It's hard to keep a clear head when the ground is shifting beneath your feet. Unless you are a fearless 12-year-old. Sankha Guha and family discover the joys, and fears, of Norway's mighty glaciers, a landscape in perpetual motion

The towers of ice that rise before us might have been multi-storey car parks in another life. They take their place among canyons and crevasses, gullies and overhangs. Water roars through hidden wormholes beneath our feet; nothing is stable; the whole defrosting edifice is sliding inexorably down the valley - terra infirma. Our crampons crunch into the vertical walls to grab a few millimetres of purchase and we use our ice axes to hack and lever ourselves upwards.

We are glacier walking in Norway, and maybe you need to be a child to take this in your stride. My 12-year-old son, Tom, is with me, axe in hand, happily yomping across the frozen waste, chattering to our guide, Arve, utterly unfazed by the surroundings. Nigardsbreen is an outflow from the Jostedal glacier, the biggest in Europe. It covers 485 sq km of mountains and valleys above the western fjords, and every once in a while a finger (if that is the appropriate image for a vast wall of mobile ice) flows down from the heights. We are specks on one of these "fingers".

Looking down the valley, way beneath us now, is the visitors' centre of the Jostedal glacier national park - for some reason shaped like a giant Roman legionnaire's helmet - where a couple of hours earlier we had gasped as we caught first sight of Nigardsbreen. It seemed impossible that London bloke and pre-teen child (whose parameters for the great outdoors are more or less defined by London's Wandsworth Common) could expect to tackle this force of nature.

The flyer for the five-hour glacier walk is bland to the point of denial - there seems little to suggest that walking on glaciers is more hazardous than a run to the corner shop. All we have to do, apparently, is be more than 12 years old, check, have windcheaters, check, warm clothes, check, hiking boots and gloves, double check. And sunglasses - which is a no-brainer on a day such as this. There is a tiny note of caution in the small print: "Glaciers are in motion! Never go on to a glacier without a local guide and NEVER venture in under any glacier." So that's fine then.

Our appointed guide, Arve, is a Viking - a blond giant straight out of central casting with all the buttons and bells except the horned helmet, which, incidentally, was never a Viking thing. He exudes the kind of outdoor can-do vigour that suggests he wrestles bears before breakfast. Polar bears, probably. Anyway he is definitely the kind of guy you want on your side when you take on glaciers.

With Arve leading the way I suspend my disbelief. Tom and I have donned harness and crampons and roped up with nonchalance worthy of Captain Oates. There was an exhilarating intake of breath as we stepped up to the ice wall - feeling the crunch of thousands of ice cubes underfoot, like walking on diamonds the size of the Star of Africa.

Now on the ice, I am suppressing the flight reflex, though something within me is still insisting this is not the natural habitat of sensible humans or their precious offspring. The light is diaphanous. The air is crisp. The sun seems closer than it should be, the sky deeper and the ridges on either side sharper than eagle talons.

Before us is an icy makeover of the Sahara - dunes fringed with black "dust". The surface is largely white but the ice below is streaked with eerie blue stains - as if a delivery of toilet cleaner has spilled from jack-knifed chariot lorries driven by demented Nordic trucker gods. The colour cast we learn later at the Norwegian Glacier Museum is because "ice absorbs more of the red and yellow light, so that more blue light passes through". It still looks about as natural as a landscape painted in Sanilav.

Next we crawl through the "Labyrinth". This is an ice canyon where, Arve explains, the glacier is moving over steeper terrain. The slab is cruising at 10m a day - unevenly. Some parts move faster than others. All is flux. We scramble through the ravines and watercourses of the Labyrinth to emerge on the ice fields of the upper valley. Here we stop for sandwiches and Arve tells us that contrary to reports that glaciers are contracting, Nigardsbreen is expanding. Every summer it pulls back but in winter it grows even further. In recent years it has advanced some 30m. Glaciers, he says, don't follow rules and this one is defying the logic of global warming.

Logic belongs to another world. For the moment we are on a river of blue ice topped by white tumuli through which we can see a vivid green lake. The views are as improbable as they are immense. Tom and I are pinching ourselves - are we really here? Or have we stumbled into an episode of Doctor Who? The possibility that we have jumped dimensions into a sci-fi fantasy is not entirely delusional: throughout the glacier walk we are ambushed by a German TV crew who keep popping out of crevasses to film actors striding purposefully across the icy expanse. Cut!

Back at Vesterland Resort near Sognedal, where the clan Guha is based, it strikes me as odd that in a country where colossal vistas seem to be the birthright of every citizen, the view from our cabin is miserly. It consists of the backs of other cabins and a few parked cars bearing GB stickers. The site seems to have been carefully chosen by an agoraphobic troll.

There is nothing much wrong with the cabin itself - if you have a high tolerance for pine. Tables, chairs, bunks, wardrobes, floors, walls and ceilings are all in the ubiquitous soft wood. But the resort is well equipped and functional if somewhat charmless. Luckily, we have not come all this way to play mini-golf, roast in the sauna or hang out in the launderette. The point of this trip is the outdoors.

Sogne Fjord is more than 200km long and is 1,400m deep in places - the biggest and deepest in the world. The network of waterways that makes up the system is criss-crossed by ferries of varying size. In the mind's eye it is easy to replace them with Viking longships appearing out of the mist, especially as we cross Luster Fjord to the tiny settlement of Urnes.

Tom and his eight-year-old brother Niko are thrilled by the dolphins riding the bow wave of the ferry. Their granny, who is also with us, is more excited by Urnes Stave Church, a Unesco World Heritage monument. The wooden church is described as "Late Viking" and has stood here in defiance of some of the meanest elements on the planet for nearly 1,000 years. The interior is tiny, dark and claustrophobic, but there is something very moving about this statement of community against the odds. The carvings on the exterior of the north wall are reused fragments from an older building - similar to the Celtic style - featuring stylish if bonkers-looking lions (Viking sculptors were not bothered much by verisimilitude) battling with serpents.

Those Vikings knew the meaning of Location, Location, Location. The church stands on top of a hill overlooking the jade-green fjord, with panoramic views of the snow-capped mountains that rise steeply from the depths. The people who worshipped here must have thought they were masters of the universe. The ever-changing play of sky, snow, earth and sea in the region is utterly intoxicating. So grand, so elemental that it should be intimidating, but in summer at least it delivers adventure on a plate and makes Viking heroes of us all - of kids and grannies and stressed-out single dads.

Carried away by these surroundings, I am persuaded to book a helicopter ride. We are instructed to turn up at the Stat Oil petrol station outside Sognedal. On arrival we find a bog-standard pump arrangement and nothing suggestive of aircraft. No H marks the spot; no barriers denote a landing area. In the shop they point us back to a car park. Sure enough, with three minutes to the appointed rendezvous a gnat-size speck appears in the sky at the head of the valley. It gets bigger and louder and suddenly it is kicking up dust, roaring in the car park. The pilot gets out, offers a jaunty hello. "Where would you like to go?"

We settle for the "normal" circuit. Tom and Niko are grinning like mad things. So is their 74-year-old granny. We hurtle into the wide avenue of Sogne Fjord before the pilot presses the up button. We shoot skywards; the horizon tilts at a silly angle; the fjords seem miles below as we clear the plateau and are in a revealed new world of gashed hanging valleys, snow fields and sparkling tarns. The pilot asks if we are up for some fun - and gets a resounding affirmation from the back. He puts the chopper into a sharp left hand bank - stomachs churn as we speed at head height over an ice field before screaming over the edge, an endless drop opens up below. Niko and granny screech in unison.

I ask my mother if she is OK but she is still grinning, or possibly grimacing. Now there is a lake below, saturated pea green and thick as soup. Above the din of the rotors the pilot explains we are flying at about 1,300m ; he is engrossed in chatting and seems oblivious to the approaching rock wall until we seem headed for inevitable contact with a lot of granite. Then at the last moment he pulls the chopper up, up and over the summit with what seems like a few feet to spare. It's his little joke. My knuckles are white - again I check to see if my mother and the boys are looking uncomfortable but they are chortling away. "Better than Colossus at Thorpe Park," offers Tom. "Better than Legoland," says Niko. And there is no praise higher than that.

My top hotel

The cosy Walaker Hotel (00 47 57 68 20 80; in the hamlet of Solvorn has been a family business for more than 300 years. It's run by one of the ninth generation of the Nitter family, the charming 36-year-old Ole Henrik and his mum Oda. The hotel is sublimely located on Luster Fjord next to the ferry jetty. It has grown organically over the centuries; avoid the 1970s block and stay in the beautifully refurbished 1650 courthouse.

My top ferry ride

An absolute must is the two-hour ferry ride from Kaupanger to Gudvangen. It crosses the wide open expanse of Sogne Fjord before turning south into Aurlandsfjord, past isolated settlements with no land access, and hangs a right into Naeroyfjorden, which is just 250m wide in places. The sheer rock faces rise 1,400m from sea level and tower over the boat. On a good day the shadow play of clouds and sunbursts will leave you mesmerised and positively charged with mystical thoughts.

Sankha Guha and family travelled with Fjordline (0191-296 1313; fjordline. and Scantours (020-7554 3530; uk). Fjordline offers return ferry crossings from Newcastle to Stavanger from £470 return for a car and up to four passengers. Scantours offers a four-night itinerary from £475 per person, based on two sharing, including flights to Bergen, transfers and b&b. Child prices (under 12) start at £249. Vesterland Resort (00 47 576 27 100; Norwegian Glacier Museum (Norsk BreMuseum) N-6848 Fjaerland, (00 47 57 69 32 88;