Sumatra

The geography of Sumatra is dominated by a mountain range called Bukit Barisan (lit: "a row of hills"). The mountain range spans nearly 1,700 km (1,100 mi) from the north to the south of the island, and it was formed by movement of the Australian tectonic plate.^[9] The plate moves with a convergence rate of 5.5 cm/year which has created major earthquakes on the western side of Sumatra including the 2004 Sumatra–Andaman earthquake.^[10][11] The tectonic movement has been responsible not only for earthquakes, but also for the formation of magma chambers beneath the island.^[9]

Only one of the 35 active volcanoes, Weh, is separated from the Sumatran mainland. The separation was caused by a large eruption that filled the lowland between Weh and the rest of the mainland with sea water in the Pleistocene epoch. The largest volcano of Sumatra is the supervolcano Toba within the 100 km (62 mi) × 30 km (19 mi) Lake Toba, which was created after a caldera collapse (est. in 74,000 Before Present).^[2] The eruption is estimated to have been at level eight on the VEI scale, the highest level for a volcanic eruption. The highest peak of the mountain range is Mount Kerinci with an elevation of 3,800 m (12,467 ft).

Sunda Strait and Java

The Sunda Strait separates the islands of Sumatra and Java with the volcanic island Krakatoa lying between them. Krakatau erupted violently in 1883, destroying two-thirds of the island and leaving a large caldera under the sea. This cataclysmic explosion was heard as far away as the island of Rodrigues near Mauritius (approx. 4,800 kilometres (3,000 mi) away).^[1] A new parasitic cone, called Anak Krakatau (or the child of Krakatau), rose from the sea at the centre of the caldera in 1930.^[14] The other Krakatau islets from the 1883 eruptions are known as Sertung, Panjang and Rakata.

Java is a relatively small island compared to Sumatra, but it has a higher concentration of active volcanoes. There are 45 active volcanoes on the island excluding 20 small craters and cones in the Dieng volcanic complex and the young cones in the Tengger caldera complex. Some volcanoes are grouped together in the list below because of their close location. Mount Merapi, Semeru and Kelud are the most active volcanoes in Java. Mount Semeru has been continuously erupting since 1967.^[15] Mount Merapi has been named as one of the Decade Volcanoes since 1995.^[16] Ijen has a unique colourful caldera lake which is an extremely acidic natural reservoir (pH<0.3).^[17] There are sulfur mining activities at Ijen, where miners collect highly concentrated sulfur rocks by hand.

Note: Height of Krakatau is of Rakata, not of the active Anak Krakatau

Lesser Sunda Islands

The Lesser Sunda Islands is a small archipelago which, from west to east, consists of Bali, Lombok, Sumbawa, Flores, Sumba and the Timor islands; all are located at the edge of the Australian continental shelf. Volcanoes in the area are formed because of oceanic crusts and the movement of the shelf itself.^[18] Some volcanoes completely form an island, for instance, the Sangeang Api island. Mount Tambora, on Sumbawa island, erupted on 5 April 1815, with a scale 7 on the VEI and is considered the most violent eruption in recorded history.^[3]

Banda Sea

The Banda Sea in the south of the Molucca archipelago includes a small group of islands. Three major tectonic plates beneath the sea, Eurasian, Pacific and Indo-Australian plates, have been converging since the Mesozoic epoch.^[19] Volcanoes in the Banda Sea are mainly islands, but some are submarine volcanoes.

Sulawesi and Sangihe Islands

Four peninsulas dominate the shape of Sulawesi island (formerly known as Celebes). The central part is a high mountainous area, but mostly non-volcanic. Active volcanoes are found in the northern peninsula and continuously stretch to the north to Sangihe Islands. The Sangihe Islands mark the border with the Philippines.

Halmahera

Halmahera island in the north of Molucca archipelago has been formed by the movement of three tectonic plates resulting in two intersecting mountain ranges, which form four rocky peninsulas separated by three deep bays. A volcanic arc stretches from north to south in the west side of Halmahera, some of which are volcanic islands, for instance, Gamalama and Tidore. Gamalama's island name is Ternate and it has been the centre for spice trading since the Portuguese Empire opened a fort in 1512. Due to its location as the centre for spice trading during the Age of Discovery, historical records of volcanic eruptions in Halmahera have been available as far back as the early 16th century.