An arctic tern coasts by at eye level. Against the blue of sky and the black and purple of distant mountains, its feathers glisten in the sun like snow. It rides the wind, tilting to catch invisible puffs and downturns. Then, as I watch, it swoops toward the dark rocks below.

Oh, boy. Sometimes it's best not to look down. The horse I am riding is picking her way along a curving trail that forms a narrow, slanted shelf on a mountain created by a volcano. Though she plants her hoofs neatly between stones the color and shape of cooling charcoal briquettes, each step loosens bits of gravel that rattle down the slope and ricochet off the boulders below, falling, falling until they're tiny, black specks and then ... are gone.

I suck in my breath. Half my brain signals, "Danger. Danger." The other half, transfixed by the sight of a meadow gleaming in the distance like a tiny emerald and a glorious, white, foamy waterfall rushing down the mountain, flashes: "Get the camera!"

Caution wins. The click of a shutter could startle my horse with genuinely dire results. Plus I can't take a picture. I'm too scared to let go of the reins. "Look straight ahead. Look straight ahead," I repeat to myself. "Isn't that what they tell mountain climbers?"

It seems to work: I can feel my heart slowing. Besides, the horse in front of us really is a beautiful animal. Dappled white and steel gray with a silky silver tail that brushes the ground, it, too, is sure-footed and strong. Its rider is from Australia and is one of the four other tourists inching along this rocky ledge. The group also includes three riders from the United States (counting me) and one from England, and as our mounts pick their way around the canyon and down an alarmingly steep slope, I think about the reasons we've come here - to Iceland.

Strangers until united on this three-night, four-day horseback riding expedition, we're all searching for adventure, great scenery and horses, horses, horses.

The ride is organized by Eldhestar, a farm southeast of Reykjavik, the capital of Iceland where nearly half of the country's 265,000 residents live. For four days we're being led over hill and dale, through rivers and lava fields by Susan Carlsson, a Swedish horsewoman of great humor and patience who has spent her last four summers guiding tourists-on-horseback.

The experience is a horse lover's dream: Each day we're in the saddle from five to seven hours and cover about 20 kilometers. At night we leave the horses at mountain farms and are driven back to Eldhestar. In the morning, we return to hit the trail again.

By the end of the trip, there's no question that we've found all that we were looking for, plus had the pleasure of facing the physical rigors of hours of trail riding - and surmounting them. (OK, I admit that I had moments of real doubt.)

The trip is neither inexpensive nor for the luxury-lover, but by its end I understand why horseback riding long has been popular among Icelanders and now represents a growing segment of the tourism industry here.

Glaciers to hot springs

Iceland is just plain cool, and I don't mean the weather. This is a land of extraordinary and disparate sights from glaciers and hot springs to black lava fields and dark green fields of moss so deep and cushiony that you literally sink into them. And as we ride through mountains and meadows, and meander along icy rivers, I spot black-and-white arctic terns, brown-speckled ptarmigan, harlequin ducks and white-tailed eagles.

Geologists think of Iceland, an island that's about the size of Virginia, as just an infant. After all, it is only 16 million to 20 million years old. It sits atop the Mid-Atlantic Ridge - the point at which the European-African and American tectonic plates are pulling apart at a rate of about an inch per year. That means the island is being slowly torn in half, and volcanoes spew magma from deep within the earth that fills the crack to overflowing - then hardens into an earthen crust.

The result is a land of peculiar and fickle beauty: Increment by it, Iceland is getting larger, and its growing pains manifest themselves as volcanoes, earthquakes, weird bubbling, boiling hot springs and geysers. In the last few centuries, Iceland has experienced some form of volcanic activity every five or so years. Some are minor - and have been dubbed "tourist eruptions" - such as the 1991 eruption of Mount Hekla. Or a 1963 eruption which began in the ocean floor near the Westmann islands and created a whole new island that is now called Surtsey. Others cause horrible destruction; in 1973, on the island of Heimaey, lava buried about one-third of the town.

One week before I came to Iceland, say the Eldhestar guides, there was an earthquake that ranked 5.3 on the Richter scale. And on the day that I landed at Reykjavik airport, there was another, smaller earthquake at the farm, but its lesser tremors weren't felt in the city.

Iceland is situated just below the Arctic Circle, which means in winter, daylight lasts only a few hours. But in summer! From June to mid-August, the light varies from bright sunlight at noon to a bluish, shadowy light at midnight - but it never goes away.

No one wants to waste a minute of it: Dinner time is around 9 p.m., late night swims are taken frequently, midnight horseback rides or golf games are common, and real partyers wouldn't dream of throwing a bash until the wee morning hours.

Despite Iceland's subarctic status, the Gulf Stream curls around the island to keep temperatures tolerable. I went in June - when temperatures range from 50 to 60 degrees - and got lucky. For five days, the sun shone and the vivid blue skies were dotted with plumed cumulous clouds. Mountain temperatures, however, did drop and rise rapidly, forcing us quickly to add or subtract clothing layers. And I frequently watched from a mountaintop as huge rain clouds pelted the valley below.

Here the horses, too, are unusual. The Icelandic horse is a breed, like the elegant, nervous thoroughbreds or enormous, showy Clydesdales. But these horses, brought to Iceland from Norway during the ninth century by Viking settlers, are relatively small (about the size of large ponies), and have personalities like none other I've ever ridden.

They seem both fey and dependable, feisty and docile. And I got a real kick out of watching them use their teeth to groom each other; three or four would line up in what looked to be a conga line of equine cooperation.

Icelanders point out that because their island-country is so isolated the breed has remained pure for centuries. In fact, when the Viking settlers established in 930 what has become the world's oldest surviving Parliament, one of its first acts was to ban all other breeds.

Horse lovers will be intrigued by the Icelandic horse for another reason, as well. Unlike many breeds, they're considered "five-gaited." In other words, while most horses walk, trot, canter and gallop, these horses have an extra gait: the tolt, a smooth gait that is a joy to experience. (There is actually a sixth gait, called the pace, but not all Icelandic horses have it.)

With their friendly personalities, sturdy little bodies, long tails and thick, tousled manes, Icelandic horses seem to embody the near-magic characteristics of storybook horses - those of Black Stallion's ilk. Or, at least it's fun to think so.

"It's like they're wild horses that have decided to let you ride them," says Anita Rayne, an attorney from Wisconsin who has come here with her 11-year-old daughter.

If I had had the money, I would have brought one home.

Attractively empty

The trip from Reykjavik to Eldhestar, which means "fire horse" in Icelandic, takes about 45 minutes. The farm sends a van to pick me up at the Hotel Saga, located near the town center. Three Swedes already sit in the back seat; they're coming for a one-day ride and then plan to go hiking.

The farm sits in a valley on a flat, green field bounded on three sides by mountains and, in the distance, by a river that leads to the sea. But to get there, we ride through miles of lava fields that were formed thousands of year ago by a volcano gushing red-hot molten rock. There are no trees in sight, and the dark lumps and bumps of the fields, which stretch on and on until they merge with the mountains, look as though they were formed by mischievous children of giants melting enormous gray and black crayons.

Emptiness, I begin to realize, is one of the great attractions of Iceland. There is something restful about riding a horse for four days and passing only one hiker. Or looking out across a valley and seeing no electrical wires, no skyscrapers, no billboards, no satellite dishes. Even the lava fields have a bizarre, barren beauty, like a desert at first glance, before its unusual colors and shapes and life forms reveal themselves. But watch for a while, and it becomes clear that "empty" or not, there is much to see in Iceland.

A dusting of green-gray lichens so thin that black shows through covers the petrified lava. Occasional clumps of lupine stand out against the sobriety of the rocks, their blossoms pointing skyward like the arms of deep-purple candelabra. Every now and then, a ewe and her lamb (or sometimes twins) perch atop a dark lump, their slender legs folded neatly under them; babies in precise imitation of mothers. And in the distance, the crisp white of glaciers stands out against the dark mountains.

At last, the road rises, falls and twists, and there, in a valley, is the farm.

No trees soften the contrast of white stucco house and barn against green grass. Two small cabins, built of wood imported from Russia, sit near the farmhouse in a garden, where the farmhands are painstakingly nurturing a few spindly, silver-green aspens. Each cabin holds bunks for nine guests, and I claim a bed by tossing my gear on it. (The five members of my group wind up sharing this cabin.)

The two Americans - Anita and daughter Kaija - arrived yesterday. As evening approaches, we are joined by the man from Australia and the Englishwoman. We gather around the dinner table to eat and to meet our guide, Susan (pronounced in Swedish Sue-sawhn.)

Meals are served boarding-house style, with the guides doing double-duty as cooks. Other tourists - returning from or leaving on rides - make appearances from meal to meal. Some are here for two days; some for a week. Conversations are a jumble of Swedish, Finnish, English and Icelandic.

I liked the food, which is a good thing because - just as when Mom cooks - if you don't care for it, you'll go hungry. Breakfasts consist of excellent coffee, soft-boiled eggs, breads, cheeses and muesli, the cereal of raw oats and nuts; dinners range from haddock and cheese casserole to lamb chops and peas. One night we eat skyr, a yogurt-like specialty that can be sweet - or not. We all sprinkle it with brown sugar and pronounce it delicious.

Over coffee, Susan questions us about our riding abilities. "It is important to be riding the right horse," she explains.

I could not agree more. As a girl, I spent every moment I could at the local stables and rode as often as I could afford it, but that was a long time ago. And who wants to be galloping through the mountains on an out-of-control horse?

Our expertise breaks down this way: Anita owned a horse as a child and is a good rider, but is out of practice. Eleven-year-old Kaija has been taking lessons recently and is quite accomplished. The Australian is a polished rider who is determined to keep up with the Englishwoman - no matter what. And the Englishwoman, who informs us that she has worked on ranches in Oklahoma and Texas, is a superb horsewoman. "I like a challenge," she says.

Already I dislike her.

Ridges and valleys

"This is Blum. It means flower. Blum doesn't like to be mounted, so I will hold him for you," says the young guide who is helping Susan saddle the horses.

Whoa. Blum sure doesn't. I'm in the saddle, but that's about all: My horse is galloping around the corral, trying to persuade me to get off now. After a few minutes of this, I swallow my pride and ask for another horse.

Maybe one that is a little less, um, sprightly.

(Humbleness pays off. Over the next four days, I'm assigned three different and wonderful horses.)

Susan, sitting on a spunky gray, beckons to us. And we're off.

For the next four days, we walk up and down the ridges and valleys of mountains known as Jorukleif, where a wicked troll is said to be hiding. From the top of one peak, we peer down at Iceland's largest lake, the serene, blue Thingvallavatn. From this height, we can also see Althing, a cleft of rocks where for hundreds of years, Icelanders held their Parliament.

Then we walk slowly and carefully through lava fields studded with boulders and gravel, or tolt through fields of grass and moss so spongy that the horses' hoofs leave deep marks - which gradually disappear. We tramp along the edges of two volcanic lakes called Kattatjarnir, kicking up fine sprays of water that glisten in the sun.

As the hours pass, we creep up razorback trails and cantor through icy rivers. When the slopes are too steep, we dismount to lead our horses carefully over paths of such sheer rock that their hoofs slip with each step, making terrifying scraping noises.

Sometimes we pass a handful of men herding as many as 40 horses into the mountains in search of fresh pastures to graze in over the summer. The horses surge around us, and the men nod, but don't stop.

Once my horse shies off the trail, snorting and tossing her head - and nearly tossing me off, as well. It's understandable: Without warning, the green earth is belching boiling, black mud as though from nowhere.

There are rewards for perseverance. We picnic - the riders on hard-boiled-egg sandwiches, the horses on grass - in a hidden canyon known as Marardalur, or the Valley of the Horses. Oddly shaped rocks surround Marardalur, giving it the feel of a protected and magical place, a spot where indeed elves or mythical beasts may be hiding. Perhaps it is: Susan explains that the meadow was formed when Odin's eight-legged horse stepped down from the heavens into the moss and left a giant hoofprint.

By our fourth day of riding, my muscles are tired, but I feel at ease with my horse and, frankly, quite pleased with my riding prowess. And our biggest reward is yet to come: On our last afternoon, we stop in yet another sunny meadow to relax. But this time we change into our bathing suits and tiptoe along a hot spring that bubbles and steams through the field. We search until we find the perfect spot: a fork in the spring where a tiny tributary feeds icy water into the hot.

We sink up to our necks in the steaming water, and, as we look around the black and white lambs grazing in the moss, at the blue sky, and the jagged charcoal mountains, Anita sighs and says, "I don't know of anything else like this."

WHEN YOU GO

Getting there: Icelandair flies direct from BWI to Reykjavik. The flight is less than six hours; round-trip airfare ranges from $500 to $879, depending on promotional rates. The "fly-bus," a shuttle that costs about $10 one way, transports visitors from the airport to most Reykjavik hotels. Eldhestar, one of many farms that organize riding expeditions, offers van service from most hotels to farm.

Tips:

* Bring a camera. Sights you'll want to capture seem to occur every minute, whether a bubbling hot spring or a field filled with newborn lambs. Photo supplies are costly: Pack plenty of film.

* Come prepared to ride. I found long underwear indispensible: After 6-7 hours in the saddle, those stirrup leathers can really chafe. A windbreaker and gloves (to protect against the rigors of riding and the sometimes chilly temperature) came in second and third on the most-handy-items-to-have list. Also, Americans like to ride in jeans, but I recommend jodhpurs or other riding pants.

* Make sure your boots are comfortable. Most of the time you'll be in the saddle, but when the slopes get very steep, you'll be walking and leading your horse.

* Be honest about your riding abilities when booking your trip. And don't be afraid to ask for a calmer horse (or, heck, a more fiery one).

* Pack a snack. If you're the kind of person who likes to nibble at odd hours, remember that the farm operates like, well, a farm. Food is only available at meals.

* Come prepared to get along. If you hate to meet new people, this adventure may not be for you.

Things to do:

If you have extra time in Reykjavik, try:

* The Pearl - an enormous dome of a restaurant built atop tanks that hold about 24 million cubic liters of hot water that's used to heat the city. It offers fine dining and a 360-degree view.

* Swimming - Icelanders really know how to design swimming pools. I took bus line No. 5 from Hotel Saga near the city center to Laugardalar Swimming Pool, a large city-run sports park. My hotel (like many) offers vouchers to guests who want to swim, and the bus costs about 120 Krona or $1.50. If you forgot your bathing suit, you can rent one at the pool. Once there, do laps in the Olympic size pool. Kick back in either the 3-foot or 5-foot hot-water pools. Or slip and slide down the three-story-high, spiral slide into the cool waters of yet another pool.

* A walk through city center. Nearly all the buildings in Reykjavik are made of white or gray stucco with colorful trim, which gives the city a quaint uniformity. Add window boxes and a lake with baby ducks, and you have real charm.

* Take a break for a carafe of strong, dark coffee at any of a number of cafes in the city center. Most public spaces have art created by local artists for sale and the cafe I chose, Cafe Salon Island, was no different. Or, if you're a night owl, stay up till midnight, then hit the bars. (The bars don't get cranking till late.)

Lodging:

* Hotel Saga, 354-552-9900 (room rates range from $192 single to $248 double).

* Hotel Borg, 354-551-1440 (room rates range from $165 single to $230 double).

Information: For maps, details about horseback riding, hiking, fishing or other tourist information, contact the Iceland Tourist Board, 655 Third Ave., New York, N.Y. 10017; 212-949-2333.

For information about Eldhestar riding tours, call Hrodmar Bjarnason, 354-483-4884 (phone); 354-486-5577 (fax); e-mail www.debugger.se/eldhestar.

AN IDEAL DAY AT ELDHESTAR

4 a.m.: Wake up to strange noises. Realize a ewe and her lambs are clickety-clacking their way across the cabin porch. Chase them, then go back to sleep.

6:30 a.m.: Up again. Since it never gets dark in Iceland during summer, it's tough to sleep in. Hustle over to the outhouse for a shower.

8 a.m.: Breakfast on cereal, cheeses and breads in the big farmhouse with the other members of your horseback-riding tour.

9 a.m.: Van leaves to drive you and four other riders in your group to a farm several miles into the mountains where the horses spent the night.

9:50 a.m.: Clamber into the saddle. Wince at sore muscles.

10:15 a.m.: Ride through lava fields where boulders and rocks make the footing tricky.

10:50 a.m.: Try, without dismounting, to take a picture of a lamb as it scampers across the fields. Contemplate the purple mountains in the distance as the trail gets steeper and becomes rockier as the horses go deeper into the mountains.

11:15 a.m.: The trail gets so steep everyone dismounts. The passage now is so narrow that there is room for only one horse and no rider. You cautiously lead your horse between boulders so large that they tower ominously over your head.

12:30 p.m.: Suddenly, there it is: Maradalur, the Valley of the Horse, a hidden meadow surrounded on all sides by weirdly shaped gray rocks. We stop here for lunch: hard-boiled-egg sandwiches and a box of juice. The grass is so thick and spongy that you could easily fall asleep.

1:30 p.m.: On the road again.

2:30 p.m.: The trail is grassy now, not rocky, and less steep than before. You ask your horse to tolt - or to move in a fast, smooth gait that is unique to Icelandic horses. She responds perfectly. Even though she's doing all the work, you feel as though you've accomplished something.

4:15 p.m.: The mountains are still superb and you're still having fun, but your muscles are telling you enough's enough.

4:30 p.m.: You've made it! You leave the horses at another farm high in the mountains for the night and head by van back to the farm.

5:30 p.m.: Go to the city swimming pool in Hveragerdi, the nearby village, to soak happily in the hot pools, dip into the cold pool and try out the sauna. A perfect end to a long day.

8 p.m.: Back at Eldhestar, it's dinner time. Tonight's menu includes lamb stew, lettuce salad and slices of cake. And, of course, strong, dark coffee.

10 p.m.: Go back to your cabin, but it's still light as day. Get out a deck of cards and play a game of hearts with your fellow horseback riders until midnight. The Australian wins.

Midnight: Tumble into bed, exhausted but happy.

Pub Date: 7/12/98