Cumbre Vieja

Cumbre Vieja (Spanish: "Old Ridge") is an active volcanic ridge on the volcanic ocean island of Isla de La Palma in the Canary Islands.

This ridge trends in an approximate north-south direction and covers the southern third of the island. It is lined by several volcanic craters. Location: 28°34′N 17°50′W / 28.567°N 17.833°W / 28.567; -17.833.

Volcanic history

La Palma is a volcanic ocean island. It is the steepest island in the world and currently the most volcanically active of the Canary Isles (since the Spanish occupation). Historical eruptions on the Cumbre Vieja occurred in 1470, 1585, 1646, 1677, 1712, 1949, and 1971.

1949 eruption

During the 1949 eruption, three vents - Duraznero, San Juan and Hoyo Negro opened and lava was erupted. Also during the eruption two earthquakes occurred with epicentres near Jedy. Following the earthquakes a fracture approximately two and half kilometres long - about 1/10th of the exposed length of the Cumbre Vieja, opened and parts of the western half of the Cumbre Vieja ridge moved about 1m sideways and 2m downwards towards the Atlantic Ocean. (Rubio Bonelli, 1950., Ortiz and Rubio Bonelli, 1951). The fracture is still visible (2008) and still has the same dimensions recorded in 1949.

It is considered that this process was driven by the pressure caused by the rising magma super-heating water trapped within the edifice of the volcano. 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 resulted in the two earthquakes that were reported as occurring during the eruption.

Proof that the water did not vapourise are the absence of phreatomagmatic explosions - steam escaping explosively from the ground is often a precursor of volcanic activity. Further evidence that vapourisation did not occur is that when the rift was visited the following day by Rubio Bonelli (Rubio Bonelli, 1950), he reported that the newly opened fissure "... Was not issuing fumes, vapour, steam, ashes, lava or other materials ..." In fact at no time during or afterwards was steam or phreatomagmatic activity reported. Thus reinforcing 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.

1971 eruption

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. No seismic activity such as that that occurred in 1949 occurred. Residual thermal activity continues.

Future threat

Satellite photo of La Palma, Canary Islands. (North is in the lower right.) The grand crater in the center is the Caldera de Taburiente. The Cumbre Vieja is a ridge to the South (upper left) of the caldera.

The British Broadcasting Coporation (BBC2 Channel) transmitted “Mega-tsunami; Wave of Destruction”, Horizon. First screened 9:30 pm, Thursday, 12th October, 2000. which suggested that a future failure of the western flank of the Cumbre Vieja would cause a "mega-tsunami."

Day et al; (1999), Ward and Day (2001)(see scientific paper: http://www.es.ucsc.edu/~ward/papers/La_Palma_grl.pdf ), hypothesise that during a future unascertained eruption, the western half of the Cumbre Vieja - approximately 500km³ (5 x 10 ¹¹ m³) with an estimated mass 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 - as a debris flow, along the ocean floor. Computer modelling indicates that the resulting initial wave may attain a local amplitude (height) in excess of 600 metres and an initial peak to peak height that approximates to 2 km, and travel at about 1000 km/h (approximately the speed of a jet aircraft), inundating the African coast in about 1 hour, the southern coast of England in about 3.5 hours, and the eastern seaboard of North America in about 6 hours, by which time the initial wave would have subsided into a succession of smaller ones each about 30 to 60 m high. These may surge to several hundred metres in height and several km apart but retaining their original speed. The models of Day et al; (1999), and Ward and Day (2001), suggest that it could inundate up to 25 km inland. This would greatly damage or destroy cities along the entire North American eastern seaboard. The physical damage would take tens if not hundreds years to repair and restore. The economies of the countries affected would likewise take several years to return to the pre-inundation levels.

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 oceanic island volcanoes) to one consisting of a single north-south rift (Carracedo 1994, 1996 and 1999). It is claimed that this structural reorganisation is a response to evolving stress patterns associated with the development of a possible detachment fault under the volcano's west flank (Day et al; 1999, Ward and Day, 2001). Siebert (1984) 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 or that there is a developing detachment plane. Research is ongoing.

There is controversy however, about the threat presented by the Cumbre Vieja (Pararas-Carayannis, 2002). 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-tsunami" 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.^[1]

Sonar surveys around many volcanic ocean islands including the Canary Islands (Rihm et al., 1998), Hawaii, Reunion etc., have mapped debris flows on the seafloor. Many of these debris flows are about 100 km long and up to 2 km thick, contain mega-blocks mixed up with finer detritus.

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

In recorded history, the Krakatau eruption 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 areas.

Approximately 3615 years ago, the volcano on Santorini exploded in a VEI estimated at 7.5. Research suggests that the eruption generated a tsunami which inundated Crete, possibly triggering the downfall of the Minoan civilization.

An earthquake and landslide in Crillon Inlet at the head of Lituya Bay, Alaska, on 9th July, 1958 generated a so called "mega-tsunami," with an initial amplitude (height) of ~524 m, which 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.

References

The following list is not exhaustive and you may need to obtain the papers via your local library.

Bonelli Rubio, J.M., 1950. Contribucion al estudio de la erupcion del Nambroque o San Juan. Madrid: Inst. Geografico y Catastral, 25 pp.

La erupción del Nambroque : (junio-agosto de 1949) / por José Romero Ortiz y Juan Mª Bonelli Rubio Madrid : Talleres del Instituto Geográfico y Catastral, 1951 100 p., 1h. pleg.;23 cm

BBC 2 TV; 2000. Transcript “Mega-tsunami; Wave of Destruction”, Horizon. First screened 21.30 hrs, Thursday, 12th October, 2000.

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.

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.

Carracedo, J. C; 1999. Growth, Structure, Instability and Collapse of Canarian Volcanoes and Comparisons with Hawaiian Volcanoes. J. Vol. Geotherm. Res. 94, 1-19.

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.

Moore, J. G; 1964. Giant Submarine Landslides on the Hawaiian Ridge. US Geologic Survey Professional Paper 501-D, D95-D98.

Pararas-Carayannis, G; 2002. Evaluation of the Threat of Mega Tsumami Generation from Postulated Massive Slope Failure of Island Stratovolcanoes on La Palma, Canary Islands, and on The Island of Hawaii, George , Science of Tsunami Hazards, Vol 20, No.5, pp 251-277.

Rihm, R; Krastel, S. & CD109 Shipboard Scientific Party; 1998. Volcanoes and landslides in the Canaries. National Environment Research Council News. Summer, 16-17.

Siebert, L; 1984. Large volcanic debris avalanches: characteristics of source areas, deposits and associated eruptions. J. Volcanol. Geotherm Res. 22, 163-197.

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

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