As a man on a mission to make geology sexy, Dr Iain Stewart isn't doing too badly. Far from hiding away in the dusty confines of academia, the Scottish-born scientist is fast becoming the poster boy for geo-chic, tagged as the "thinking women's crumpet" and amassing an adoring legion of female fans. Googling his name brings up one website where a group of besotted ladies announce their love for him. "I want to marry Dr Iain Stewart" is one such declaration (although, it must be said, the comments quickly descend into territory not really appropriate for a family newspaper). Later, when I send Stewart the link to the page, he responds with: "Oh my. Will need to keep this well hid." It would appear that the study of rocks has never been more sensual.

Still, as Stewart would be the first to point out, there's a lot more to geology than that. "That's probably the biggest misconception about geology, that it's all about stones," he says, with a chuckle. "People think we love stones; rocks too. There is some truth in that, although those stones and rocks are really just the windows into the past. It's the stories they tell us that are interesting. The key thing about geology is understanding how the planet works." He is sure, however, that people will come round to his way of thinking. "Geology is sexy," he says. "It's got dinosaurs, earthquakes and volcanos. I can't understand why some people might think it's not."

Born in East Kilbride and now based in Plymouth, Stewart's passion for geology was kindled while studying at the University of Strathclyde. His specialist areas of interest include neotectonics (that's earth crustal movements to you and I) and geo-hazards (earthquakes, tsunamis, volcanic eruptions) with much of his field work centring on the volatile Mediterranean region. Of late, the 42-year-old has been juggling his academic career as a lecturer at the University of Plymouth with TV presenting (hence his ever-growing band of geology groupies).

His new TV series, Earth: The Power of the Planet, will see Stewart combine geology with high-octane adventure. The show charts the story of the planet from its birth 4.5 billion years ago, covering the major events that have shaped Earth's history and allowed life to bloom. Stewart travelled to some of the most remote areas of the planet for the five-part series, abseiling into a lava lake in Ethiopia and cave-diving in underwater caverns in Mexico. He will also show how volcanoes saved the planet, how a rainstorm lasted for thousands of years and how the swamp lakes of Siberia are key to global warming.

"I wanted to show what an amazing planet we live on," says Stewart. "The earth is robust and a real survivor. It has endured the most extreme things yet come through with amazing diversity of life - things we take for granted, really. Many of these things are from incredible flukes and good fortune rather than it being destiny that we ended up with this populated place. There have been a few times when life nearly didn't make it and the planet almost became a wasteland."

Stewart is also keen to show how geology impacts on almost every area of our lives. "If we can't grow it, it comes from the rocks and the earth," he says. "Geologists have always had the job of providing resources to society. You can chart the rise and fall of a lot of human history alongside the rise and fall of resources. Geologists understand how the planet works and are therefore in a good position to advise on ways to avert global warming and some of the other threats that are looming in the future."

Knowing how the earth has behaved for the past 4.5bn years, says Stewart, has brought the realisation that it's not the planet that needs saving from humans - rather it's us who are in peril. "The problem is that we humans as a species have probably out-run our time on this planet," he says. "The average species on earth lasts for a couple of million years. We've been on it for that now, so as a statistic it wouldn't be a surprise if we disappeared. The irony of climate change and global warming is that we're getting rid of the environmental conditions that we evolved into. The ice caps are melting and we are cutting down the rainforests. The planet doesn't care - it hasn't had ice caps for most of its history and tropical rainforests come and go - but as humans what we mean by saving the planet is actually saving the world we know and have evolved into."

Despite such insight, Stewart professes not to be a huge eco-warrior. "Like most people, I'm probably somewhere in the middle," he says. "I recycle as well or as badly as anyone else. I take lots of flights and then feel bad about it. Filming this series, however, there were occasions, particularly in Siberia and Greenland, when I looked around me and realised: S***, my kids, or perhaps their kids will never see this.' Those were moments when climate change all seemed very real."

Geology, of course, has always had proud Scottish roots. "Most of the people who have pushed geology along have either been Scottish or Scottish-based," says Stewart. "To a large extent, Scotland is seen as the home of geology. The geology you get here is remarkable. For such a small place it has everything, although I think it is very much under-rated. People love the Highlands and the landscapes, but that diversity of geology and how it has played a fundamental role in understanding our planet worldwide is something I don't think most people really appreciate."

He may appear a dyed-in-the-wool scientific boffin, but Stewart almost followed another career path entirely. In his teens he studied at the Royal Scottish Academy of Music and Drama (RSAMD) in Glasgow. An equity card followed, along with various TV and repertory work - he was cast as Napoleon in the BBC children's series Huntingtower - but Stewart's acting aspirations were cut short at 16 as his O-level exams loomed.

"I wanted to be an actor for a long while but there were always far better people than me," he says. "John Hannah was part of the same rep company in East Kilbride and to see someone like that, I'd think, Oh s***, he's good,' and I knew I wasn't that good. So that made the decision as to whether to go on to drama college or university much easier. John went on to drama college and I went to university to study geography, a nice clean-cut decision that I don't think either of us has regretted."

Married to Paola, 43, Stewart has two daughters, Cara, 10, and Lauren, six. Unlike his army of female admirers, however, his offspring are somewhat less entranced by their father's work. "My youngest daughter gets embarrassed when she sees me on television," says Stewart. "My eldest, she doesn't mind so much because it gives her extra street-cred at school." He laughs. "They like it a little bit but if I start talking about geology too much they will roll their eyes. They also come out with all these geology words - all used completely wrongly, of course."

High on Stewart's wish-list is a trip to Patagonia in South America following in the footsteps of Charles Darwin's Beagle expedition. "Most people are familiar with his theories on evolution, but Darwin started out as a geologist and many of his fundamental observations on that trip are as valid today as they were then," he says.

So, all roads lead back to geology? "Absolutely," says Stewart, laughing again. "Two hundred and fifty years ago it was the main talking point: the age of the earth and how it was formed. We have lost track of some of those questions but even today they are still pretty big questions and in many respects we still don't know the answers to them."

  • Earth: The Power of the Planet starts on BBC2 on November 20 at 9pm.

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