Celebes shows some evidence of being squeezed between the conflicting forces of the more stable surrounding masses of the Sunda and Sahul shelves. Its complex shape somewhat resembles a capital K, with an extremely long peninsula running northeast from its north-south backbone. There are, therefore, three large gulfs: Tomini (or Gorontalo) to the north, Tolo to the east, and Bone to the south. The coastline is long in relation to the size of the island. The land consists of ranges of mountains cut by deep rift valleys, many of which contain lakes. The island is fringed by coral reefs and is bordered by oceanic troughs in the south. Its northeastern arm, the Minahasa Peninsula, is volcanic and structurally different from the rest of the island, which is composed of a complex of igneous and metamorphic rocks.

The Moluccas consist of a group of roughly 1,000 islands with a combined area that is about two-thirds the size of Java. Halmahera Island is the largest of the group, followed by Ceram and Buru. The Moluccas lie in the same geologically unstable zone as Celebes, although the northern islands are associated more with the Sahul Shelf. Halmahera Island, in the north, is volcanic, as are the islands of the Banda Sea, which are frequently rocked by earthquakes. Most of the northern and central Moluccas have dense vegetation and rugged mountainous interiors where elevations often exceed 3,000 feet (900 metres). Once commonly known as the “Spice Islands,” the Moluccas—especially Ternate, Tidore, Ambon, and Banda Besar—were a source of cloves, nutmeg, and mace, particularly during the 16th and 17th centuries.

Volcanoes

There are over 100 active volcanoes in Indonesia and hundreds more that are considered extinct. They run in a crescent-shaped line along the outer margin of the country, through Sumatra and Java as far as Flores, then north through the Banda Sea to a junction with the volcanoes of northern Celebes. Volcanic eruptions are by no means uncommon. Mount Merapi, which rises to 9,551 feet (2,911 metres) near Yogyakarta (Jogjakarta) in central Java, erupts frequently—often causing extensive destruction to roads, fields, and villages but always greatly benefiting the soil. Mount Kelud (5,679 feet [1,731 metres]), near Kediri in eastern Java, can be particularly devastating, because the water in its large crater lake is thrown out during eruption, causing great mudflows that rush down into the plains and sweep away all that is before them.

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Perhaps the best-known volcano is Krakatoa (Krakatau), situated in the Sunda Strait between Sumatra and Java, which erupted disastrously in 1883. All life on the surrounding island group was destroyed. The eruptions caused tidal waves throughout Southeast Asia, killing tens of thousands of people, and ash clouds that circled the Earth decreased solar radiation and produced spectacular sunsets for more than a year. Another major incident occurred in 1963, when Mount Agung on Bali erupted violently after having been dormant for more than 140 years. In 2006 the drilling of an exploratory petroleum well triggered the eruption of an unusual mud volcano in a heavily populated region of eastern Java. Hot mud flowed voluminously from the well for the next several years, ultimately engulfing dozens of villages, obstructing roads and railways, and displacing tens of thousands of residents. In 2010 Mount Sinabung, in northern Sumatra, erupted after more than 400 years of dormancy, forcing tens of thousands to evacuate their homes.