A boyhood fascination with the natural world sparked Richard Fortey's interest in the geologies that have shaped human development.

Harriet Swain meets a hands-on adventurer.

Richard Fortey leads the way - past a patch of rainforest, past simulated dinosaurs shrieking from computer screens, past crowds of excited schoolchildren, through an unprepossessing door and into the bowels of the Natural History Museum. This is where the hidden work happens that drives the collection of treasures above. And here are the researchers who, dipping into files and computer records, pulling open cabinets of specimens, classifying and consulting, attempt to give shape to the wider natural world.

Fortey, a senior palaeontologist, has been researching his favourite extinct animals, trilobites, at the Natural History Museum for more than 30 years. But this is neither as narrowly focused nor as static an occupation as it sounds. For Fortey is a believer in the value of detailed knowledge to inform the big picture, and vice versa. The subject of his last but one book was his beloved trilobite, although it also explored questions of evolution and extinction, scientific endeavour and time; the book before that tackled life. "I like to think of it as looking through the same optical instrument," he says, "but the wrong way round."

Similarly, although he has spent three decades at the museum, he is preoccupied with movement - from the shifting nature of science, in which fresh discoveries succeed one another, to movement and change in the landscape and the way this influences human experience.

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His latest book, The Earth: An Intimate History , explores the impact of movement at the deepest level, showing how the slow shifting of tectonic plates in the earth's crust over 3.5 billion years has been responsible for mountain ranges, earthquakes, volcanoes, chains of islands, the shapes of continents, and how the landscapes, rock-types and water courses that result have, in turn, affected every aspect of our lives. "Whatever the surface decoration, geological truths determine much of the reality and character of cities; the stone that builds them, the height to which towers can rise," he writes. "The geology, in turn, is often related to tectonic plates: a deeper reality still, arbiter of the shape of the world."

The premise of the book also demanded a good bit of movement from Fortey himself. Conscious that most people respond to geology only at the local level, through its manifestation in the natural world, he decided to visit places where movements in "the motor" of the earth have affected its surface in particularly obvious ways. Hence, he travels to the Hawaiian islands, "where you can go to see the world being made"; to the Alps, "a place where nature has apparently relished stirring up the strata on such a scale as to make wriggling rock conundra to torment the minds of scientists"; to the Grand Canyon; to the Deccan Traps in India; to Scourie Dyke in Scotland.

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His aim is to identify areas not only with striking geologies, but where that geology has clearly influenced human culture. His starting point, therefore, is the Bay of Naples and Mount Vesuvius, where volcanic eruptions have been responsible both for destroying cities and for sustaining them through fertile lava-rich soil.

But he also wants to tell the story of the study of geology, which, as he relates it, is again about movement. "The story of geology has been one of letting go of permanence," he writes. "From a world created just as it should be by God, we now have a world in flux." He describes the scientists and scientific controversies that have succeeded each other over the years and that have laid the foundations for current theories, which, in turn, are likely to be succeeded themselves. For him, all questions in science are merely "journeys towards the right answer".

Some of these journeys appeal to Fortey more than others. Geology is now conducted in a much more theoretical way, with greater reliance on computers, remote sensors and mathematical modelling. "There is a feeling sometimes in the community that old-fashioned geologists like me, looking for fossils and making maps, have become a bit passe," he says. "The old image of the grizzled, incredibly tough geologist is perhaps a bit dated."

He admits that much useful work has come out of the theoretical approach, but one reason for writing the book was his belief that the new kind of geology - resembling chemistry or physics more than hands-on science - is less generally accessible. He feels passionately that geology is something that should excite everyone. "I won't name names," he says, "but there are some scientists who feel geology should become more, not less, reductionist and who want to recruit research students from mathematics." Many students are all too eager to avoid getting stuck into fieldwork. "A lot of them are quite wimpy these days," he says.

Fortey is certainly not that. He was brought up in Ealing, west London, but has been fascinated by the natural world in a hands-on way since childhood.

He started bird-watching and mushrooming - hobbies he continues today - as a schoolboy and was soon taking off on trips to Wales to hunt for fossils.

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His book Life: An Unauthorised Biography begins with a description of a field trip he took, just after leaving Cambridge, to the Arctic island of Spitsbergen, where he spent weeks chipping away at rocks in freezing gales by day and trying to keep warm in a tiny tent by night, surviving on porridge and dried meat. It would be many people's idea of hell, but it is clear that he was in his element - indeed it was then that his future career path was set - and he still loves field trips today.

Even if not everyone shares his enthusiasm for standing on blustery coasts looking for ancient rocks, he believes that most people relate to the world as natural historians rather than theoreticians. "It seems to me that quite a lot of science writing fails because scientists find theory so riveting that they expect this mythical figure, "the intelligent layman", to be as fascinated by it as they are," he says.

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Instead, he wants to make his books engage people as if they were reading a novel. He employs a poetic and gossipy style, with vivid descriptions of landscapes and witty accounts of the scientists and scientific controversies that have come to the fore over the years. He admits that he enjoys "the process of writing as writing" - he has written short stories and poetry, humour and pastiche - and his books aim to relate what is already known and what still remains to be known rather than pushing a particular scientific line or theory.

The Earth , he says, is an "anti-textbook" in that, instead of starting by propounding a broad theory, which is then explored through more detailed examples, he begins with examples on the ground and only in the final chapter, with a sweeping bird's-eye view of the world, does he explore a "unified theory of the earth". He is anxious that the "I" in his books is not "I, the great scientist", but "I, the observer" - the kind of person you want by your side if you are going around the Bay of Naples. "I do have the urge to explain, but also to amuse, entertain and charm the reader," he says.

Yet there is more to his books than charm. He also feels strongly about the natural world - "I revel in the complexity of things," he says - and it is a soapbox he finds himself keener to climb on the older he gets. His all-encompassing approach, his attempt to relate geology to human culture and history and present the big picture is, at the most basic level, an attempt to encourage people to appreciate the landscape more, to realise they cannot "**** it about too much". It is, he says, "enormously complex and should be admired for its complexity". He would be happy if he could make people realise exactly what extinction means, to make them feel the absence of things lost, just as he feels, "quite viscerally", the loss of habitats he knew when he was young. He is sad that his children will never experience, as he did, the sight of chalk downland filled with orchids.

These habitats still have a special hold over him - "There is something about familiar places that demands my affection, although it is strengthened by travelling somewhere new," he says - and one of his favourite pastimes is still mucking about on the Welsh borderland in Shropshire, where he used to spend time with his father while he was fly-fishing.

His book notes that human beings seem programmed to love their home territory but also that "our patch is actually linked to every other patch.

Perhaps realising that we are all small creatures riding piggy-back atop our own tectonic plates located within the irregular chequerboard of the earth might enforce a proper sense of humility in our arrogant little species."

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The Earth: An Intimate History is published by HarperCollins, ?25.00.