Unlike the volcanoes of Hawaii, where the magmas flow freely and accumulate gradually as spreading lava sheets, Indonesian magmas are highly viscous and move far less readily. On reaching the surface, these slow-moving magmas have time to cool, periodically blocking release of the pent up forces beneath. Thus trapped, great reservoirs of liquid magma accumulate within the volcano, building up pressure until the earth can contain it no longer. Without warning the top of the volcano gives way and the contained magma bursts forth with unimaginable violence.
With the passage of time the volcano, its core now emptied, slowly collapses back within itself to form a giant sunken crater or caldera. Eventually, as fresh magma reaches the surface, new volcanic vents may appear within the caldera, slowly occluding it as discharged ash and lava gradually continue to accumulate.
Mount Batur today
Mt. Batur as we know it today was formed by an eruption in 1917; it is a still active, secondary volcanic cone sporting numerous subsidiary vents around its flanks. Measured from the floor of the crater it rises within the circling embrace of the caldera to a height of ca. 700 meter (1,717 meter / 5,633 feet above sea level).
The eruptions of Mt Batur have been registered since 1804 and since that time 22 eruptions of the volcano have occurred.
Since 1917 Mt Batur erupted three more times (1926, 1974, 1994), on each occasion shifting a little more to the west, creating new sub craters that are referred to as Batur I, II and III respectively.
