Volcanoes erupted to make the dramatic terrain of Arusha National Park in Tanzania, just east of the huge fault known as the Great Rift Valley. They spewed fire and lava into the air 20 million years ago, then collapsed into two big calderas, the Ngurdoto and Meru craters, which mark the western and eastern boundaries of the 52-squre-mile park. Depressions in boiling mud became the Momela Lakes. Today, these lakes are fed by underground streams that leach salt from the alkaline soil and support algae, a few small fish, and thousands of birds, especially grebes and flamingos.

Mount Meru remains a volcano, though dormant since its last conniption in 1910, a lava-streaked peak that rises to 14,979 feet encased by lush forests and bare rock. The fifth highest mountain in Africa, Mt. Meru is favored by climbers who want to avoid the heavier foot traffic on Mt. Kilimanjaro less than 50 miles away. To ensure that climbers have time to acclimatize and to enjoy the most scenically beautiful route up the mountain, Thomson Safaris leads its Mt. Meru expedition in four days, three nights.

The park teems with wildlife, largely because human population pressures are increasing outside the confines. People are not permitted to live in Arusha National Park, although it is named for the Arusha tribe, whose people who once lived there. During the 1900s, large sections of the park were used as a cattle ranch and then converted to wildlife sanctuary. The park was formed in 1960 and called Ngurdoto Crater National Park until Mt. Meru was added in 1967.

Each of the three main areas, Ngurdoto Crater, the Momela Lakes, and Mt. Meru, contains plants and animals suited to its particular altitude and geology. Ngurdoto Crater is ringed by montane forests, but swamps cover the crater floor. The forests host a wide range of trees, from common cedars to varieties that are so rare outside of Africa that no popular names exist for them in English. Some of the trees bear fruit like mangoes, olives, dates and figs. Primates rule the branches and, of course, eat the fruit. Olive-colored baboon mothers carry their babies like little jockeys, while mature males perch pensively nearby, wisely watching from poodle-nosed faces circled by full manes. Colobus monkeys groom each other's black bodies and long, white capes. Down below, in cedar groves, hide the shy bushbuck and the little red duiker, a mostly nocturnal antelope. A herd of Burchell's zebras lives in Serengeti Ndogo, the "little Serengeti" west of Ngurdoto Crater.

Look down to the swampy crater floor, 4,838 feet below and nearly two miles in diameter, to see a criss-cross pattern of animal trails. Buffaloes are attracted by permanent water sources and mud wallows that keep them cool, and klipspringers climb the steep cliffsides. Mosses, ferns, and even delicate orchids thrive in the misty atmosphere of the swamps. Lichens hang from the trees, along with a filmy combination of fungus and algae that are so intimately intertwined that they are classified, together, as a single species. Buffaloes and elephants pose a danger in some areas of the park, so visitors on foot may need to be accompanied by an armed ranger.

Giraffes commonly roam around the Momela Lakes, as do hyenas and elephants. Little troops of banded mongooses follow the trails of elephants and buffaloes to eat the dung beetles that collect in their wake. Hippopotamuses peek out of the muddy water, sun themselves on the shore, and wallow in the mud.

Near Mt. Meru, watch for warthogs. You may see a mother and her baby swine, or a male with curved tusks, emerging from a burrow stolen from some other animal.

Within this wide range of habitats, almost 400 species of birds have been recorded. Some migrate from Europe and live in the park from October to April, while others are permanently resident in the forests. Magnificent raptors like Verreaux's eagles, nest on cliffs during the rainy season. White-necked ravens, and peregrine falcons hover above the crater rims. The lakes attract water birds from cormorants to grebes, herons, egrets, ducks, plovers, ibis and plovers. The crowned crane and saddle-bill stork may put in an appearance.

Among Africa's national parks, Arusha ranks as one of the most rewarding for a short visit. Its small size and dense concentration of ecosystems make it a microcosm of African diversity and beauty.