Eruption of Piip Crater (Kamchatka)

In autumn of 1966 on the northern slope of Kliuchevskoy volcano a chain of new adventive craters broke out at the height of about 2200 m. Eighty-four hours before the beginning of the eruption a swarm of preliminary volcanic earthquakes had appeared. The number of preliminary shocks was 457 with total energy of 4 × 10¹⁷ erg. With the beginning of the lava flow the earthquakes stopped and a continuous volcanic tremor appeared. The total energy of volcanic tremor amounts to 10¹⁶ erg. During the eruption numerous explosive earthquakes with the energy of 10¹⁵–10¹⁶ erg were recorded and besides the microbarograph of the Volcanostation recorded 393 explosions with an energy more than 10¹³ erg and their total energy was equal to 10¹⁷ erg. All together it has been formed 8 explosive craters and the lowest 9th crater was effusive. The slag cone was formed round this effusive crater, the lava effusion of basaltic-andesite composition (52,5% SiO₂) tooke place from the lava boccas at the cone base and from the crater. The lava flow covered a distance of 10 km along the valley of the Sopochnoy river and descended to a height of about 800 m. The lava flow velocity at the outflow reached 800 m/hr, the lava temperature was 1050°C. The effused lava volume amounts to 0.1 km³. The eruption stopped on December 25–26, 1966.     

Prediction and characteristics of the 1975 eruption of Tolbachik volcano,Kamchatka

According to a long-term prediction. Tolbachik volcano was expected to erupt with a 0.7 probability, some time in the period 1964–1978. An eruption of Tolbachik commenced at 21.45 GMT on July 5, 1975. It took place on the southwestern Hank of the volcano at an altitude of 880 m a.s.l. about 18 km from the central crater. An earthquake swarm preceded it. The place and time of eruption were predicted three days belore it began on the basis of epicenter locations and characteristics of recorded seismic activity. During the period July 5–28 gases and incandescent magma were continuously ejected to a height of 2,000 m above ground level. Ash clouds rose to a height of 6 to 8 km, with a train of ash extending over a horizontal distance of 300 km. The velocity of jets from the crater was about 200 m/see. During the first days of the eruption the quantity of materials erupted and the eruption power amounted to 1.25 · 10⁵ kg/see and 2.1 · 10¹¹W, respectively. The vertical growth of the scoria cone was consistent with the lawH=2.15³√vt, where the time and height are expressed in seconds and meters, respectively. The mouth of the volcano conduit was estimated to be 12 m in diameter. Lava began to erupt at 22^h23^m on July 28. During the period July 5–31 about 3 · 10¹¹ kg of magmatic material, consisting of ash, scoria and lava, was erupted onto the earth’s surface. The energy released over the period of eruption accounted for 5 · 10¹⁷ J.     

The complex filling of alae crater,Kilauea Volcano,Hawaii

Since February 1969 Alae Crater, a 165-m-deep pit crater on the east rift of Kilauea Volcano, has been completely filled with about 18 million m³ of lava. The filling was episodic and complex. It involved 13 major periods of addition of lava to the crater, including spectacular lava falls as high as 100 m, and three major periods of draining of lava from the crater. Alae was nearly filled by August 3, 1969, largely drained during a violent ground-cracking event on August 4, 1969, and then filled to the low point on its rim on October 10, 1969. From August 1970 to May 1971, the crater acted as a reservoir for lava that entered through subsurface tubes leading from the vent fissure 150 m away. Another tube system drained the crater and carried lava as far as the sea, 11 km to the south. Much of the lava entered Alae by invading the lava lake beneath its crust and buoying the crust upward. This process, together with the overall complexity of the filling, results in a highly complicated lava lake that would doubtless be misinterpreted if found in the fossil record.     

Seismological studies of the Raoul Island eruption, 1964

Raoul Island, the largest of the Kermadec Group, is about 8 km across and is situated about 1100 km north-east of Auckland, New Zealand. An eruption of steam and mud occurred on the island on 1964 November 21 (local date); this was preceded by an earthquake swarm which started on 10 November. Records made with the Willmore seismograph at the Meteorological Station on the north coast of the island show that within four hours of the start of the swarm, the frequency of shocks had risen to a peak of more than 80 an hour. Most shocks had sharp beginnings with S-P intervals of 1 to 2 seconds. Tremor was noticeable on 11 November and increased until by 12 November it was continuous, masking all but the largest of the discrete shocks. The level of tremor and the number of recorded shocks then decreased, until by the time the largest earthquake occurred on November 14, only 30 to 40 shocks were being recorded per hour. The largest earthquake was assigned a magnitude of 5.7 from recordings made in New Zealand, and was felt on Raoul Island at intensity 7 on the Modified Mercalli scale. Further shocks and tremor were recorded, and on 15 November the tremor was particularly active for several hours. On 21 November, an eruption occurred, throwing steam, mud, and rocks to an estimated height of 800 m from a crater on the edge of Green Lake, about 2 km from the Meteorological Station. During the eruption the seismograph recorded a peak vibration 30 to 40 times the amplitude of the normal background level. By two hours after the eruption, the level of vibration had stabilised at double that before the eruption. The island was evacuated from 23 November until 6 December, during which time the seismograph was inoperative. From 6 December to 11 December three portable seismographs were recording in addition to the permanent station. By this time the frequency of recorded shocks had dropped to about two per hour. The earthquakes were located in the vicinity of Denham Bay, some kilometres to the west of the main crater. Volcanic activity has been observed previously in Denham Bay, and it is thought that the Bay may be a former crater.     

Integrated field, satellite and petrological observations of the November 2010 eruption of Erta Ale

Erta Ale volcano, Ethiopia, erupted in November 2010, emplacing new lava flows on the main crater floor, the first such eruption from the southern pit into the main crater since 1973, and the first eruption at this remote volcano in the modern satellite age. For many decades, Erta Ale has contained a persistently active lava lake which is ordinarily confined, several tens of metres below the level of the main crater, within the southern pit. We combine on-the-ground field observations with multispectral imaging from the SEVIRI satellite to reconstruct the entire eruptive episode beginning on 11 November and ending prior to 14 December 2010. A period of quiescence occurred between 14 and 19 November. The main eruptive activity developed between 19 and 22 November, finally subsiding to pre-eruptive levels between 8 and 15 December. The estimated total volume of lava erupted is ??0.006?km³. The mineralogy of the 2010 lava is plagioclase?+?clinopyroxene?+?olivine. Geochemically, the lava is slightly more mafic than previously erupted lava lining the caldera floor, but lies within the range of historical lavas from Erta Ale. SIMS analysis of olivine-hosted melt inclusions shows the Erta Ale lavas to be relatively volatile-poor, with H₂O contents ??1,300?ppm and CO₂ contents of ??200?ppm. Incompatible trace and volatile element systematics of melt inclusions show, however, that the November 2010 lavas were volatile-saturated, and that degassing and crystallisation occurred concomitantly. Volatile saturation pressures are in the range 7?C42?MPa, indicating shallow crystallisation. Calculated pre-eruption and melt inclusion entrapment temperatures from mineral/liquid thermometers are ??1,150?°C, consistent with previously published field measurements.     

Gigantic directed blast at Shiveluch volcano (Kamchatka)

On November 12, 1964, after a long swarm of preliminary earthquakes a gigantic directed blast took place at Shiveluch Volcano. The Crater top of the volcano with five large domes was completely destroyed. The deposits of the directed blast fell on an area of 98 sq. km, at a distance up to 10 km from the crater. The volume of the deposits is 1.5 km³ at least. A new crater was formed, its size is 1.5 × 3 km. Numerous pyroclastic flows were poured out the new crater. The eruption lasted only one hour, its thermal energy is 1,3 × 10²⁵ ergs, kinetic energy of the blast ? 1 × 10²⁴ ergs, air wave energy ? 1,8 × 10²¹ ergs. Initial velocity of the explosion: 280–310m/sec, pressure: 800–1000atm. The eruption of Shiveluch volcano belongs to the « Bezymianny type » eruption.     

Overview of the 1990–1995 eruption at Unzen Volcano

Following 198 years of dormancy, a small phreatic eruption started at the summit of Unzen Volcano (Mt. Fugen) in November 1990. A swarm of volcano-tectonic (VT) earthquakes had begun below the western flank of the volcano a year before this eruption, and isolated tremor occurred below the summit shortly before it. The focus of VT events had migrated eastward to the summit and became shallower. Following a period of phreatic activity, phreatomagmatic eruptions began in February 1991, became larger with time, and developed into a dacite dome eruption in May 1991 that lasted approximately 4 years. The emergence of the dome followed inflation, demagnetization and a swarm of high-frequency (HF) earthquakes in the crater area. After the dome appeared, activity of the VT earthquakes and the summit HF events was replaced largely by low-frequency (LF) earthquakes. Magma was discharged nearly continuously through the period of dome growth, and the rate decreased roughly with time. The lava dome grew in an unstable form on the shoulder of Mt. Fugen, with repeating partial collapses. The growth was exogenous when the lava effusion rate was high, and endogenous when low. A total of 13 lobes grew as a result of exogenous growth. Vigorous swarms of LF earthquakes occurred just prior to each lobe extrusion. Endogenous growth was accompanied by strong deformation of the crater floor and HF and LF earthquakes. By repeated exogenous and endogenous growth, a large dome was formed over the crater. Pyroclastic flows frequently descended to the northeast, east, and southeast, and their deposits extensively covered the eastern slope and flank of Mt. Fugen. Major pyroclastic flows took place when the lava effusion rate was high. Small vulcanian explosions were limited in the initial stage of dome growth. One of them occurred following collapse of the dome. The total volume of magma erupted was 2.1×10⁸ m³ (dense-rock-equivalent); about a half of this volume remained as a lava dome at the summit (1.2 km long, 0.8 km wide and 230–540 m high). The eruption finished with extrusion of a spine at the endogenous dome top. Several monitoring results convinced us that the eruption had come to an end: the minimal levels of both seismicity and rockfalls, no discharge of magma, the minimal SO₂ flux, and cessation of subsidence of the western flank of the volcano. The dome started slow deformation and cooling after the halt of magma effusion in February 1995.     

The November 2002 eruption at Piton de la Fournaise volcano,La Réunion Island: ground deformation,seismicity, and pit crater collapse

An eruption on the eastern flank of Piton de la Fournaise volcano started on 16 November, 2002 after 10 months of quiescence. After a relatively constant level of activity during the first 13 days of the eruption, lava discharge, volcanic tremor and seismicity increased from 29 November to 3 December. Lava effusion suddenly ceased on 3 December while shallow earthquakes beneath the Dolomieu summit crater were still recorded at a rate of about one per minute. This unusual activity continued and increased in intensity over the next three weeks, ending with the formation of a pit crater within Dolomieu. Based on ground deformation, measured by rapid-static and continuous GPS and an extensometer, seismic data, and lava effusion patterns, the eruptive period is divided into five stages: 1) slow summit inflation and sporadic seismicity; 2) rapid summit inflation and a short seismic crisis; 3) rapid flank inflation, onset of summit deflation, sporadic seismicity, accompanied by stable effusion; 4) flank inflation, coupled with summit deflation, intense seismicity, and increased lava effusion; and finally 5) little deflation, intense shallow seismicity, and the end of lava effusion. We propose a model in which the pre-intrusive inflation of Stage 1 in the months preceding the eruption was caused by a magma body located near sea level. The magma reservoir was the source of an intrusion rising under the summit during Stage 2. In Stage 3, the magma ponded at a shallow level in the edifice while the lateral injection of a radial dike reached the surface on the eastern flank of the basaltic volcano, causing lava effusion. Pressure decrease in the magmatic plumbing system followed, resulting in upward migration of a collapse front, forming a subterranean column of debris by faulting and stoping. This caused intense shallow seismicity, increase in discharge of lava and volcanic tremor at the lateral vent in Stage 4 and, eventually the formation of a pit crater in Stage 5.     

Seismicity of an andesitic volcano during block-lava effusion: Volcán de Colima, México, November 1998–January 1999

The block-lava effusion at Volcán de Colima, México began on November 20, 1998, after 12 months of seismic activity, and ended about 80 days later. Three types of seismic events were observed during the lava effusion. Volcano—tectonic earthquakes occurred mainly at the very beginning and after the termination of lava effusion. Explosion earthquakes occurred frequently during the period of the maximum rate in lava effusion. The remainder of the seismic signals were associated with pyroclastic flows and rockfalls from the lava dome. These latter signals increased sharply in number at the onset of lava effusion. The rate of occurrence remained high when the lava discharge rate decreased but gradually decreased after the termination of lava effusion. Maximum daily durations of seismic signals are proportional to the daily volumetric output of lava, indicating the dependence of the number of pyroclastic flows on the rate of lava output. A log-log plot of seismic signal duration vs. number of events with this duration displays a linear relationship. The short-period seismic signals can be divided into three categories based on duration: short events with durations less than 100 s; intermediate events with durations between 100 and 250 s; and long events with durations longer than 250 s. We infer that long events correspond to pyroclastic flows with mean deposit volume 2×10⁵ m³, and intermediate events represent pyroclastic flows with mean deposit volume 1×10³ m³.Editorial responsibility: J McPhie     

The 1919–1920 eruption of Mauna Iki,Kilauea: chronology,geologic mapping,and magma transport mechanisms

Utilizing historical accounts, field mapping, and photogeology, this paper presents a chronology of, and an analysis of magma transport during, the December 1919 to August 1920 satellitic shield eruption of Mauna Iki on the SW rift zone of Kilauea Volcano, Hawaii. The eruption can be divided into four stages based on the nature of the eruptive activity. Stage 1 consisted of the shallow injection of a dike from the summit region to the eventual eruption site 10 km downrift. During stage 2, a low ridge of pahoehoe formed in the vent area; later a large a'a flow broke out of this ridge and flowed 8.5 km SW at an average flow front velocity of 0.5 km/day. The eruption continued until mid-August producing almost exclusively pahoehoe, first as gas-rich overflows from a lava pond (stage 3), and later as denser tube-fed lava (stage 4) that reached almost 8 km from the vent at an average flow-front velocity of 0.1 km/day. Magma transport during the Mauna Iki eruption is examined using three criteria: (1) eruption characteristics and volumetric flow rates; (2) changes in the surface height of the Halemaumau lava lake; and (3) tilt measurements made at the summit of Kilauea. We find good correlation between Halemaumau lake activity and the eruptive stages. Additionally, the E-W component of summit tilt tended to mimic the lake activity. The N-S component, however, did not. Multiple storage zones in the shallow summit region probably accounted for the decoupling of E-W and N-S tilt components. Analysis of these criteria shows that at different times during the eruption, magma was either emplaced into the volcano without eruption, hydraulically drained from Halemaumau to Mauna Iki, or fed at steady-state conditions from summit storage to Mauna Iki. Volume calculations indicate that the supply rate to Kilauea during the eruption was around 3 m³/s, similar to that calculated during the Mauna Ulu and Kupaianaha shield-building eruptions, and consistent with previously determined values of long-term supply to Kilauea.     

The December 4, 1983 to February 18, 1984 eruption of Piton de la Fournaise (La Reunion, Indian Ocean): Description and interpretation

On December 4, 1983 an eruption started at vents located 1.5 km southwest of the summit of Piton de la Fournaise at the base of the central cone. After 31 months of quiescence this was one of the longest repose period in the last fifty years. The eruption had two phases: December 4 to January 18 and January 18 to February 18. Phase 1 produced about 8 × 10⁶ m³ of lava and Phase II about 9 × 10⁶ m³. The erupted lava is an aphyric basalt whose mineralogical and geochemical composition is close to that of other lavas emitted since 1977.The precursors of the December 4 outbreak were limited to two-week shallow (1.5–3 km) seismic crisis of fewer than 50 events. No long-term increase was noted in the local seismicity which is very quiet during repose periods and no long-term ground inflation preceded the eruption. Outbreaks of Phases I and II were preceded by short (2.5 hours and 1.5 hours) seismic swarms corresponding to the rise of magma toward the surface from a shallow reservoir. Large ground deformation explained by the emplacement of the shallow intrusions, was recorded during the seismic swarms. A summit inflation was observed in early January, before the phase II outbreak, while the phase I eruption was still continuing.Piton de la Fournaise volcanological observatory was installed in 1980. Seismic and ground deformation data now available for a period of 4 years including the 1981 and the 1983–1984 eruptions, allow us to describe the physical behavior of the volcano during this period. These observations lead us to propose that the magma transfer from deep levels to the shallow magma reservoir is not a continuous process but a periodic one and that the shallow magma reservoir was not resupplied before the 1981 and 1983–1984 eruptions. Considerations on the eruptive history and the composition of recent lavas indicate that the reservoir was refilled in 1977.     

The 1991 eruption of Hekla,Iceland

The eruption that started in the Hekla volcano in South Iceland on 17 January 1991, and came to an end on 11 March, produced mainly andesitic lava. This lava covers 23 km² and has an estimated volume of 0.15 km³. This is the third eruption in only 20 years, whereas the average repose period since 1104 is 55 years. Earthquakes, as well as a strain pulse recorded by borehole strainmeters, occurred less than half an hour before the start of the eruption. The initial plinian phase was very short-lived, producing a total of only 0.02 km³ of tephra. The eruption cloud attained 11.5 km in height in only 10 min, but it became detached from the volcano a few hours later. Several fissures were active during the first day of the eruption, including a part of the summit fissure. By the second day, however, the activity was already essentially limited to that segment of the principal fissure where the main crater subsequently formed. The average effusion rate during the first two days of the eruption was about 800 m³ s^–1. After this peak, the effusion rate declined rapidly to 10–20 m³ s^–1, then more slowly to 1 m³ s^–1, and remained at 1–12 m³ s^–1 until the end of the eruption. Site observations near the main crater suggest that the intensity of the volcanic tremor varied directly with the force of the eruption. A notable rise in the fluorine concentration of riverwater in the vicinity of the eruptive fissures occurred on the 5th day of the eruption, but it levelled off on the 6th day and then remained essentially constant. The volume and initial silica content of the lava and tephra, the explosivity and effusion rate during the earliest stage of the eruption, as well as the magnitude attained by the associated earthquakes, support earlier suggestions that these parameters are positively related to the length of the preceeding repose period. The chemical difference between the eruptive material of Hekla itself and the lavas erupted in its vicinity can be explained in terms of a density-stratified magma reservoir located at the bottom of the crust. We propose that the shape of this reservoir, its location at the west margin of a propagating rift, and its association with a crustal weakness, all contribute to the high eruption frequency of Hekla.     

Genetic aspects of the Jan Mayen fissure volcano group on the mid-oceanic submarine Mohns Ridge,Norwegian Sea

During six recent expeditions, of which four were led by the author, to the mainly basaltic island of Jan Mayen (length 53.6 km; mean width 7 km; area 380 km²), evidence has been gathered for at least six distinct volcanic phases, coupled with rythmic magmatic variations in the oceanite-trachybasalt-trachyandesite-trachyte lava suite. There are also certain intermediate types and associate pyroclasts, and effusive or explosive uprise of these lavas through two fissure-swarms, intersecting at about 12°, produced a subaerial volcano-group of several hundred cones, elongate north-east — south-west on the north-west margin of a large submarine pedestal possibly capped by a drowned plane of marine erosion at 100–200 m below present mean sea level. These rocks appear to range in age between Tertiary and Recent. Jan Mayen grows from the north-west flank of the submarine Mohns Ridge close to its axial rift within a markedly seismic zone, at a likely junction of crustal fractures immediately north of a sharp east-west flexure in the rift which may indicate a major strike-slip fault. The lavas have affinities with corresponding lavas in Scottish Hebrides and with the basalt-trachyte associations on the islands of Ascension, St Helena, Tristan da Cunha and Gough on the mid-Atlantic Ridge. Both form and structure suggest the island mass has the configuration of a volcanic dome (or possibly two coalescent domes diverging slightly south-west) at least 70×30 km in area and about 1.5 km in height. In the mass are two distinct major volcanic foci: an earlier South Jan or Rudolftoppen « dispersed » or « plexiform » vent, ascribable to numerous «drilled out» fissure-intersections within an area of more than 25 km², and a later North Jan or Beerenberg central vent. A third focus of indeterminate relative age may lie beneath Straumflaket, in the shallow sea off South Cape. Magmas rose through individual fissures and their intersections, to form linear cones of tuff and lava, and extensive basalt floods. Most are vertical dikes but, in places, highly inclined sheets and sills tend to follow bedding and other planes of weakness in tuff and sometimes fed lava flows. Basaltic magma invaded a complex system of intersecting master fissures and subsidiary fractures in tuff near the surface, inflated the mass, distorted and generated local joint systems in the tuff and finally gave rise to meshworks of basaltic sheets in it. Following a long period of repeated fissure eruption, ten of the main basaltic throats at the South Jan dispersed focus, and one near the junction between North Jan and South Jan, were plugged by trachyte, after which there was volcanic quiescence with contemporaneous deep glacial, fluvial and marine erosion. During the subsequent resumption of volcanic activity the North Jan focus of central eruption rose to importance at the expense of the South Jan focus, which remained sealed by trachyte, but numerous small basaltic fissure volcanoes erupted on the seaward edges of the South Jan plateau and through the coastal platform beneath its cliffs, at or near sea level.     

Voluminous lava flows at Oldoinyo Lengai in 2006: chronology of events and insights into the shallow magmatic system

The largest natrocarbonatite lava flow eruption ever documented at Oldoinyo Lengai, NW Tanzania, occurred from March 25 to April 5, 2006, in two main phases. It was associated with hornito collapse, rapid extrusion of lava covering a third of the crater and emplacement of a 3-km long compound rubbly pahoehoe to blocky aa-like flow on the W flank. The eruption was followed by rapid enlargement of a pit crater. The erupted natrocarbonatite lava has high silica content (3% SiO₂). The eruption chronology is reconstructed from eyewitness and news media reports and Moderate Resolution Imaging Spectroradiometer (MODIS) satellite data, which provide the most reliable evidence to constrain the eruption’s onset and variations in activity. The eruption products were mapped in the field and the total erupted lava volume estimated at 9.2 ± 3.0 × 10⁵ m³. The event chronology and field evidence are consistent with vent construct instability causing magma mixing and rapid extrusion from shallow reservoirs. It provides new insights into and highlights the evolution of the shallow magmatic system at this unique natrocarbonatite volcano.     

Seismic activity related to the 1991 eruption of Colima Volcano,Mexico

Ten years after the last effusive eruption and at least 15 years of seismic quiescence, volcanic seismic activity started at Colima volcano on 14 February 1991, with a seismic crisis which reached counts of more than 100 per day and showed a diversity of earthquake types. Four other distinct seismic crises followed, before a mild effusive eruption in April 1991. The second crisis preceded the extrusion of an andesitic scoriaceous lava lobe, first reported on 1 March; during this crisis an interesting temporary concentration of seismic foci below the crater was observed shortly before the extrusion was detected. The third crisis was constituted by shallow seismicity, featuring possible mild degassing explosion-induced activity in the form of hiccups (episodes of simple wavelets that repeat with diminishing amplitude), and accompanied by increased fumarolic activity. The growth of the new lava dome was accompanied by changing seismicity. On 16 April during the fifth crisis which consisted of some relatively large, shallow, volcanic earthquakes and numerous avalanches of older dome material, part of the newly extruded dome, which had grown towards the edge of the old dome, collapsed, producing the largest avalanches and ash flows. Afterwards, block lava began to flow slowly along the SW flank of the volcano, generating frequent small incandescent avalanches. The seismicity associated with the stages of this eruptive activity shows some interesting features: most earthquake foci were located north of the summit, some of them relatively deep (7–11 km below the summit level), underneath the saddle between the Colima and the older Nevado volcanoes. An apparently seismic quiet region appears between 4 and 7 km below the summit level. In June, harmonic tremors were detected for the first time, but no changes in the eruptive activity could be correlated with them. After June, the seismicity decreasing trend was established, and the effusive activity stopped on September 1991.     

The Hekla eruption 1980–1981

The sixteenth eruption of Hekla since 1104 began on August 17th, 1980, after the shortest repose period on record, only ten years. The eruption started with a plinian phase and simultaneously lava issued at high rate from a fissure that runs along the Hekla volcanic ridge. The production rate declined rapidly after the first day and the eruption stopped on August 20th. A total of 120 million m³ of lava and about 60 million m³ of airborne tephra were produced during this phase of the activity. In the following seven months steam emissions were observed on the volcano. Activity was renewed on April 9th 1981, and during the following week additional 30 million m³ of lava flowed from a summit crater and crater rows on the north slope. The lavas and tephra are of uniform intermediate chemical composition similar to that of earlier Hekla lavas. Although the repose time was short the eruptions fit well into the behaviour pattern of earlier eruptions. Distance changes in a geodimeter network established after the eruptions are interpreted as due to inflation of magma reservoirs at 7–8 kilometers depth.     

Preliminary report on the activity at Unzen Volcano (Japan), November 1990-November 1991: Dacite lava domes and pyroclastic flows

The eruption of Unzen Volcano commenced on 17 November 1990. Phreatic and phreatomagmatic eruptions occurred by early May 1991. No large-scale explosive eruptions preceded the extrusion of lava domes. Lava domes appeared in a summit crater on 20 May 1991, and they grew on the steep slope of Mt. Fugen at Unzen Volcano. Rockfalls from the margins of the domes frequently generated pyroclastic flows. Major pyroclastic flows occurred on 3 June, 8 June, and 15 September 1991. The 3 June pyroclastic flow killed forty-three persons. Many of the pyroclastic flows seem to have resulted from the simple rockfalls, except one flow on 8 June, which was accompanied by an explosion from the crater. Many of the rockfalls that generated pyroclastic flows were witnessed. As of November 1991. Unzen Volcano was still active with a nearly constant magma-supply rate of about 0.3 × 10⁶ m³/d. The total magma output exceeded 45 × 10⁶ m³ by the beginning of November 1991. The volume of the lava domes is more than 23 × 10⁶ m³.     

The 1963–65 eruption of Lopevi Volcano (New Hebrides)

The eruption commenced on July 7th 1963 with activity at the summit crater which had been dormant for at least 50 years. Production of lava spatte r characterised the opening stages of the eruption, and although hot lava blocks avalanched down the north-eastern slope no flows were produced. In August a crater opened at a height of approximately 1,000 metres at the head of a north-west trending fissure, the site of the 1960 eruption. Intermittent lava fountaining up to a height of 600 feet took place at the crater which was active throughout the remainder of the eruption, and viscous steep-sided tongues of «aa» lava flowed from it. A new east-west trending fissure 200 feet deep and 400 feet wide opened in September at a height of approximately 240 metres and extended up the slope to a point approximately 660 metres above sea level. From this fissure lavas of more fluid character though identical in mineral composition to tongues issuing from the flank crater flowed into the sea until mid November when activity at the fissure ceased. Whilst the fissure was active gas issued from a vent located immediately beyond it’s uper end. The slopes above the anchorage at Tematu were the site of subsidiary activity. Four small fissures opened at heights of up to 180 metres above sea level from mid-October to February 1964 producing short tongues of «aa» lava which flowed into the water. Emission of small ash clouds at sporadic intervals was noted at a crater situated in the highest fissure during a visit in December, 1963. There was a change from activity of «Strombolian type» with associated production of lava flows at the flank crater from November 1963 when the proportion of ash emitted increased. Ash emission became the predominant type of activity throughout the remainder of the eruption. Although the interval between successive outbursts lengthened progressively during 1964 the activity reached a climax on April 8th when the ash column attained a height of 30,000 feet, the maximum recorded during the course of the eruption. There was also an increase in July culminating in the production of a dense ash cloud 15 miles in diameter on the 26th. The activity entered a new phase in July 1964 when fissures producing lava tongues opened not only on the northern slopes but on the east side of the volcano as well. Activity continued on the opposite side to the north-west quadrant in which it had previously been localised when a fissure with a small crater at it’s head appeared in September on the south-east slopes a few hundred metres above sea level. The infrequency of outbursts during 1965 suggests that the present cycle of activity is waning, and that the volcano will soon become quiescent once more. Structures of interest in the lava flows include channels and tunnels. Hypersthene andesite was produced simultaneously with tholeiitic olivine bearing basalt during the opening stages of the eruption although the lavas produced later were all of the latter type. It is suggested that the hypersthene andesite was formed by magmatic differentiation of an olivine-bearing basalt parent magma, the lighter more acid fraction being tapped first at the beginning of the eruption. Such differentiation could account for similar basalt-andesite associations in older volcanic sequences within the central area.     

Seismological studies of the soufriere of St. Vincent, 1953-79: Implications for volcanic surveillance in the lesser antilles

The Soufriere of St. Vincent has been monitored for more than 25 years as part of a regional programme in the Lesser Antilles. In that time the volcano has erupted twice but our studies have shown no discernible change in regional seismicity before either event. However, very small seismic events were observed in the crater during the 1971–1972 eruption and were detected before the start of the 1979 explosive eruption; we believe that they were generated by thermally induced hydraulic fracturing within the lava mass inside the crater lake. We conclude that seismographic monitoring of Lesser Antillean volcanoes can give ambiguous results but that at least one instrument must be placed within 1 km of the vent if the earliest signs of activity are to be detected.     

A comparison of the recent eruptions of Mt. Pelée,Martinique and Soufrière,St. Vincent

The simultaneous eruption of Mt. Pelée, Martinique and Soufrière, St. Vincent are regarded as the first recognized examples of Pelean-type and St. Vincent-type pyroclastic eruptions. Both produced nuées ardentes, the former usually laterally directed because of the presence of a dome and the latter vertically directed from an open crater. Both volcanoes have subsequently erupted for a second time this century. The 1902–05 and 1929–32 eruptions of Mt. Pelée produced andesite lava of almost identical composition and mineralogy. Both contain two generations of plagioclase, orthopyroxene, Fe-Ti oxide, corroded brown amphibole and olivine rimmed by pyroxene. In contrast, the Soufrière material is more basic in composition varying from basaltic andesite to basalt in 1902–03 and basaltic andesite in 1971–72. The Soufrière material contains two generations of plagioclase (with those of 1971–72 having additional zones of labradorite), clinopyroxene, orthopyroxene, olivine and Fe-Ti oxide. The pyroclastic deposits are strikingly different, those from the Pelean-type eruption are termed «block and ash deposits» being characterised by poorly vesicular lava blocks up to 7 m in diameter, while the St. Vincent-type eruption produced «scoria and ash deposits» containing vesicular ropey blocks or bombs no larger than 1 m in diameter. The differences in styles of eruption are attributed to differences in viscosity and mechanism of eruption of the magmas. Stratigraphic studies of Mt. Pelée reveal that the volcano has produced basaltic andesite scoria and ash deposits from St. Vincent-type eruptions. It is concluded that the recent eruptions of Pelée tapped a deep level magma during both eruptions releasing magma of similar composition, while the 1971 Soufrière magma is thought to be a remnant of the 1903 basaltic magma which remained at a high level within the volcano where it underwent enrichment in plagioclase and loss of olivine and oxide.