Location Of Eruption Research Articles

Migration of an Unerupted Second Molar to the Condyle: Report of a Case With Sequential Radiographs

Unerupted molars are generally found at or close to their usual site of eruption, but occasionally these teeth are encountered in more distant locations.1 Tooth dislocation may occur as a result of migration or may be induced by pathology.2 The etiology and mechanism of intraosseous tooth migration is not fully understood, but it is an uncommon natural event that primarily affects unerupted mandibular second premolars, canines, and lateral incisors.2-4 Occurrences of migrated canines, second premolars, and lateral incisors appearing generally near their original eruption location are frequently described in the literature.

Geophysical survey of the intra-caldera icefield of Mt Veniaminof, Alaska

AbstractMt Veniaminof is a large active stratovolcano located on the Alaska Peninsula (56.2° N, 159° W). We present results of the first geophysical survey of the icefield that fills much of the 10 km×8 km caldera that was most recently modified during the last major eruption roughly 3700 BP. The subglacial topography and ice volume are derived from an 8MHz radio-echo sounding survey conducted in July 2005. Prominent internal reflectors are assumed to be isochronal ash/acid deposits related to local eruptions. Accumulation rates and basal melt rates are calculated using a Nye one-dimensional steady-state accumulation model applied at a location that approximates an ice divide and calibrated by matching internal reflectors with published records of recent local volcanic eruptions. The model yields order of magnitude estimates of the accumulation rate of 4ma–1 water equivalent and 2 ma–1 of basal melt. The subsequent geothermal flux of ∽19Wm–2 is similar to active hydrothermal vents in volcanic lakes. We suggest that these values represent an upper limit for the geothermal flux within the ice-covered regions of the main caldera. We also analyze likely subglacial meltwater flow paths to examine the implications of recent eruption activity at an active intra-caldera cinder cone. Two lava-producing eruptions from the cinder cone in 1983–84 and 1993–94 melted roughly 0.17km3 of ice. The lack of significant deformation of the internal stratigraphy to the south and east of the melt hole suggests that any subglacial drainage in those directions was entirely within subglacial deposits. We suggest that the more likely drainage route was northwest into a large outlet glacier.

P-wave velocity structure of Usu volcano: Implication of structural controls on magma movements and eruption locations

The three-dimensional P-wave velocity structure of Usu volcano, Japan, is investigated by using first arrival time data of the 2001 active seismic survey. The velocity structure down to the Pre-Neogene basement below the volcano is revealed. The most prominent feature of the velocity model is the deepening of the basement toward the south-southwest. The resultant velocity model contributes to the relocations of 2000 eruption precursory earthquakes. The precursory seismic activity, which reflects pre-eruptive magma movements, is divided into three parts: (1) a sub-vertical distribution indicating magma ascent to beneath the summit of the volcano, (2) a northward migration that indicates magma movement resulting in the subsequent eruptions at the northwestern foot, and (3) a horizontal southward migration possibly indicating intrusion of sill. The northward and southward migrating earthquakes are constrained by the dipping basement structure such that most of the events occur within the middle Miocene or Pre-Neogene layer. The geographical distribution of craters and the upheavals of past eruptions are also constrained by the subsurface basement. At the northern foot of Usu Volcano, the distribution of these craters and upheavals is such that they are enclosed by the subsurface basement, which becomes shallower toward the north. On the other hand, no traces of eruptions are found in the southern and southwestern foots where the basement deepens toward the south-southwest.

A novel technique for particle tracking in cold 2-dimensional fluidized beds—simulating fuel dispersion

This paper presents a novel technique for particle tracking in 2-dimensional fluidized beds operated under ambient conditions. The method is applied to study the mixing mechanisms of fuel particles in fluidized beds and is based on tracking a phosphorescent tracer particle by means of video recording with subsequent digital image analysis. From this, concentration, velocity and dispersion fields of the tracer particle can be obtained with high accuracy. Although the method is restricted to 2-dimensional, it can be applied under flow conditions qualitatively resembling a fluidized-bed combustor. Thus, the experiments cover ranges of bed heights, gas velocities and fuel-to-bed material density and size ratios typical for fluidized-bed combustors. Also, several fluidization regimes (bubbling, turbulent, circulating and pneumatic) are included in the runs. A pattern found in all runs is that the mixing pattern of the tracer (fuel) solids is structured in horizontally aligned vortexes induced by the bubble flow. The main bubble paths always give a low concentration of tracer solids and with the tracer moving upwards, while the downflow of tracer particles in the dense bottom bed is found to take place in zones with low bubble density and at the sidewalls. The amount of bed material (bed height) has a strong influence on the bottom bed dynamics (development and coalescence of bubbles) and, consequently, on the solids mixing process. Local dispersion coefficients reach maximum values around the locations of bubble eruptions, while, in the presence of a dense bottom bed, an increase in fluidization velocity or amount of bed material enhances dispersion. Dispersion is found to be larger in the vertical than in the horizontal direction, confirming the critical character of lateral fuel dispersion in fluidized-bed combustors of large cross section.

Probabilistic Assessment of Vent Locations for the Next Auckland Volcanic Field Event

Auckland, New Zealand’s most populous Region is centred on the Auckland volcanic field (AVF), which contains an estimated 49 small-volume basaltic volcanoes. Although these volcanoes are considered monogenetic, a number of centres may have been active either simultaneously or within a short period of time. After reviewing the characteristics of the field, known vent locations and a provisional eruption sequence were used to estimate likely eruption locations for the next event. The term event is used here to describe a short period of activity comprising one or more eruptions. Point pattern analysis and temporal consideration allow vents related by location and age to be grouped, suggesting that as few as 18 separate events may have produced the AVF. Probability distributions describing previous event attributes were used to generate Monte Carlo simulations, each defining likely vent locations for the next AVF event. Simulations were based on the assumption that distance from Rangitoto, the youngest volcano in the field, to the next event is best approximated using distances between the centroids of previous consecutive events. The maximum extent of the field is constrained only by faulting on its eastern side. The simulations show that 69% of events involved at least one eruption centred either on land or within 1 km of the coast. These events would result in substantial damage to buildings and infrastructure, significant business interruption and, potentially, loss of life. A simple risk assessment is presented considering only spatial eruption probability contingent upon a future event and building density within a 2 km grid. Work in progress aims to develop a more realistic assessment of risk for the region.

A tentative study for the prediction of the CME related geomagnetic storm intensity and its transit time

Using 80 CME-ICME events during 1997.1–2002.9, based on the eruptive source locations of CMEs and solar magnetic field observation at the photosphere, a current sheet magnetic coordinate (CMC) system is established in order to study the propagation of CME and its geoeffectiveness. In context of this coordinate system, the effect of the eruptive source location and the form of heliospheric current sheet (HCS) at the eruptive time of CME on the geomagnetic storm intensity caused by CME and the CME’s transit time at the Earth is investigated in detail. Our preliminary conclusions are: 1) The geomagnetic disturbances caused by CMEs tend to have the so-called “same side-opposite side effect”, i.e. CMEs erupt from the same side of the HCS as the earth would be more likely to arrive at the earth and the geomagnetic disturbances associated with them tend to be of larger magnitude, while CMEs erupting from the opposite side would arrive at the earth with less probability and the corresponding geomagnetic disturbance magnitudes would be relatively weaker. 2) The angular separation between the earth and the HCS affect the corresponding disturbance intensity. That is, when our earth is located near the HCS, adverse space weather events occur most probably. 3) The erupting location of the CME and its nearby form of HCS will also affect its arrival time at the earth. According to these conclusions, in this context of CMC coordinate we arrive at new prediction method for estimating the geomagnetic storm intensity (Dstmin) caused by CMEs and their transit times. The application of the empirical model for 80 CME-ICME events shows that the relative error of Dst is within 30% for 59% events with Dstmin≤−50 nT, while the averaged absolute error of transit time is lower than 10 h for all events.

Volcanic risk ranking for Auckland, New Zealand. II: Hazard consequences and risk calculation

In a companion paper, a methodology for ranking volcanic hazards and events in terms of risk was presented, and the likelihood and extent of potential hazards in the Auckland Region, New Zealand investigated. In this paper, the effects of each hazard are considered and the risk ranking completed. Values for effect are proportions of total loss and, as with likelihood and extent, are based on order of magnitude. Two outcomes were considered – building damage and loss of human life. In terms of building damage, tephra produces the highest risk by an order of magnitude, followed by lava flows and base surge. For loss of human life, risk from base surge is highest. The risks from pyroclastic flows and tsunami are an order of magnitude smaller. When combined, tephra fall followed by base surge produces the highest risk. The risks from lava flows and pyroclastic flows are an order of magnitude smaller. For building damage, the risk from Mt. Taranaki volcano, 280 km from the Auckland CBD, is largest, followed by Okataina volcanic centre, an Auckland volcanic field eruption centred on land, then Tongariro volcanic centre. In terms of human loss, the greatest risk is from an Auckland eruption centred on land. The risks from an Auckland eruption centred in the ocean, Okataina volcanic centre, and Taupo volcano are more than an order of magnitude smaller. When combined, the risk from Mt. Taranaki remains highest, followed by an Auckland eruption centred on land. The next largest risks are from the Okataina and Tongariro volcanic centres, followed by Taupo volcano. Three alternative situations were investigated. As multiple eruptions may occur from the Auckland volcanic field, it was assumed that a local event would involve two eruptions. This increased risk of a local eruption occurring on land so that it was equal to that of an eruption from Mt. Taranaki. It is possible that a future eruption may be of a similar, or larger size, to the previous Rangitoto eruption. Risk was re-calculated for local eruptions based on the extent of hazards from Rangitoto. This increased the risk of lava flow to greater than that of base surge, and the risk from an Auckland land eruption became greatest. The relative probabilities used for Mt. Taranaki volcano and the Auckland volcanic field may only be minimum values. When the probability of these occurring was increased by 50%, there was no change in either ranking.

Magma–tectonic interactions in Nicaragua: the 1999 seismic swarm and eruption of Cerro Negro volcano

A low-energy (Volcanic Explosivity Index [VEI] 1), small-volume (0.001 km 3 Dense Rock Equivalent [DRE]) eruption of highly crystalline basalt occurred at Cerro Negro volcano, Nicaragua, August 5–7, 1999. This eruption followed three earthquakes (each ∼ M w 5.2) with strike-slip and oblique-slip focal mechanisms, the first of which occurred approximately 11 h before eruptive activity and within 1 km of Cerro Negro. Surface ruptures formed during these events extend up to 4 km from Cerro Negro, but concentrate ∼1 km south of Cerro Negro. Surface ruptures did not occur within 300 m of the cone, however, three new vents formed on the south flank and base of Cerro Negro and on trend with the Cerro La Mula–Cerro Negro volcanic alignment. Earthquake swarms were located northwest and southeast of Cerro Negro and seismicity was elevated for up to 11 days after the initial event. The temporal and spatial patterns of earthquake swarms, surface ruptures, and the eruption location can be explained using the Hill [J. Geophys. Res. 82 (1977) 1347] model for earthquake swarms in volcanic regions, where an eruption is triggered by tectonically induced changes in the regional stress field. In this model, tectonic strain, rather than magmatic overpressure causes dilation of the conduit for magma ascent. Numerical simulations for the 1999 eruption illustrate that the observed velocities (up to 75 m s −1) and fountain heights (50–300 m) can be achieved by eruption of magma with little excess magmatic pressure, in response to changes in Coulomb stress along the Cerro La Mula–Cerro Negro alignment. These observations and models show that 1999 Cerro Negro activity was a tectonically induced small-volume eruption in an arc setting, with the accommodation of extensional strain by dike injection.

Magnetic Field–Minimum Intensity Correlation in Sunspots: A Tool for Solar Dynamo Diagnostics

Within a sunspot umbra, the continuum intensity is known to be inversely proportional to the magnetic field strength. Studied less is the relationship between the minimum continuum intensity and the maximum field strength of different sunspots. We conduct a test of this global relationship, using brightness ratios and magnetic field data from the Advanced Stokes Polarimeter and the Michelson Doppler Imager (MDI) for 10 sunspot umbrae of various sizes observed 1998 May-2003 June. We determine that the peak field strengths of sunspots can be ascertained from a fit to their corresponding brightness ratios with an accuracy of ≈100 G, nearly twice the accuracy that a fit to the MDI magnetogram values can provide. We then analyze continuum intensity data from the MDI to characterize the distribution of sunspots as a function of latitude. We hand-select 331 and 321 umbrae, respectively, in the northern and southern hemispheres during Carrington rotations 1910-2003. Although the average location of sunspot eruption moves equatorward throughout the solar cycle, the northern hemisphere shows darker umbrae located systematically closer to the equator, while brighter umbrae are found at higher latitudes. These findings confirm the results of simulations that show strong flux emerging radially while weak flux emerges nonradially, causing weak flux to emerge poleward of its original toroidal field position. The average umbral intensity decreased in the north through the solar cycle, reaching a minimum intensity around sunspot maximum, possible evidence of the toroidal field strength peaking at solar maximum. This finding is in opposition to previous observations suggesting an increase late in the cycle. The southern hemisphere umbral distribution appears more disorganized and periodic in nature.

Impacts of volcanism on pre-European inhabitants of Taveuni, Fiji

Taveuni is a Fijian ocean-island volcano that sporadically erupted throughout the Holocene. The 437-km2 island is an active monogenetic volcanic field with a constantly shifting locus of activity along a single apparent rift axis. Although the eruptions were not large ( ≤VEI 2), unexpected shifts in Taveuni volcanism had the potential to affect habitation sites. Since known human settlement of the Fiji Group (ca. 950–750 BC), there have been at least 58 eruptions on Taveuni. Up to 25 of these eruptions potentially affected pre-European inhabitants of the island and at least four former occupation sites are known to have been affected by volcanic products. Despite apparent earliest settlement of Taveuni post-dating other nearby islands by up to 600 years, volcanism probably did not hinder or stall settlement of Taveuni compared with neighbouring islands. However, a period of voluminous eruptions between 300 and 500 AD covered much of south Taveuni with lava and/or thick tephra, apparently causing abandonment of at least this portion of Taveuni until approximately 1100 AD. Most eruptions were not of catastrophic proportions and, due to their localised effects, re-settlement was rapid in marginal unaffected areas. Localised stories and a relict place name survive to describe former eruption locations and effects since approximately 120–320 AD. Knowledge of the impacts on Taveuni's past inhabitants forms the basis of volcanic disaster-mitigation strategies to minimise future effects on the current 14,500 residents.

Comparison and cross-checking of historical, archaeological and geological evidence for the location and type of historical and sub-historical eruptions of multiple-vent oceanic island volcanoes

Abstract Oceanic island volcanoes, like many others, have many small vents scattered over their flanks in addition to, or in place of, large summit vents. These small vents are commonly monogenetic and many eruptions of this type of volcano involve activity at more than one such vent: lines of volcanic cones are often produced by eruptions fed by dykes, and if more than one dyke is emplaced during an eruption these vents can be along different alignments, and many kilometres apart. Identifying which vents were produced in which eruptions is an important problem in reconstructing the development of multiplevent volcanoes. Reconstruction of historical and sub-historical eruptions of two oceanic island volcanoes, the Cumbre Vieja volcano on La Palma in the Canary Islands and Cha das Caldeiras volcano on Fogo, Cape Verde Islands, has indicated that historical eyewitness accounts and archaeological evidence can be extremely valuable adjuncts to detailed geological studies. In contrast, secondary accounts including folk memories and earlier accounts in the scientific literature are commonly inconsistent both with the eyewitness accounts and with the results of detailed geological mapping, archaeological evidence or other historical documents. A common source of error is confusion of the vents of historical eruptions with older but larger or more prominent volcanic vents that lie along the same line of sight as viewed from adjacent settlements or from convenient viewpoints. Examples of this are provided by mis-locations of the vents of the AD 1677 eruption on La Palma and of some of the vents of the AD 1951 eruption on Fogo. Another problem arises when the location or style of eruption on a volcano has changed in early historical time, as has occurred on Fogo. The differences between early historical accounts of eruptions on this volcano and the more detailed accounts of more recent eruptions has led to the discrediting of the former by some researchers, whereas geological studies have supported the early historical accounts.

Visualization of a mechanism for three-dimensional interaction and near-wall eruption

An experimental configuration has been found which allows detailed observation of three-dimensional vortex–vortex and associated vortex–surface interactions which appear similar to those observed in fully turbulent flow. Hydrogen bubble visualization illustrates a complicated intertwining, or braiding, of two initially co-rotating vortices. It is observed that the three-dimensional interactions of the braided vortices induces a pair of local surface-fluid eruptions reminiscent of the ‘bursting’ behaviour characteristic of the near-wall regeneration process of fully turbulent boundary layers.

Paleosecular variation during sequential geomagnetic reversals from Hawaii

Paleomagnetic directions measured from 175 flows in four volcanic sequences on the island of O'ahu (Hawaii) cover about 0.3 m.y. of geomagnetic field changes and include detailed records of the successive Gilbert-Gauss (3.57 Ma) and Lower Mammoth reversals (3.33 Ma). The dominant pattern of the directional changes is the presence of large inclination variations with increasing amplitude and steep values during the transitional periods. The morphology of these recurring features during different transitions is reminiscent of the characteristics of the non-dipole field inferred from studies of paleosecular variation in the Pacific. These observations are consistent with transitional mechanisms yielding the emergence of non-dipole components with characteristics similar to those during periods of stable polarity. The three different records of the lower Mammoth reversal show that clusters of transitional pole positions are not reproducible between parallel sections and are thus linked to local eruptions and not to long-lived states of the transitional field.

Reactive Flow in Nova Outbursts

Thermonuclear burning in hydrogen envelopes is studied using two-dimensional simulations, in a broader manner than that presented recently by Glasner & Livne. The evolution of the convective burning process before and during thermonuclear runaway (TNR) is discussed. We study the coupling between the burning and the hydrodynamics, which determines the nature of the convective flow, and analyze the relevant timescales. The consequences of the combustion and the nucleosynthesis are investigated in detail. The combustion process is composed of many local eruptions, scattered over the entire hydrogen envelope (at its base). Several mechanisms regulate the process in such a way that the TNR occurs over a time interval of several hundred seconds. These mechanisms are pressure waves, buoyancy, and turbulent diffusion. The TNR is terminated by a nearly spherically symmetric expansion of the outer layers of the envelope. For a 1 M☉ core, the enrichment of the hydrogen envelope by convective overshoot is approximately 30%, in agreement with observations of novae.

Importance of gravitational spreading in the tectonic and volcanic evolution of Mount Etna

THE interaction between gravity and thermal effects largely determines the structural and magmatic evolution of volcanic constructs at scales spanning several orders of magnitude, from small cones to the oceanic crust1–3. Although gravitational spreading is a direct consequence of this interaction and a fundamental process in volcano growth, it is rarely recognized as such. Here we describe a striking example of this process, at Etna volcano, in Italy. The volcano and its clay-rich substratum are slowly spreading towards the east and south, driven by gravity. Spreading produces extensional structures in the summit region and contpressional structures at the base of the volcano. Eastward movement of the volcanic edifice over a stationary magma supply may be the cause of an apparent westward migration of volcanic activity. As gravitational spreading seems to control the location and magnitude of shallow seismicity and flank eruptions, an appreciation of its effects could become an essential element of future volcanic hazard evaluation. Our model proposed here for Etna may also be relevant to a reinterpretation of the geological history of a number of other volcanoes.

Case study of a volcanic geothermal system, Mount Pelée, Martinique

Mount Pelée is one of the active volcanoes of the Lesser Antilles arc, with more than 20 eruptions during the last 5000 years. Despite this abundant potential heat flow, there are few surface manifestations indicative of the existence of a large high-temperature hydrothermal system. A few Na-HCO 3 -SO 4 -type hot springs with emergence temperatures not exceeding 65°C occur on the south and west flanks of the volcano. Two small hydrothermal eruptions occurred in 1792 and 1851 on the southwestern flank, on the site of now-extinct sulfur springs. The heat source provided by the Mount Pelée magma chamber should occur, according to petrologic data, at shallow depth (5–8 km) beneath the summit region. We assume abundant groundwater recharge to take place within the central calderas where faults and magmatic conduits provide vertical channelways for deep infiltration of cold meteoric water. This would be expected to depress a presumed NaCl hydrothermal system located deep in the core of the volcano. Upwelling of high-temperature fluids and gases is thought to be found at the periphery of the central calderas, along the arcuate caldera-related faults and/or along regional faults and any permeable stratigraphic levels connected to the central caldera structure. Steam heating of shallow meteoric waters by a degassing NaCl system could produce a Na-HCO 3 -SO 4 -type system, which would develop preferentially on the periphery of the central caldera structure or even outside it in an adjacent zone of fracture permeability (caldera or regional faults), where cold meteoric flows are less abundant. This Na-HCO 3 -SO 4 system is assumed to be located beneath the south flank of the volcano, and the major upflow zone seems to be the southern limit of the intermediate caldera, as evidenced by the location of hot springs and hydrothermal eruptions. The scarcity of thermal manifestations on Mount Pelée is not surprising in view of its groundwater hydrogeology. The Na-HCO 3 -SO 4 fluids which ascend will be masked wherever a highly permeable, subhorizontal aquifer occurs near the surface, and they commonly mix with cold waters before discharging.

Basaltic volcanism in Ethiopia: Constraints on continental rifting and mantle interactions

Middle to late Cenozoic mafic lavas from the Ethiopian volcanic province exhibit considerable chemical and isotopic diversity that is linked to eruption age and eruption location. These variations provide a geochemical framework in which continental rifting can be examined. Trace element and Sr, Nd, and Pb isotopic data are interpreted to indicate involvement of up to two depleted and two enriched mantle reservoirs throughout Cenozoic rift development in Ethiopia. Superimposed on the characteristics imparted by varying degrees of melting of these distinct reservoirs are the effects of crystal fractionation and, in some instances, crustal contamination. Initial stages of Oligocene rifting and volcanism, as manifested by the rift‐bounding plateau flood basalts, are attributed to asthenospheric upwelling and melting of a heterogeneous, enriched subcontinental lithospheric mantle. Mildly alkaline lavas were produced from an enriched source with characteristics similar to those of the inferred source of other mantle‐derived lavas and xenoliths from east Africa (LoNd array, EMI to HIMU). Contemporaneous tholeiitic lavas were derived from a source similar to that producing oceanic basalts from Samoa and the Society Islands (EMII). As lithospheric thinning and rifting continued into the Miocene, upwelling depleted asthenosphere (depleted OIB reservoir, PREMA) interacted with the lithospheric sources producing lavas with hybrid elemental and isotopic characteristics (11–6 Ma plateau and rift margin basalts). Crustal contamination is most evident in the Oligocene to Miocene plateau basalts and is suggested to have taken place primarily at middle to lower crustal levels during initial stages of continental rifting. By 4–5 Ma b.p. continental breakup had begun in Afar, with basalts during this period being derived almost entirely from a depleted PREMA‐type reservoir. In the Main Ethiopian Rift, where continental breakup is less advanced, young rift basalts retain a geochemical signature consistent with enriched (LoNd)‐depleted (PREMA) mantle hybridization. During the Holocene, proto‐oceanic crust and oceanic crust characterize the Afar and Red Sea/Gulf of Aden, respectively, and input from a depleted MORB source first becomes apparent. Chronologic and tectonic control on mantle melting, mantle reservoir interactions, and crust‐mantle interactions is a theme common to many extensional regions. Another common feature is the apparent role of a depleted PREMA‐type reservoir in these regions, supporting the idea that this reservoir is located at depth within the convecting asthenosphere. Involvement of enriched mantle during continental extension‐related magmatism is also prevalent, but the geochemical signature of this component varies from region to region, suggesting a strong link to local crust formation history and local enrichment events such as subduction‐driven lithospheric recycling.

Explosive rhyolitic volcanism in the Jemez Mountains: Vent locations, caldera development and relation to regional structure

The Jemez volcanic field straddles the western margin of the Rio Grande rift where the rift is intersected by the Jemez lineament in north central New Mexico. The field has a record of volcanism extending back to before 13 Ma. Initial basaltic activity was related to active rifting, with minor rhyolitic eruptions occurring along N‐S rift‐bounding faults. Between 10 and 7 Ma, voluminous andesitic volcanism took place in the central Jemez Mountains area, overwhelming contemporaneous basaltic and rhyolitic magmatism. An apparent tectonic lull took place from 7 to 4 Ma, accompanied by lower eruption rates. During this interval, dacitic magmas were erupted to form the Tschicoma volcanic center, but mafic and rhyolitic volcanism virtually ceased. Since 4 Ma, accompanying resumption of rifting, a growing silicic magma system has been present under the central part of the Jemez Mountains, ultimately evolving to the magma body that produced the voluminous rhyolitic Bandelier tuffs. Explosive rhyolitic eruptions from this large magma body have occurred many times since 3 to 4 Ma. Early eruptions, 3.6–2.8 Ma, produced high‐silica rhyolite ignimbrites restricted to the southwest part of the Jemez Mountains; formation of these units may have been accompanied by caldera collapse. These events were followed by the two caldera‐forming Bandelier Tuff ignimbrite eruptions, 1.45 and 1.12 Ma. Post caldera explosive and effusive rhyolite eruptions have also tapped the magma body from vents generally located along ring fractures after both Bandelier events. Vent and caldera locations for the rhyolitic eruptions during 3–4 Ma have been inferred from grain size characteristics, dispersal patterns, and facies variations in the Plinian deposits and ignimbrites. Lithic breccia zones of the pre‐Bandelier ignimbrites indicate possible caldera sources in the southwest part of the present Valles caldera. During the eruption of both Bandelier tuffs, initial plinian falls and early pyroclastic flows emanated from vents centrally located in the Jemez Mountains. In the lower Bandelier Tuff eruption a transition to ring fracture vents occurred before the emission of later pyroclastic flows, but there is no strong evidence to suggest such a transition occurred during the upper Bandelier eruption. Calderas associated with the lower and upper Bandelier tuffs (Toledo and Valles, respectively) are almost identical in location, as are the Plinian vent sites for these two large eruptions. The Toledo embayment, northeast of the Valles caldera, contains lava domes from up to 3.6 Ma and may be a caldera or crater associated with early explosive dacitic volcanism in the Tschicoma volcanic center. Post‐1.4 Ma lava domes also fill this depression. The main volcanic features of the Jemez Mountains field, including the Valles caldera complex, eruption vents, and the apical graben of the post‐Valles‐Redondo resurgent block, appear to be aligned along the NE‐SW trending Jemez fault zone. This zone, the local expression of a Precambrian basement feature (the Jemez lineament), has exerted strong control on the location and style of eruptions from the Bandelier rhyolitic magma system.

Acid and basic late neogene volcanism in central aegean sea: Its nature and geotectonic significance

Local eruptions of acid volcanic rocks occurred in the Central Aegean region around the Miocene-Pliocene boundary. Rhyolites outcrop on the Island of Antiparos, located in the central part of the Attic-Cycladic Massif. The age of these volcanic rocks ranges from 4.0 to 5.4 m.y., and chemical and Sr isotopic data suggest they were generated by partial melting of the continental crust. At the same time, along the border of the Attic-Cycladic Massif,i.e. on the island of Patmos and Caloyeri, local eruptions of Naalkaline basalts occurred. The whole of the eruptive activity is interpreted as an expression of the marked tensional tectonic phase which has affected the Central Aegean area since Middle Miocene. The contrasting nature of the erupted volcanic rocks (crustal and sub-crustal) is attributed to the different thermal state of the lithosphere beneath the two areas, as emphasized also by the presence of a wide granitic belt, of mainly Miocene age, which developed in the median sector of the Attic-Cycladic Massif.

Ridge Crest Hot Springs: The story so far

The first evidence that required some form of hydrothermal activity at oceanic ridge crests came from the discovery about 15 years ago of iron‐manganeserich sediments close to the spreading axis of the East Pacific Rise [Bostrom and Peterson, 1966]. Subsequent exploration has shown that these deposits are a common feature on fast‐spreading ridges. Furthermore, they often form the basal section in sediment cores recovered by DSDP (Deep Sea Drilling Project). However, the scale of the activity necessary to produce the iron and manganese—whether surficial, i.e., associated with local eruptions, or pervasive, i.e., penetrating deep into the intrusion zone—can be ion vaguely specified.