Tuff Cones Research Articles

Volcanism in the city of Naples

A twenty years lasting geo-volcanological survey allowed us to reconstruct the eruptive history of the city of Naples which is part of the active Campi Flegrei (Phlegraean Fields) volcanic field. Naples hills are mainly modelled by volcanic and volcano­tectonic processes, partly masked by historical floods that buried the lower part of the city as well as the main roman buildings. The ancient core of these hills is made by coalescent tuff cones and a lava dome older than 78 ka. Remnants of this ancient activity are draped by a thick succession of pyroclastic deposits comprising large, caldera-forming ignimbrites (Campanian Ignimbrite, 39 ka and Neapolitan Yellow Tuff, 15 ka), phlegraean tephra and monogenetic vents (e.g. Trentaremi and Nisida volcanoes). The western area of Naples collapsed during both the ignimbrite episodes. Caldera faults bound Camaldoli hill, San Martino hill, Capodimonte hill and Posillipo hill. Stratigraphical and geochronological data show that the volcanic activity in the city of Naples has lasted at least 78 ka and probably longer on the basis of the undated ancient tuffs at the base of the neapolitan succession. This persistent volcanic activity indicates that this urban sector of the Campi Flegrei volcanic field, could be considered as a likely eruption location for the next event.

Mecanismos eruptivos y procesos depositacionales del Grupo Choiyoi en el área de Las Caletas, Cordillera Frontal de San Juan, Argentina

The Permo-Triassic Choiyoi rhyolite province of Chile and Argentina is by its extension, volume and geological significance, the record of one of the most important magmatic events of the southwestern margin of Gondwana. However, aspects dealing with the physical volcanology and tectonic setting are poorly understood. In order to characterize the eruptive activity and style of deposition, a stratigraphic and litofacial study was carried out in the Horcajo Formation, a thick silicic volcanic succession representative of the upper section of the Choiyoi Group cropping out in the Cordillera Frontal of San Juan (western Argentina). A volcanic cycle defined by four eruptive episodes is recognized in the lower and middle sections of the unit: 1. pre-collapse effusive volcanism; 2. phreatomagmatic explosive volcanism; 3. explosive volcanism and volcanotectonic collapse; and 4. post-collapse effusive volcanism and syn-eruptive sedimentation. The emplacement of the eruptive units of the Horcajo Formation was controlled by volcano-tectonic subsidence processes associated with large magnitude ignimbrite forming eruptions. Although this eruptive mechanism would have been dominant, it alternate with important effusive phases which lead to the emplacement of voluminous rhyolite lava flows. Minor phreatomagmatic activity was responsible for the construction of silicic tuff cones or rings also related to the formation of local alluvial and lacustrine environments during brief intervals of syn-eruptive sedimentation. Stratigraphic and structural evidence suggests that the recognized deposits are remnants of an ancient caldera-like eruptive center, although without limits or geometry clearly exposed. The processes involved in the genesis of Horcajo Formation, a widespread unit of Cordillera Frontal, can be considered as some of the most common of the Choiyoi volcanism.

Deposits of pyroclastic mass flows at Bibby Hill (Pliocene, James Ross Island, Antarctica)

Sedimentological study of the southern slopes of Bibby Hill (relic of a Pliocene tuff cone) allows recognition of twelve lithofacies and three facies associations. Deposits of pyroclastic currents (both low- and high density pyroclastic currents) dominate over the deposits of pyroclastic flows. Products of suabaerial resedimented pyroclastic deposits play minor role. Vertical distribution of facies associations within the studied succession is not uniform. These differences in the distribution of facies associations are interpreted as response to variations in the intensity and type (Surtseyan, Taalian) of phreato-magmatic eruptions, water availability and morphology of the cone.

Brittle deformation and slope failure at the North Menan Butte tuff cone, Eastern Snake River Plain, Idaho

The manifestation of brittle deformation within inactive slumps along the North Menan Butte, a basaltic tuff cone in the Eastern Snake River Plain, is investigated through field and laboratory studies. Microstructural observations indicate that brittle strain is localized along deformation bands, a class of structural discontinuity that is predominant within moderate to high-porosity, clastic sedimentary rocks. Various subtypes of deformation bands are recognized in the study area based on the sense of strain they accommodate. These include dilation bands (no shear displacement), dilational shear bands, compactional shear bands and simple shear bands (no volume change). Measurements of the host rock permeability between the deformation bands indicate that the amount of brittle strain distributed throughout this part of the rock is negligible, and thus deformation bands are the primary means by which brittle strain is manifest within this tuff. Structural discontinuities that are similar in appearance to deformation bands are observed in other basaltic tuffs. Therefore deformation bands may represent a common structural feature of basaltic tuffs that have been widely misclassified as fractures. Slumping and collapse along the flanks of active volcanoes strongly influence their eruptive behavior and structural evolution. Therefore characterizing the process of deformation band and fault growth within basaltic tuff is key to achieving a more complete understanding of the evolution of basaltic volcanoes and their associated hazards.

Pyroclastic density currents and local topography as seen with the conveyer model

Pyroclastic density currents (PDCs) are multiphase flows generated during explosive volcanic eruptions, and they move down the volcano, and over the surrounding topography. The flow–topography interaction can play a fundamental role in the sedimentary processes, and in the resulting deposit facies architecture, as well as can play a dramatic role in the flow behavior, and in the associated volcanic hazard. This paper aims at discussing the PDC–topography interaction theme from the viewpoint of both deposits and flow structure, by accounting for appropriate literature, and revising the concepts in light of the theoretical conveyer model of Doronzo and Dellino (2013) on sedimentation and deposition in particulate density currents. First the effects, then the causes of the flow–topography interaction are discussed, in order to follow the historical development of theme concepts. The discussion is relative in terms of inertial and forced currents, which are defined on the basis of a dimensionless quantity (SD) representing the conservation of mass. Momentum equation relating depositional unit thickness, flow shear velocity, and density contrast shows that the flow is the cause of PDC motion, whereas the density contrast sustains the momentum, and the deposits are the process effect. In particular, the flow structure is described into three parts, flow–substrate boundary zone, boundary layer (lower part), and wake region (upper part) of the current. The facies architecture of PDC deposits, and the volcanic hazard depend on fluid dynamic and hydraulic behavior represented, in light of the conveyer model, by the balance of sedimentation and deposition rates through transport and erosion (“sedimentation–deposition” ratio, SD). This balance acts between flow–substrate boundary zone and boundary layer. The paper discussion mainly applies to small-to-intermediate volume eruptions. Field and modeling examples of Vulcano tuff cone and Colli Albani maar (Italy) constrain the conveyer model, whereas the literature of very large, ignimbrite-forming eruptions, and stratovolcanism is accounted for theme completeness. The main findings are some relative guidelines on PDC–topography interaction that can be used when modeling the flow, and interpreting the pyroclastic deposits: low SD is typical of inertial currents, whereas high SD is typical of forced currents, which can vary depending on topography.

Eocene to Quaternary mafic-intermediate volcanism in San Luis Potosí, central Mexico: The transition from Farallon plate subduction to intra-plate continental magmatism

The San Luis Potosí Volcanic Field (SLPVF) of central Mexico includes volcanic sequences of felsic, intermediate and basic compositions that were erupted as discrete episodes from the Eocene to the Pleistocene. Volcanism was dominated by widespread and voluminous rhyolitic ignimbrites of the mid-Tertiary Ignimbrite Flare-up. However, the complete volcanic history must consider basaltic and andesitic Eocene–Pleistocene volcanic successions that provide key evidence for understanding the geochemical evolution of the volcanism in the SLPVF during this time span. Five sequences are recognized according to their geochemical characteristics, each comprising a volcano-tectonic episode. The first episode comprises basaltic andesites and andesites erupted during three intervals, 45–42Ma, 36–31Ma, and 31–30Ma. The oldest was derived from subduction magmatism, whereas the youngest has an intra-plate magmatic signature and this represents the transition from the end of a long lasting subduction regime of the Farallon plate to the initiation of intra-plate continental extension in the North American plate. The second episode, at 29.5–28Ma, comprises a bimodal succession of high-silica rhyolites and alkaline basalts (hawaiites) that are interpreted as magmatism generated in an intra-plate continental extension regime during the Basin and Range faulting. The third episode, at 21Ma, is characterized by trachybasalts and trachyandesites that represent mantle basaltic melts that were contaminated through assimilation of the lower crust during advanced stage of intra-plate extension that started at Oligocene. The fourth episode includes 12Ma alkaline basalts and andesites that were erupted from fissures. These mantle derived magmas evolved to andesites by crustal anatexis and crystal fractionation within a continued, extensional, intra-plate regime. Lastly, the fifth episode comprises 5.0 to 0.6Ma alkaline basalts (basanites) containing mantle xenoliths, that were erupted from maars and tuff cones, which are the youngest manifestations of mantle-derived intra-plate extensional events. Based upon this volcanic record, the last subduction manifestations of the extinct Farallon plate occurred at about 42Ma, this was followed by a transition to intra-plate magmatism between 42 and 31Ma, and an extensional, intra-plate tectonic setting from 31Ma to almost Present.

Fractures and faults in volcanic rocks (Campi Flegrei, southern Italy): insight into volcano-tectonic processes

The present study was focused to analyze fractures and faults in the Campi Flegrei calderas presently hosting several volcanic edifices, such as lava domes, scoria, and tuff cones. A complex network of fractures and faults affects the volcanic rocks, mostly as planar with highly variable density. Frequently faults appearing as conjugate structures showing normal kinematics often associated with ductile deformation such as drag folds and deflexed layers, suggesting a syn-eruption deformation. However, the most of faults, mainly hosted along the caldera/crater rims, are very steep with dominant normal and secondary reverse movements. The fracture pattern indicates a slight prevalence of NE–SW and NW–SE directions, but N–S and E–W trends also occur. Fractures and faults found in rocks older than 15 ka (Neapolitan Yellow Tuff included), measured in western and eastern sectors of the study area, indicate a rotation of ca. 30° of the main directions among these two sectors. For the faults occurring along the caldera/crater rims, we suggest a kinematic evolution characterized by the reactivation of tensile fractures previously formed in response to both regional extension and locale resurgent dome. Finally, normal faults located in the central sector of caldera, between La Starza and Accademia localities, cutting the youngest volcanic deposits, indicate a constant NNE–SSW extension probably related to the caldera resurgence.

Opaque Mineralogy as a Tracer of Magmatic History of Volcanic Rocks: an Example from the Neogene‐Quaternary Harrat Rahat Intercontinental Volcanic Field, North Western Saudi Arabia

Abstract:The Neogene‐Quaternary Harrat Rahat volcanic field is part of the major intercontinental Harrat fields in western Saudi Arabia. It comprises lava flows of olivine basalt and hawaiite, in addition to mugearite, benmorite, and trachyte that occur mainly as domes, tuff cones and lava flows. Based on opaque mineralogy and mineral chemistry, the Harrat Rahat volcanic varieties are distinguished into Group I (olivine basalt and hawaiite) and Group II (mugearite, benmorite and trachyte). The maximum forsterite content (∼85) is encountered in zoned forsteritic olivine of Group I, whereas olivine of Group II is characterized by intermediate (Fo=50), fayalitic (Fo=25) and pure fayalite in the mugearite, benmorite and trachyte, respectively. The more evolved varieties of Group II contain minerals that show enrichment of Fe2+, Mn2+ and Na+ that indicates normal fractional crystallization. The common occurrence of coarse apatite with titanomagnetite in the benmorite indicates that P5+ becomes saturated in this rock variety and drops again in trachyte. Cr‐spinel is recorded in Group I varieties only and the Cr# (0.5) suggests lherzolite as a possible restite of the Harrat Rahat volcanics. The plots of Cr# vs. the forsterite content (Fo) suggest two distinct trends, which are typical of mixing of two basaltic magmas of different sources and different degrees of partial melting. The bimodality of Harrat Rahat Cr‐spinel suggests possible derivation from recycled MORB slab in the mantle as indicated by the presence of high‐Al spinel. It is believed that the subcontinental lithospheric mantle was modified by pervious subduction process and played the leading role in the genesis of the Harrat Rahat intraplate volcanics. The trachytes of the Harrat Rahat volcanic field were formed most probably by melting of a lower crust at the mantle‐crust boundary. The increase in fO2 causes a decrease in Cr2O3, and Al2O3, and a strong increase in the proportion of Fe3+ and Mg# of spinel crystallizing from the basaltic melt at T ∼1200°C. The olivine‐pyroxene and olivine‐spinel geothermometers yielded equilibrium temperature in the range of 935–1025°C, whereas the range of <500–850°C from single‐pyroxene thermometry indicates either post crystallization re‐equilibrium of the clinopyroxene, or the mineral is xenocrystic and re‐equilibrated in a cooling basaltic magma.

The sub-Antarctic Antipodes Volcano: a

The sub-Antarctic Antipodes Islands group, located on the edge of the Campbell Plateau, is composed of Surtseyan-like tuff cones, porphyritic lavas and dykes, overlain by scoriaceous cinder cones. The youthful nature is indicated by well-preserved cones and craters, raised boulder beaches, pollen and spores comparable to the present-day vegetation in peat underlying one lava flow, and published Ar/Ar ages of ≤0.5 Ma. The alkaline crystalline rocks record a fractionation lineage that was controlled by progressive removal of clinopyroxene and olivine at shallow depths with minimal interaction with continental crust. A compilation of isotopic data confirms that the magmas were derived from a mantle source containing a high-time integrated U/Pb (HIMU)-like component. Although this source component has been recognised in many Zealandia intraplate volcanoes, the Antipodes Volcano lavas contain consistently high 206Pb/204Pb and low 87Sr/86Sr ratios, making the geochemical suite an excellent baseline for HIMU studies. The distinctive isotope ratios and chemistries are comparable with those of some anhydrous Pb- and K-depleted peridotitic mantle xenoliths from intraplate volcanoes in Zealandia. If the lithospheric mantle contributed to the source of Antipodes Volcano, the Pb and K anomalies in the lavas may not require residual amphibole/phlogopite and/or sulphide in their source region. Furthermore, comparison with the peridotite suite indicates that the Zealandia lithospheric mantle is isotopically heterogeneous and thus not all the Zealandia Cretaceous–Cenozoic intraplate volcanoes need have tapped a HIMU-like reservoir.

Genesis and evolution of mafic and felsic magmas at Quaternary volcanoes within the Main Ethiopian Rift: Insights from Gedemsa and Fanta 'Ale complexes

This paper presents the results of an investigation carried out on young volcanic rocks from the Gedemsa and Fanta 'Ale complexes, located in the Main Ethiopian Rift, the site of an intense magmatism since Eocene–Oligocene. The earlier NW–SE direction of extension of the Rift, which generated NE–SW trending faults, rotated around E–W in Quaternary times, and produced the still active N to N–NE Wonji Fault System. The Gedemsa volcano is located in the central part of the Ethiopian Rift, about 100km SE of Addis Ababa. It is characterized by a wide central caldera, about 8km in diameter. The general stratigraphic sequence in the area includes, from base upwards, rift-floor ignimbrites, pantelleritic and subordinate trachytic pyroclastic deposits and lava flows and domes, and widespread basaltic deposits. The Fanta 'Ale volcanic complex is located in the northern part of the Main Ethiopian Rift, where the Afar depression begins. It is characterized by a summit caldera of which the diameter is about 4km. This volcano erupted trachytic and rhyolitic lavas, whereas the most diffuse unit is an ignimbrite related to the caldera collapse. Explosive activity has occurred inside and outside the caldera, forming tuff cones and thick pumice-fallout deposits. The only mafic unit is represented by a basaltic eruption that occurred in 1870AD. Historical eruptions and intense fumarolic activity are evidence for the persistence activity of the Fanta 'Ale in this part of the Main Ethiopian Rift.New geochemical and Sr–Nd–Pb isotope data on representative samples from Gedemsa and Fanta 'Ale volcanoes are presented and discussed in order to shed light on the genesis of mafic and felsic magmas, the genetic link between them, and their possible interaction with the local crust. Volcanic rocks show a typical mafic–felsic bi-modal distribution with few intermediate terms (Daly Gap), as observed at regional scale along the Main Ethiopian Rift as well as on the plateau. Geochemical data and modeling suggest that magmas evolved mainly through fractional crystallization processes, accounting for the entire mafic–felsic compositional variation. However, Sr–Nd–Pb isotope data reveal also open-system evolution processes. The most differentiated, Sr-poor rhyolites suffered important low temperature contamination by shallow fluids of hydrothermal and/or meteoric origin. This affected mostly the Sr isotopic composition of whole-rocks, and much less that of separated feldspars that provide more reliable 87Sr/86Sr values. Mafic rocks, as well as the least contaminated felsic rocks, provide evidence for two components involved in the genesis and evolution of mafic magmas: a mantle component, carrying the isotopic composition of the Afar plume, and a crustal component, likely Pan-African sialic lower crust, that might have been added in small amounts, about 2%, to mafic magmas. The origin of the primary magmas is inferred to have occurred by 7% partial melting of a mixed source region including both depleted and enriched mantle components.

Hydrovolcanic tuff rings and cones as indicators for phreatomagmatic explosive eruptions on Mars

AbstractHydrovolcanism is a common natural phenomenon on Earth and should be common on Mars, too, since its surface shows widespread evidence for volcanism and near‐surface water. We investigate fields of pitted cones in the Nephentes/Amenthes region at the southern margin of the ancient impact basin, Utopia, which were previously interpreted as mud volcanoes. The cone fields contain pitted and breached cones with associated outgoing flow‐like landforms. Based on stratigraphic relations, we determined a Hesperian or younger model age. We test the hypothesis of a (hydro)volcanic origin. Based on a detailed morphological and morphometrical analysis and an analysis of the regional context, an igneous volcanic origin of these cones as hydrovolcanic edifices produced by phreatomagmatic eruptions is plausible. Several lines of evidence suggest the existence of subsurface water ice. The pitted cones display well‐developed wide central craters with floor elevations below the preeruptive surface. Their morphometry and the overall appearance are analogous to terrestrial tuff cones and tuff rings. Mounds that are also observed in the same region resemble terrestrial lava domes. The hydrovolcanic interaction between ascending magma and subsurface water and/or water ice may explain the formation of the pitted cones, although other scenarios such as mud volcanism cannot be ruled out. Together with the mounds, the cones might represent effusive and explosive edifices of a monogenetic volcanic field composed of lava domes, tuff rings, tuff cones, and possibly maars.

Stratigraphy, sedimentology and eruptive mechanisms in the tuff cone of El Golfo (Lanzarote, Canary Islands)

The tuff cone of El Golfo on the western coast of Lanzarote (Canary Islands) is a typical hydrovolcanic edifice. Along with other edifices of the same age, it was constructed along a fracture oriented NEE–SWW that coincides with the main structural trend of recent volcanism in this part of the island. We conducted a detailed stratigraphic study of the succession of deposits present in this tuff cone and here interpret them in light of the depositional processes and eruptive dynamics that we were able to infer. The eruptive sequence is represented by a succession of pyroclastic deposits, most of which were emplaced by flow, plus a number of air-fall deposits and ballistic blocks and bombs. We distinguished five different eruptive/depositional stages on the basis of differences in inferred current flow regimes and fragmentation efficiencies represented by the resulting deposits; the different stages may be related to variations in the explosive energy. Eight lithofacies were identified based on sedimentary discontinuities, grain size, components, variations in primary laminations and bedforms. The volcanic edifice was constructed very rapidly around the vent, and this is inferred to have controlled the amount of water that was able to enter the eruption conduit. The sedimentological characteristics of the deposits and the nature and distribution of palagonitic alteration suggest that most of the pyroclastic succession in El Golfo was deposited in a subaerial environment. This type of hydrovolcanic explosive activity is common in the coastal zones of Lanzarote and the other Canary Islands and is one of the main potential hazards that could threaten the human population of this archipelago. Detailed studies of these hydrovolcanic eruptions such as the one we present here can help volcanologists understand the hazards that this type of eruption can generate and provide essential information for undertaking risk assessment in similar volcanic environments.

Highly Refractory Lithospheric Mantle beneath the Tanzanian Craton: Evidence from Lashaine Pre-metasomatic Garnet-bearing Peridotites

Here we report the first occurrence of ultra-depleted pyrope garnets with rare earth element (REE) patterns similar to those of hypothetical garnets proposed to have formed in the subcontinental lithospheric mantle prior to those of metasomatic origin. These unique ultra-depleted garnets have not previously been identified in global suites of mantle xenoliths or diamond inclusions. They occur in certain harzburgite members of the xenolith suite from the Lashaine tuff cone, northern Tanzania. The Lashaine ultra-depleted garnets are characterized by low concentrations of CaO (<0·35 wt %), Cr2O3 (2·5 wt %), incompatible trace elements (e.g. Ba, Sr, Ti, Zr and Y), and have chondrite-normalized REE patterns with steep positive middle-to-heavy REE slopes and flat-to-positive, light-to-middle REE slopes [i.e. they are ‘tick’ (√)-shaped]. The ultra-depleted garnets have high Mg# (92·5) and coexist in chemical and textural equilibrium with highly refractory olivine (Fo95·4) and orthopyroxene (Mg# = 96·4), which are all more magnesian than encountered in most global mantle harzburgites and diamond inclusions. The ultra-depleted garnets occur in interconnecting networks around grains of orthopyroxene, which give the rocks a banded appearance. We propose that the ultra-depleted garnets formed by isochemical exsolution from orthopyroxene following a change in geothermal gradient (decrease in temperature and increase in pressure) associated with Archean lithospheric thickening. Metasomatism of refractory mantle is recorded in the Lashaine xenoliths by an almost continuous variation from ultra-depleted to sub-calcic (harzburgitic) to calcic (lherzolitic) garnet compositions. Overall this systematic trend correlates with increasing depth and is accompanied by a change in chondrite-normalized REE patterns, from ‘√’-shaped to sinusoidal to normal (i.e. light REE depleted). We attribute these compositional variations in garnet, together with a general decrease in Mg# in all mineral phases with depth, to a gradual change from low-temperature high-density fluid metasomatism to high-temperature silicate melt-related enrichment. Pressure and temperature estimates suggest that this metasomatism is concentrated at the base of the Tanzanian lithospheric mantle (i.e. between 125 and 160 km depth), which is consistent with the results of previous studies. Harzburgites containing ultra-depleted garnets were entrained from shallower depths (∼125 km) in the lithosphere than most Lashaine lherzolites and we suggest that long-term survival of this refractory mantle reflects its location above the level reached by ascending metasomatic agents. We propose that preservation of widespread highly refractory, low-density and high-viscosity lithospheric mantle at shallow depths beneath the Tanzanian Craton may have ensured its long-term stability and resistance to delamination. The presence of refractory mantle beneath both the core and eastern margin of the Tanzanian Craton may account for the steep gradient in lithospheric thickness relative to other regions of thick stable Archean lithosphere where thinning beneath cratonic margins is more gradual.

제주도에 분포하는 제4기 단성화산체의 형태적 분류 및 분포 특성 연구

우리나라에서 가장 큰 화산섬인 제주도에는 약 360여 개의 단성화산체가 분포되어 있다고 알려져 있으나, 본 연구를 통해 기존에 일려진 것보다 100여 개가 더 많은 총 455개의 단성화산체가 분포하고 있음을 확인하였다. 총 455개의 단성화산체를 형태학적으로 분석해 본 결과, 분석구가 373개로 전체의 82.0%를 차지하여 가장 높은 비율로 나타나며, 분석구 외에도 정상부가 뾰족한 형태이면서 용암으로 구성된 것이 9개(2.0%), 순상화산체가 27개(5.9%), 응회환이 17개(3.7%), 응회구가 3개(0.7%), 마르가 1개(0.2%), 용암돔이 25개(5.5%)가 분포하고 있다. 또한 이들 중 알오름의 형태로 나타나는 것이 15개가 있다. 단성화산체의 지역별 분포를 살펴보면 전체적으로 제주도의 서쪽에 비해 동쪽에 더 우세하게 분포하고 있음을 알 수 있다. 또한 강수량과 같은 풍화 요인에 의해 분석구의 형태가 영향을 받는다면, 강수량이 월등히 더 많은 남부 지역에 한 방향으로 터진 말발굽형 화구를 가진 분석구나 초승달형 분석구, 불규칙한 형태의 분석구 등이 더 많이 분포해야 할 것이다. 그러나 실제 제주도에서는 오히려 강수량이 더 적은 북부 지역에 이러한 분석구들이 더 많이 분포하는 것으로 나타나, 제주도의 남북 간의 기후적인 요소의 차이가 분석구의 형태나 분포에 크게 영향을 끼친 것은 아니라고 생각된다. 단성화산체 중 응회환, 응회구와 마르는 주로 제주화산섬의 지하 또는 해안가의 저지대에 위치하며, 분석구는 대부분이 해안에서 떨어진 섬의 내륙부에 위치한다. 이는 제주도 단성화산체를 형성한 화산활동이 물(지하수 또는 얕은 바닷물)과의 접촉 유무에 따라 수성화산활동(수증기마그마분화)을 하거나, 마그마성화산활동(스트롬볼리안분화 혹은 하와이안분화)을 한 것임을 알 수 있다. 또한 이들 단성화산체의 고도별 분포를 살펴보면 고도 300 m 이하의 해안저지대에 253개(55.6%), 고도 300~600 m의 중산간지대에 110개(24.2%), 600 m 이상의 산악지대에 92개(20.2%)가 분포하고 있어 과반수 이상이 해안저지대에 분포하고 있음을 알 수 있다. 제주화산섬에 분포하는 단성화산체들은 응력장 안에서 생긴 단층이나 틈을 따라서 틈새 분출을 통해 선상으로 배열되어 나타나는 것과 이러한 틈새와는 무관하게 독립된 단일 화구를 통한 중심 분출을 통해 생성된 것이 함께 나타남을 알 수 있다. Jeju island is the biggest volcanic island in Korea and there are over 455 Quaternary monogenetic volcanoes, of which approximately 373 volcanoes(82.0%) are cinder cones. Other volcanic forms in the island include sharp-pointed lava cone without crater(9 volcanoes; 2.0%), shield volcanoes(27 volcanoes; 5.9%), tuff rings(17 volcanoes; 3.7%), tuff cones(3 volcanoes; 0.7%), a maar(1 volcano; 0.2%) and lava domes(25 volcanoes; 5.5%). The monogenetic volcanoes include 15 small nested cinder cones(aloreum). The monogenetic volcanoes are more abundant in the eastern part of the island than in the western part. If the main cause of the weathering such as precipitation affected the shape of the monogenetic volcanoes, more monogenetic volcanoes(BC, CC, DC, etc.) are supposed to be present in the southern part that have more precipitation than in the northern part. But the distribution of the monogenetic volcanoes shows no difference between the southern and the northern parts. So we suggest that the difference of the climatic conditions did not affect the distribution or the shape of cinder cones. Tuff rings, tuff cones and a maar are distributed beneath the island or in the low-altitude areas along the shore although cinder cones are distributed in the interior of the island. This means that the volcanic activity which formed the monogenetic volcanoes resulted from either phreatomagmatic eruption or magmatic (hawaiian or strombolian) eruptions depending on the reaction with water (underground water or shallow waters). The distribution of the monogenetic volcanoes according to the altitude shows that 253(55.6%) volcanoes occur in low-lying coastal areas at an altitude below 300 m, 110(24.2%) in a middle mountainous area at an altitude between 300~600 m and 92(20.2%) in a high mountainous area at an altitude above 600 m. So more than half of monogenetic volcanoes are distributed in low-lying coastal areas.

40Ar/39Ar geochronology of the Neogene-Quaternary Harrat Al-Madinah intercontinental volcanic field, Saudi Arabia: Implications for duration and migration of volcanic activity

New 40Ar/39Ar ages, based on incremental heating techniques for groundmass separates of 25 samples, are presented for the Harrat Al-Madinah volcanic field, part of Harrat Rahat in the north western part of the Arabian plate. This area is an active volcanic field characterized by the occurrence of two historical eruptions approximately in 641 and 1256AD. Field investigations of the main volcanic landforms indicate dominantly monogenetic strombolian eruptions, in addition to local more explosive eruptions. The lavas consist mainly of olivine basalt and hawaiite flows with minor evolved rocks of mugearite, benmoreite, and trachyte that occur mainly as domes, tuff cones and occasionally as lava flows. Previous K/Ar dating shows that the Harrat Al-Madinah lava flows and associated domes comprise seven units spanning an age range of ca. 1.7Ma–Recent. The new 40Ar/39Ar age determinations confirm, to a great extent, the previously obtained K/Ar ages in the sense that no major systematic biases were found in the general stratigraphy of the different flow units. However, the 40Ar/39Ar plateau ages show that volcanism in this area began in the Neogene (∼10Ma) and continued to Recent, with the most voluminous eruptions occurring in the Quaternary. Neogene volcanism occurred in at least three pulses around 10, 5 and 2Ma, whereas Quaternary volcanism produced at least seven units reflecting lava flow emplacement in the time period of 1.90Ma–Recent. Thus, the whole duration of volcanic activity in the Harrat Al-Madinah (10Ma–Recent) appears much longer than that previously identified. The longevity of volcanism in the same part of the moving Arabian plate and absence of evidence for uni-directional migration of volcanic activity indicate that there is no fixed plume beneath this region. The NNW-trending distribution of the volcanic vents is parallel to the Red Sea, and suggests their origin is related to periodic extensional episodes along the reactivated Red Sea fault system.

New seismo-stratigraphic and marine magnetic data of the Gulf of Pozzuoli (Naples Bay, Tyrrhenian Sea, Italy): inferences for the tectonic and magmatic events of the Phlegrean Fields volcanic complex (Campania)

A detailed reconstruction of the stratigraphic and tectonic setting of the Gulf of Pozzuoli (Naples Bay) is provided on the basis of newly acquired single channel seismic profiles coupled with already recorded marine magnetics gathering the volcanic nature of some seismic units. Inferences for the tectonic and magmatic setting of the Phlegrean Fields volcanic complex, a volcanic district surrounding the western part of the Gulf of Naples, where volcanism has been active since at least 50 ka, are also discussed. The Gulf of Pozzuoli represents the submerged border of the Phlegrean caldera, resulting from the volcano-tectonic collapse induced from the pyroclastic flow deposits of the Campanian Ignimbrite (35 ka). Several morpho-depositional units have been identified, i.e., the inner continental shelf, the central basin, the submerged volcanic banks and the outer continental shelf. The stratigraphic relationships between the Quaternary volcanic units related to the offshore caldera border and the overlying deposits of the Late Quaternary depositional sequence in the Gulf of Pozzuoli have been highlighted. Fourteen main seismic units, both volcanic and sedimentary, tectonically controlled due to contemporaneous folding and normal faulting have been revealed by geological interpretation. Volcanic dykes, characterized by acoustically transparent sub-vertical bodies, locally bounded by normal faults, testify to the magma uprising in correspondence with extensional structures. A large field of tuff cones interlayered with marine deposits off the island of Nisida, on the western rim of the gulf, is related to the emplacement of the Neapolitan Yellow Tuff deposits. A thick volcanic unit, exposed over a large area off the Capo Miseno volcanic edifice is connected with the Bacoli-Isola Pennata-Capo Miseno yellow tuffs, cropping out in the northern Phlegrean Fields.

Susceptibility modelling of hummocky terrain distribution using the information value method (Deception Island, Antarctic Peninsula)

The hummocky terrains of Deception Island (Antarctic Peninsula) are continuous surfaces with decimetre to metre wide and decimetre depth bumps located mainly in the lower section of sloping lapilli and scoria terrains. A detailed study site between Cerro Caliente and Crater Lake was selected for the detailed mapping of hummocky terrains and for modelling their spatial distribution according to controlling geographical factors. A model of the susceptibility of occurrence of the hummocky terrains was created using the information value method, together with five independent variables: elevation, slope, global summer radiation, total curvature and lithology. Success and prediction rate curves were used for model validation and the Area Under the Curve index was used to quantify the levels of performance and prediction. The results were of high quality with a success rate of 88% and a prediction rate of 78%. The classes of the independent variables with more relevance in the occurrence of hummocky terrains were: elevation between 20–30m and 60–70m; concave or rectilinear/flat areas; slopes between 8 and 12º; tuff cones and maar deposits and global summer radiation between 1.8 and 2.0 TJm−2. The good quality of the modelling results supports its use for assessing the future potential for formation of new hummocky terrain areas, or even to estimate the spatial distribution of buried ice within the permafrost environment of Deception Island.

Ilchulbong tuff cone, Jeju Island, Korea, revisited: A compound monogenetic volcano involving multiple magma pulses, shifting vents, and discrete eruptive phases

Ilchulbong (Sunrise Peak) tuff cone is a United Nations Educational, Scientific, and Cultural Organization (UNESCO) World Heritage site that owes its scientific importance to the outstanding coastal exposures that surround it. It is a type locality for the sedimentary evidence of pyroclastic transport and depositional processes that occur during phreatomagmatic basaltic eruptions. Its classically shaped cone morphology was long considered to result from a single short-lived eruption. Reanalysis of the sedimentary sequence has now revealed two subtle paraconformities in the deposits, indicating that the eruption occurred in three separate phases. The break between phases I to II was "long," i.e., days to weeks, during which erosion and compaction of the lower cone occurred, leading to a volcanowide discontinuity surface and reworked deposits in the lower flanks. Possibly following cooling and solidification of the initial shallow conduit system, the phase II eruption resumed from a new vent location ∼600 m away, before being terminated by a very brief interval and final eruption of phase III from the same general site. The second pause led to a more subtle discontinuity, marked by differences in deposit character above and below it, and it was associated with a period of seismicity-induced slumping and slipping of deposits. Detailed geochemical study of the juvenile clasts through the sequence reveals that three separate alkali basaltic magma pulses were erupted, corresponding directly to the three eruption phases. Magmas 1 and 3 may be genetically related, with the former showing evidence for longer periods of shallow-level fractionation, whereas magma 2 originated from a slightly different mantle domain, although at a similar depth. These results demonstrate that breaks in monogenetic eruption sequences may represent deep magma system processes, and they raise fundamental questions concerning the mechanisms of triggering and eruption of multiple magma pulses in small-volume basaltic volcanoes. This study shows the importance of recognizing sedimentary evidence of time breaks in such eruption sequences as potential indications of shifts in eruption chemistry and vent location. These have important implications for the controls of vent migration at other monogenetic volcanoes and for hazard planning during future similar eruptions.

Spring Water and Water Culture on Jeju Island

Introduction Jeju (Cheju) Island is often called the Hawaii of Korea. The island, the country’s largest, is located approximately 140 km south of the Korean mainland. Mt. Halla, the central peak of the island, is the highest mountain in South Korea at 1950 m above sea level. A variety of volcanic landforms, excellent geologic exposures, favorable climate, and beautiful scenery make the island an exceptional place for Koreans and foreigners alike. In particular, several geological features of the island, including Mt. Halla, the Seongsan Ilchulbong tuff cone, and the Geomunoreum Lava Tube System, received the UNESCO World Natural Heritage designation in 2007 (Jeju Island Geopark 2009). The annual precipitation on the island is 1975 mm, the highest rainfall in Korea, but there are no perennial streams. Because of the highly permeable geological structures of the volcanic rocks, surface waters are rare (Koh et al. 2006, Won et al. 2006). Therefore, it was very difficult to obtain water on the island before modern water-supply systems using groundwater wells were introduced. Spring water and bongcheonsu (rain water) were the main water sources (Yoo 1991). Today, the availability of water is no longer a problem. A water industry has been promoted to develop products such as bottled water, beverages, alcoholic drinks, and even water therapy services through the use of the island’s clean and abundant groundwater. The water culture of Jeju Island is quite distinct from that of the mainland. Spring water has long played an important role in the island’s culture as the water of life, and it used to be at the heart of the water culture of Jeju. It is still considered highly valuable, and it continues to

Geochemistry and Sr–Nd–Pb isotopic composition of the Harrat Al-Madinah Volcanic Field, Saudi Arabia

Whole-rock geochemical and Sr, Nd and Pb isotope data are presented for the Harrat Al-Madinah volcanic field, in the north western part of the Arabian plate, aiming to understand their origin and the composition of their mantle source. This area is an active volcanic field characterized by the occurrence of two historic eruptions approximately in 641 and 1256A.D. Field investigation of the main volcanic landforms indicates dominantly monogenetic strombolian eruptions, in addition to local phreatomagmatic eruption style. The lavas consist mainly of alkali olivine basalt, olivine transitional basalt, and hawaiite with ocean island basalt (OIB)-like characteristics. Evolved rocks, represented by mugearites, benmoreites, and trachytes, occur mainly as domes, tuff cones and occasionally as lava flows. Chemical variations in the evolved rocks indicated their evolution by low pressure crystal fractionation of olivine, plagioclase, clinopyroxene, and Fe–Ti oxides from the relatively primitive basalts. The isotopic compositions of 143Nd/144Nd (0.512954–0.512995), 87Sr/86Sr (0.702899 to–0.702977) and Pb (206Pb/204Pb=18.5515–18.7446, 207Pb/204Pb=15.5120–15.5222, 208Pb/204Pb=38.1347–38.4468), show restricted variations suggesting only minor crustal contamination. They defined an array consistent with mixing of two geochemically distinct components of depleted MORB-mantle (DMM) and high 238U/204Pb ratio (HIMU). The variations in Tb/Yb, La/Yb and Sm/Yb ratios in the relatively primitive basalts (MgO>6wt.%) indicated garnet peridotite source. However, the positive Nb, Sr, Ba and Ti anomalies in the primitive mantle-normalized incompatible element patterns and the significant variation between Zr/Nb vs. Ce/Y and La/Yb vs. Yb suggest contribution of an amphibole-bearing spinel lherzolite source. Moreover, the negative correlations between SiO2 vs. 87Sr/86Sr and Th vs. 143Nd/144Nd are interpreted as an indication of mixing melts derived from two end-members; one is garnet bearing asthenospheric source with OIB characteristic and the other is amphibole-bearing spinel lherzolite. The Harrat Al-Madinah volcanic field occurs near the Red Sea Rift System and its origin reflects a strong lithospheric control on the loci of partial melting. The dominantly NNW alignment patterns of the volcanoes, which is similar to the regional Red Sea trend, may suggest that the magmas were produced by decompression partial melting triggered by lithospheric extension related to the Red Rift.