High Quality Content by WIKIPEDIA articles! Java is almost entirely of volcanic origin, and contains numerous volcanoes, 20 of which have been active in recent historical times. As is the case for many other Indonesiain islands, volcanoes have played a vital role in the geological and human history of Java. Indeed, land is created on Java as a result of lava flows, ash deposits, and mud flows (lahars). Volcanoes are a major contributor to the immense fertility of Java, as natural erosion transports volcanic material as alluvium to the island`s plains, forming thick layers of fertile sediment. The benefit is not just in the immediate vicinity of the volcano, with fine ash emitted from eruptions being dispersed over wide areas.