GEOLOGY OF MT.AGUNG


Mt.Agung is a gigantic monoconic strato volcano which rises to an elevation about 3142 m above sea level.
The volcano is located in the eastern part of the Bali island. The northeast slope developed freely as far as the coast of the Indian ocean and Lombok strait. The east slope is bordered by the old volcanic cone of Seraja. The northwest and west slopes are separated by a narrow valley from the active volcano Batur while the south part is bordered by an old volcanic complex.
Mt.Agung is a young composite volcano. The last eruption was in 1963, after reposing for about 120 years, characterized by vulcanian type eruption that produced lavas, pyroclastic flow, fall and lahar (mud flow) as a secondary products. The volcanic products show andesitic to basaltic composition.
In general, lithology of Mt.Agung area can be divided into Tertiary deposits, Quaternary old volcanic products, and Mt.Agung Quaternary deposits.

Tertiary deposits of the Ulakan formation spread out south part of Mt.Agung. It shows old undulating topography and consists of coral reef, basaltic andesite lava that mostly weathered, volcanic breccia, calcareous sandstone that was deposited in Upper Miocene, while lava was deposited in Upper Pliocene.

Topographically, Quaternary old volcanic products is contrast from Mt.Agung and clearly recognized surrounding Mt.Agung. The products consist of lava and pyroclastic rocks of andesite to basaltic andesite in composition. Self et al. (1979) assumed these are a remnant of an old caldera, then covered by young volcanic products of Mt.Agung.

Mt.Agung Quaternary products consist of lava, pyroclastic and associated lahar. In the south flank, Old lava flows crop out in valleys approximately 1000 m above sea level. In eastern and northern parts however, the younger lava distributed in a large area and some flowed to the sea.

Petrographic study indicates that the rocks are mostly porphyritic, containing plagioclase, two pyroxenes, minor olivine, amphiboles and opaque minerals set up in ground mass of glass and microlites of plagioclase, pyroxene and olivine.

1963's Mt.Agung eruption

The first activity of Mt.Agung started on February 18, 1963. The inhabitants heard a loud crash and noticed cloud rising from the crater, then followed by pyroclastic products thrown to the air.
On February 24, 1963, lavas which were highly viscous flowed out over the northern slope, moving down with a length of 7 km in about 18 to 20 days, reaching a height of 510 m above sea level. They have a wide of about 0.5-0.8 km and 30-40 m in height. Rough estimated shown that the total volume of emitted lava was about 50 million cubic meters.
Since then, the eruption continued pored out a combination of explosive and effusive activities.
On March 17, 1963 however, the paroxysmal eruption took place, throwing out pyroclastic products 8-10 km into the air, look like a cauliflower. The lower portion of the explosive pillars, due to the weight of the fragments broke away and moved down the slopes of volcano, generating pyroclastic flows (nuee ardentes). They traveled up to about 12-15 km from the crater to the south and east valleys, with the speed for about 60 km/hour in a high temperature. The pyroclastic flows destroyed many villages around the volcano and caused the death of many people living near the river valleys.


After May paroxysmal eruption, the south peak of the crater wall broke and decreased 200 meters.
The lowest crater wall recorded presently is the upper end of Langon river (2600 m). It seems the pyroclastic flow or lava will pour out through and along the Langon river if Mt.Agung's eruption occurs.

 

Thicker air fall deposits mostly covered west ridge areas (50-70 cm thick) that could be caused by wind direction during 1963's Mt.Agung eruption. The deposits mostly eroded by rainfall seasonal rivers. Those outcrops are still exposed on cross road of Besakih area.
Mt.Agung's lahar contain more than 50% of sand size materials and smaller, the others are mainly boulder and gravel. In a few places lahar contain lower than 50% of fine materials. These may be formed soon and long after eruption, when condition meet: sources of fragmental materials, adequate water to mobilize and slope.

Lahar of Mt.Agung may also be formed by: a collapse of unconsolidated materials from the peak of crater wall during heavy rain fall, and entry of pyroclastic materials into the rivers and mixing of flow materials with water.
Pyroclastic flow and lahar were the most destructive elements of the eruption.