Lava lake activity in two summit craters - Mbwelesu and Benbow.
A large amount of gas was emitted from Ambrym volcano from the beginning of October until December 2009. Volcanic gas measurements 17th December 2009 show that there were 15,000 tonnes of sulphur dioxide emitted. During an expedition to Ambrym volcano by John Seach from 5-14 December 2009, there were large amounts of gases visible in the craters of Marum, Benbow, Niri Mbwelesu, and Mbogon Niri Mbwelesu. A decent into Benbow crater by John Seach on 11th December showed lava visible in one vent, and also in Mbwelesu crater on the eastern side of Marum the following day. On 6th October 2009 a MODIS satellite image showed volcanic fog (vog) extending over 800 km NW of Ambrym volcano into the Coral Sea. On 4th December 2009 John Seach observed vog over the island of Espiritu Santo in northern Vanuatu, 120 km NW of Ambrym Island, while on an Air Vanuatu flight. Acid rain is a potential hazard for communities living on Ambrym island.
Beginning December 2008, there has been increased emission of volcanic gases particularly sulphur dioxide causing acid rain over surrounding areas. Acid rain and ash fall threaten water safety as well as crops and vegetation. More than 9000 people from 40 villages in Ambrym have been affected. Currently, there is no monitoring station on the island and reports are only based from Ambrym communities who are affected by acid rain. Almost 95% of the population is dependent on rainwater collection or underground water as an alternative source of water which has been contaminated by acid rain. The underground water was also reported to be left open and exposed to acidic rain fall. In West Ambrym there is only one water supply system in operation. Other water supply systems have stopped operations because sources have dried up, pipe systems were damaged, and have not been maintained due to limited funds.
Lava lake activity was present at Ambrym volcano in 2008.
2007. Continued activity at Ambrym volcano with lava in Mbwelesu crater.
2005. Strong ash emissions damage gardens and burn inhabitants of Ambrym. Wednesday 20th April 2005 Ambrym volcano has produced the strongest point source of sulphur dioxide on the planet for the first months of 2005. The volcano has been erupting in a more destructive manner than usual for the past year, and producing high sulfur dioxide emissions for at least six months. During the first ten days of March 2005, data collected by the Ozone Monitoring Instrument on NASA’s Aura satellite showed high concentrations of sulfur dioxide drifting northwest from the volcano. Visual observations of the volcano at the end of March by John Seach recorded a strong night glow at Benbow, and the sound of bubbling lava could be heard deep in the crater, accompanied by continuous light-brown ash emissions. The volcano still poses a hazard to the local population with ashfall and acid rain affecting food crops and drinking water. During January 2005 many residents were burnt from acid rain and required medical assistance.
2003 Ambrym Expedition by John Seach
John Seach previously reported his observations of the Ambrym caldera made during a visit in December 2002. This report contains his observations of the caldera during a 7-11 September 2003 visit and flyovers on 6 and 13 September. The level of activity during September 2003, with visible lava in six vents, was higher than that during his previous visit.
Observations of Benbow. During the 6 September flyover, two white plumes were rising 200 m above the crater rim and drifting NW. On the evening of 7 September, orange glows were seen from the caldera edge (3 km SE). A strong glow originated N of the crater and the central crater pit produced a less intense fluctuating glow. During the 13 September flyover, both pits continued to emit white and light-brown plumes to 200 m above the rim.
Observations of Mbogon Niri Mbwelesu. Large white vapor emissions from the collapse pit formed mushroom-shaped clouds on 6 September that drifted W and attained a height of 300 m. A visit to the S rim on 7 September showed a weak orange glow and copious gas emissions. On 8 September, observations from the N rim showed the pit full of swirling brown and white vapor. The NW wall was stained with yellow and red deposits, and pungent sulfurous gases were being emitted. Loud, rhythmic degassing sounds were heard every few seconds. The bottom of the pit was visible on 10 September, allowing views of two glowing red holes 150 m below the rim separated by a small wall a few meters wide. The two vents degassed simultaneously, but the E vent emitted larger amounts of brown ash.
Observations of Niri Mbwelesu. During the 6 September overflight, the pit of Niri Mbwelesu crater was filled with white vapor. The crater was climbed on 8 September and observations from the S rim showed the crater still filled with vapor; no sounds were heard. During that evening, an orange glow was observed. Excellent visibility on 10 September enabled sighting of a 10-m-diameter, crusted lava pond. Red lava was visible through surface cracks, and lava spatter rose 10 m above them at infrequent intervals.
Loud cannon-like explosions about every 20 minutes shook the ground and were accompanied by the sounds of cracking rock. During the evening, glowing projectiles were ejected into the air, although none fell outside the crater. Loud, roaring degassing noises like a jet engine at take-off were also heard. The roar would gain intensity over 30 seconds, cease for 15 seconds and then re-start. During periods of intense roaring, red lava was observed through cracks in the crusted surface.
Both types of intense degassing were accompanied by gentle emissions of brown vapor. A pit, 6 m in diameter, located N of the crusted pond in the crater wall, emitted brown ash. Fumaroles were high on the N inner crater wall. Brown ash was emitted from the S crater floor.
Observations of Mbwelesu. Mbwelesu crater was observed for 3 hours during mid-day on 8 September from a position on the SW rim. At times, the crater was filled with vapor, but observation of the lake surface was only possible about 60% of the time. The lava lake showed remarkable similarities in location, size, and dynamics compared to December 2002. The 50-m-diameter lava lake was contained inside a circular funnel-shaped pit 100-120 m in diameter. Violent agitation of the surface occurred most of the time. Lava splashed onto the pit walls and drained back vertically 25 m into the pit.
Large 10-m-diameter gas bubbles burst in the SE half of the lava lake with up to eight bubbles visible at the same time. Jets of lava were ejected every few seconds, created by wave intersections from the bursting bubbles. During periods of low activity, lasting tens of seconds, lava drained back into the middle of the pit. Surface crusting occurred after as little as one minute during quiet periods. Subsequently, the crust was broken up by a resumption of degassing from the SW side of the pit. On several occasions, up to 80% of the lava lake surface was covered by darker crust.
Acid rain was experienced on the edge of the crater and observers felt minor burning on the face. White, light-brown, and blue-tinged vapors smelling of sulfur were emitted from the crater.
Mbwelesu was scaled again on 10 September and observations of the lava lake were made over eight hours. The crater was clear, enabling detailed observations. At times 80% of the lake surface was deformed by bubbling. The SE portion of the pit contained the most degassing. Violent explosions regularly sprayed orange lava mixed with black crust in all directions. At one point the whole lake surface rotated clockwise and lava drained back into the middle of the pit. This whirlpool was followed by an avalanche on the W side of the pit that threw black material into the lake. A second pit with a diameter of 75 m NE of the lava lake was separated by an unstable 10-m-wide wall from which numerous avalanches occurred during the day; red lava spatter was ejected once.
An afternoon flyover on 13 September enabled excellent views of the active lava lake. The smaller pit NE of the lava lake contained a small lava pond with a diameter of ~ 8-10 m.
Observations of Marum. Two areas of fumarolic activity were seen at the edge of the 1953 crater (between Marum and Mbwelesu). Brown ash was being emitted from the ground at these locations.
2002 Ambrym Expedition by John Seach
Observations of Ambrym were made by John Seach during a climb to the caldera during 11-15 December 2002. Lava lakes were visible in both Mbwelesu and Benbow craters that had been absent during a visit in February 2000 (BGVN 25:02) . Reports from local guides indicated that two lava lakes appeared in Mbwelesu crater during February 2001 and joined to form a single lava lake in August 2001. A lava lake reappeared in Benbow crater during June 2002. During November 2002 acid rain, for the third consecutive year, destroyed the mango crops between Sanesup and Lalinda on the W coast of Ambrym.
Activity at Mbwelesu Crater, 12 December 2002. Perfect visibility into the crater enabled detailed observations of the lava lake over 5 hours from the S side of the crater at an elevation of 950 m and over 300 m above the lava lake. The lava lake, located at the bottom of Mbwelesu Crater inside a circular pit (figures 6 and 7), had a diameter of 40-50 m, was in constant motion, and made continuous loud crashing sounds like waves at the beach. The lava lake was much more active than during previous visits in 1998 and 1999. Pele's hair littered the observation area, and white lithic blocks up to 30 cm in diameter were scattered on the rim.
The surface of the lava lake was continuously disrupted by degassing. Bubbles caused the lake surface to blister and finally burst, splashing lava into the air. Up to eight large bubbles formed at any one time and covered over 80% of the lake surface. The cycle of bubble formation and rupture took about 3 seconds. Waves up to 10 m high formed due to the degassing and crashed onto the side of the pit. After lava waves hit the side of the pit there was a drain-back of lava into the main lake much like ocean waves receding off a beach. Jets of lava were regularly expelled from the lake surface and directed both vertically and at an angle towards the pit side. Fountains reached up to 40 m high. Blobs of molten lava spattered onto the side of the pit up to 20 m from the lava lake edge. This spatter was more erratic than lava fountains and sprayed over a greater area. When large amounts of lava were thrown onto the pit wall, some would cascade back into the lake via a lava stream, lava fall, or a wide curtain of orange flowing lava.
Crusting of the surface was observed when parts of the lake had a lower level of activity, most often in the NE part of the pit opposite the area of most vigorous degassing. Sometimes a lava fountain would burst through the crust, throwing darker pieces of lava high into the air. At times the orange lava lake surface was covered with black pieces of broken crust. Crusting lasted for only a few minutes at a time before it was disrupted by fountains or waves. Lava disappeared into the lava lake surface by subducting under layers of other lava. Some lava disappeared into overhangs on the side of the pit. Lava lake activity continued out of view for an unknown distance past these overhangs.
The lava lake level rose and fell over a period of less than an hour in response to changes in the surface degassing rate. When the rate of degassing was high the lake level was raised by 10 m. The changes appeared to be caused by inflation of the lake due to gas rather than any change in lava eruption rate. During a period of low lava lake activity, the whole lake surface tilted 5 m towards the N and then back to the S over a two-second period. Violent intra-crater winds were observed around the lava lake as reflected in their effects on gas emissions. These were also felt beside the lava lake in Benbow crater. Vapors emitted from the lake surface were white tinged with blue.
Two 15-m-diameter vents 100 m N of the lava lake and 60 m higher were separated by a thin wall. The W vent did not show any activity. The E vent made almost continuous loud degassing noises, and larger explosions ejected black ash 50 m into the air. Mbwelesu was approached again on 15 December, but rain the previous day and low clouds had filled the crater with white vapor, allowing only brief views of the still constantly active lava lake.
Activity at Mbogon Niri Mbwelesu, 12 December 2002. This small collapse pit has been re-named (formerly Niri Mbwelesu Taten) after a request by local residents. The new name comes from the local Port Vato language of W Ambrym, as did the previous name, but is more culturally appropriate. The translation of the new name is "mouth of the wild young pig" (Mbogon = mouth, Niri = son, Mbwelesu = wild pig).
On 12 December excellent visibility enabled detailed observations into Mbogon Niri Mbwelesu. Observations were made from the N side of the pit. Loud crashing, degassing sounds were heard inside the pit, and a 10-m-diameter vent was observed on the floor about 180 m below. The pit glowed bright orange, but lava was not directly observed. This was the first time in 2002 that guides had observed the presence of lava in this pit. Loud degassing occurred every few seconds, and the larger explosions were accompanied by light brown emissions and ground shaking. Pungent sulfurous fumes were emitted from the pit, forcing the observer to use a respirator at times. Strong degassing of brown vapors was coming from the E side of the pit, 50 m below the rim. The W inside wall of the pit was coated with red and yellow deposits.
Activity at Niri Mbwelesu Crater, 12 December 2002. On 12 December excellent views were obtained into Niri Mbwelesu. A recent large landslide on the W wall of the crater had covered the previously lava-filled vent. Rockfalls were heard regularly inside the crater and degassing occurred about every 30 seconds. About every 20 minutes larger explosions were heard at the crater; some were audible over 3 km away.
Activity at Benbow Crater, 13 December 2002. Benbow was climbed from the S on 13 December. The observer free-climbed 165 m down to the floor of the first level, and then another 45 m further down to the edge of the lava lake pit in the N of the crater. Inside Benbow there were two active pits. The larger pit, in the middle of the crater, contained a crusted lava lake and two active vents. The SW vent was 25 m in diameter and was full of vapor but emitted no sounds. The NW vent was 10 m in diameter, glowed red, and loudly degassed. The N crater in Benbow contained an active lava lake. The observer climbed to the rim and was able to view the lake surface, ~50 m below, for a few seconds before retreating. The lava lake was in constant motion and lava was ejected in to the air. Violent winds (over 80 km/hour) were generated inside the pit and made observations on the edge dangerous. At times the pit was filled with white and blue-tinged vapors which made breathing difficult. The lava lake made continuous rumbling and sloshing noises. On a wall next to the lava lake pit there was dripping water with a pH of 3.5 and 700 ppm total dissolved solids.
2000 Ambrym Expedition by John Seach
John Seach investigated Ambrym caldera during a 19-28 January 2000 climb. Lava lakes had disappeared from both Benbow and Mbwelesu craters and a new vent had opened inside the previously inactive 1953 crater. A series of earthquakes were registered around Ambrym Island on 27 November 1999. The largest of these was magnitude 7.1. The earthquakes were followed by a month of reduced activity during which there were no reported observations of lava lakes. Landslides were visible in the caldera and ground cracking visible at Benbow, Mbwelesu, and Niri Mbwelesu craters.
Activity at Benbow Crater. Four vents were active inside Benbow. On 19 January a white plume tinged with blue and yellow rose 1,000 m above the crater rim. Twin plumes were visible the next day rising from the S end of the crater at 15 m/s and from the N end of the crater, where they were tinged with brown. Each time the crater was climbed from the S on 22, 23, and 24 January the pit was full of vapor and no sounds were heard. On 25 January the observers lowered themselves into Benbow using 200 m of rope. The floor of the first level was covered with fine brown ash and a shallow brown pond was present in the SW end of the crater. The inner crater was climbed and observations made from its rim. Below the observers was a ledge 120-140 m down covered with ash and containing a 10-m circular vent emitting white vapor. The main vent was 50 m farther down and 40 m in diameter. This was the vent that contained the lava lake in January 1999. No lava was observed inside this vent and it made no sound. At 1300 a large roar from the vent was followed by brown ash emission. At the NE end of the inner crater was a plume emission from an unseen vent.
The N end of Benbow crater (on the first level) contained another vent that could not be directly observed but regularly emitted light brown ash. On 26 January a loud continuous 30-second degassing heard from the N vent was followed by brown ash emission and rain of small cinders on observers at the S crater edge. From the central pit the vapor was rising at 5 m/s. During the late afternoon two visible atmospheric perturbations were observed above the main vent. The first followed a loud degassing sound and rose at 40 m/s to a height of 200 m above the vent. Rockfalls were also heard during the afternoon. During the night of 26 January twin skyglows of fluctuating intensity were visible above Benbow followed by a large brown ash emission that rose 1,400 m above the crater in 3 minutes.
Activity at Mbogon Niri Mbwelesu. On both 19 and 20 January light brown or red/brown ash was emitted from the collapse pit and rose 200-250 m. On 21 January a brown pond of water 150 m NE of the pit was bubbling from both fixed and random locations. Active fumaroles were present on a ledge 60 m down. There were large cracks on the SE side and evidence of wall collapse since August 1999. Ash fell on observers in the area N of the pit. On 23 January larger ash emissions occurred about every hour.
On 24 January the collapse pit was entered using ropes. Fumaroles on the ledge 60 m down averaged 64°C. The pit bottom was 120-140 m below the ledge covered in brown ash. Small clouds of ash were emitted occasionally from two large fissures. Bubbles of hot blue vapors, 6 m in diameter, rose past the observer. Continual degassing sounds were heard in the pit, like the sound of waves crashing on the beach. On 26 January from 0600 to 1100 dark gray ash clouds were continually being emitted from the pit. Plumes rose at 8 m/s to a height of 200 m above the pit, filling the caldera in all directions. During the afternoon the pit returned to a low level of activity. On 27 January a continuous emission of brown ash occurred all day to a height of 800 m above the pit.
Activity at Niri Mbwelesu. On 20 January white vapor tinged with blue was constantly emitted to 600 m above crater. During the evening a very intense pulsating night glow was visible. The glow would brighten (sometimes flicker), then rapidly drop to a lower level of illumination. The bright/dim cycle would repeat every 10-15 seconds. On 21 January in the afternoon degassing was heard from the crater rim and during the evening clouds were illuminated 250 m above the crater. Observers on the crater edge felt hot vapor. When the crater was climbed on the evening of 25 January a clearing of the vapor enabled the bottom to be seen 280 m down. A 40-m-diameter vent was visible emitting bright yellow burning gas, radiant heat was felt on the faces of observers, and moderate degassing was heard.
Activity at Mbwelesu. Observations were made of Mbwelesu crater on 21 January. The two lava lakes observed in August 1999 had disappeared (BGVN 24:08). A brown pond surrounded by fumaroles was in the Vent B location, with large amounts of ash and rock to the SE. The sill on the SE edge of the crater had large craters and several large sections (over 10 m) that had broken off and fallen into the crater. The fumarole field 40 m SE of the crater rim had a temperature of 72.7°C. Heavy rains caused waterfalls and rockfalls inside the crater. The crater was otherwise quiet with some vapor emissions from many fumaroles on the floor. Fumaroles were also present in the location of the former lava lake at Vent C.
Activity in the 1953 Crater. The 1953 crater contained two levels. The higher (W half) contained a brown pond. The lower (E half) had developed a deep smoking vent. This was in the location of the green pond observed in August 1999.
February 2000 Observations of Ambrym by John Seach
Ambrym was observed during an aircraft overflight on 25 February 2000. Pilot reports indicated that lava was beginning to reappear in some vents. As previously reported, the long-standing lava lakes in Benbow and Mbwelesu craters had disappeared after the 26 November 1999 earthquake. Activity increased slightly from January indicating the magma column may be rising again. Brown vapor was being emitted from Mbwelesu crater. The bottom of the crater could be seen but no lava was observed. Four craters, each ~8 m in diameter, were located on a 40 x 10 m section of the sill on the E rim of the crater. This rim had been weakened by the November 1999 earthquake and appeared ready to fall. Both vents inside Benbow crater emitted white vapor, which rose 1.5 km as one plume. The bottom of the vents could not be seen.
1999 Ambrym Expedition by John Seach
Ambrym Volcano was investigated by John Seach during a climb into the caldera 1-8 January 1999. A lava lake in Benbow cone was present during 1-3 January but was covered by deposits from an avalanche that occurred overnight 4-5 January. Fumarolic and Strombolian activity was observed at other craters. Activity at Benbow. Benbow crater was climbed from the S, after which observers lowered themselves using ropes 200 m down from the crater rim to a point where they could observe the crater interior. In the center of the crater, an active lava lake was seen 220 m below the observation point. The lava lake was ~40 m in diameter and constantly in motion. Large explosions caused lava fountains that reached 100 m high. Bombs glowed for up to one minute in daylight and radiated great heat. Bombs could be heard landing on the side of the pit where they caused glowing avalanches.
At night a strong glow from the lava lake was visible in the sky over Benbow. Elsewhere inside Benbow crater, Pele's hair covered the ground and fumaroles were active on the NE crater wall. Acid rain burned eyes and skin. Heavy rainfall caused many waterfalls to form inside the crater rim and a shallow brown pond formed on the floor of the first level. During 4-5 January violent Strombolian explosions could be heard almost hourly. Each series of explosions lasted 5-10 minutes and produced dark ash columns above the crater. At some time during these explosions an avalanche on the W side of the lava lake crater completely covered the lava lake. No night glow was visible above the crater after the night of 5 January. On 6 January Benbow crater was entered again. The wall collapse that covered the lava lake was confirmed visually. In the location of the former lava lake was a depression of rubble with two small, glowing vents nearby. The entire crater was clear of magmatic gases. Three violent Strombolian eruptions were viewed from the crater rim in the afternoon. Bombs were thrown 300 m into the air and dark ash clouds were emitted. Activity at Niri Mbwelesu Taten. This small collapse pit continuously emitted white, brown, and blue vapors. Red deposits covered the crater walls. A small amount of yellow deposits covered the S wall. Fumarole temperatures were 66 to 69°C at a point 40 m SE of the pit.
On 6-7 January numerous deep, loud degassings were heard from a distance of 4 km. Activity at Niri Mbwelesu. Pungent, sulfurous-smelling white vapor was emitted from this crater. Periods of good visibility enabled views 200 m down from the crater rim, but the bottom could not be seen. Rockfalls were heard inside the crater. Activity at Mbwelesu. Excellent visibility to the bottom of this crater enabled detailed observations of the lava lake. Night observations were also obtained. The lava lake was in constant motion and splashing lava out over the sides of the pit. The lake was at a lower level than during observations made three months earlier. Large explosions sent lava fountains up to 100 m in height and threw lava onto the sides of the pit causing glowing avalanches. During one night observation a 20 x 5 m section of the crater wall broke off and fell into the lava lake. The 60-m-wide lake radiated heat that could be felt from the viewing area 380 m away. North of the lava lake was a circular vent 20 m in diameter that glowed brilliantly from magma inside and huffed out burning gasses every 20 seconds. Foul gas, smelling of rotten fish, was emitted from the crater. South of the lava lake was an elongated vent (40 x 10 m) that spattered lava every 5-10 seconds and sent showers of glowing orange lava spray 150 m high. On the S side of Mbwelesu, fumarole temperatures averaged 43°C at 10 m from the crater edge. On the SE side, 40 m from the crater edge, fumaroles measured 57°C. On 4 January ashfall occurred on the S side of the caldera.
August 1999 Ambrym Expedition by John Seach
Observations within Ambrym's summit crater were made by John Seach during a 22-30 August 1999 climb. There are two active cones in the caldera complex, Benbow and Marum. The Marum cone has three active sub-craters: Mbwelesu, the main crater; Niri Mbwelesu, a secondary crater close to Mbwelesu's rim; and Mbogon Niri Mbwelesu, a small collapse pit to the S of Niri Mbwelesu. Seach found increased activity compared to a previous visit in January 1999, with one new vent inside the Benbow crater and a second lava lake in Mbwelesu crater. Temperature measurements were made with a hand-held digital thermometer with an 8-cm probe. All depth estimates are visual; distances were estimated in comparison to a climbing rope. A Mw 6.5 earthquake on 22 August was centered ~18.6 km NW of Ambrym volcano. Seach reported that the earthquake was felt with an intensity of MM VI with large ground movements in a NW-SE direction for 10 seconds. The earthquake caused landslides in the caldera and opened large cracks in the rim of Benbow crater. Coconut trees were knocked down on the W coast of Ambrym Island, and there was infrastructure damage in southern Pentecost Island. Fourteen aftershocks were felt during the next 8 hours. Activity at Benbow.
The crater was full of vapor when climbed on 22 August, but bubbling lava sounds were heard. A brown ash emission at 1740 was followed that night by moderate glow. Occasional crater explosions were heard between 1400 and 2100 on 26 August. Good visibility into the crater the next day showed that a new vent had opened on the N crater floor that was continually emitting light brown ash that rose 1,500 m above the crater floor (~2,300 m altitude) before being blown to the SE. Thicker ash emissions about every 5 minutes were accompanied by vigorous degassing; these emissions rose at a rate of ~13.5 m/s. From 1615 to 1625 on 27 August there were 66 discrete explosions or loud venting noises heard from the crater. At one location on the NW rim of the central crater, 200 m from the new vent, a recent ash deposit was ~50 m thick. Fumaroles were active on the inside N wall of the central crater, with yellow and red deposits at their bases.
The lava lake vent that was covered by an avalanche in January had reopened and exhibited orange glow from the pit at night; white vapor tinged with blue was emitted, and brown ash was ejected every 15-30 minutes. Both active vents glowed a brilliant orange color at night. Loud degassing was heard at 0620 on 28 August from a distance of 4 km. Activity at Mbwelesu. Excellent visibility at times during 26-28 August allowed detailed observations of the crater bottom. Continuous load roaring and crashing sounds were heard coming from the pit. Explosions shook the ground and bombs could be heard striking inside the crater. An observer on the E edge of the crater, 350 m from the lava, felt radiant heat. Scattered blocks 20-30 cm in diameter and Pele's hair covered the edge of the crater. A fumarole field 60 m SE of the crater had a measured temperature of 53°C. A brown pond was present on the floor of the Marum crater. The central floor of the 1953 crater (in Marum adjacent to Mbwelesu) contained a green pond surrounded by active fumaroles. Intense night glow was visible on the evening of 26 August.
Three vents were active within the Mbwelesu crater. Vent A, in the SE part of the crater, was ~10 m wide and ejected sprays of lava 150 m high every 5-10 seconds. Occasional lava spatter fell onto the vent wall. Lava spray was generally directed S, and formed black marks on the wall of Mbwelesu crater. Vent B, in the central part of the crater, was roughly circular and ~45 m in diameter. It was ~15 m from the rim to the lava lake. During this visit the lava lake exhibited 20-m-high waves moving across the pit. Vigorous boiling of the surface and incandescent lava fountaining to 150 m heights was observed. Some black bombs were ejected. At 1220 on 28 August a large part of the S rim of the pit fell into the lava lake, generating a large brown cloud that filled Mbwelesu crater and triggering Strombolian explosions for the next 10 minutes. Vent C within the Mbwelesu crater (figure 4) was located NW of Vent B. When seen in January 1999 this vent emitted burning gas, but it had deepened, widened, and contained a lava lake by 28 August. The elongate ~75 x 45 m pit had a steep N wall (~75 m high), but was open to the S (rim ~15 m above the lava lake).
The lava lake surface was directly visible and boiling violently, ejecting showers of dark bombs every 1-2 seconds. Lava fountaining occurred from a fixed point along the edge from the NE to the E side of the pit. Some bombs remained incandescent for 20 seconds in the daylight. Lava was splashing onto the SW walls and sides of the pit after emerging from the NW edge of the lake. Horizontal current flow was estimated at 8 m/s towards the SE shore across at least 75% of the diameter of the lake. Three simultaneous 20-m-diameter lava bubbles were seen that almost filled the entire lake surface.
Activity at Niri Mbwelesu. White sulfurous-smelling gas was emitted from the pit in late August. Mild degassing was observed from the south part of the crater, and fumaroles were active on the N crater wall.
Activity at Mbogon Niri Mbwelesu. Compared to January 1999, in late August there was an increase in activity and substantial morphological changes. On 25 August at 1045 a thick brown ash emission continued for 5 minutes, rising 300 m and then being carried 4 km NW by the wind. Similar ash emissions occurred at 10-20 minute intervals during the afternoon. The E rim was littered with new rocks. Recent cowpat bombs were present 10-20 m from the E side of the crater. Mild degassing and sloshing lava sounds were heard. Fumaroles were active in two small collapse pits on the SE side of the crater. The fumarole field 40 m SE of the main pit had a temperature measured at 67-69°C, the same as in January 1999. Views into the crater to a depth of 150 m were obtained on 28 August, but the bottom was not seen. The south inside wall was covered with yellow deposits. Red-brown ash was building a raised rim on the NW side of the pit. The pit was degassing every 1-2 seconds. Fumaroles on a sill 50 m down were venting in unison with the main pit. Bubbles of hot gas 20 m in diameter were observed rising from the vent at 30 m/s.
1998 Ambrym Expedition by John Seach
This volcano was visited by John Seach during 4-7 September 1998. At Mbogon Niri Mbwelesu, a small collapse pit, strong degassing was observed as well as yellow sulfurous deposits on the NW wall. During the night, degassing was heard from a distance of 4 km and white vapor tinged with blue was constantly emitted from the pit. Niri Mbelesu crater was constantly full of vapor resulting in poor visibility. But bubbling lava was heard and at night the clouds reflected a red glow from the crater. At Mbwelesu crater, an active elongated lava lake (~100 x 30 m) was observed. The larger explosions threw lava high into the air and onto the crater wall. To the east of the lava lake a smaller elongated vent contained lava. On the NW wall of the crater was a circular vent 20 m in diameter from which no lava was extruded. Benbow crater was climbed from the S. The sound of bubbling lava was heard but not observed, and there was a very intense night glow.
1997. At Mbwelesu crater the lava lake surface was continuously overturned by fountains which were tens of metres high.
1994. An overflight on 7th December revealed normal level of volcanic activity with lava lakes present in both Marum and Benbow craters.
1989. Intra caldera lava flow.
1988. On 27th May there was a 50 m diameter lava lake present in Mbwelesu crater, and the crater emitted dark grey clouds. Intra caldera lava flow.
1986. On 8th March a pilot reported an ash eruption to 3000 m elevation drifting 30km downwind.
1981. Marum and Benbow were active from 10th February to 18th March with the emission of small clouds. A strong eruption in May caused damage to gardens in surrounding villages.
1979. An eruption cause significant damage to vegetation and acid burns over an area of 90 sq km.
1973. Lava lake in Benbow
1972. A strong eruption commenced on 21st April. Powerful explosions were reported at Marum a few hours after a large earthquake on evening of 4th May. Between 27th July and 6th August there was a strong short-lived eruption at Ambrym. This was the strongest eruption since 1951.
Light ash fell on Craig Cove in January from an eruption at Benbow.
1966. Benbow crater showed mildly explosive activity, and ejections of ash and incandescent blocks of lava from a central vent.
1965. Volcanic activity was lower than usual during the year. An earthquake and tsunami in August, followed by an eruption of Ambrym.
1964. Marum crater exploded violently on the 3rd March with a mushroom cloud.
1962-63. In January an overflight revealed an eruption in Benbow, which contained a lava lake with fire fountaining and large mushroom clouds of ash. The eruption of December 1962 continued into 1963 with a paroxysm on 19th January 1963.
1961. Small eruption on 15th August and explosions at Benbow on 30-31 August.
1960. A moderate precursor earthquake in the vicinity of the volcano preceded the moderate eruption of 1960.
1957. Towards the end of August, the inhabitants of Ambrym heard explosions.
1955. Benbow started a new eruptive phase, which lasted for 11 months, with emission of ash, abundant scoria, and some peripheral lava flows which were not very dangerous.
1954. Aerial reconnaissance established that Benbow was in eruption while Marum was quiescent.
1953. The May 1953 eruption of Ambrym was preceded 5 months by a magnitude 6.5 earthquake.
1952. In August explosive activity resumed after nine months quiescence. Lava was present in two craters.
1950. On 6th December violent tremors preceded a major eruption of Benbow, which remained active until November 1951.
1944. Lava lake activity.
1942. In 1942 there was an eruption of Ambrym, which was preceded by a moderate earthquake in the vicinity of the volcano. A large lava flow from Benbow crater entered the sea five miles south of Ranon.
1938. The entire south-west coast of Ambrym in the throes of a gigantic volcanic outburst.
1937. The glare from the volcano illuminates the earth and sky and sea.
1935. An eruption of Ambrym was preceded by a magnitude 7.1 earthquake.
1931. Tremendous volumes of smoke and ash bursting out of the crater and ascending to a great height.
1930. A visitor to the island reported: “Mount Benbow, the volcano in Central Ambrim, was unusually active, and the volcanic outbursts continued without intermission, all the time we were there. Occasionally, the explosions succeeded each other with the rapidity of a machine gun, frightening everyone out of their wits. Day after day we were smothered with falls of ash”
1929. For two months the eruptions were of exceptional violence, going on without intermission and smothering the island beneath a mantle of ash and cinders.
1927. A small eruption sent ash SE over Paama Island.
1918. Violent eruptions reported by a missionary.
1916. In December and eruption turned day into night.
1913. Great eruption began in December. This was one of the largest eruptions in Melanesia in the past 400 years. A missionary reported "In December 1913, the age-long sleep of these extinct craters was broken, and the imprisoned giants awoke to life. From numerous thunder-throated vents the island was rent and torn by convulsive explosions. The outburst was heralded by a series of earthquake shocks which increased in frequency and severity until the solid earth reeled and tottered. The hospital buildings rocked like a ship at sea; the natives, in their picturesque mode of speech, saying that Ambrim danced...Then, from the newly-formed vents, was seen to rise, dark as the blackest London fog, a dense cloud which shot up like a pillar and spread out in all directions like a gigantic mushroom. In a short time ash and cinders began to fall, making noise like hailstones and smothering Ambrim and the adjacent islands in a thick layer of volcanic ash."
1910. Lava from the base of Marum flowed 5 miles NNW and reached the sea.
1909. Strombolian eruptions. Report from a missionary "Towards the end of February there was a succession of great outbursts. Our house shook from top to bottom, and it seemed as if a volcanic vent was going to form underneath us. In the darkness of the evening the sight was truly magnificent. The volcano played like a huge fountain with streams of molten lava instead of the jets of water. The lava was thrown to a greater height than I ever saw it before. The whole district was illuminated.”
1907. Strong georthermal activity in SE Ambrym. A missionary reported “All around are great jets of steam issuing out of the ground, and the whole scene is calculated to strike terror into the heart of a stranger”
1895. An eruption in February accompanied by earthquakes.
1894. The whole island trembled and plunged like a frightened horse. Earthquakes accompanied the eruption of Ambrym. A fissure extended the whole length of the island. A mile wide river of lava flowed to the west of the island. The eruptions of 1894 caused an uplift of half a metre of the ground around Dip Point.
1888. Violent eruption of Ambrym Volcano. Lava flowed to the SE on the SE side of the volcano.
1884. Missionary died from lung problems made worse by volcanic ash.
1886. Volcano was reported to contain two active craters and two dormant ones. The largest was eight miles in circumference with fire burning hundreds of feet below.
1883. First ascent of Ambrym by Europeans. Flame and smoke were ascending in considerable volume according to the explorers.
1876. The volcano was unusually active and nightly illuminated the horizon.
1875. Reports from a passing ship noted a huge volume of smoke arising from the craters.
1871. In 1871, the wooden sloop “Rosario” reached Ambrym Island.“The islands of Aurora, Pentecost and Ambrym seem rich and fertile, and abound with cocoa-nut trees. There is an active volcano on the latter island, from which several brilliant explosions were observed during the night, a loud rumbling noise being heard at each explosion.”
1870. Moderate activity was noted by officers of the mission ship Southern Cross.
1820. An old man related a story about a disastrous eruption, which occurred on Ambrym about 1820. Lava poured into the sea at Craig Cove.
1774. Captain Cook sailed past Ambrym on his second round the world voyage. He noted in his journal “[Ambrym] is 20 Leagues in Circuit, its Shores are low and the land rises with a gentle assent and forms a tolerable high Mountain in the Middle of the isle, from which assended great Columns of Smoak, but we were not able to determine with any degree of certainty whether it was occasioned by a Volcano or no."
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