The great majority of Earth's volcanoes are found along subduction zones, where one tectonic plate is dipping beneath another; the lower plate begins to melt as it approaches the mantle, magma rises and a chain of volcanoes develops near the edge of the overriding plate. The famous Ring of Fire refers to the numerous subduction zone volcanoes that have developed along the Pacific Rim; these include the Andes, the volcanoes of Central America, the Cascades, the Aleutian Islands, Japan, Taiwan and the Philippines, among others.
But the most concentrated and active swath of subduction volcanoes lies along the western and southern edge of Indonesia, where the Australian Plate is subducting beneath the Eurasian Plate. Having developed within the past 15 million years, the volcanic islands of Sumatra, Java, Bali and others harbor more than 150 active volcanoes and have been the site of some of the most powerful volcanic eruptions in Earth's history: Krakatoa, Toba and Tambora. Over the past week, Merapi, on Java, has begun erupting for the 27th time since 1930, the most active and, in light of the regional population, one of the more deadly volcanoes on our planet.
This concentration of volcanism in Indonesia is, of course, merely a reflection of Earth's current geography and topography. Since it formed, 4.6 billion years ago, our planet's surface has undergone constant change; oceans have opened and closed, continents have merged and rifted apart and the sites of active subduction have shifted across the globe. The intense volcanism in Indonesia and elsewhere is evidence that the evolution of Planet Earth continues to unfold.
