Merapi (literally: Fire Mountain; Elevation: 2,968 m; Geolocation: 7.5420S110.4420E) is the most active stratovolcano in Indonesia and has erupted regularly since 1548. Smoke can be seen emerging from the mountaintop at least 300 days a year. Officials said about 500 volcanic earthquakes had been recorded on the mountain over the weekend of 23–24 October, and that the magma had risen to about 1 km (3,300 ft) below the surface due to the seismic activity. On the afternoon of 25 October 2010 Mount Merapi erupted lava from its southern and southeastern slopes. To Top
The subduction zone southwest of Sumatra is part of a long convergent belt that extends from the Himalayan front southward through Myanmar, continues south past the Andaman and Nicobar Islands and Sumatra, south of Java and the Sunda Islands (Sumba,Timor), and then wraps around to the north. This trench accommodates the northward motion of the Australian plate into Eurasia. (The trench is known by several local names but here called the Sunda subduction zone in general and sometimes the Sumatran subduction zone when referring to the stretch offshore Sumatra.)
West of Sumba, the dense lithosphere of the Indian Ocean seafloor subducts beneath the continental Sunda shelf, whereas to the east, the lighter continental Australian lithosphere thrusts beneath the oceanic lithosphere.
The along-strike change in the average density of lithosphere being subducted coincides with changes in the character of the tectonics.
The plate kinematics of the Sumatran region is, in a broad sense, the simple interaction of the Indian-Australian and Eurasian plates. However, in detail it is much more complex
than that. The subducting Indian-Australian plate comprises two separate plates that are bounded by a broad, slowly deforming region currently being subducted at the Sunda trench. The internal deformation of the subducting plate may lead to a centimeter per year or so of uncertainty in the convergence at the Sunda trench.
The active volcanoes of Sumatra are generally parallel to the subduction zone and above the ~100 to 150 km depth contours of the subducted plate. This pattern of subduction-related volcanism has persisted since at least Oligocene time. The modern arc rocks are andesite, dacite, and rhyodacite to rhyolite, and their strontium ratios indicate variable amounts of crustal contribution. The volcanic rocks reveal a long history of subduction along the entire southwestern margin of Sunda.
Merapi is the youngest in a group of volcanoes in southern Java. It is situated at a subduction zone, where the Indo-Australian Plate is sliding beneath the Eurasian Plate. It is one of at least 129 active volcanoes in Indonesia, part of the Pacific Ring of Fire – a section of fault lines stretching from the Western Hemisphere through Japan and South East Asia. Stratigraphic analysis reveals that eruptions in the Merapi area began about 400,000 years ago, and from then until about 10,000 years ago, eruptions were typically effusive, and the out flowing lava emitted was basaltic. Since then, eruptions have become more explosive, with viscous andesitic lavas often generating lava domes. Dome collapse has often generated pyroclastic flows, and larger explosions, which have resulted in eruption columns, have also generated pyroclastic flows through column collapse. Typically, small eruptions occur every two to three years, and larger ones every 10–15 years or so.
The tomographic image of the subducting slab below Sumatra, the Andaman Islands and Burma.
An improved iterative regional–global tomographic method was applied to the Sumatra–Andaman and adjacent regions to better constrain the 3-D mantle velocity heterogeneity in the region. Constructed tomographic model
model for the P-wave velocity structure of the western Indochina subduction
zones along the Sumatra and Andaman Islands, and Burma. The resulting model illustrates the complex slab geometry in the upper-mantle and transition zone regions along the Sumatra, Andaman and Burma subduction zones.
Data and regional tectonic setting map. Cross-section locations (A–D) for all subsequent figures, plate boundaries,
bathymetric features [Wharton FossilRidge (WFR) and Investigator Fracture
Zone (IFZ)] and convergence rates are also shown.
(b) (A) Cross-sections (See data and regional tectonic setting map) showing perturbations (2 per cent scale) relative to ak135 for the final model (IT5r). Cross-section lengths (km) are listed at the top right of each section. Volcanoes (triangles) are shown within 100 km on either side of each section. (B) Synthetic slab restoration tests (4 per cent scale) for the same cross-sections
Sources of information:
1. McCaffrey R. 2009 The Tectonic Framework of the Sumatran Subduction Zone. Annu. Rev. Earth Planet. Sci. 2009. 37:345–66
2. Pesicek J. D., Thurber C. H., Widiyantoro S., Zhang H, DeShon H. R. and E. R. Engdah 2010 Sharpening the tomographic image of the subducting slab below Sumatra, the Andaman Islands and Burma. Geophys. J. Int. (2010) 182, 433–453