Emplacement and arrest of sheets and dykes in central volcanoes

Sheet intrusions are of two main types: local inclined (cone) sheets and regional dykes. In Iceland, the inclined sheets form dense swarms of (mostly) basaltic, 0.5–1 m thick sheets, dipping either at 20–50° or at 75–90° towards the central volcano to which they belong. The regional dykes are (mostly) basaltic, 4–6 m thick, subvertical, subparallel and form swarms, less dense than those of the sheets but tens of kilometres long, in the parts of the volcanic systems that are outside the central volcanoes. In both types of swarms, the intrusion intensity decreases with altitude in the lava pile. Theoretical models generally indicate very high crack-tip stresses for propagating dykes and sheets. Nevertheless, most of these intrusions become arrested at various crustal depths and never reach the surface to supply magma to volcanic eruptions. Two principal mechanisms are proposed to explain arrest of dykes and sheets. One is the generation of stress barriers, that is, layers with local stresses unfavourable for the intrusion propagation. The other is mechanical anisotropy whereby sheet intrusions become arrested at discontinuities. Stress barriers may develop in several ways. First, analytical solutions for a homogeneous and isotropic crust show that the intensity of the tensile stress associated with a pressured magma chamber falls off rapidly with distance from the chamber. Thus, while dyke and sheet injection in the vicinity of a chamber may be favoured, dyke and sheet arrest is encouraged in layers (stress barriers) at a certain distance from the chamber. Second, boundary-element models for magma chambers in a mechanically layered crust indicate abrupt changes in tensile stresses between layers of contrasting Young’s moduli (stiffnesses). Thus, where soft pyroclastic layers alternate with stiff lava flows, as in many volcanoes, sheet and dyke arrest is encouraged. Abrupt changes in stiffness between layers are commonly associated with weak and partly open contacts and other discontinuities. It follows that stress barriers and discontinuities commonly operate together as mechanisms of dyke and sheet arrest in central volcanoes.     

Injection and arrest of dykes: implications for volcanic hazards

Dykes are the principal channels through which magma reaches the surface in volcanic eruptions. For this reason dykes observed in the field are commonly assumed to be feeders to lava flows. The actual proportion of dykes reaching the surface is, however, poorly known. In order to develop models for the purpose of estimating volcanic hazard, this proportion must be known. This follows because such models should not only consider the probability of dykes being injected from magma chambers during periods of unrest in the associated volcanoes, but also the probability of the injected dykes being arrested. This paper presents field data on several thousand dykes from Iceland and Tenerife (Canary Islands) indicating that many, and probably most, dykes become arrested at various crustal levels and never reach the surface to feed eruptions. Using the results of analytical and numerical models, it is shown that, for common loading conditions, the stress field in the vicinity of a magma chamber may favour the injection and propagation of dykes while the stress field at a certain distance from the chamber favours dyke arrest. This means that many dykes that are injected from the chamber propagate only for a very limited distance from the chamber to the point where they become arrested. The implication is that during periods of unrest in volcanoes, the probability of volcanic eruption is only a small fraction of the probability of dyke injection from the source magma chamber.     

Cyclic flank-vent and central-vent eruption patterns

 Many basaltic and andesitic polygenetic volcanoes have cyclic eruptive activity that alternates between a phase dominated by flank eruptions and a phase dominated by eruptions from a central vent. This paper proposes the use of time-series diagrams of eruption sites on each polygenetic volcano and intrusion distances of dikes to evaluate volcano growth, to qualitatively reconstruct the stress history within the volcano, and to predict the next eruption site. In these diagrams the position of an eruption site is represented by the distance from the center of the volcano and the clockwise azimuth from north. Time-series diagrams of Mauna Loa, Kilauea, Kliuchevskoi, Etna, Sakurajima, Fuji, Izu-Oshima, and Hekla volcanoes indicate that fissure eruption sites of these volcanoes migrated toward the center of the volcano linearly, radially, or spirally with damped oscillation, occasionally forming a hierarchy in convergence-related features. At Krafla, terminations of dikes also migrated toward the center of the volcano with time. Eruption sites of Piton de la Fournaise did not converge but oscillated around the center. After the convergence of eruption sites with time, the central eruption phase is started. The intrusion sequence of dikes is modeled, applying crack interaction theory. Variation in convergence patterns is governed by the regional stress and the magma supply. Under the condition that a balance between regional extension and magma supply is maintained, the central vent convergence time during the flank eruption phase is 1–10 years, whereas the flank vent recurrence time during the central eruption phase is greater than 100 years owing to an inferred decrease in magma supply. Under the condition that magma supply prevails over regional extension, the central vent convergence time increases, whereas the flank vent recurrence time decreases owing to inferred stress relaxation. Earthquakes of M≥6 near a volcano during the flank eruption phase extend the central vent convergence time. Earthquakes during the central eruption phase promote recurrence of flank eruptions. Asymmetric distribution of eruption sites around the flanks of a volcano can be caused by local stress sources such as an adjacent volcano. Received: 18 March 1996 / Accepted: 14 January 1997     

大兴安岭焰山、高山火山——一种新的火山喷发型式

在野外地质资料基础上,利用火山形态学方法,探讨了大兴安岭焰山、高山火山的喷发型式。结果表明,大兴安岭哈拉哈河-绰尔河火山群中的焰山和高山火山不同于斯通博利式喷发形成的火山,其早期爆破喷发的火山碎屑形成火山渣锥、空降火山碎屑席和小型火山碎屑流,晚期溢出大量熔岩。两火山具有较高大的锥体(标高200～300m以上),在结构上,松散火山砾、火山弹等构成下部的降落锥,熔结集块岩构成上部的溅落锥。由火山砾和火山灰组成的空降火山碎屑席分布在火山锥体周围。两火山溢出的熔岩经历了从结壳熔岩→翻花石→渣状熔岩的演变。根据喷发产物可推断焰山和高山火山具有以下喷发特征:爆破喷发形成持续的喷发柱→斯通博利式喷发→熔岩喷泉喷溢,其中以持续时间较长的喷发柱区别于典型的斯通博利式喷发。类似焰山、高山火山的喷发特征,在龙岗第四纪火山群、镜泊湖全新世火山群中也都有个例,这是中国大陆火山作用中一种新的喷发型式。     

The effect of stress on faulting and minor intrusions in the vicinity of a magma body

The stress field in the vicinity of a body of fluid of simple geometry contained within a non-homogeneously stressed solid has been calculated and the result applied to the case of a magma body within a region of the crust subject to triaxial stresses. The types of faulting and minor intrusion which result are described. The theory indicates that the regional stresses in the crust together with the magma pressure control the type of faulting, the form of the minor intrusions and the occurrence of eruptions from the magma body. The stress conditions favouring caldera formation, the intrusion of radial dykes, dyke swarms and cone sheets are described.     

Cyclic magma storages and transfers at Piton de La Fournaise volcano (La Réunion hotspot) inferred from deformation and geochemical data

Piton de La Fournaise is in a period of intense volcanic activity since 1998. To constrain the magma dynamics responsible for this activity, we combined GPS ground deformation monitoring interpreted through numerical modelling and geochemistry. Two cycles of continuous volcano inflation are evidenced for the May 2004–December 2005 period, with a rest from March to October 2005. These inflations are consistent with two cycles of compatible major element enrichment in the emitted lavas. Numerical models indicate that the pressurization of a single magma reservoir may be responsible for the observed pre-eruptive inflations of the volcano. The reservoir, located at 2300 m depth, has a radius of  500 m. At the beginning of each cycle, dykes propagate from the roof of the reservoir and yield eruptions of differentiated basalt near the summit. At the end of the cycle, dykes propagate from the eastern sidewall of the reservoir and yield distal eruptions of primitive magmas away from the summit. The volumes of magma emitted during the primitive eruptions seem too large to explain the surface deformations and therefore suggest some refill of the reservoir by deeper magmas. Our results may be used to predict the location and lava volume of future eruptions at Piton de La Fournaise volcano, depending on the timing of these eruptions within a cycle of volcanic activity.     

Major changes in the post-glacial evolution of magmatic compositions and pre-eruptive conditions of Llaima Volcano,Andean Southern Volcanic Zone,Chile

Llaima is one of the most active volcanoes of the Chilean volcanic front with recent explosive eruptions in 2008 and 2009. Understanding how the volcano evolved to its present state is essential for predictions of its future behavior. The post-glacial succession of explosive volcanic eruptions of Llaima stratovolcano started with two caldera-forming eruptions at ～16 and ～15 ka, that emplaced two large-volume basaltic-andesitic ignimbrites (unit I). These are overlain by a series of fall deposits (unit II) changing from basaltic-andesitic to dacitic compositions with time. The prominent compositionally zoned, dacitic to andesitic Llaima pumice (unit III) was formed by a large Plinian eruption at ～10 ka that produced andesitic surge deposits (unit IV) in its terminal phase. The following unit V represents a time interval of ～8,000 years during which at least 30 basaltic to andesitic ash and lapilli fall deposits with intercalated volcaniclastic sediments and paleosols were emplaced. Bulk rock, mineral, and glass chemical data constrain stratigraphic changes in magma compositions and pre-eruptive conditions that we interpret in terms of four distinct evolutionary phases. Phase 1 (=unit I) magmas have lower large ion lithophile (LIL)/high field strength (HFS) element ratios compared to younger magmas and thus originated from a mantle source less affected by slab-derived fluids. They differentiated in a reservoir at mid-crustal level. During the post-caldera phase 2 (=units II–IV), relatively long residence times between eruptions allowed for increasingly differentiated magmas to form in a reservoir in the middle crust. Fractional crystallization led to volatile enrichment and oversaturation and is the driving force for the large Plinian eruption of the most evolved (unit III) dacite at Llaima, although replenishment by hot andesite probably triggered the eruption. During the subsequent phase 3 (=unit V >3 ka), frequent mafic replenishments at mid-crustal storage levels favored shorter residence times limiting erupted magma compositions to water-undersaturated basaltic andesites and andesites. At around 3 ka, the magma storage level for phase 4 (=unit V <3 ka to present) shifted to the uppermost crust where the hot magmas partly assimilated the granitic country rock. Although water contents of these basaltic andesites were low, the low-pressure storage facilitated water saturation before eruption. The change in magma storage level at 3 ka was responsible for the dramatic increase in eruption frequency compared to the older Llaima history. We suggest that the change from middle to upper crust magma storage is caused by a change in the stress regime below Llaima from transpression to tension.     

Geometry of the September 1971 eruptive fissure at Kilauea volcano,Hawaii

A three-dimensional model has been used to estimate the location and dimensions of the eruptive fissure for the 24–29 September 1971 eruption along the southwest rift zone of Kilauea volcano, Hawaii. The model is an inclined rectangular sheet embedded in an elastic half-space with constant displacement on the plane of the sheet. The set of best model parameters suggests that the sheet is vertical, extends from a depth of about 2 km to the surface, and has a length of about 14 km. Because this sheet intersects the surface where eruptive vents and extensive ground cracking formed during the eruption, this sheet probably represents the conduit for erupted lava. The amount of displacement perpendicular to the sheet is about 1.9 m, in the middle range of values measured for the amount of opening across the September 1971 eruptive fissure. The thickness of the eruptive fissure associated with the January 1983 east rift zone eruption was determined in an earlier paper to be 3.6 m, about twice the thickness determined here for the September 1971 eruption. Because the lengths (12 km for 1983 and 14 km for 1971) and heights (about 2 km) of the sheet models derived for the January 1983 and September 1971 rift zone eruptions are nearly identical, the greater thickness for the January 1983 eruptive fissure implies that the magma pressure was about a factor of two greater to form the January 1983 eruptive fissure. Because the September 1971 and January 1983 eruptive fissures extent to depths of only a few kilometers, the region of greatest compressive stress produced along the volcano's flank by either of these eruptive fissures would also be within a few kilometers of the surface. Previous work has shown that rift eruptions and intrusions contribute to the buildup of compressive stress along Kilauea's south flank and that this buildup is released by increased seismicity along the south flank. Because south flank earthquakes occur at significantly greater depths, i.e., from 5 to 13 km, than the vertical extent of the 1971 and 1983 eruptiv fissures, the depth of emplacement of these eruptive fissures cannot be the main factor in controlling the hypocentral depths of south flank earthquakes. Two possible explanations for the occurrence of south flank earthquakes in the depth range of 5–13 km are (1) a deeper pressure source, possibly related to deeper magma storage within the rift zone, and (2) a lowstrength region located between 5 and 13 km beneath Kilauea's south flank, possibly at the interface between oceanic sediments and the base of the Hawaiian volcanics.     

Three-dimensional structure of inclined sheet swarms: Implications for crustal thickening and subsidence in the volcanic rift zones of Iceland

Inward-dipping (cone) sheet swarms and an associated central volcano are well-exposed in the deeply-eroded Tertiary crust of Vatnsdalur, Skagi Peninsula region, northern Iceland. Spatially registered orientations of 389 mafic sheets, mapped in three distinct sheet swarms define both the overall shape and magmatic source of each swarm. The Vatnsdalur sheet swarms consist of planar inward-dipping sheets that collectively define a conical shape rather than a bowl- or trumpet-shape as have been found in swarms in other locations. In the best exposed swarm, three-dimensional projection of mafic sheets into the subsurface defines two distinct foci, which are interpreted as the magmatic sources of two temporally distinct sub-swarms. These results help to establish the influence of inclined sheet intrusion on crustal accretion at central volcanoes. The geometry of the swarm constrains the thickness of material that was added to the crust during sheet intrusion. When combined with estimates of surface relief, we calculate that 2.2 to 4.1 km of subsidence were required beneath the central volcano in order to accommodate the intrusion of the sheet swarm. Similar processes of crustal thickening and subsidence likely occur in a wide variety of both continental rift and mid-ocean ridge systems where magmatic activity is focused at central volcanoes.     

What makes flank eruptions? The 2001 Etna eruption and its possible triggering mechanisms

Most flank eruptions within a central stratovolcano are triggered by lateral draining of magma from its central conduit, and only few eruptions appear to be independent of the central conduit. In order to better highlight the dynamics of flank eruptions in a central stratovolcano, we review the eruptive history of Etna over the last 100 years. In particular, we take into consideration the Mount Etna eruption in 2001, which showed both summit activity and a flank eruption interpreted to be independent from the summit system. The eruption started with the emplacement of a ~N-S trending peripheral dike, responsible for the extrusion of 75% of the total volume of the erupted products. The rest of the magma was extruded through the summit conduit system (SE crater), feeding two radial dikes. The distribution of the seismicity and structures related to the propagation of the peripheral dike and volumetric considerations on the erupted magmas exclude a shallow connection between the summit and the peripheral magmatic systems during the eruption. Even though the summit and the peripheral magmatic systems were independent at shallow depths (<3 km b.s.l.), petro-chemical data suggest that a common magma rising from depth fed the two systems. This deep connection resulted in the extrusion of residual magma from the summit system and of new magma from the peripheral system. Gravitational stresses predominate at the surface, controlling the emplacement of the dikes radiating from the summit; conversely, regional tectonics, possibly related to N-S trending structures, remains the most likely factor to have controlled at depth the rise of magma feeding the peripheral eruption.     

Seismic energy releases from volcanoes

Seismic energy release during the precursory, eruptive and declining stages of volcanic activities provides various information about the mechanisms of volcanic eruptions and the temporary developments of their activities. Hitherto the energy release patterns from precursory earthquake swarms were used to predict the eruption times, especially of andesitic or dacitic volcanoes. In this paper the discussion is expanded to quantify the total amount of seismic energy released at the threshold of volcanic eruptions, with reference to the results observed at several volcanoes. The results generally indicate that the cumulative seismic energy release from the precursory earthquake swarms exceed 10¹⁷¹⁸ergs before eruptions at any andesitic or dacitic volvanoes. This allows the seismic efficiency, or the ratio of energy radiated seismically, and the energy required for the volumetric expansion to be estimated by incorporating available deformation data with the seismic data. The dependency of seismic efficiency on the type of volcanic activity, i. e. non-explosive outbreaks, phreatic and magmatic eruptions, dome formation, etc., was evaluated from observations at a few volcanoes that provided a variety of examples.     

Geophysical surveys of the Joya Honda maar (México) and surroundings; volcanic implications

Joya Honda (JH) is a Quaternary maar excavated in Mesozoic limestone. It is located in central Mexico and belongs to the Ventura volcanic field (VVF), which is composed by cinder cones and maars made of intraplate-type mafic alkalic rocks. Volcanoes in the region form  N20W lineaments, roughly parallel to a regional set of normal faults, but there is no obvious relation between these faults and vent distribution in the exposed geology around the maar. The volcanic rock volume is small in the VVF, and most volcanoes and their products are scattered in a region where outcrops are dominated by limestone.The near-vent tephra associated to the JH maar lies north of the crater. This relation suggests that the crater was formed by directed hydromagmatic explosions and may indicate an inclined volcanic conduit near the surface. The tephra stratigraphy suggests that the initial explosions were relatively dry and the amount of water increased during the maar forming eruption. Therefore, the existing model of the maar–diatreme formation may not be applicable to Joya Honda as it requires the formation of a cone of depression in the aquifer and deepening of the focii of the explosions as the crater and underlying diatreme grew. Thus, it is unlikely that there is a diatreme below Joya Honda.Aeromagnetic data shows a boundary between two regional magnetic domains near the elongated volcanic cluster of the VVF. The boundary is straight, with a distinct kink, from NE- to NW-trend, near JH. The limit between the domains is interpreted as fault contacts between mid-Tertiary volcanic rocks and marine Mesozoic sedimentary rocks. Hence, magma ascent in the area may have been facilitated by fractures near the surface.Magnetic and gravimetric ground surveys show that the anomalies associated with the maar are not centered in the crater, which could be consistent with an inclined volcanic conduit. A magnetic profile measured on exposed limestone across the volcanic lineament failed to show an anomaly such as that caused by a connecting dike between the volcanoes. Therefore, either the dike does not exist or it is so deep or so thin that it is beyond the limit of detection of the method and/or equipment used. Thus, the volcanic conduit immediately below Joya Honda can be reasonably modeled in the shape of a plug. A 2-D model of the crater anomaly is consistent with a roughly tabular deposit formed by fall-back pyroclasts and slump deposits near the surface. Based on this result we propose an alternative model for the formation of maar-type volcanoes excavated in hard rock, where there is no evidence of a gradual decrease of the water/magma ratio as the eruption advanced.     

Tyatya Volcano, southwestern Kuril arc: Recent eruptive activity inferred from widespread tephra

Abstract Tyatya Volcano, situated in Kunashir Island at the southwestern end of Kuril Islands, is a large composite stratovolcano and one of the most active volcanoes in the Kuril arc. The volcanic edifice can be divided into the old and the young ones, which are composed of rocks of distinct magma types, low‐ and medium‐K series, respectively. The young volcano has a summit caldera with a central cone. Recent eruptions have occurred at the central cone and at the flank vents of the young volcano. We found several distal ash layers at the volcano and identified their ages and sources, that is, tephras of ad 1856, ad 1739, ad 1694 and ca 1 Ka derived from three volcanoes of Hokkaido, Japan, and caad 969 from Baitoushan Volcano of China/North Korea. These could provide good time markers to reveal the eruptive history of the central cone, which had continued intermittently with Strombolian eruptions and lava flow effusions since before 1 Ka. Relatively explosive eruptions have occurred three times at the cone during the past 1000 years. We revealed that, topographically, the youngest lava flows from the cone are covered not by the tephra of ad 1739 but by that of ad 1856. This evidence, together with a report of dense smoke rising from the summit in ad 1812, suggests that the latest major eruption with lava effusion from the central cone occurred in this year. In 1973, after a long period of dormancy, short‐lived phreatomagmatic eruptions began to occur from fissure vents at the northern flank of the young volcano. This was followed by large eruptions of Strombolian to sub‐Plinian types occurring from several craters at the southern flank. The 1973 activity is evaluated as Volcanic Explosivity Index = 4 (approximately 0.2 km³), the largest eruption during the 20th century in the southwestern Kuril arc. The rocks of the central cone are strongly porphyritic basalt and basaltic andesite, whereas the 1973 scoria is aphyric basalt, suggesting that magma feeding systems are definitely different between the summit and flank eruptions.     

Mode of eruption of the deccan trap basalts

The Deccan Trap basalts have long been considered to be products of fissure eruptions and the dykes intruding them have been supposed to represent the fissures of eruption. However, the question of the mode of eruption of Deccan Trap lavas seems to need more careful consideration in view of the features brought to light by detailed field work in Western Maharashtra. Detailed studies of dykes suggest that majority of the dykes could not have acted as feeders as previously supposed. When examined in detail the basalt flows have more often been found to have only a limited lateral extent and are not always quite horizontal. Thin irregular flows with ropy surfaces, dipping in different directions and piled up into a chaotic mass are frequently met with, indicating eruption from local vents of the central type. Volcanic vents are found at a number of widely separated localities. All this suggests that many lavas are products of central type of volcanicity. However, central type of volcanoes would be inadequate to account for the vast amounts of lavas, and as the known dykes are not likely to have acted as feeders, and dykes still remain to be reported from large portions of the Deccan Trap area, the question of how the lavas came to the surface largely remains unanswered. Extensive beds of volcanic breccia traceable over a few miles and upto 50 feet thick are met with. As such extensive beds are more likely to be associated with fissure cruptions the question arises whether these fissure eruptions were accompanied by considerable explosive activity.     

Small scale arcuate intrusions on Saint Helena,South Atlantic

Sixteen arcuate intrusions have been emplaced at extremely high levels into the basaltic shield volcanoes of Saint Helena. These intrusions are of special interest because of their small size and modes of emplacement. The arcuate masses are of three distinctive types:

1. Irregular, steeply inward-dipping, basic sheets with diameters of 150 m to 450 m infill tensional fractures originating at depths of about 500 m beneath the volcano surface.
2. Strongly curved sheets from 25 m to 750 m in diameter are cross-sections of inclined intrusions which in three dimensions resemble single sticks of celery. These intrusions, varying from basalt to trachyte in composition, are infilled tensional fractures originating at « point » pressure sources, inclined to the horizontal, at depths of about 500 m below the surface.
3. Salic intrusions with near-vertical sides and gently inclined roof-infillings have outer diameters of 350 m to 1070 m. Pressure exerted onto the flanks of the volcano by domed, convex upwards, areas of a magma chamber roof, at a depth of about 2 km, caused near-vertical ring fractures to form. Formation of a sub-horizontal cross fracture and subsequent intrusion of magma produced the « roof-infillings » by updoming the overlying basalts or sinking of the enclosed block, or combinations of the two processes. Two intrusions of this third type are multiple.

     

Precursors to dyke-fed eruptions at basaltic volcanoes: insights from patterns of volcano-tectonic seismicity at Kilauea volcano,Hawaii

To investigate the physical controls on volcano-tectonic (VT) precursors to eruptions and intrusions at basaltic volcanoes, we have analyzed the spatial and temporal patterns of VT earthquakes associated with 34 eruptions and 23 dyke intrusions that occurred between 1960 and 1983 at Kilauea, in Hawaii. Eighteen of the 57 magmatic events were preceded by an acceleration of the mean rate of VT earthquakes located close to the main shallow magma reservoir. Using a maximum-likelihood technique and the Bayesian Information Criterion for model preference, we demonstrate that an exponential acceleration is preferred over a power-law acceleration for all sequences. These sequences evolve over time-scales of weeks to months and are consistent with theoretical models for the approach to volcanic eruptions based on the growth of a population of fractures in response to an excess magma pressure. Among the remaining 40 magmatic events, we found a significant correlation between swarms of VT earthquakes located in the mobile south-flank of Kilauea and eruptions and intrusions. The behaviour of these swarms suggests that at least some of the magmatic events are triggered by transient episodes of elevated rates of aseismic flank movement, which could explain why many eruptions and intrusions are not preceded by longer-term precursory signals. In none of the 57 cases could a precursory sequence be used to distinguish between the approach to an eruption or an intrusion, so that, even when a precursory sequence is recognized, there remains an empirical chance of about 40% (24 intrusions from 57 magmatic events) of issuing a false alarm for an imminent eruption.     

2017年全球活动火山时空分布以及阿贡火山喷发前后的形变特征

归纳总结2017年度全球81座活火山的活动情况,共计活动1058座次,平均每周记录20座活火山的活动信息。根据火山潜在喷发的危险性和火山活动的强弱程度对上述火山进行分级描述,火山活动主要反映了地球表层的构造活动,其中大角度俯冲带的弧后火山最为强烈,小角度的俯冲带、拉张裂谷和走滑为主的板块边界火山活动较为平静,火山活动频繁的印度尼西亚岛链是受灾最为严重的区域。预计全球火山活动将进一步加剧,印尼岛链受火山灾害威胁的程度依然较大。位于印尼岛链巴厘岛上的阿贡火山自2017年9月开始活动以来,整个喷发过程极具代表性,监测阿贡火山喷发过程可为全球典型火山喷发事件研究提供参考。     

Volcanism in Iceland in historical time: Volcano types,eruption styles and eruptive history

The large-scale volcanic lineaments in Iceland are an axial zone, which is delineated by the Reykjanes, West and North Volcanic Zones (RVZ, WVZ, NVZ) and the East Volcanic Zone (EVZ), which is growing in length by propagation to the southwest through pre-existing crust. These zones are connected across central Iceland by the Mid-Iceland Belt (MIB). Other volcanically active areas are the two intraplate belts of Öræfajökull (ÖVB) and Snæfellsnes (SVB). The principal structure of the volcanic zones are the 30 volcanic systems, where 12 are comprised of a fissure swarm and a central volcano, 7 of a central volcano, 9 of a fissure swarm and a central domain, and 2 are typified by a central domain alone.Volcanism in Iceland is unusually diverse for an oceanic island because of special geological and climatological circumstances. It features nearly all volcano types and eruption styles known on Earth. The first order grouping of volcanoes is in accordance with recurrence of eruptions on the same vent system and is divided into central volcanoes (polygenetic) and basalt volcanoes (monogenetic). The basalt volcanoes are categorized further in accordance with vent geometry (circular or linear), type of vent accumulation, characteristic style of eruption and volcanic environment (i.e. subaerial, subglacial, submarine).Eruptions are broadly grouped into effusive eruptions where >95% of the erupted magma is lava, explosive eruptions if >95% of the erupted magma is tephra (volume calculated as dense rock equivalent, DRE), and mixed eruptions if the ratio of lava to tephra occupy the range in between these two end-members. Although basaltic volcanism dominates, the activity in historical time (i.e. last 11 centuries) features expulsion of basalt, andesite, dacite and rhyolite magmas that have produced effusive eruptions of Hawaiian and flood lava magnitudes, mixed eruptions featuring phases of Strombolian to Plinian intensities, and explosive phreatomagmatic and magmatic eruptions spanning almost the entire intensity scale; from Surtseyan to Phreatoplinian in case of “wet” eruptions and Strombolian to Plinian in terms of “dry” eruptions. In historical time the magma volume extruded by individual eruptions ranges from ∼1 m³ to ∼20 km³ DRE, reflecting variable magma compositions, effusion rates and eruption durations.All together 205 eruptive events have been identified in historical time by detailed mapping and dating of events along with extensive research on documentation of eruptions in historical chronicles. Of these 205 events, 192 represent individual eruptions and 13 are classified as “Fires”, which include two or more eruptions defining an episode of volcanic activity that lasts for months to years. Of the 159 eruptions verified by identification of their products 124 are explosive, effusive eruptions are 14 and mixed eruptions are 21. Eruptions listed as reported-only are 33. Eight of the Fires are predominantly effusive and the remaining five include explosive activity that produced extensive tephra layers. The record indicates an average of 20–25 eruptions per century in Iceland, but eruption frequency has varied on time scale of decades. An apparent stepwise increase in eruption frequency is observed over the last 1100 years that reflects improved documentation of eruptive events with time. About 80% of the verified eruptions took place on the EVZ where the four most active volcanic systems (Grímsvötn, Bárdarbunga–Veidivötn, Hekla and Katla) are located and 9%, 5%, 1% and 0.5% on the RVZ–WVZ, NVZ, ÖVB, and SVB, respectively. Source volcano for ∼4.5% of the eruptions is not known.Magma productivity over 1100 years equals about 87 km³ DRE with basaltic magma accounting for about 79% and intermediate and acid magma accounting for 16% and 5%, respectively. Productivity is by far highest on the EVZ where 71 km³ (∼82%) were erupted, with three flood lava eruptions accounting for more than one half of that volume. RVZ–WVZ accounts for 13% of the magma and the NWZ and the intraplate belts for 2.5% each. Collectively the axial zone (RVZ, WVZ, NVZ) has only erupted 15–16% of total magma volume in the last 1130 years.     

The plinian fallout associated with Quilotoa's 800 yr BP eruption,Ecuadorian Andes

Large volcanic eruptions at dacitic or rhyolitic volcanoes often generate exceptional volumes of fine ash that mantles an area up to a million km². These eruptions are characterized by extreme fragmentation of the magma and hence extraordinary dispersal of ash and are categorized as plinian, ultraplinian, or phreatoplinian events. Large-volume co-ignimbrites or co-plinian ashes are often produced by such eruptions. High fragmentation indices of > 90% are attributed to the violent eruption of silicic magma, especially if augmented by fuel-coolant reactions produced when abundant external water interacts with the magma. The present study documents a case where the fine ash (≤ 1 mm diameter) fall deposit related to the plinian phase of the eruption comprises the overwhelming bulk – about 87 wt.% of the eruptive products. This is another example demonstrating the predominance of a widespread, fine-grained, co-plinian ash which follows the initial coarser lapilli fall. Historical eruptions at two other Andean volcanoes Quizapu, (Chile) and Huaynaputina, (Peru), and at Santa Maria, (Guatemala) and Novarupta, (Alaska) produced similar ash fall sequences.     

Volcanic history of Mount Sidley,a major alkaline volcano in Marie Byrd Land,Antarctica

Mount Sidley is a complex, polygenetic stratovolcano composed primarily of phonolitic and trachytic lavas and subordinate pyroclastic lithologies at the southern extremity of the Executive Committee Range, a linear chain of volcanoes in central Marie Byrd Land, Antarctica. Detailed field investigation coupled with 14 high precision ⁴⁰Ar/³⁹Ar age determinations reveal a 1.5 million year life span between 5.7 and 4.2 Ma in which three major phonolitic central vent edifices (Byrd, Weiss and Sidley volcanoes) and their calderas were developed (5.7–4.8 Ma). This was followed (4.6–4.5 Ma) by the eruption of trachytic magmas from multiple vent localities further south, and then by small volume benmoreite-mugearite lavas and tephras around 4.4–4.3 Ma at the southern end of Mount Sidley. The final phase of activity was the eruption of basanite cones at approximately 4.2 Ma. The southward migration of volcanic activity was accompanied by distinct changes in magma composition and is best explained by the sequential release of magmas stored within an intricate system of conduits and chambers in the crust by tectonically driven (magma assisted?) fracture propagation. The style of volcanic migration at Mount Sidley is emulated on a larger scale by other volcanoes in the Executive Committee Range, in which progressive southward displacement of volcanic activity corresponds with significant petrological variations between major centers.