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Sumatra: ERS-SAR Mosaic

The image below is a large area mosaic made up of 26 passes (more than 600 scenes) of ERS SAR images. It shows the complete Sumatra island of Indonesia with East Malaysia and Singapore to its north.

This mosaic image beautifully illustrates the utility of radar in understanding the basic geological features in an area which is normally under heavy cloud cover and vegetation. The major fault system on the island, the Semangko Fault, is seen trending the entire length of Sumatra. Most volcanoes are located on or very near this fault system including the large depression of Toba towards the northwest. The Semangko Fault system and its associated volcanoes have given rise to the Barisan Mountains which form the backbone of Sumatra.

© ESA 1996 . Sumatra, Indonesia . ERS-SAR Mosaic | The Semangko Fault is of strike-slip variety and the associated convergent plate margin subduction lies to the southwest below the Indian Ocean. The islands off the west coast of Sumatra are over the fore-arc basin of this subduction zone. Eastward from the Barisan Mountains lies a rolling landscape which terminates at a very wide swampy coastal plain next to the Malacca Strait.

Lake Toba is a complex and very large tectonic-volcanic depression. The past volcanoes of the area erupted several times releasing huge quantities of ash which has deposited a special type of rock called ignimbrite in this area. The eruptions were so large that Toba ash has been found as far west as Sri Lanka and India. Violent eruptions and tectonic collapses created the huge depression which later was filled up with water to form Lake Toba. One of the eruptions which took place about 73 000 years ago was gigantic. In this super-eruption an estimated 2800 km3 of material was released. In contrast, Mount St. Helens in 1980 and Pinatubo in 1991 ejected 0.35 km3 and 5 km3 of material respectively.

© ESA 1996 . West Coast | © ESA 1996 . Lake Toba | © ESA 1996 . South Sumatra