Zone has been divided into three segments, with a central rhyolitic segment, dominated by explosive caldera associated with more typical Island Arc type andesite - dacite stratovolcanoes in either surrounding segment. In

1479-599: The fault activity is normal faulting . While continental intraarc rifts such as this, and those associated with Mount Aso in Japan, and the Trans-Mexican Volcanic Belt result from a different tectonic process from the more studied intracontinental (intraplate) rifts it has been shown that the Taupō Rift displays all of the three modes of evolution. These are narrowing, lateral migration, and along-strike propagation, as found with intracontinental rifts. The Taupo Rift

1530-567: The form of tension cracks and small scale thrusting, which is not seen in most ignimbrites. An adaptation of the en masse theory suggests that the ignimbrite records progressive aggradation from a sustained current and that the differences observed between ignimbrites and within an ignimbrite are the result of temporal changes to the nature of the flow that deposited it. Rheomorphic structures are only observed in high grade ignimbrites. There are two types of rheomorphic flow; post-depositional re-mobilization, and late stage viscous flow. While there

1581-512: The formation of Lake Rotorua and eruption of the Mamaku ignimbrite. The Ohakuri Caldera lies mainly to the east and north of the Ātiamuri Dam and extends almost to the Ōhakuri Dam . Its borders are ill-defined, particularly the northern and eastern borders, possibly because later volcanotectonic activity has completely replaced landforms that could have at one stage included a lake extending almost from Lake Rotorua to this caldera. Its western border

Ohakuri Caldera - Misplaced Pages Continue

1632-540: The hundreds of faults and their segments, some have associations with volcanism, but most fault activity is tectonic. The rift is in that part of the continental Australian Plate associated with the largely underwater Zealandia continental tectonic plate region. The rate of spread of the rift varies from effectively zero, at its southern inland end where the South Wanganui Basin is forming an initial back-arc basin, and volcanic activity has not yet begun, to in

1683-484: The ignimbrites, like all felsic rocks, and the resultant mineralogy of phenocryst populations within them, is related mostly to the varying contents of sodium, potassium, calcium, the lesser amounts of iron and magnesium. Some rare ignimbrites are andesitic, and may even be formed from volatile saturated basalt , where the ignimbrite would have the geochemistry of a normal basalt. Large hot ignimbrites can create some form of hydrothermal activity as they tend to blanket

1734-451: The land surface. More rarely, clasts are cognate material from the magma chamber. If sufficiently hot when deposited, the particles in an ignimbrite may weld together, and the deposit is transformed into a 'welded ignimbrite' , made of eutaxitic lapilli-tuff . When this happens, the pumice lapilli commonly flatten, and these appear on rock surfaces as dark lens shapes, known as fiamme . Intensely welded ignimbrite may have glassy zones near

1785-425: The later case, the strike of the basaltic dyke of the 1886 eruption of Mount Tarawera follows that of faults to the south and north, confirming other hints that orientation of volcanism is preserved. The modern Taupō Volcanic Zone started forming 61,000 years ago but the modern Taupō Rift appears to only have intra-rift fault activity after the immensely disruptive Oruanui eruption . Earthquake activity in

1836-567: The magmatism followed the end of the Laramide orogeny , when deformation and magmatism occurred far east of the plate boundary. Additional eruptions of ignimbrite continued in Nevada until roughly 14 million years ago. Individual eruptions were often enormous, sometimes up to thousands of cubic kilometres in volume, giving them a Volcanic Explosivity Index of 8, comparable to Yellowstone Caldera and Lake Toba eruptions. Successions of ignimbrites make up

1887-405: The mean fault strike occurs just south of the Ōkataina Caldera . The normal fault trends range from N20°E in the south to N45°E in the central and northern sectors. There is good evidence that the orientation of intra-arc strike and extension processes has been maintained for 4 million years in this region of New Zealand. The modern active rift ranges in width from 15 kilometres (9.3 mi) in

1938-951: The minimum welding temperature to below the temperature of the glassy particles; this is secondary welding. This secondary welding is most common and suggests that the temperature of most pyroclastic density currents is below the softening point of the particles. The factor that determines whether an ignimbrite has primary welding, secondary welding or no welding is debated: Landscapes formed by erosion in hardened ignimbrite can be remarkably similar to those formed on granitic rocks . In Sierra de Lihuel Calel , La Pampa Province , Argentina, various landforms typical of granites can be observed in ignimbrite. These landforms are inselbergs , flared slopes , domes , nubbins , tors , tafonis and gnammas . In addition, just like in granite landscapes landforms in ignimbrites may be influenced by joint systems . Ignimbrites occur worldwide associated with many volcanic provinces having high-silica content magma and

1989-463: The most significant eruption of the Caldera. This ignimbrite deposit has been reported to extend to about 15 km (9.3 mi) to the north and east. To the north east there is definite presence 17 km (11 mi) away. There is now good evidence that the 240,000 years ago Ohakuri eruption was a paired eruption within days/weeks of the very slightly earlier, slightly larger, northerly eruption from

2040-426: The north represents a now mainly inactive old Taupō Rift margin. Further south, where more of the old Taupō Rift faults appear to be inactive, the active and very complex Taupō Fault Belt is orientated north-north-east. This is trending with the modern Taupō Rift alignment, which is not always quite parallel with the old rift alignment. Beyond Lake Taupō to the south, there is a relatively narrow rifting segment in

2091-464: The northern Bay of Plenty sector, to 40 kilometres (25 mi) beyond Lake Taupō . Significant faults may be separated by as little as 100 metres (330 ft) in the north but in the south increase to up to 10 kilometres (6.2 mi) separation. There are breaks in the intra-rift fault systems in the recently active central rhyolitic caldera segments at the Taupō Volcano and Ōkataina Caldera. In

Ohakuri Caldera - Misplaced Pages Continue

2142-527: The older Whakamaru Caldera almost certainly crosses the present river course and overlaps the Ohakuri Caldera to a degree. The Waikato River course follows roughly the borders of these two caldera but the thermal area of Orakei Korako to the east is likely more related to the Maroa Caldera. There is evidence of local volcanic activity before 240,000 years ago and not all might have been due to events in

2193-589: The process the materials that made up this mixture fused together into a very tough rock of medium density. Ignimbrite also occurs in the Coromandel region of New Zealand , where the striking orange-brown ignimbrite cliffs form a distinctive feature of the landscape. The nearby Taupō Volcanic Zone is covered in extensive flat sheets of ignimbrite erupted from caldera volcanoes during the Pleistocene and Holocene. The exposed ignimbrite cliffs at Hinuera (Waikato) mark

2244-500: The quartz polymorphs cristobalite and tridymite are usually found within the welded tuffs and breccias . In the majority of cases, it appears that these high-temperature polymorphs of quartz occurred post-eruption as part of an autogenic post-eruptive alteration in some metastable form. Thus although tridymite and cristobalite are common minerals in ignimbrites, they may not be primary magmatic minerals. Most ignimbrites are silicic, with generally over 65% SiO 2 . The chemistry of

2295-571: The resulting explosive eruptions. Ignimbrite occurs very commonly around the lower Hunter Region of the Australian state of New South Wales . The ignimbrite quarried in the Hunter region at locations such as Martins Creek, Brandy Hill, Seaham ( Boral ) and at abandoned quarry at Raymond Terrace is a volcanic sedimentation rock of Carboniferous age (280–345 million years). It had an extremely violent origin. This material built up to considerable depth and must have taken years to cool down completely. In

2346-448: The rift walls. Between 2016 and 2020 there was low volcanic activity in the rift except at Whakaari / White Island , and the areas of maximal satellite measured subsidence were confined to a small areas of about 30 mm (1.2 in)/year near the 2012 Te Māri eruptions site, or the rift geothermal power stations, while from Lake Taupō to the coast subsidence more usually peaked at about 15 mm (0.59 in)/year. The majority of

2397-502: The same mush body feeding the Rotorua Caldera . Ignimbrite , up to 180 metres (590 ft) thick was deposited in the surrounding area to the south of Rotorua. Between Rotorua and Ohakuri crosssections of the ash and ignimbrite from the two eruptions have been able to be sequenced completely and have relationships that can only be explained by a sequence of eruptions separated on occasions by days or less (e.g. no rainfall between eruptions). The pairing separated by 30 kilometres (19 mi)

2448-661: The structures such as imbricate fiamme. Ignimbrite is primarily composed of a matrix of volcanic ash ( tephra ) which is composed of shards and fragments of volcanic glass, pumice fragments, and crystals. The crystal fragments are commonly blown apart by the explosive eruption. Most are phenocrysts that grew in the magma, but some may be exotic crystals such as xenocrysts , derived from other magmas, igneous rocks, or from country rock . The ash matrix typically contains varying amounts of pea- to cobble-sized rock fragments called lithic inclusions. They are mostly bits of older solidified volcanic debris entrained from conduit walls or from

2499-479: The volcanic source, ignimbrites often contain thick accumulations of lithic blocks, and distally, many show meter-thick accumulations of rounded cobbles of pumice. Ignimbrites may be white, grey, pink, beige, brown, or black depending on their composition and density. Many pale ignimbrites are dacitic or rhyolitic . Darker-coloured ignimbrites may be densely welded volcanic glass or, less commonly, mafic in composition. Two main models have been proposed to explain

2550-469: The wet soil and bury watercourses and rivers. The water from such substrates will exit the ignimbrite blanket in fumaroles , geysers and the like, a process which may take several years, for example after the Novarupta tuff eruption. In the process of boiling off this water, the ignimbrite layer may become metasomatised (altered). This tends to form chimneys and pockets of kaolin -altered rock. Welding

2601-476: Was possibly through tectonic coupling of separate magma bodies that co-evolved from a lower in the mantle common mush body, as paired events are being increasingly recognised. The maximum outflow dense-rock equivalent (DRE) of the Ohakuri ignimbrite is 100 cubic kilometres (24 cu mi) which means the combined eruptions produced 245 cubic kilometres (59 cu mi) of material. It has been postulated that

SECTION 50

#1732869300634
#633366