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  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 high population densities of Java and Bali.