section_path: Geography/Geology
section_name: Geology
paragraph_text: Geology
  
  Tectonically, most of Indonesia's area is highly unstable, making it a site of numerous volcanoes and frequent earthquakes. It lies on the Pacific Ring of Fire, where the Indo-Australian Plate and the Pacific Plate are pushed under the Eurasian plate, where they melt at about 100 kilometres (62 miles) deep. A string of volcanoes runs through Sumatra, Java, Bali and Nusa Tenggara, and then to the Banda Islands of Maluku to northeastern Sulawesi. Of the 400 volcanoes, around 130 are active. Between 1972 and 1991, there were 29 volcanic eruptions, mostly on Java. Volcanic ash has made agricultural conditions unpredictable in some areas. However, it has also resulted in fertile soils, a factor in historically sustaining the high population densities of Java and Bali.
  
  A massive supervolcano erupted at present-day Lake Toba around 70,000 BCE. It is believed to have caused a global volcanic winter and cooling of the climate and subsequently led to a genetic bottleneck in human evolution, though this is still in debate. The 1815 eruption of Mount Tambora and the 1883 eruption of Krakatoa were among the largest in recorded history. The former caused 92,000 deaths and created an umbrella of volcanic ash that spread and blanketed parts of the archipelago and made much of the Northern Hemisphere without summer in 1816. The latter produced the loudest sound in recorded history and caused 36,000 deaths due to the eruption itself and the resulting tsunamis, with significant additional effects around the world years after the event. Recent catastrophic disasters due to seismic activity include the 2004 Indian Ocean earthquake and the 2006 Yogyakarta earthquake.