Chaitén bulletin no. 107 (1 October 2009) 7 October 2009Posted by admin in activity reports, Chaitén, Chile, eruptions.
Tags: Chaitén, Chile, South America, volcanic activity reports, volcanic eruptions
SERNAGEOMIN bulletin no. 107 on the Chaitén eruption, covering the period 16-30 September 2009, has been published and reports the results of an overflight carried out on 29 September, running to a remarkable five pages in length. Among the interesting facts reported: the emergence of a new third lava dome, the appearance of an elongated depression in the central area of the dome complex, and a collapse event on 29 September possibly triggered by the Samoa earthquake.
The original document is available as a PDF via the informes page of the Observatorio Volcanológico de los Andes del Sur (OVDAS) website, and in a shortened version on the main SERNAGEOMIN site. Translation as follows.
TECHNICAL BULLETIN NO. 107
16-30 SEPTEMBER 2009
1. Visual monitoring
SERNAGEOMIN and ONEMI personnel, carrying out an overflight on 29 September, observed that the volcanic system continues in eruption, with a changing morphology that reflects the growth and collapse of the dome complex (figures 3 and 4). The appearance of a third dome towards the central-southwest portion is notable. The columns of gas and ash, which on occasion reach 2.0 km altitude above the dome complex, show two principal sources of gas and ash emission located in the central part and towards the east of the dome complex. The block-and-ash flows still persist, indicating that the dome complex is very active and has continued to grow. In the course of this period, the images observed through the DGAC camera located in Chaitén to the south of the volcano, show that the gas column has changed its dimensions both in diameter and height, because of the activity itself and the atmospheric conditions.
Fig. 1. Image taken by the DGAC [camera] at dawn on 28 September.
2. Seismic activity
Between 16 and 30 September the seismological monitoring network of Chaitén volcano registered the following seismicity:
Earthquake type: HB Hybrid
Average: 15 daily
Magnitude: maximum 4.2, minimum 1.5
RSAM: equal to or less than 140,000 units
The majority of the seismicity shows very similar wave forms, which suggests that they have their origin in the same source and reflect the same mechanism of origin.
3. Event of 29 September 2009
On 29 September information was received from the Los Lagos regional office of ONEMI of a ‘reactivation of Chaitén volcano’. The information provided corresponded with visual observations carried out by people within the town of Chaitén, who had reported an increase in eruptive activity from 14:00, shown by an increase in the size of the eruptive column emitted from the dome complex located within the caldera. These observations appear to correspond to the phenomenon observed in the images from the monitoring camera of the DGAC at 14:42 (fig. 2), where a small anomaly can be seen, perhaps related to a gravitational collapse of the dome towards the south-west (see more below, extent of collapse, figure 3) and subsequently an increase in the emissions of gas and ash.
Fig. 2. Images taken from the town of Chaiten on 29 September: top, column of gases beforehand; bottom, at 14:42, the box and red arrow indicating the possible collapse at that time.
The seismic data observed and processed by OVDAS-SERNAGEOMIN at that time showed a distant earthquake of magnitude 8.0, located at the island of Samoa, South Pacific, characterized by the presence of low frequencies and large amplitudes. The event, which was captured by the network of monitoring stations, apparently generated small disturbances in the uppermost parts of the volcanic system, affecting the most unstable areas of the growing complex of domes, which indicates the degree of instability which is found there. There are documented examples at national and international levels of earthquakes of considerable magnitude (greater than 7.0 on the Richter scale) generating a volcanic disturbance or an eruption.
4. Evolution of the dome complex
The overflight conducted on 29 September in a Chilean Air Force aircraft and in the company of the Regional Director of ONEMI and representatives of the regional government of Los Lagos revealed that the eruption continues, and observed notable changes in the morphology, instability and structure and in the dynamics of the construction of the dome complex, with important changes compared with observations after the collapse of 19 February of this year (fig. 3). This reflects the continuation of the growth of the dome due to the effusion of magma.
The observations indicate that in the south-western sector a third phase [‘Phase III’] of the dome complex has been constructed, which has filled the depression and ridge formed by the collapse of 19 February, and which has generated a new debris fan superimposed on that of February, Unlike the latter, which had a whitish colouring, this new fan has a greyish colouring. Also, it was verified that the central pinnacle has disappeared and a depression, elongated in a north-north-western direction, has appeared in the central section of the dome complex, to the east of Phase III of the complex, which has an even greater depression in its north-central sector with small pinnacles growing (fig. 4). It appears that this depression is not present towards the northern sector of the complex, where Phase II of the dome complex is located. The observations did not permit verification of the origin of this depression, which may represent either an internal collapse, possibly partial, relatively slow and structurally controlled, or significant explosion during the evolution of recent times.
An irregular eruption column, somewhat vigorous, has not exceeded 2.0 km altitude and essentially conformed to coming from two sectors, forming occasional whorls: one on the summit of the Phase III dome where water vapour predominates over gas and pyroclastics, and the other in the elongated depression, particularly in the central section, where pyroclastics and gases predominate over water vapour, which causes the orange-brown colour of the emissions. On that day [29 September] the dispersal plume was directed to the south-south-east. The bottom of the circular valley (the bottom of the caldera) which surrounds the dome complex has been partially filled by volcanic materials, including that highlighted as forming part of the debris fan associated with the construction of Phase III of the dome complex (fig. 3). Additionally, it was verified that there are significant obstructions in its north, north-eastern and south-eastern sectors, with a small gap in the north-western sector.
Finally, as a result of the transport of volcanic material, the delta deposited in the mouth of the Río Blanco (or Río Chaitén) has also grown significantly during the winter, extending several hundred metres from the coastline.
Fig. 3. View towards the north-east showing (A) the morphology and structure of the dome complex in February (photograph by Paul Duhart) compared to that observed (B) on the overflight of 29 September 2009 (photograph and interpretation by Jorge Muñoz), demonstrating the significant evolution of the dome complex and the surrounding deposits.
Fig. 4. View towards the north showing the new elongated depression in the central sector, with its principal ridge, its active emission centres and small active pinnacle, with part of Phase III of the dome complex, which fills the collapse scar generated on 19 February (photograph and interpretation by Jorge Muñoz).
5. Conclusions and interpretation
The eruptive activity continues with the growth of the dome complex, notable for the presence of a third [dome], which raises the latent danger of collapse because of the growth of the domes, with possible explosions and generation of block-and-ash flows, which may affect the valleys adjacent to the volcano. Moreover, the quantity of pyroclastic material from fallen volcanic rock originating from both block-and-ash flows and lateral explosions has formed important accumulations in the adjacent valleys and especially towards the valley of the Chaitén river, from which the fresh occurrence of lahars towards Chaitén during torrential rains cannot be ruled out. In consequence, given that the seismic activity remains elevated – a result of the growth of the dome complex – and the eruptive activity continues, directly confirmed by the overflight, with the possibility of the generation of block-and-ash flows in random directions which may affect the surrounding valleys including the generation of new lahars, SERNAGEOMIN suggests maintaining Volcanic Red Alert.
1 October 2009
[End of SERNAGEOMIN bulletin. Thanks to Werner Luis for letting me know about the appearance of bulletin 107.]
For all our Chaitén coverage: Chaitén « The Volcanism Blog.
Global Volcanism Program: Chaitén – summary information for Chaitén (1508-41)
SERNAGEOMIN – Servicio Nacional de Geología y Minería (Spanish)
Erupción del Volcán Chaitén – extensive coverage of the Chaitén eruption