Earthen barriers to control lava flows in the 2001 eruption of Mt. Etna

Preceded by four days of intense seismicity and marked ground deformation, a new eruption of Mt. Etna started on 17 July and lasted until 9 August 2001. It produced lava emission and strombolian and phreatomagmatic activity from four different main vents located on a complex fracture system extending from the southeast summit cone for about 4.5 km southwards, from 3000 to 2100 m elevation (a.s.l.). The lava emitted from the lowest vent cut up an important road on the volcano and destroyed other rural roads and a few isolated country houses. Its front descended southwards to about 4 km distance from the villages of Nicolosi and Belpasso. A plan of intervention, including diversion and retaining barriers and possibly lava flow interruption, was prepared but not activated because the flow front stopped as a consequence of a decrease in the effusion rate. Extensive interventions were carried out in order to protect some important tourist facilities of the Sapienza and Mts. Silvestri zones (1900 m elevation) from being destroyed by the lava emitted from vents located at 2700 m and 2550 m elevation. Thirteen earthen barriers (with a maximum length of 370 m, height of 10–12 m, base width of 15 m and volume of 25 000 m³) were built to divert the lava flow away from the facilities towards a path implying considerably less damage. Most of the barriers were oriented diagonally (110–135°) to the direction of the flow. They were made of loose material excavated nearby and worked very nicely, resisting the thrust of the lava without any difficulty. After the interventions carried out on Mt. Etna in 1983 and in 1991–1992, those of 2001 confirm that earthen barriers can be very effective in controlling lava flows.     

Geology of the Heimaey volcanic centre, south Iceland: early evolution of a central volcano in a propagating rift?

Heimaey is the southernmost and also the youngest of nine volcanic centres in the southward-propagating Eastern Volcanic Zone, Iceland. The island of Heimaey belongs to the Vestmannaeyjar volcanic system (850 km²) and is situated 10 km off the south coast of Iceland. Although Heimaey probably started to form during the Upper Pleistocene all the exposed subaerial volcanics (10 monogenetic vents covering an area of 13.4 km²) are of Holocene age. Heimaey is composed of roughly equal amounts of tuff/tuff-breccias and lavas as most eruptions involve both a phreatomagmatic and an effusive phase. The compositions of the extrusives are predominantly alkali basalts belonging to the sodic series. Repeated eruptions on Heimaey, and the occurrence of slightly more evolved rocks (i.e. hawaiite approaching mugearite), might indicate that the island is in an early stage of forming a central volcano in the Vestmannaeyjar system. This is further substantiated by the development of a magma chamber at 10–20 km depth during the most recent eruption in 1973 and by the fact that the average volume of material produced in a single eruption on Heimaey is 0.32 km³ (dense rock equivalent), which is twice the value reported for the Vestmannaeyjar system as a whole. We find no support for the previously postulated episodic behaviour of the volcanism in the Vestmannaeyjar system. However, the oldest units exposed above sea level, i.e. the Norðurklettar ridge, probably formed over a 500-year interval during the deglaciation of southern Iceland. The absence of equilibrium phenocryst assemblages in the Heimaey lavas suggests that magma rose quickly from depth, without long-time ponding in shallow-seated crustal magma chambers. Eruptions on Heimaey have occurred along two main lineaments (N45°E and N65°E), which indicate that it is seismic events associated with the southward propagation of the Eastern Volcanic Zone that open pathways for the magma to reach the surface. Continuing southward propagation of the Eastern Volcanic Zone suggests that the frequency of volcanic eruptions in the Vestmannaeyjar system might increase with time, and that Heimaey may develop into a central volcano like the mature volcanic centres situated on the Icelandic mainland.     

The submarine eruption of the Bombarda volcano,Milos Island,Cyclades, Greece

The Milos volcanic field includes a well-exposed volcaniclastic succession which records a long history of submarine explosive volcanism. The Bombarda volcano, a rhyolitic monogenetic center, erupted ∼1.7 Ma at a depth <200 m below sea level. The aphyric products are represented by a volcaniclastic apron (up to 50 m thick) and a lava dome. The apron is composed of pale gray juvenile fragments and accessory lithic clasts ranging from ash to blocks. The juvenile clasts are highly vesicular to non-vesicular; the vesicles are dominantly tube vesicles. The volcaniclastic apron is made up of three fades: massive to normally graded pumice-lithic breccia, stratified pumice-lithic breccia, and laminated ash with pumice blocks. We interpret the apron beds to be the result of water-supported, volcaniclastic mass-How emplacement, derived directly from the collapse of a small-volume, subaqueous eruption column and from syn-eruptive, down-slope resedimentation of volcaniclastic debris. During this eruptive phase, the activity could have involved a complex combination of phreatomagmatic explosions and minor submarine effusion. The lava dome, emplaced later in the source area, is made up of flow-banded lava and separated from the apron by an obsidian carapace a few meters thick. The near-vertical orientation of the carapace suggests that the dome was intruded within the apron. Remobilization of pyroclastic debris could have been triggered by seismic activity and the lava dome emplacement. Published online: 30 January 2003 Editorial responsibility: J. McPhie     

陆相渐新统划分中的问题及新进展

近年来国际上关于始新世－渐新世界线的研究取得了重大进展，其同位素年龄被确认为３４Ｍａ，北美哺乳动物年代表已据此重新厘定。通过对哺乳动物群的分析和对比，论证了我国原认为是早渐新世的乌兰戈楚期和呼尔井期的时代应改归晚始新世，原认为是中渐新世的三达河期的时代应为早渐新世。在亚洲始新世－渐新世的界线不是在乌兰戈楚期之末，而是在呼尔井期与三达河期之间。     

长白山天池火山地质学研究的若干进展与灾害分析

通过以减轻火山灾害为目的的天池火山锥体顶部地区地质填图工作，发现了天池火山锥体附近不同期次火山泥石流，部分火山泥石流显示的高温定位特征指示了其与千年大喷发的成因联系。这些火山泥石流构成了严重的火山泥石流灾害，天池火山锥体近顶部大型滑坡体的发现则指示了天池火山另一种重要的灾害类型。滑坡体堆积物结构上可分为3种类型。天池火山千年大喷发时不同成分与物性的岩浆混合作用十分发育，指示了天池火山喷发前不同岩浆批的混合与共喷发机理。本文还论述了天池火山近代历史记录喷发物的分布与鉴别特征。     

Earthquake swarms preceding the 2000 eruption of Miyakejima volcano, Japan

The first sign of magma accumulating beneath Miyakejima, an island volcano in the northern Izu islands, Japan, came at around 18:00 on 26 June 2000, when a swarm of earthquakes was detected by a volcano seismic network on the island. Earthquakes occurred initially beneath the southwest flank near the summit and gradually migrated west of the island, where a submarine eruption occurred the next morning. Earthquakes then migrated further to the northwest between Miyakejima and Kozushima, another volcanic island and developed to the most intense earthquake swarm ever observed in and around Japanese archipelago. To better image how the initial magma intrusion occurred, we relocated hypocenters by using a station-correction method and a double-difference method. The relocated epicenters are generally concentrated near the upper bound of dyke intrusions inferred from geodetic studies throughout the initial stages of the 2000 eruption at Miyakejima from 26 to 27 June 2000. As for seismic activity westward off Miyakejima in the morning on 27 June, hypocenters from both a nationwide seismic network that were relocated by the double-difference method, and those from the volcano seismic network relocated by the station-correction method, formed a very shallow cluster that ascended slowly with time as it propagated northwestward from Miyakejima. This suggests that the dykes have both a radial and upward component of movement.Editorial responsibility: S. Nakada, T. Druitt     

A complex, Subplinian-type eruption from low-viscosity, phonolitic to tephri-phonolitic magma: the AD 472 (Pollena) eruption of Somma-Vesuvius, Italy

The combined use of field investigation and laboratory analyses allowed the detailed stratigraphic reconstruction of the Pollena eruption (472 AD) of Somma-Vesuvius. Three main eruptive phases were recognized, related either to changes in the eruptive processes and/or to relative changes of melt composition. The eruption shows a pulsating behavior with deposition of pyroclastic fall beds and generation of dilute and dense pyroclastic density currents (PDC). The eruptive mechanisms and transportation dynamics were reconstructed for the whole eruption. Column heights were between 12 and 20 km, corresponding to mass discharge rates (MDR) of 7×10⁶ kg/s and 3.4×10⁷ kg/s. Eruptive dynamics were driven by magmatic fragmentation of a phono-tephritic to tephri-phonolitic magma during Phases I and II, whereas phreatomagmatic fragmentation dominated Phase III. Magma composition varies between phonolitic and tephritic-phonolitic, with melt viscosity likely not in excess of 10³ Pa s. The volume of the pyroclastic fall deposits, calculated by using of proximal isopachs, is 0.44 km³. This increases to 1.38 km³ if ash volumes are extrapolated on a log thickness vs. square root area diagram using one distal isopach and column height.Editorial responsibility: R Cioni     

Seismic signature of a phreatic explosion: hydrofracturing damage at Karthala volcano, Grande Comore Island, Indian Ocean

Karthala volcano is a basaltic shield volcano with an active hydrothermal system that forms the southern two-thirds of the Grande Comore Island, off the east coat of Africa, northwest of Madagascar. Since the start of volcano monitoring by the local volcano observatory in 1988, the July 11th, 1991 phreatic eruption was the first volcanic event seismically recorded on this volcano, and a rare example of a monitored basaltic shield. From 1991 to 1995 the VT locations, 0.5<M_l<4.3, show a crack shaped pattern (3 km long, 1 km wide) within the summit caldera extending at depth from –2 km to +2 km relative to sea level. This N-S elongated pattern coincides with the direction of the regional maximum horizontal stress as deduced from regional focal mechanism solutions. This brittle signature of the damage associated with the 1991 phreatic eruption is a typical pattern of the seismicity induced by controlled fluid injections such as those applied at geothermal fields, in oil and gas recovery, or for stress measurements. It suggests the 1991 phreatic eruption was driven by hydraulic fracturing induced by forced fluid flow. We propose that the extremely high LP and VT seismicity rates, relative to other effusive volcanoes, during the climax of the 1991 phreatic explosion, are due to the activation of the whole hydrothermal system, as roughly sized by the distribution of VT hypocenters. The seismicity rate in 1995 was still higher than the pre-eruption seismicity rate, and disagrees with the time pattern of thermo-elastic stress readjustment induced by single magma intrusions at basaltic volcanoes. We propose that it corresponds to the still ongoing relaxation of pressure heterogeneity within the hydrothermal system as suggested by the few LP events that still occurred in 1995.Editorial responsibility: H Shinohara     

Active Fault and Volcanic Activity in the Longhai-Zhangpu Coastal Area, Fujian Province

INTRODUCTION TheLonghai ZhangpucoastalareaofFujianProvinceliesonthesouthernsideoftheoutletofthe JiulongjiangRiver.Tectonically,itislocatedonthesouthernsegmentoftheChangle Zhao’anfault zone.Previously,alotofseismogeologicresearchworkhasbeencarriedoutinthi…     

Study on the Relationship between Large Volcanic Eruptions and Temperature Variation Based on Tree-Ring Data in the Eastern Tibetan Plateau during the Past Millennium

Volcanic eruptions can significantly cool the global troposphere on the time scales from several months up to a decade due to reflection of solar radiation by sulfate aerosols and feedback mechanisms in the climate system. The impact of volcanic eruptions on global climate are discussed in many studies. However, few studies have been done on the impact of volcanic eruption on climate change in China in the past millennium. The 1300-year and 600-year temperature series were reconstructed based on the six tree-ring temperature proxy data in northeastern and southeastern Tibetan Plateau, respectively. Three warm periods occurred in 670-920,1000-1310 and 1590-1930, and three cold periods happened at 920-1000,1310-1590 and 1930-2000 in the northeastern Tibetan Plateau. There were two obviously warm periods (1385-1450 and 1570-1820) and two cold periods (1450-1570 and 1820-2000) in southeastern Tibetan Plateau. Contrasting with volcanic eruption chronology, we analyzed the relationship between volcanic activity and temperature variation in the eastern Tibetan plateau during the past millennium using Superposed Epoch Analysis (SEA) method. The results indicated that the temperature decreased one year after large volcanic eruptions located beteen 10°S and 10°N in latitude in northeastern Tibetan Plateau and two years in southeastern Tibetan Plateau. The volcanic eruptions occurred at different latitudes have different impacts on the temperature variations, which may be caused by regional difference, the nature of the eruption, the magnitude of the resulting change in incoming solar radiation, prevailing background climate and internal variability, season, latitude, and other considerations.