Ready to Blow?
This Is Grenada by Frances Kay, © 1971,
Carenage Press, St. George's, Grenada
As if having a hurricane season isn't enough excitement! The Caribbean
also has volcanoes, and now the prospect of tsunamis has again reared its
Tsunamis are waves formed when huge masses of water are displaced by undersea volcanic eruptions or earthquakes. A single wave is normally less than a few meters high in the open ocean, but can extend more than 750 kilometers (465 miles) in length in the open ocean. This creates a sea-surface slope so gentle that the wave usually passes unnoticed in deep water. But as the waves transport seismic energy from their source, they travel up to 800 kilometers (500 miles) per hour and may build up to devastating heights when they approach land.
The recent major eruptions at Montserrat have made people more aware than ever that this is a volcanic region. The islands of the Eastern Caribbean lie along the Lesser Antilles volcanic arc. Volcanoes occur where the various moving plates which make up the Earth's surface meet. The volcanoes in the Lesser Antilles arc are located along the edge where the South American Plate is pushing itself under the Caribbean Plate.
Now cruising in the Caribbean, Ian Deas of the yacht Manx Cat relates: "We experienced a tsunami in Yugoslavia in 1978 resulting from an earthquake in Greece. We were in a lagoon with a narrow entrance, rafted alongside a friendly larger yacht and having a convivial drink when we sank! The quay rose by one and a half meters or so, then dropped by the same. We were only on our second gin and tonic and realized that it was no hallucination as we sank and rose a couple more times.
"Two days later we sailed into Korcula to find it a disaster area, with big fishing boats high and dry on the quay, and shops with their fronts smashed in and high water marks up to their first floors. The sea had run out of the harbour, grounding all the boats. The ensuing tidal wave had snapped mooring lines, and even boats that had held their moorings were wrecked. Korcula harbour is at the head of an estuary; a wave which was small at the entrance was magnified, so that when it arrived at the quay it must have reached ten meters. We were lucky that our lagoon had a small entrance."
The tsunami in Grenada, described by Frances Kay in the quotation on page one, was caused by an earthquake in the Virgin Islands in 1867. The reason for the current tsunami awareness in the region is Kick 'em Jenny, an underwater volcano off Grenada's north coast, named after and not to be confused with the small nearby island (Diamond Islet on some charts). Grenadian yachtsman Laddie McIntyre is quoted in the book Isles of the Caribbees (by Carleton Mitchell, © 1966, National Geographic Society, Washington DC, USA): "Nobody knows where the name comes from. Maybe it's a corruption of the French, cay que gêne, `the troublesome cay,' because the currents around it gave the old sailing ships such a hard time. Some say it's Kick 'em Jenny because it kicks like a mule."
The Kick 'em Jenny volcano is active, there is no doubt about that.
According to the Global Volcanism Program of Smithsonian Institution, it
has erupted ten times since 1939, with the most recent eruption occurring
"Marine and aviation warnings have been out for years, but yachts, schooners and ferries continue to go right over it," says Tony Buxo, a member of Grenada's National Emergency Advisory Council.
Ian Deas recalls: "A few years ago the Grenada Coast Guard gave out a security warning to give this area a wide berth of at least six miles. We came past the island two days later and gave it an offing of over a mile compared to our usual half mile, with nothing to be seen, felt or smelt. We were rather disappointed not to be given a whiff of sulphur. Perhaps we were foolhardy."
Perhaps. Although most eruptions of this volcano have been identified by the detection of strong underwater acoustic signals at regional seismograph stations, the 1939 eruption sent a black cloud up to 885 feet (270 meters) above sea level, and during the 1974 eruption the sea above the volcano boiled turbulently and spouted steam; shoals of fish were found "belly-up" west of the site.
Kick 'em Jenny volcano is located at approximately 12·18'N, 61·38'W, just west of The Sisters rocks and Ile de Ronde, and about 6 miles (9 kilometers) north of Grenada. Nearby Ile de Caille is made up of two youthful craters and lava flows. Kick 'em Jenny is the southernmost active volcano in the Lesser Antilles volcanic arc and, although at a number of scuba diving sites such as "Champagne" in Dominica and one area of "Mayreau Gardens" near the Tobago Cays, bubbles trickle up from the bottom signalling local volcanic activity, Kick 'em Jenny is the only active submarine volcano in the arc.
Kick 'em Jenny has a basal diameter of about 3 miles (5 kilometers) and rises about 4,300 feet (1,300 meters) above the sea floor. Its summit grew from about 235 meters (770 feet) below the sea surface in 1962 to about 150 meters (492 feet) in 1982.
Editor Jack Dausend wrote in The Boca, July '99 "researchers have estimated that at the current rate of ascent the volcano could emerge above sea level early in the 21st century. In 1988 a researcher dove on Kick 'em Jenny, documenting its size and contour on video, and at that time verified its depth as 150 meters below sea level. It last erupted in 1990, and it is thought that its top blew off with that eruption." (because the most recent measurement placed the summit 27 meters [88 feet] lower, at a depth of 177 meters [581 feet].)
The following table, provided by the Seismic Research Unit (SRU), shows
depth measured by various vessels since 1962:
DATE SHIP DEPTH
1966 June HMS Lynx 192m
1972 May HMS Hecla 190m
1976 May R/V Gillis 190m
1978 April R/V Endeavor 160m
1981 October N.O. Noroit 160m
1985 April R/V Conrad 160m
1989 April Submersible 150m
1997 May R/Malcolm Baldridge 177m
The SRU plans to make another survey soon. The distance from the volcano's summit to the sea surface is critically important, and the consistent diminishing of this distance (except for the most recent recording) is one of the reasons Kick 'em Jenny has recently received so much attention.
What's happening, explains Dr. John Shepherd who is Head of Seismic Research at the SRU and who first surveyed Kick 'em Jenny in 1974, is that the periodic eruptions since the first was recorded on 24 July, 1939, have resulted in the summit of the volcano climbing closer and closer to sea level. The weight of the sea, when the summit remains deep, dampens the energy of the explosions, although some have already broken the surface. But as the summit gets nearer to the surface, the explosions will become more violent, with the resultant increased possibility of a tsunami.
News of the volcano's growth toward the surface has spawned some hysterical reports in the media. This past February, Grenada resident Dave Hadley was told by his dentist in Canada that Grenada was about to be swamped by a 100-foot wall of water. "We had been delayed in our return to Grenada," says Dave, "and he felt that I should be glad not to be in the danger zone."
Apparently a Montreal newspaper had picked up a wire-service report indicating that Kick 'em Jenny was being monitored for future eruptions and a possible tsunami and the newspaper added its own prediction that this would occur within 48 hours of the story's publication!
Other alarms have been generated by sailors unfamiliar with the rough
seas often encountered in the vicinity of Kick 'em Jenny. "The number of
rumors from passing mariners has even managed to trickle as far as Washington,
DC," says Rick Wunderman of the Global Volcanism Program of the Smithsonian
Institution. "And we have received calls from Venezuela. "
But Compass correspondent Norman Faria reports that, speaking at a public lecture sponsored by the Central Emergency Relief organisation (CERO) in Barbados in early June, Dr. Shepherd said there's no need to worry at this time. "It's not about to blow up." Dr. Shepherd emphasises that no eruption is imminent and there are no signs of unusual activity at Kick 'em Jenny although, because future eruptions are expected, the SRU is strengthening its monitoring of any renewal of activity.
Monitoring of volcanic activity throughout the English-speaking Lesser Antilles has been conducted for decades by the SRU. Dr. Shepherd, who joined the SRU in 1964, says, "My first field project on a real volcano was the Kick 'em Jenny eruption in 1965. We'd suspected there was a volcano in that area so we went to Sauteurs and the people there knew all about it. In fact, Father Divas, a Jesuit priest in Grenada, had written a detailed account of the 1939 explosion." Dr. Shepherd says Kick 'em Jenny has been his "pet project" since then.
He says that a particular concern, as the volcano's summit approaches the sea surface and thus the weight of overlying water decreases, is that samples of rocks collected from eruptions over the past 20 years show that Kick 'em Jenny's magma contains a high proportion of dissolved water (water in molecular form), which is an indicator of high explosivity. "The water molecules are like the gas molecules in a bottle of champagne," he explains. They'll stay dissolved until the pressure is released, then POP! Dr. Shepherd notes that Kick 'em Jenny is one of the very few submarine volcanoes in the world having this particular combination of molten rock with a high water content, and near-surface depth, making it especially explosive.
According to a May 1999 article in Scientific American magazine, 82 tsunamis have reported worldwide since 1990. This report rate higher than the historical average of 57 a decade is due to improved global communications. The majority of tsunamis affect the Pacific Ocean, and most of those are the products of undersea earthquakes around the Pacific Rim. Tsunamis in the Caribbean have been relatively rare.
Nevertheless, a recent Caribbean Development Bank newsletter warns, "With regard to Kick 'em Jenny, studies have indicated that in the event of a major eruption, tsunamis may be generated which could reach as far as Puerto Rico to the north and Trinidad and Venezuela to the south." Dr. Shepherd says that the islands most directly affected in the case of a tsunami would be Grenada, the Grenadines, St. Vincent, Tobago and Barbados, although some effects would be felt further north and as far south as the north coast of Venezuela. He notes that tsunamis do not radiate evenly from their source, like rings from a pebble dropped in a pond, but are affected by islands and the shape of the ocean floor. "A tsunami from Kick 'em Jenny would be funneled by the Aves Ridge toward St. Maarten, Anguilla and the Virgin Islands," he says.
Recent alarmist accounts in the media quoted out of context from a hypothetical "worst case scenario" which had been developed for a published doctoral thesis by one of Dr. Shepherd's students. This worst case was based on a theoretical assumption of an eruption at Kick 'em Jenny being of the same intensity as the 1883 eruption of Krakatoa in Indonesia, the strongest volcanic explosion in human history. Is such a scenario likely? Not very. The erupted volume (one measure of a volcano's intensity) of Krakatoa was about 9 square kilometers of dense rock; the erupted volume of Kick 'em Jenny's historic eruptions are estimated not to have exceeded one-tenth of a square kilometer of rock, giving Krakatoa's famous eruption 90 times the erupted volume of a typical Kick 'em Jenny eruption.
"Kick 'em Jenny's eruptions are frequent, but small," say Shepherd. "Our hypothetical worst case scenario was for forty-meter waves in the Grenadines, but the realistic worst case scenario is for fifteen-meter waves and in the most likely scenario, waves would probably not be much worse than the swell from a near-miss hurricane." The SRU's "high probability" predictions (based on Kick 'em Jenny's 1939 eruption, which caused a 1- to 2-meter tsunami in Barbados) forecast a 3 meter wave at Grenada's north coast, diminishing to under one meter by the time it reaches Port of Spain, Trinidad.
"Nevertheless, we have to be prepared," warns Shepherd. Eruptions of Kick 'em Jenny can be predicted, says the SRU, since the volcano needs time to build up to full eruption. Therefore volcano-generated tsunamis can be predicted by monitoring the volcano, and a progressive series of warnings could be issued as the eruption approaches.
The Caribbean Development Bank (CDB) has recently provided a grant of up to US$170,600 to the SRU for the establishment of means for continuous monitoring of the volcano, and USAID/OFD have provided an additional US$60,000 toward a public information campaign. For this project, the University of the West Indies, through the SRU, in collaboration with the Caribbean Disaster Emergency Response Agency (CDERA), will engage professional, technical, and support staff and services. The CDB financing will meet consultants' fees, and the costs of equipment, sensitization workshops and involvement of regional disaster preparedness agencies. Public lectures have already been given in Barbados, St. Vincent, Grenada and Trinidad.
Inundation models showing areas likely to be flooded will be generated by computer, and recommendations on evacuation procedures will be made. Some volcano monitoring devices are already in place at Mount St Catherine, and additional equipment will be installed at Ile de Ronde, Sauteurs (on Grenada) and Carriacou.
The schematic diagram above illustrates the technical details of the proposed tsunami warning center. All data are transmitted to a center where they are recorded and monitored in real-time or near real-time. Data is fed to a computer which logs and displays the incoming information. At the local warning center a seismometer (an instrument which measures the direction and force of eruptions) will provide accurate recordings of volcanic activity. Another instrument will be used to provide continuous visual recordings of information from the Ronde Island station. The third workstation is to retrieve and process data from the acquisition computers. Each acquisition machine will be equipped with "event triggers" which will be tuned to recognize events which could generate a tsunami.
Still, as the article "Tsunami!" in May's Scientific American admonished: "Predicting where a tsunami may strike helps to save lives and property only if coastal inhabitants recognize the threat and respond appropriately. technology alone cannot save lives. Coastal inhabitants must be [ready to] seek higher ground immediately." As Tony Buxo of Grenada's National Emergency Advisory Council recommends, "If you feel a big rumble, don't bother to pick anything up; just head for higher ground. If you're on a boat, head offshore."
"Part of the education programme about the potential dangers from Kick 'em Jenny is to warn residents not to go into the dry seabed to collect stranded fish shortly before the tsunami strikes. That happened in the Pacific, and the people weren't fast enough to get back to shore when the tsunami did arrive," says Dr. Shepherd.
He also warns, "Marine advisories on Kick 'em Jenny are genuine. Imagine
a meter-diameter rock fired directly upward at a hundred miles per hour."
Ian, who experienced that tsunami in Yugoslavia, suggests: "Look around your present environment. Would it be safe?" Keep in mind that a tsunami wave may carry huge quantities of sand, rocks, vessels and debris, as well as water.
And after the eruption which may occur when Kick 'em Jenny reaches the point estimated by Dr. Shepherd to be about 150 meters or less from the surface of a potentially tsunami-causing explosion?
"Once Kick 'em Jenny gets above the surface, it will form an island which will, most likely, look a lot like Ile de Caille. It could grow to the surface soon" Dr. Shepherd says, "or it could just blow the top off itself and be quiet again for a century."
Information from the Seismic Research Unit of the University of the West Indies, the Global Volcanism Program of the Smithsonian Institution, Volcano World website (http://volcano.und.nodak.edu/), correspondents Norman Faria, Dave Hadley and Ian Deas, the CDB News (Caribbean Development Bank newsletter), Scientific American magazine (May 1999) and the Journal of Volcanology and Geothermal Research (1995) was used in this report.
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