Tsunamis are poorly understood by the general populace and by early scientists. Many questions have been asked about the origin of the name, the causes of tsunamis, what a tsunami looks like in deep water and as it approaches land. Most people think of tsunamis as a threat for the Pacific and Indian oceans. Contrary to popular belief, tsunamis are also a threat to the Eastern Seaboard of the United States and South America. This threat comes, in part, from the possible instability of the Canary Islands.

The word tsunami actually comes from two Japanese characters. The first character is ‘tsu’, which means harbor. The second character is ‘nami’, which means wave. Together they form harbor wave. That is a better term than "tidal wave", used by the general public, or "seismic sea wave", used by early scientists. Both of these terms are misleading. Tsunamis are not caused by tides, and more than just seismic events can cause them.

Many processes can cause tsunamis: cosmic collisions, earthquakes, volcanic eruptions, and landslides (www.geophys). Cosmic collisions are composed of mainly meteor and comet strikes. These are, fortunately, rare events. The last three are more common and interrelated than the cosmic collision. Undersea volcanoes erupt and quickly expel large amounts of gas and rock into the ocean. The gas and rock take up space and displace the water. Earthquakes occur along plate boundaries and other fault locations. In an earthquake, the sudden movement of land will displace the water. The amount of displacement depends on the severity of the earthquake. Landslides displace large amounts of water when they occur. They can be caused by volcanoes, earthquakes (volcanic in origin or not), or by anything else that can cause a slope to fail.

To the naked eye, a tsunami in deep water is indistinguishable from any other ocean wave. Normal waves and tsunamis, though, are very different physically. Normal waves have a wavelength of 150 m, tsunamis have wavelengths in excess of 100 km . The period of a normal wave is about 10 sec, and the period of a tsunami is about an hour. Normal waves usually only affect the ocean surface and are relatively slow. Tsunamis’ influence the ocean down to the deepest seafloor and can travel over 700 km/hr, or 420 miles/hour . Unlike normal waves, tsunamis can travel long distances with little energy loss (www.geophys).

As a tsunami approaches land, an observer will start to notice a difference between it and normal waves. Tsunami energy is related to its speed and water depth, and as the depth decreases, the speed decreases. The problem is that the energy stays constant. This means that due to the shoaling effect, tsunamis will appear to grow several meters, or more as they approach land. Tsunamis can appear as: rapidly rising or falling tides, a series of breaking waves, or a bore (a single large wave)(www.geophys).

The Canary Islands are located off the coast of Morocco in northwest Africa. The islands are a series of volcanoes fed by a hot spot beneath the African Plate (http://volcano). The island of La Palma, on the western side of the island chain, is the main source of concern for landslide-induced tsunamis. La Palma is composed of three composite volcanoes (from north to south): Taburiente, Cumbre Nueve, and Cumbre Vieja. Cumbre Vieja is still active. The highest elevation on the island is 2426 m and the island covers about 728 sq km (Scarth and Tanguy, 2001).

The ground dangers of La Palma are a result of its geology. The lower unit consists of pillow basalts (about 10 to 350 m thick) that are cut by basaltic dikes. The upper layer is composed of basaltic lavas and pyroclastic rocks (http://volcano). A rift developed at Taburiente and extended south through Cumbre Nueve and Cumbre Vieja (Scarth, 2001). There are east west trending faults that radiate off of the rift all along its length and the western flank of La Palma is steep and unstable (Pararas-Carayannis, 2002).

There is ample historical evidence of major landslides on La Palma (and other Canary Islands) and large tsunamis hitting the east coast of North and South America (Lockridge et al, 2002). A study done by Ward & Day (as cited by Pararas-Carayannis, 2002) predicts that a major gravitational collapse could occur with the next eruption. The resultant landslide would contain about 500 cubic km of material, create a 50 m wave in Florida and the Caribbean, a 40 m wave in Brazil, and destructive waves would hit along the western slope of Europe (Pararas-Carayannis, 2002).

George Pararas-Carayannis (2002) states that there are some major problems with current modeling of tsunamis from La Palma and other places. Massive flank failures are extremely rare. The why and how of these failures is still unknown. The dike swarm intrusions that have occurred will likely decrease the chances of a landslide. The dikes act like nails and can stabilize an area. Pararas-Carayannis also states that the results of tsunamis, for distant coasts, are exaggerated.

In conclusion, without knowing how and why these massive slope failures occur, it is hard to predict when one will occur. We know that major slope failures have occurred in the Canary Islands and La Palma, and that the east coast of North and South America had been impacted by tsunamis. More studies need to be done on the geologic history of La Palma and the other Canary Islands. More studies also need to be run on the accuracy of destruction for distant coasts by current tsunami models.
 
 

References

Lockridge, Patricia A., Whiteside, Lowell S., Lander, James F., Tsunamis and Tsunami-Like Waves of the Eastern United States, Science of Tsunami Hazards, 20, 120-148, 2002.

Pararas-Carayannis, George, Evaluation of the Threat of Mega Tsunami Generation From Postulated Massive Slope Failures of Island Stratovolcanoes on La Palma, Canary Islands, and on the Island of Hawaii, Science of Tsunami Hazards, 20, 251-277, 2002.

Scarth, Alwyn, Tanguy, Jean-Claude, Volcanoes of Europe, Oxford University Press, 125-127, 2001.

University of Washington’s Geophysics program, Welcome to Tsunami, March, 24 2003, www.geophys.washington.edu/tsunami/general/physics/characteristics.html

Volcano World, La Palma, Canary Islands, January, 28 2003 http://volcano.und.nodak.edu/vwdocs/volc_images/africa/lapalma.html

A fun site with links to lots of other tsunami sites: www.pmel.noaa.gov/tsunami/tsu_links.html

The top two have some interesting videos.