The 1950 eruption of Santorini volcano (Nea Kameni)
It followed 8 1/2 years after the previous eruption that ended with the formation of the Niki lava dome in 1941, and occurred very close to it.
Georgalas suggests that the 1950 eruption represents the effusion of a rest amount of magma from the same chamber than the 1939-41 eruption, which was prevented from erupting then, because the lavas had cooled too quickly and had temporarily blocked the vents, until 8 and a half year later, gas pressure was large enough to un-block the way. When this happened, the same fractures were used as pathways.
Precursors of the eruptionEarthquakes were noted by inhabitants of the island on 15 December 1949, and in the days preceding and during the eruption on 10, 11, 13 and 16 January 1950.
At that time, there was no seismic monitoring, so only quakes of magnitude significantly above 3 have been recorded, but there could have been many more small ones.
The eruption starts: 10 Jan 1950The first explosion took place at 01h00 on 10 January 1950, accompanied by weak noises and a felt quake. 11 other weak explosions could be observed until 8h35. The activity originated from the eastern flank of the Georgios cone, about 175 m away from its summit.
Until noon, 7 more weak explosions and one stronger one were noted, the latter produced a loud nois and a dense column of sandy ash. Between 13h50 until 16h50, 6 weak explosions took place with white-gray clouds, and 3 larger explosions that erupted dark gray clouds of ash.
Georgalas, who reports the events during this eruption, notes that the normally weak fumaroles in other parts of Nea Kameni suddenly were much more intense since the eruption started.
Residents in Fira noted lightnings and small flames at night in the ash cloud erupted from the eruption site.
Second day of the eruption (11 Jan 1950)Explosions increased and occurred all day long. Most of the produced mainly steam clouds, the more violent ones gray ash plumes, some lasting up to 2 minutes. Lightnings were again visible at night.
Between 11h00 and 15h00, Georgalas counted 17 eruptions.
At night, the eruption intensified, and amoderate quake (probably 3-4 magnitude) was felt by all inhabitants. 5 minutes later, a violent explosion occurred.
Day 3-5 of the eruptionOn 12 January, the eruption continues in the same way as the previous day. At the vent on the eastern flank of the Georgios cone, small to moderate explosions are near-continuous.
13 January: a light quake was felt by some in Fira. It was follewed by a large explosion that produced a "very large" ash cloud. From 11am to 3pm, 17 explosions occur.
14 January sees a slight increase in explosivity. Blue flames can be seen at night from Fira. Incandescent bombs are seen being ejected to 150-200 height and reaching about the same horizontal distance.
Continous steaming from the vent. It is noted that this degassing is completely silent. 17 explosions counted between 11 am and 3 pm.
13-14 Jan: a new vent opens
15 Jan: paroxysmal eruptionAt 04h30 on 15 Jan, a powerful (paroxysmal) eruption occurred, producing an ash column of 1000 m height, and ejecting incandescent bombs up to 850 distance, easily visible from Fira.
Georgalas counted 8 explosions between 12h35 and 13h55. Some of them produced gray ash clouds that reached 250-300 m height, and small ballistic projectiles.
At night, incandescence was again visible from Fira.
On 16 Jan, a magnitude 3 quake is felt by some in Fira at 4 am, and is followed by a strong explosion.
28 weak explosions were counted between 8h55 and 17h55. It is noted that most of these explosions were steam and contained little ash, while some were gray and had a cauliflower shape (typical for weak phreatic activity), and were mixed with smaller and larger solid ejecta.
Ballistic ejecta were reaching 150-200 m height and landed up to 200 m away from the vent. Many of the ash-explosions were preceded immediately by steam explosions (Georgalas calls it "partial explosions").
Sulfur smellGeorgalas notes the smell of sulfur when he was on Nea Kameni under the plume. Between single explosions, there was complete silence and absence of activity.
A low lava dome of viscous blocky lava had formed at the second vent SE of the Georgios cone. It reached a longest diameter of 195 m and had its highest point at 108 m above sea level. It was described as looking like a shield of large angular blocks, cut by numerous irregular fractures. Between these fractures, the explosions took place.
The volume of lava erupted was very small. On the southern end, a weak flow of the blocky lava was observed. At night, incandescence within cracks of this lava dome was visible from Fira.
Fumaroles had appeared at the north side of the new lava dome. They were producing mostly steam and some sulfur dioxide were found in up to 1000 m distance to the north. The temperatures measured at them was between 50 and 105 deg C.
17-18 Jan: weak explosions22 explosions were counted between 9h45 and 15h25 on 17 Jan, and a smell of H2S is noted in addition to SO2. The lava dome increases visibly in size. Small orange-red flames seen at night from Fira.
18 Jan: 20 explosions in 5 hours betwene 7h45 to 12h45, clouds of 200-250 m height. 4 weak, 5 strong and 2 very strong explosions between 16h40 and 21h00, with plumes reaching 300 m height, and ejecta reaching 100 m height and 100-150 m distance.
19 Jan: 10 explosions between 17h15 until 20h00 (4 weak, 3 strong and 3 very strong ones). 14 explosions between 9h30 until 15h34. Slight growth of the lava dome observed.
No significant changes on 20-21 Jan: explosions roughly every 30 minutes, flames visible at night, and sometimes bluish gas (SO2). SO2 can be smelled at Fira.
22 Jan: explosions are becoming less frequent and weaker, producing cluods of steam and little ash reaching 50-80 m height.
23-31 Jan: eruption wanesLesser eruptions than the previous days, and only weak steam plumes are observed. In a 3 hours interval on 23 Jan, only 2 weak explosions are observed.
1-2 Feb: the eruption ends4-5 explosions occur on 1 Feb, and 4 during the night to 2 Feb. On 2 February, all explosive and effusive activity stops.
Observations on the eruptionGeorgalas writes about the explosive phase of the eruption (mainly during the 4 first days) that in comparison with the explosive phases of the previous eruptions at the Nautilus come (1928) and the Triton dome (1939), that:
a) the 1950 eruption exceeded the mentioned earlier eruptions, and
b) its duration was larger than the 1939 eruption of the Triton lava dome.
Note: This should be surprising, as commonly the 1950 eruption is described as a minor event in comparison, but the above is from eye-witness of a trained observer.
Comment on the effusive phase of the eruption by Georgalas:
The gas pressure penetrated the overlaying strata at 2 vent locations, "opening the way" for magma to erupt. Out of the 2 vents, the lower and second vent "conentrated the gas pressure", and as a result, the earlier (higher) vent quickly "fell into a solfataric stage".
Due to the inclination of the ground and the small volume, the lavas erupted did not form a typical steep dome, but rather a flat, cake-like carpet, reaching only 9-13 m height, and is coalescent with its lava flow that descended into the valley between the Georgios cone and the 1941 Niki dome, and partly climbed the western slope of the latter. 2 small tongues of lava spread also from the N and S side of the new dome. The total surface of the new lavas was 7312 square meters.
No summit depression (crater) formed on top of the dome.
Fumaroles at the lava dome:
At the western end of the come, low-temperature fumaroles of 50-60 deg C were present, emitting mostly steam. At the NW side on the summit of the lava dome (Georgalas climbed it soon after it had stopped erupting) fumarole temperatures were 190-210 deg C and contained minor quantities of SO2 and HCl.
Observations on the location of the ventThe site of the eruption was no coincidence, but tied to the volcano-tectonic situation of the area. It was located on a zone of weakness, faults cutting through the strata overlying the magma chamber. These existed already since the 1925 explosions at the Dafni craters, and are related to the ferruginous hot sources commonly called Kokkina Nera ("red waters"), that were abundant until 1925 in the bay of the same name (which was then buried by the 1925 lava flows, but continue to day in many locations around the island).
4 groups of fissures had been created after the eruption in 1925 and 1939:
1) parallel (in NE-SW direction) to the 1925 lava flows
2) perpendicular to these flows, from the explosion crater of Dafni until the Nautilus dome
3) diagonal in N-S direction. An extension of this fracture cuts through the 1866-1870 flow field and the eastern part of the lava plateau below Georgios cone. These fractures developed during the activity between 1939-41. They define the locations of the fumaroles on the eastern side of the Georgios crater and the north end of the 1950 lava dome.
4) The fissures of Nautilus ("Nautilusspaltensystem") in an average NE-SW direction.
The latter fissures were almost exactly pointing to the site of the new vents in 1950, Georgalas concludes.
The above is a summarizing (and slightly commented) translation of the original work by G.C. Georgalas, who visited the island during and after the eruption, and published a detailed account in the Bulletin of Volcanolgy in 1953:
G. Georgalas (1953) "L' éruption du volcan de Santorin en 1950", Bull. Volcanolgy, v 13, pp. 39-55