“You’re driving very fast,” Jenny said to Mac when they were in the car. “I always drive fast when I’m trying to save the world,” said Mac. “Well, when you put it like that,” Jenny said, holding on to the dash as the car swerved. “But I’m not going to lie, MacGregor, I’ve had better dates.”

There’s plenty of dialogue like this in the new thriller novel Eruption, in which a plucky crew of understandably self-important volcanologists are in a race against time to stop lava from transforming Hawaii into a latter-day Pompeii. All while trading witty banter. But who wrote it?

Eruption is based on an unfinished manuscript left by Michael Crichton. The Chicago-born writer, who died of cancer in 2008 at the age of 66, was one of the most successful novelists of all time, selling 200m books, among them titles that became blockbuster franchises, from Jurassic Park to Westworld and ER. The manuscript was discovered by his widow Sherri and has now been completed by the no-less-successful writer James Patterson, renowned for several thriller series of his own, from Alex Cross to Women’s Murder Club.

Patterson challenges readers of Eruption to spot where he took over. “There is a definite moment,” he says. “Wonder if your readers can spot it?” Is it when hunky Mac and sexy demolition expert Rebecca Cruz start flirting? “I’m not going to tell you!” snaps Patterson.

Clearly, though, there are passages Crichton could not have written, such as the scene referring to then US president Barack Obama watching the capture of Osama bin Laden in 2011, three years after Crichton’s death. But otherwise, it’s hard to tell.

Patterson, speaking via video call from his Florida home, has previous experience in big collaborations, having written novels with Bill Clinton and Dolly Parton. But neither was like this: in Eruption, two of the world’s bestselling thriller writers, boasting combined sales of 675m, have equal billing on the flame-filled cover. “What Michael created needed someone of his brilliance to complete,” writes Sherri, an actor and producer, in the afterword. “For years, I considered possible collaborators, holding out for that perfect partner. Then I was introduced to James Patterson.”

Why did he accept? For one thing, he says there was, “just the idea of, ‘Oh my God! There’s another Crichton novel! That’s a very cool thing.’ Imagine if there was another Hitchcock movie – who would not want to see that? But the short reason was, ‘Now that I’ve read what I’ve read, I’ve got to know how the damn thing ends!”

But wasn’t it risky? What if he’d messed it up and it left his reputation in tatters. “I don’t think about that stuff,” says Patterson. “I wouldn’t say 100% I don’t care – but 95% I don’t care. I’m not competitive with other writers. I’m competitive with myself.” There were other reasons: “Firstly, I was one of his biggest fans. I had read everything. Secondly, I was intrigued that he had two ticking clocks, which is very unusual. Usually, you have one ticking clock.”

The first concerns Mauna Loa, which is just days away from erupting. That’s terrifying enough, certainly if you can see it from your bathroom window. But it’s the second that throws volcanologists, civil defence experts and the US army into a tizzy. Some military numpty has buried nuclear waste under the volcano. If the lava reaches the canisters, it’s the end of the world.

“That’s based on reality,” says Patterson, “because for a while, the US, Russia and maybe China were dumping a lot of material in the ocean. Until somebody said, ‘Hey, that’s not good.’ So then they started storing this stuff underground. Which isn’t good either.”

Sherri, joining us from her home in Hollywood, says: “You see it in a lot of Michael’s work – the fragility and yet the intensity of nature, and how human interference can be weaponised in the wrong hands.” Take the terrifying moment in Spielberg’s 1993 adaptation of Crichton’s Jurassic Park, in which a cloned velociraptor, on the hunt for human flesh, figures out how to work a door handle.

Jurassic Park was set on a fictional island off Central America but was filmed in Hawaii, which seemed a perfect fit. “When Michael discovered Hawaii,” says Sherri, “it became a very special place for him. He’d go scuba-diving. Then it became a special place for us. We lived there. We were always hiking and he would just throw things out – interesting facts about volcanic activity, ash, the composition of the land.”

Volcanology became his obsession. “When we went to Italy, he visited Pompeii. He had volumes of scientific notes and did video interviews with volcanologists.” His volcano project, given the working title The Black Zone, was one of the few book ideas Crichton discussed with his wife: “He rarely shared what he was working on. His publishers didn’t even know he was writing a dinosaur novel.”

Patterson reckons he and Crichton were similar, in that they were both driven by their parents to achieve. “If I got a 98, my parents would say, ‘Why didn’t you get 100?’ Push, push, push, push. And Michael was one of those people who really just kept pushing himself.” Crichton always wanted to be a writer, says Sherri, but trained as a doctor after learning that there were only a couple of hundred professional novelists earning a good living in America. He didn’t fancy his chances of joining them. “Michael hated medical school,” says Sherri, “but what came out of it were many novels written under the name John Lange. Only eight were published.” There are other unpublished manuscripts in the archive from this period that, Sherri hints, may see the light of day.

Crichton also wrote a little thing called Emergency Department, based on his time at a Boston hospital. The story was optioned by Warner Brothers. “It sat on their shelves for 25 years before it was made into a TV series,” says Sherri. When it was finally turned into ER, the genre-defining juggernaut that ran from 1994 to 2009, Crichton’s creation didn’t just propel George Clooney and Julianna Margulies into the acting stratosphere. It also included a thinly veiled portrait of the writer himself as an unhappy ER ingenue, in the form of Dr John Carter.

The first novel Crichton wrote under his own name, 1969’s The Andromeda Strain, was about the outbreak of an extraterrestrial microbe from a facility in Arizona. It could kill a human in two minutes. The novel established him as the father of the techno-futuristic thriller and he went on to write pacy stories, lightly dusted with science and boasting enough visual flair to make them ready fodder for Hollywood.

Eruption is very Crichton: a thriller about standup Americans struggling heroically to stop the world from ending, while we page-turning readers salute them for their sacrifice and learn a little volcanology in the process. Crichton started writing it in 1994. “He was looking to the future,” says Sherri. “He was always a future-thinker. He wrote Jurassic Park 30 years ago – and now we’re actually cloning things.”

In 1973 he directed Westworld, working from his own screenplay about an adult amusement park with three themed worlds: the wild west, medieval Europe and ancient Pompeii. It starred Yul Brynner as an android gunslinger who starts to malfunction – with terrifying consequences that perfectly prefigure our current fears about AI. And all this half a century ago.

“He was always a prescient thinker,” says Sherri, giving another example: the 1994 novel Disclosure, which was made into a film starring Demi Moore and Michael Douglas. Disclosure is about a woman who makes false sexual harassment allegations about a male colleague. I wonder how that would play in #MeToo era? Then there’s his 1996 novel Airframe, about deaths on an airliner that experiences severe pitch oscillations. It suddenly seems very contemporary, given the recent spate of turbulent flights. Perhaps Disney, which paid $10m for the rights, will finally make the movie.

Eruption, too, has contemporary resonances, not least in its suggestion that past ecological follies will return to bite humanity on the backside. But would Crichton himself have wanted to see it published? Did he leave any instructions? “No, he did not,” Sherri says. “I think Michael was not prepared to go. At the time, we were pregnant, and we felt we had beat the cancer. It was tragic because it was very unexpected.

“My concern is always to do the right thing, to be particular about what gets published and how – and to protect his legacy.” One reason she wanted Eruption to be completed was to honour John Michael, the son the writer never got to see. That placed an additional responsibility on Patterson. “Completing the book is an epilogue for their love story,” he says. “That motivated me to do the best job I could.”

There is, they both admit, a great deal of interest in adapting Eruption for the big screen. That’s understandable for the spectacular location alone: Hawaii’s thrillingly cinematic backdrops worked for the Jurassic Park films, which have grossed more than $6bn globally. But neither want the book to be marred by a hack adaptation. “Jurassic Park elevated the genre,” says Patterson. “We want to elevate the genre with this. We wouldn’t want it to be just another disaster movie. It could be very special.”

Especially if Spielberg directed? “That’d be good!” says Patterson. “You can spread that around.”

• Eruption by Michael Crichton and James Patterson is published on 6 June by Cornerstone. To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply