Indonesian Plate Boundaries (week 2)

Indonesia falls on the meeting point of several tectonic plates. As shown in the photo above, Indonesia is located between two continental plates, the Eurasian plate and the Indo-Australian plate, and two oceanic plates, the Philippine plate and the Pacific plate. This makes Indonesia a very complicated topic to study, since it is affected by both convergent plate boundaries (moving towards each other) and transform plate boundaries (moving past each other). As a result, Indonesia experiences a high occurrence of earthquakes, tsunamis and volcanoes. 

Convergent boundaries are formed when an oceanic plate subducts underneath a continental plate, which results in the continental plate folding/buckling upwards to form a mountain range, and the rock that melts (from the subducting oceanic plate) inside the earth's interior can cause earthquakes and eventually volcanic eruptions as it rises up. In the case of Indonesia, the subduction of the Indo-Australian plate under the Eurasian plate formed the volcanic arc in the Sumatra, Java and Bali region (western Indonesia). The two plates meet about 200 km off the west coast of Sumatra and 5 km below the ocean floor. Two famous examples of the effects of these convergent plates is the Krakatoa volcano and its eruption of 1883, which erupted due to the subduction of the Indo-Australian plate (moving north) under the Eurasian plate, and the Mount Tambora eruption of 1815 (Sumbawa, Indonesia), which was the largest recorded in history.

Indonesia's greatest example of a transform boundary is the Great Sumatran fault. A fault is caused by the tectonic plates sliding past each other horizontally. If enough stress is created as part of the plates are stuck and the rest is still moving, this will cause a fracture in the plate and push the plates forward, causing an earthquake. The break itself in the plate is called a fault. The Great Sumatran fault runs the entire length of the island of Sumatra, which happens to be Indonesia's largest and usually experiences the most earthquake activity in the country. It is a strike-slip fault, meaning that the movement is horizontal where the plates grind past each other, as opposed to a thrust fault that experiences vertical movement and is more likely to cause a tsunami. Indonesia experiences both strike-slip earthquakes and thrust earthquakes. The correlation between the volcanic activity and the Sumatra fault line are shown below. 

References:

Chapter 2 book summary 

http://www.virtualuppermantle.info/TransformFaults.htm

https://tectonicplatesindonesia.weebly.com/causes.html

http://www.tectonics.caltech.edu/outreach/highlights/sumatra/why.html

https://www.earthmagazine.org/article/sumatran-strike-slip-earthquakes-challenge-seismologists