Cumbre Vieja

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Cumbre Vieja
La palma volcano-close.jpg
Satellite photo of Caldera de Taburiente and Cumbre Nueva, La Palma, Canary Islands (south is above, north is below).
Elevation 1,945 m (6,381 ft)[1]
Location
Location La Palma, Canary Islands,  Spain
Coordinates 28°34′N 17°50′W / 28.567°N 17.833°W / 28.567; -17.833
Geology
Type Stratovolcano
Last eruption 1971[2]

Cumbre Vieja (Spanish: Old Summit) is an active though dormant volcanic ridge on the volcanic ocean island of Isla de La Palma in the Canary Islands, Spain, that erupted twice in the 20th century – in 1949 and again in 1971.

This ridge trends in an approximate north-south direction and covers the southern third of the island. Several volcanic craters are located on the summit ridge and flanks.

Volcanic history[edit]

La Palma is a volcanic ocean island. It is currently the second most volcanically active of the Canary Islands [3] Historical eruptions on the Cumbre Vieja occurred in 1470, 1585, 1646, 1677, 1712, 1949 and 1971.

1949 eruption[edit]

The eruption started on 24 June 1949 – the feast day of St John, which is why in Spanish texts the eruption is referred to as "la erupcion del Nambroque o San Juan," which in English is "The Eruption of the Nambroque or St John's volcano." During the 1949 eruption, eruptive activity was located at three vents —Duraznero, Llano del Banco, and Hoyo Negro — mild strombolian activity occurred at the Duraznero vent. Lava was erupted from the Llano del Banco vents, whilst only mild phreatomagmatic emissions occurred at the Hoyo Negro vent. Then on the 30th - the last day of eruptive activity, lava was erupted at the Duraznero fissure and vent. During the eruption on 1 and 2 July, two strong earthquakes with an estimated intensity of VIII on the Modified Mercalli Scale also occurred, the epicentre was calculated to be near Jedey. Following the earthquakes a fracture was noted and it had a length of approximately one and half kilometres. It was traceable to the Hoyo Negro and Duraznero vents, making a total length of about two and half kilometres or about 1/10 of the exposed length of the Cumbre Vieja, and parts of the western half of the Cumbre Vieja ridge had apparently moved about 1 metre sideways and 2 metres downwards towards the Atlantic Ocean.[4][5] As of 2008, the fracture is still visible and still has the same dimensions recorded in 1949.[6]

The timeline for the eruption, according to Bonelli-Rubio,[4] is as follows: The first reported seismic activity was noted on the rim of the Caldera de Taburiente on 23 July 1936, with further activity noted over the next two days. During the following years periodic seismic activity occurred, but due to the absence of monitoring equipment, the only reports are those recorded in the media. Then at about 09:00 (local time) on the 24 June 1949, the Duraznero vent opened with mildly explosive activity, venting of gases, and rocks; with eruptive activity continuing in this manner until 6 July. During this phase a strong earthquake occurred on 1 July and again on 2 July with an estimated intensity of VIII on the Modified Mercalli Scale. Visits to the summit region revealed a crack about 1.5 km (~1 mile) long which extended in a northerly direction from the Hoyo Negro (Black Hole) and south to the Duraznero vent making a total length of about 2.5  km (~1.6 mile), (this crack is the subject of research and heated debate as to whether it indicates initial failure of the western flank - or not). Later analysis placed the epicentre north of the township of Jedey. No eruptive activity occurred on 7 July. On 8 July eruptive activity commenced at the Llano del Banco vents as lava was erupted and flowed down the western flank. The vents opened progressively up the barranco (ravine), forming a series of en-echelon (diagonally side by side), vents. On 10 July the westward flow of lava from the Llano del Banco vents reached the coast at Puerto de Naos and entered the Atlantic Ocean, forming a lava delta, the velocity is estimated at ~14 metres (approximately 46 feet) per sec. On 12 July mildly explosive activity commenced at the Hoyo Negro (Black Hole) with emissions of rocks, fumes and some phreatomagmatic activity indicating that the eruption encountered ground waters. Activity at the Hoyo Negro ceased on 22 July, but continued at the Llano del Banco vents until 26 July. Only residual fumarolic activity and thermal emissions occurred until 30 July when the Duraznero vent and fissure re-activated. Lava then flowed from the Duraznero vent and fissure, filling the adjacent crater of El Fraile and created a lava lake. This subsequently overflowed and the lava flowed down the eastern flank towards the ocean. It finally stopped about 1 km (about 0.5 mile) from the ocean. Also on 30 July all eruptive activity ceased and only residual fumarolic activity continued until 4 August; thereafter there was only thermal emissions. It is estimated that approximately 60 million cubic metres of lava was erupted during the eruption.[4][7]

The earthquakes of 1 and 2 July

The process creating the earthquakes of 1 and 2 July is considered to have been driven by the pressure caused by the rising magma super-heating water trapped within the edifice of the volcano.[8] It is unlikely that the trapped waters could vapourise due to being under considerable pressure. What is postulated is that the waters were heated to a point where they could not absorb further thermal energy in the available space. Continuing heating required the water to expand further and the only way it could do so was to move the flank of the volcano. This caused the two earthquakes that were reported as occurring during the eruption along with the development of the crack.

That the entrapped (within the edifice) water did not vapourise is shown by the absence of phreatomagmatic explosions except the eruptive activity that occurred at the Hoyo Negro vent from 12 July to 22 July: steam escaping explosively from the ground is often a precursor of volcanic activity. Further evidence that vapourisation did not occur is that when Rubio Bonelli visited the rift the following day, the newly opened fissure "... Was not issuing fumes, vapour, steam, ashes, lava or other materials ..."[4] In fact at no time during or after was steam or phreatomagmatic activity reported. This reinforces the claim that the waters trapped within the edifice never vapourised, which they would do if the pressure had fallen sufficiently to allow the super-heated water to flash into steam. Only at the Hoyo Negro did any phreatomagmatic activity occur.

1971 eruption[edit]

The 1971 eruption occurred at the southern end of the Cumbre Vieja at the Teneguia vent. The eruption was mainly strombolian in style. Lava was also erupted. Seismic activity did occur before and during the 1971 eruption, but was not on the scale associated with the 1949 eruption. Residual thermal emissions continue.

Future threats[edit]

BBC's megatsunami[edit]

Satellite photo of La Palma, Canary Islands (north is in the lower right). The crater in the centre is the Caldera de Taburiente. The Cumbre Vieja is the ridge to the south (upper left) of the caldera and between them is the Cumbre Nueva.

In 1999 a paper was published which indicated that the Cumbre Vieja may be in the initial stages of failure. In the paper the authors showed that the geological development of La Palma had undergone changes due to the southerly migration of the hotspot, and the collapse of the earlier volcanoes. Subsequent to this a triple arm rift system had evolved with the eventual closing down of volcanic activity associated with two of the arms - the north-west and north-east rifts. The reasons can only be hypothesised. This caused the southern arm - the Cumbre Vieja to be the sole source of volcanic activity.[8]

In October 2000, the British Broadcasting Corporation (BBC) transmitted a "Horizon" programme called "Mega-tsunami; Wave of Destruction",[9] which suggested that a future failure of the western flank of Cumbre Vieja would cause a "mega-tsunami".

Day et al; (1999)[8] and Ward and Day (2001)[10] hypothesize that during an eruption at some unascertained future date, the western half of the Cumbre Vieja—approximately 500 km3 (5 x 1011 m3) with an estimated mass of 1.5 x 1015 kg—will catastrophically fail in a massive gravitational landslide and enter the Atlantic Ocean, generating a so-called 'mega-tsunami'. The debris will continue to travel along the ocean floor as a debris flow. Computer modelling indicates that the resulting initial wave may attain a local amplitude (height) in excess of 600 metres (2,000 ft) and an initial peak to peak height that approximates to 2 kilometres (1 mi), and travel at about 720 kilometres per hour (450 mph) (approximately the speed of a jet aircraft), inundating the African coast in about 1 hour, the southern coastlines of the British Isles in about 3.5 hours, and the eastern seaboard of North America in about 6 hours, by which time the initial wave will have subsided into a succession of smaller ones each about 30 metres (100 ft) to 60 metres (200 ft) high. These may surge to several hundred metres in height and be several kilometres apart while retaining their original speed. The models of Day et al.[8] and Ward and Day[10] suggest that the event could inundate up to 25 kilometres (16 mi) inland. If the model is correct, then this scale of inundation would greatly damage or destroy cities along the entire North American eastern seaboard e.g. Boston, New York City, Miami, etc., and many other cities located near the Atlantic coast.

Detailed geological mapping shows that the distribution and orientation of vents and feeder dykes within the volcano have shifted from a triple rift system (typical of most volcanic ocean islands) to one consisting of a single north-south rift.[3][11][12] It is hypothesised that this structural reorganisation is in response to evolving stress patterns associated with the development of a possible detachment fault under the volcano's west flank.[8][10] Siebert (1984)[13] showed that such failures are due to the intrusion of parallel and sub-parallel dykes into a rift. Eventually the structure becomes unstable and catastrophic failure occurs. There is no evidence that the 1949 section of the rift extends in a north-south direction beyond its surface expression, nor that there is a developing detachment plane. Research is ongoing.

Criticism[edit]

There is controversy, however, about the threat presented by the Cumbre Vieja. Current indications are that recent landslides may have been gradual, and therefore may not generate tsunamis unless they increased in magnitude. Studies of possible local 'mega-tsunamis' in the Hawaiian Islands draw distinctions between the tsunami wave periods caused by landslides and subduction-zone earthquakes, arguing that a similar collapse in Hawaii would not endanger Asian or North American coastlines.[14]

Sonar surveys around many volcanic ocean islands including the Canary Islands,[15] Hawaii, Réunion etc., have mapped debris flows on the sea floor. Many of these debris flows are about 100 kilometres (60 mi) long and up to 2 kilometres (1 mi) thick, contain mega-blocks mixed up with finer detritus.

Moore (1964)[16] was the first geologist to interpret such features depicted on a United States Navy bathymetric chart. The chart showed two features that seem to originate from the Hawaiian islands of Oahu and Molokai.

Moss et al; (1999),[6] reported that the western flank is static and there is no indication that it has moved since 1949, confirming the dimensions provided by Bonelli-Rubio(1950)[4][5]

Carracedo et al; (2001),[17] state that they consider the crack to be a surface expression which is of a shallow and inactive nature. They also indicate that it should be monitored, but consider the possibility that the edifice is unstable as being almost non-existent.

Ward and Day (2003),[18] reported on the only documented flank collapse of a volcanic island - Ritter Island, that occurred on 13 March 1888. Approximately 5 X 109 m3 of material generated a landslide which entered the ocean and generated a westerly directed tsunami. The tsunami inundated adjoining islands and may have killed several hundreds of people. According to Cooke [19] damage was inflicted on islands several hundreds of kilometres from Ritter Island. There is no record of inundation occurring at trans-oceanic distances. The tsunami was witnessed by several Europeans living on many of the islands.

Murty et al.; (2005)[20] claim that it is almost impossible for a trans-oceanic tsunami to be generated in the basin of the Atlantic Ocean, which if correct supports the work by many other researchers that if the western flank of the Cumbre Vieja did fail it is unlikely to generate a "mega-tsunami."

Scientists at T U Delft in Holland, reported in 2006 that the section of the western flank of the Cumbre Vieja that was conjectured as potentially failing and falling into the Atlantic Ocean to create the hypothesised La Palma mega-tsunami was too small in volume and mass, and currently far too stable to break away within the next 10,000 years.[21][22]

In recorded history, the explosive eruption of Krakatau in 1883 generated devastating tsunami, yet the damage was local and did not propagate across long distances. This may have been due to the confining geography of the area.

The Tsunami Society has issued a statement of caution regarding the Ward and Day research. This is supported by research at Delft (Maarten Keulemans). The Keulemans work and the Tsunami Society paper, along with several other studies, all disagree with the geological model of Ward and Day, favouring a different collapse.[23] The Ritter Island collapse would seem to support the claim that the collapse of the western flank on La Palma is unlikely to trigger a tsunami with the potential to cause inundation at trans-oceanic distance.

In support of the above criticisms, is the fact that the Atlantic Ocean has a surface area of ~106 million square kilometres, and a volume of ~425 million cubic kilometres - or about 425 x 1015 m3, and has a mass of approximately 482 x 1017 kilograms (~482 x 1014 metric tonnes). In comparison La Palma has an area of ~706 square kilometres which equates to about 1/10000 of the surface area, the volume likewise also equates to less than 1/10000 of the volume of the Atlantic Ocean. This infers that the Cumbre Vieja does not contain sufficient mass and therefore insufficient potential energy to trigger the mega-tsunami that has been postulated.

Historical megatsunamis[edit]

On 9 July 1958 a 7.9 magnitude earthquake and landslide released ~3.1 x 107 m3 of rock and debris in Crillon Inlet at the head of Lituya Bay, Alaska. The mass impacted the water with a force of ~8.8 x 1010 N m2 and this generated a massive surge (which has since become known as the Lituya Bay 'mega-tsunami'), with an initial amplitude (height) of ~300 metres (980 ft). The wave surged to a height of ~520 metres (1,710 ft), and stripped trees and soil from the opposite headland and inundated the entire bay, destroying three fishing boats anchored there and killing two people. Once the wave reached the open sea, however, it rapidly dissipated.

On 9 October 1963, a mass of rock, soil and other debris estimated at 2.6x108 m3, slid in a massive landslide off Monte Toc infilling the newly constructed Vajont Dam or Vaiont Dam in north-east Italy. It displaced ~3.0x107 m3 - about 1/5th of the water present in the reservoir, which surged over the dam wall and flooded the Piave Valley below the dam, killing over 2000 people. From the onset of the landslide to impact on the opposite side of the gorge took an estimated 45 seconds - according to seismic records.[24] It was not strictly a tsunami, but rather a massive displacement of water with a resultant surge., but does serve to indicate how massive volumes of debris can displace water. In the case of the disaster, the water behind the dam could only be displaced over the dam.

During the second millennium BC, the volcano on Santorini exploded with a VEI estimated at 7. Research suggests that the eruption generated a tsunami which inundated Crete, possibly triggering the downfall of the Minoan civilization.

Lateral collapse events at stratovolcanoes, similar to the current threat posed by the western flank of Cumbre Vieja, could increase due to the physical effects of global warming on the Earth from increases in deviatoric stress from post-glacial rebound, while the size and frequency of eruptions are also likely to increase.[25][26]

References[edit]

  1. ^ "La Palma: Synonyms and Subfeatures". Global Volcanism Program. Smithsonian Institution. 
  2. ^ "La Palma: Eruptive History". Global Volcanism Program. Smithsonian Institution. 
  3. ^ a b Carracedo, J.C. 1996. A simple model for the genesis of large gravitational landslide hazards in the Canary Islands. In McGuire, W: Jones, & Neuberg, J. P. (eds). Volcano Instability on the Earth and Other Planets. Geological Society, London. Special Publication, 110, 125-135.
  4. ^ a b c d e Bonelli Rubio, J.M., 1950. Contribucion al estudio de la erupcion del Nambroque o San Juan. Madrid: Inst. Geografico y Catastral, 25 pp.
  5. ^ a b Ortiz, J.R., Bonelli Rubio, J.M., 1951. La erupción del Nambroque (Junio-Agosto de 1949). Madrid: Talleres del Instituto Geográfico y Catastral, 100 p., 1h. pleg.;23 cm
  6. ^ a b Moss, J. L; McGuire, W. J; & Page, D. 1999 - Ground deformation monitoring of a potential landslide at La Palma, Canary Islands. J. Volcanol. Geotherm. Res. 94, 251–265
  7. ^ Klügel, A; Schmincke, H –U; White, J. D. L. and K. A. Hoernle, 1999. Chronology and volcanology of the 1949 multi-vent rift-zone eruption on La Palma (Canary Islands). J. Volcan. Geotherm. Res. 94, 267-282
  8. ^ a b c d e Day, S. J; Carracedo, J. C; Guillou, H. & Gravestock, P; 1999. Recent structural evolution of the Cumbre Vieja volcano, La Palma, Canary Islands: volcanic rift zone re-configuration as a precursor to flank instability. J. Volcanol. Geotherm Res. 94, 135-167.,
  9. ^ BBC 2 TV. 2000. Transcript "Mega-tsunami; Wave of Destruction". Horizon. First screened 21.30, Thursday, 12 October 2000.
  10. ^ a b c Ward, S. N. & Day, S. J; 2001. Cumbre Vieja Volcano; potential collapse and tsunami at La Palma, Canary Islands. Geophys. Res. Lett. 28-17, 3397-3400. http://www.es.ucsc.edu/~ward/papers/La_Palma_grl.pdf
  11. ^ Carracedo, J.C. 1994. The Canary Islands: an example of structural control on the growth of large oceanic-island volcanoes. J. Volcanol. Geotherm Res. 60, 225-241.
  12. ^ Carracedo, J. C; 1999. Growth, Structure, Instability and Collapse of Canarian Volcanoes and Comparisons with Hawaiian Volcanoes. J. Vol. Geotherm. Res. 94, 1-19.
  13. ^ Siebert, L; 1984. Large volcanic debris avalanches: characteristics of source areas, deposits and associated eruptions. J. Volcanol. Geotherm Res. 22, 163-197.
  14. ^ Pararas-Carayannis, G; 2002. Evaluation of the Threat of Mega Tsunami Generation from Postulated Massive Slope Failure of Island Stratovolcanoes on La Palma, Canary Islands, and on The Island of Hawaii. Science of Tsunami Hazards, Vol 20, No.5, pp 251-277.
  15. ^ Rihm, R; Krastel, S., CD109 Shipboard Scientific Party. 1998. Volcanoes and landslides in the Canaries. National Environment Research Council News. Summer, 16-17.
  16. ^ Moore, J. G. 1964. Giant Submarine Landslides on the Hawaiian Ridge. US Geologic Survey Professional Paper 501-D, D95-D98.
  17. ^ Carracedo, J. C; Badiola, E. R; Guillou, H; de la Nuez, J; and Pérez Torrado, F. J; 2001. Geology and Volcanology of La Palma and El Hierro, Western Canaries. Estudios Geol. 57, (5-6) 175-273.
  18. ^ Ward, S. N. & Day, S; 2003. Ritter Island Volcano - lateral collapse and the tsunami of 1888. Geophys. J. Int. 154, 891 - 902
  19. ^ Cooke, R. J. S; 1981. Eruptive history of the volcano at Ritter Island, in Cooke-Ravian Volume of Volcanological Papers, pp. 115-123, ed. Johnson, R. W; Geological Survey Papua New Guinea, Memoir 10.
  20. ^ Murty, T. S; Nirupama, N; Nistor, I; and Rao, A. D. 2005. Why the Atlantic Generally cannot generate trans-oceanic tsunamis? ISET J. of Earhquake Tech. Tech. Note., 42, No. 4, pp 227-236.
  21. ^ http://home.tudelft.nl/fileadmin/UD/MenC/Support/Internet/TU_Website/TU_Delft_portal/Actueel/Magazines/Delft_Integraal/archief/2006_DI/2006-3/achtergrond/doc/LaPalma.pdf.
  22. ^ ({cite web|last=Berlo|first=Janneke|title=New research puts 'killer La Palma tsunami' at distant future|url=http://phys.org/news77977989.html%7Caccessdate=23 October 2014})
  23. ^ name=maartenkeulemans>Keulemans, Maarten. "The day the world ended: La Palma Mega tsunami suffers a slight delay" (PDF). Maarten Keulemans. Retrieved 13 April 2014. 
  24. ^ Ward, S. N; & Day, S; 2011. The 1963 Landslide and Flood at Vaiont Reservoir, Italy. A tsunami ball simulation. Ital. J. Geosci. (Boll. Soc. Geol. It. 130, 1. 16-26. doi:10.3301/IJG.2010.21
  25. ^ Tuffen, H. (May 2010). "How will melting of ice affect volcanic hazards in the twenty-first century?". Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 368 (1919): 2535–58. Bibcode:2010RSPTA.368.2535T. doi:10.1098/rsta.2010.0063. PMID 20403841. 
  26. ^ Deeming, K. R.; McGuire, B.; Harrop, P. (May 2010). "Climate forcing of volcano lateral collapse: evidence from Mount Etna, Sicily". Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 368 (1919): 2559–77. Bibcode:2010RSPTA.368.2559D. doi:10.1098/rsta.2010.0054. PMID 20403842. 

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