06 November 2010

Wedhus Gembel dan Merapi

Cyberedhoy, Yogyakarta

Banyak yang kepingin tahu seperti apa wajah dan bentuk "wedhus" yang ganas ini. Wedhus gembel ini dikenal oleh volcanologist sebagai awan panas. Namun jangan dibayangkan seperti awan putih bersih seperti biri-biri Australi yang lucu.

"Wedhus gembel" merupakan sebuah fenomena khusus dari aktifitas Gunung Merapi di sebelah utara Jogjakarta. Minggu ini berita tentang Merapi dahsyat banget. Meletus berulang-ulang, korban berjatuhan, diawali dengan meninggalnya Mbah Marijan sang Penakluk Gunung Merapi di awal meletusnya 26 Oktober 2010 lalu.

(in memoriam 'Mabah Marijan') 

General View of Merapi Volcano

Merapi volcano is situated above the subduction zone between the Eurasian and Indo-Australian tectonic plates. This subduction extends for about 2000 km from Sumatra to the Lesser Sunda Islands. In the Java-Bali sector, subduction occurs between an oceanic plate 10-km in thickness (Indo-Australian) and an intermediate plate of 20-km thickness (Eurasian). This Mesozoic subduction has formed a trench of 6 to 7 km deep.

Merapi volcano was born in period of late Pleistocene - Early Holocene. It is situated at the intersection between two main volcanic alignments Ungaran-Telomoyo - Merbabu - Merapi (N164E) and Lawu - Merapi -Sumbing-Sindoro-Slamet or between two main quartenary faults, Semarang fault (north south) and Solo fault (east west). Among them, Merapi volcano is the youngest one.

Merapi volcano has suffered an evolution. The early stage of its growth, it had a basaltic magma with effusive eruptions. The characteristics of magma then change to more silicic and more viscous. Lava extrusion, thus, may be effusive or explosive. Recently, magma is quite viscous so that it extrudes and accumulates at the crater surface as a lava dome. Since magma is poor in gas, eruption is usually less than 3 in VEI-scale.

Merapi activity is characterized by a very frequent eruption ranging from 1 to 5 years of time duration, weak explosion, and low gas pressure. Eruption is usually accompanied by pyroclastic flows, or "awan panas" in local term or a familiar term "wedhus gembel". There are two types of awan panas: awan panas guguran and awan panas letusan.

Awan panas "guguran" results from collapse of lava domes in the crater of the volcano. Occasionally, a dome might grow so large that it becomes unstable and collapses into several drainage catchments. The distance traveled by the flow and its extent depend strongly upon the volume of the destroyed lava, gas pressure and the slope angle of the flank.

Awan panas "letusan" results from collapse of debris that is erupted vertically. Flow direction depends to some extent on the summit morphology.

Most of the Merapi eruption has low explosivity and the awan panas usually reaches about 6 to 7 km from the summit. Some awan panas have traveled as far as 13 km from the summit, such as the deposit generated during the 1969 eruption. Velocity of pyroclastic flow can reach up to 110 km/hour. Infrared spectrometry of carbonized woods sampled from pyroclastic deposits indicates that awan panas have a temperature range of about 3000 - 4000 C.

Some eruptive activities at of the Merapi are characterized by dome formation. A slow upflow of andesitic magma leads to an extrusion of viscous magma, which accumulate and construct a dome in the crater. Magma is almost completely degassed and flows out of the conduit with a rate of up to 20.000 m3/day a normal dome formation. The shape and evolution of the dome depend on the crater configuration. The 1994’s dome stays in a slope of about 350 in the horseshoe shaped crater open to the southwest. The core of the dome is viscous and incandescent (looks glowing at night), while the dome crust is solid.
Written & photos by Edhoy
© 2010 by Cyberedhoy Inc, Allrights Reserved

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