Bali is continually being formed by volcanic action. The island lies over a major subduction zone where the Indo-Australian plate collides with the rigid Sunda plate with explosive results. A violent eruption of Mt. Agung (3,142 m before the eruption; 3,014 in now) in 1963 showered the mountain's upper slopes with ash and debris that slid off as mudflows, killing thousands of people and laying waste to irrigation networks and rice fields that had been built up over many years. Mt Batur (1,717 in) to the west is also active, with greater frequency but less violence.

A mild, equatorial climate

Lying between 8 and 9 degrees south of the equator, Bali has a short, hot wet season and a longer, cooler dry season. The mountains are wet year round, averaging 2,500 to 3,000 mm (100 to 120 inches) of rain annually, with warm days and cool nights. The lowlands are hotter and drier, but fresh and persistent winds make the climate less oppressive here than elsewhere in the equatorial zone.

The wet season lasts from November to March, and though there are only five or six hours of sunshine a day, this is also the hottest time of year (30-31" C by day, 24-25o C at night). The dry season is from April to October, when southeasterly winds blow up from the cool Australian interior (28-29o C by day, and a pleasant 23" C at night), with seven or eight hours of sunshine daily.

By itself, the rainfall in the lowlands is not enough for wet rice cultivation. In other parts of Indonesia, particularly Java, flood waters following heavy rains can be collected behind dams, but the steep, narrow valleys of Bali offer no good dam sites. Over the centuries, the Balinese have instead devised many sophisticated irrigation systems which optimize the water available from rain and rivers.

Bali's volcanic soils are in fact not naturally well-suited to wet rice cultivation. They are deep, finely textured and well-drained, so water soaks through them rapidly. While this reduces the risk of floods, it wastes precious water. Paradoxically, the solution is vigorous and repeated plugging, which actually renders the soils less permeable. Irrigated areas, moreover, receive a supply of nutrients from river water enriched by domestic effluents.

Man has extensively modified the natural vegetation of Bali. The moist primary forest which is its natural vegetation now covers only 1,010 sq km or 19 percent of Bali's total area, mainly in the western mountains and along the arc of volcanic peaks from Agung to Batukau. About a quarter of the forest is protected in four nature reserves, the largest of which is Bali Barat National Park (763 sq km. Further reserves are planned to protect another quarter of the island's forests.