'I knew, of course, that my luck might run out one day. But even though I accepted that possibility, I never actually believed it would happen to me. People in high-risk professions are confident they can beat the odds. I always felt the same way. Otherwise I'd never have gone near a crater."

Illinois-born Stanley Williams (48) is a volcanologist, a self-confessed thrill-seeker who hikes up the sides of active volcanoes to take samples of corrosive sulphuric gases. He takes educated chances. His just published book, Surviving Galeras recounts an expedition in 1993 to Galeras, the most active volcano in Colombia, that literally blew up in his face. Williams nearly lost his right foot and suffered brain damage which has left him with a speech impediment and depression. Of the group of 16 colleagues and three tourists who were on the volcano that day, nine were killed in the blast.

Williams looks calm when I meet him in Dublin's Shelbourne Hotel, a far cry from the terrain he is accustomed to exploring.

He admits that the experience of a book tour is "a different world" from his beloved, unpredictable "geological beasts" - so described by a former associate Maurice Kraft, whose passion for photographing erupting volcanoes cost him his life (alongside his wife Katia) at Unzen, in Japan, in 1991.

Nevertheless, Williams's speech is still affected by the accident and, as we sip our tea, he downs a handful of pills. He has new psychiatrists, he explains, who have diagnosed depression and are helping him "come out from under the black cloud".

His marriage did not survive the personality changes brought on by the accident. Although he and his wife Lynda still live in Phoenix, Arizona, and work at Arizona State University (where she is a geologist), they have recently divorced: "We still have family dinners twice a week and get on well," he muses. "But the only reason we both stick in the area is so we can be near the kids."

And although his book, which he dictated to co-author Fen Montaigne, is a vivid and fascinating account that has already been successful in the US, Williams's life continues to be dogged by Galeras: "Oh, a new book has been published in the US by a woman called Victoria Bruce, claiming to be the true account of what happened at Galeras." He says she quotes only one scientist, Bernard Chouet who "believes I have stolen his limelight".

Chouet wrote a report for the US Geological Survey in 1991 about the gases being emitted at Galeras, showing that as long as the volcano kept regularly "de-gassing" it was relatively safe. An end to de-gassing accompanied by a particular type of tremor which indicates magmatic fluids and gases are on the rise (known as long period earthquakes), along with Williams looks weary. Although he claims he feels no guilt for having led the expedition, it is clear he has endured years of having to justify leading a journey that went horribly wrong. As he recalls his visits to the families of the dead, his eyes fill up with tears.

"At least I could tell them the deaths had been instant." Lyudmila, the widow of Igor Menyailov (a highly-regarded Russian volcanologist), had become a recluse, giving up her own work on volcanology in her grief over her lost husband. Monica, the widow of Jose Arles Zapata (who worked for Colombia's Geological Survey) could not understand why Jose had adored this work that had killed him: "I was able to tell her it was valuable work and that was a relief to her," says Williams.

He makes return visits to Galeras accompanied by Marta Calvache, his former PhD student and one of the team who rescued him after the blast. But Galeras is now "finished". He is currently most interested in a volcano in Mexico, Popocatepetl ("The Smoking Man"), which is located about 35 miles from Mexico city: "I think Popo (as it is known locally), is most in need of monitoring right now."

Williams helped local research teams to procure a COSPEC (correlation spectrometer), which measures emissions of suphur dioxide from the magma: "You point it above your head towards the plume of hot gas," explains Williams (unlike a seismometer, used in earthquake research, a COSPEC needs a person to operate it).

"Anything over 1,000 tonnes of sulphuric gas being released per day is dangerous". When Pop erupted last year, preparations had been streamlined. The eruption had been anticipated thanks to COSPEC sampling, observation of deformation , and the know-how of earthquake experts ( who did not want to be caught out again after the earthquake which devastated Mexico city in 1985). Because of a simple volcano warning system, most of the tens of thousands of local residents were evacuated in time.

The death toll over the last 225 years due to volcanic eruptions has been more than 220,000, estimates Williams. While some die engulfed in the pyroclastic flow of gas and ash, others are drowned in massive waves created by the eruption, or swept away by mudflows. "Today," warns Williams, "500 million people live within reach of an eruption." And yet he says proudly: "I love my work. I go to unique places where I'm at the edge of knowledge and experience. We scientists are detectives, struggling to solve a three-dimensional puzzle. You have to make your best judgements about the missing pieces. You are led by your experiences with previous volcanoes. The learning is transferred from Etna to Guatamala."

As a result of the fatal eruption at Galeras in January 1993 - and subsequent blasts that year - more potential volcanic warning signs have been recognised, notably tornillos (screw-shaped signals on seismographs). And yet this clue is not always reliable either, as Galeras did not erupt on two occasions after tornillos appeared.

Will there be another blast somewhere on the planet on the scale of the one in Siberia, 248 million years ago, when eruptions lasting thousands of years led to the extinction of 95 per cent of all ocean and 70 per cent of all land species?

Williams can't say. "We are very unsure about when big scale eruptions will happen."

He hopes that volcanology will attract funding for more research.

The world's few hundred committed and fearless volcanologists must climb into caters to carry out their research knowing that volcanoes have no season and can blow at any time.

Williams concludes" Volcanologists are emergency room doctors. We clamber on volcanoes because it is the best way to understand their behaviour. But we're also hooked on the thrill of climbing into the crater, of confronting so monumental a force. No place on earth leaves me feeling as alive as a volcano does".