The island of Sumatra, in Indonesia, features regularly in both the news and the history books for what seems to be an unending sequence of natural disasters in the form of earthquakes, volcanic eruptions and their consequences. These disasters are the consequence of the island’s geological setting.
Sumatra: Tectonic Setting
The earth’s surface is composed of a number of large plates which move around, driven by the planet’s internal heat. Rates of movement are extremely slow but the distance moved is considerable over long periods of time. As a consequence, plates move against each other: depending on the type of crust involved, mountain ranges may be created or one plate may over-ride another.
Sumatra lies on the boundary between the Indian (sometimes called the Indo-Australian) Plate to the south and east and the Eurasian Plate to the north and west. It is part of the so-called Pacific Ring of Fire, the seismically-active boundary of the Pacific Plate (which comprises most of the Pacific Ocean) and is linked to the massive forces involved in building the Himalayas by way of the northwards-trending Kaladan Fault (Open University).
Where the two plates meet, the denser Indian Plate is forced beneath its neighbour, creating a long, deep, curved trench (the Sunda Trench) along the ocean floor. The geological processes involved are complex: in essence the island of Sumatra has been created by the melting of subducted rock under immense pressure. This rock then rises to form new crust through the eruption of volcanoes.
Along with the neighbouring island chain to the east (which includes Java) Sumatra forms part of a typical example of the landforms created in such a situation - a volcanic island arc, in which a line of volcanic islands lies at a distance behind a deep ocean trench.
Geology of Sumatra: Earthquakes and Volcanoes
A map of Indonesia shows a long line of volcanoes along the plate margin as it passes through both Java and Sumatra (Smithsonian Institute). The Institute’s map shows 37 volcanoes in Sumatra and a further 42 in neighbouring Java. At the time of writing (September 2010) five of Sumatra’s volcanoes are currently displaying an alert assigned by the authorities, the highest being Level 4 for Mount Sinabung, which erupted in August 2010 (Idahnesia website).
Sumatra’s volcanoes include some of the most explosive and destructive on the planet, including Toba (which produced what is thought to be the largest eruption in the geological record), Tambora, the environmental impacts of whose 1815 eruption created the ‘year without a summer’ and, most famously, the 1883 eruption of Krakatoa (Open University).
Earthquakes are also a significant hazard at plate boundaries and subduction zones are characterised by high levels of seismic activity. These zones are the location of nine of the world’s ten largest earthquakes since 1900 (USGS) and in Sumatra itself major earthquakes of magnitude 7 or more are far from uncommon.
Indeed, the USGS list shows that of the largest earthquakes in the past 110 years, four have taken place in Sumatra and three have occurred since 2004. These include the notorious Boxing Day earthquake of 2004 which, with its associated tsunami, killed an estimated 227,000 people. This was the third largest recorded and had a magnitude of 9.1. There were earthquakes of magnitude 8.5 and 8.6 in 2007 and 2005.
Sources and Further Information
California Institute of Technology “Sumatra Plate Boundary Project”, tectonics.caltech.edu
Angela Colling et al Earth and Life: The Dynamic Earth Open Univesity 1997
Indahnaesia website, “Volcanic Activity”, indahnesia.com
Smithsonian Institute Global Volcanism Program volcano.si.edu
United States Geological Survey (USGS) Earthquake Hazards Program earthquake.usgs.gov
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