The geography of Indonesia is covered with volcanoes.  

Indonesian Landscape

http://balianyartour.com/batur-lake-and-batur-volcanokintamani-bali-indonesia/image-4/

Indo-Australian and Erusian Plates

http://www.savu-raijua.com/geology.htm

These volcanoes have formed from subduction zones that have formed between the Indo-Australian plate and the Eruasian plate.  Some Indonesian volcanoes are still active and erupting.  The Indo-Australian plate is moving forward which is causing it to slide under the Eurasian plate. The Indo-Australian plate is moving at a fairly constant speed of 1mm per year, but there is a rare occasion when the plate may slide at a great rate. This causes an earthquake which can affect the country’s volcanoes (Savu-Raijua). 

All of the volcanoes in Indonesia are within what is called the Pacific Ring of Fire.  There is a current scientific belief that the earthquake and resulting tsunami in 2004 changed the eruption pattern of the Pacific Ring of Fire volcanoes.  It is believed Mount Sinabung in Indonesia erupted direct because of the changes the earthquake and tsunami brought to the underground geography of Indonesia.  This volcano had not erupted since the 1600’s (Associated Press). However, this is just scientific speculation at this point.

Eruption of Mount Sinabung

http://www.cbc.ca/news/world/indonesia-s-mount-sinabung-erupts-forcing-6-000-to-flee-1.1855595

Indonesia has the most active volcanoes of any country with over 130 active volcanoes.  The volcanoes are mostly located along the Indonesian island arc system.  This is on an arc of the plates that are primarily located on the islands of Java and Sumatra (Geology.sdsu.edu).

Indonesian Island Arc System

http://www.geology.sdsu.edu/how_volcanoes_work/Krakatau.html

This arc, known as the Sunda Arc, is over 3000 km from NW Sumatra to the Banda Sea.  The arc is formed by the subduction of Indian Ocean crust that lies beneath the Asian plate.  The volcanoes on either end of the arc are more complex as the plates on the ends are more complex (Smithsonian Institution). 

Works Cited

Associated Press. "Indonesia's Mount Sinaburg Erupts, Forcing 6,000 to Flee." CBC News.

          N.p., 16 Sept. 2013. Web. 12 Oct. 2014.

Bali Anyar Tour and Travel. "Batur Lake and Batur Volcano, Kintamani Bali-Indonesia." Bali

          Anyar Tour and Travel. N.p., n.d. Web. 12 Oct. 2014.

Geology.sdsu.edu. "How Volcanoes Work Krakatau, Indonesia." Geology.sdsu.edu. N.p.,

          n.d. Web.

Savu-Raijua. "Tectonic Evolution of Savu." Savu Raijua. N.p., n.d. Web. 12 Oct. 2014.

Smithsonian Institution. "Indonesia." Smithsonian Institution National Museum of Natural

          History Global Volcanism Program. N.p., 2013. Web. 12 Oct. 2014.