Cumbre Vieja (Spanish: Old Summit) is an active volcanic ridge on the volcanic ocean island of Isla de La Palma in the Canary Islands, Spain.
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
La Palma is a volcanic ocean island. It is currently the second most volcanically active of the Canary Islands Historical eruptions on the Cumbre Vieja occurred in 1470, 1585, 1646, 1677, 1712, 1949 and 1971.
1949 eruption
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, three vents â€"Duraznero, Llano del Banco, and Hoyo Negro â€"opened and lava flowed from the Duraznero and Llano del Banco vents. Two earthquakes also occurred during the eruption, with epicentres near Jedey. Following the earthquakes a fracture approximately two and half kilometres long, about 1/10 of the exposed length of the Cumbre Vieja, opened and parts of the western half of the Cumbre Vieja ridge moved about 1 metre sideways and 2 metres downwards towards the Atlantic Ocean. As of 2008, the fracture is still visible and still has the same dimensions recorded in 1949.
This process is considered to have been 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.
That the water did not vapourise is shown by 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 Rubio Bonelli visited the rift the following day, the newly opened fissure "... Was not issuing fumes, vapor, steam, ashes, lava or other materials ..." 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.
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. Such seismic activity did not occur during the 1949 eruption. Residual thermal activity continues.
Future threats
BBC's Megatsunami
In October 2000, the BBC aired “Mega-tsunami; Wave of Destructionâ€, which suggested that a future failure of the western flank of Cumbre Vieja would cause a "mega-tsunami".
Day et al. (1999) and Ward and Day (2001) hypothesize that during an eruption at some unascertained future time, the western half of the Cumbre Viejaâ€"approximately 500 km3 (5 x 1011 m3) with an estimated mass of 1.5 x 1015 kgâ€"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 coast of Great Britain 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. and Ward and Day 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 oceanic island volcanoes) to one consisting of a single north-south rift. 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. 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 beyond its surface expression, nor that there is a developing detachment plane. Research is ongoing.
Criticism
There is controversy however, about the threat presented by 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.
Sonar surveys around many volcanic ocean islands including the Canary Islands, Hawaii, Réunion etc., have mapped debris flows on the seafloor. 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) 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.
Dutch scientists at T U Delft found in 2006 that the flank required to fall into the Atlantic to create the purported megatsunami was both far too small to create such an energetic event, and far too stable to be broken away within the next 10,000 years.
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.
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, disagree with the geological model of Ward and Day, favouring a different collapse.
Historical Megatsunamis
An earthquake and landslide in Crillon Inlet at the head of Lituya Bay, Alaska, on 9 July 1958 generated a 'mega-tsunami' with an initial amplitude (height) of ~300 metres (980Â ft). The wave surged to a height of ~520 metres (1,710Â ft), 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.
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.
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.
References
External links
Information and sources
- Benfield Hazard Research Center: Why the only certainty about the La Palma tsunami is that it will happen
- Benfield HRC Q&A
- A rebuttal of Cumbre Vieja
- Another rebuttal of Cumbre Vieja
- Cumbre Vieja Volcano--Potential collapse and tsunami at La Pama, Canary Islands
- Evidence for a mega-tsunami from flank collapse of Mauna Loa
- US Geological Survey geologist Uri ten Brink says that Cumbre Vieja tsunami is unlikely, and would only be a few feet high when it reached the US East Coast.
Press articles
- CNN: Scientists warn of massive wave
- BBC: Expert says threat is being ignored.
- BBC: Threat is stated to be a worse case scenario; it might not happen.
- NBC News: Risk is low, but US East Coast faces a variety of tsunami threats.
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