The islands of Indonesia are among the newer pieces of real estate on the globe. They were formed only about fifteen million years ago., as a result of shifts of land under the ocean and volcanic eruptions. Indonesia has some 155 centres of active volcanism.
Java and Bali are the most volcanically active islands in the world with 20 of their volcanoes having been active in historical times.
In addition, 13 older volcanoes have active solfatara (vents emitting hydrogen sulphide and other sulphurus gases) and fumaroles (vents emitting high pressure steam and other gases.
Some peaks such as Semuru, Merapi Agung, and Ciremai have classic shapes, and some, such as Tengger and Batur, have dramatic calderas where the peak has been blown off leaving a plain or a lake with smaller peaks within.
The Batur complex has been described as one of the world's largest and finest caldera.
Volcanoes have played a crucial role in the geological and human history of Indonesia. Their impact has been positive because they create land through lava flows, ash deposits, and mud flows, forming thick layers of fertile sediment.
But at the same time they have hurt, when they occured.
The frequent geological activity is generating deadly events. Earthquakes are by far the cause of most of the causalties, either directly or indirectly with the tsunamis.
Every body will remember the tsunami of December 2004, where 80,000 Indonesian of Sumatra, in the Aceh province, were found dead.
