The Dieng geothermal field is located in central Java Island approximately about 80 kilometers northwest of the city of Yogjakarta surrounded by mountainous terrain. The geothermal area at Dieng is heavily farmed (rice and vegetables) and highly populated. The nearest sizable town to the project area is Wonosobo, located about 25 kilometers to the south.
The Island of Java has numerous volcanic centers along its E-W axis which is located near the boundary of two major lithospheric plates, the Indian Ocean-Australian plat on the south and the Eurasian plate to the north (Figure 1). Collision of these two plates has resulted in the formation of a large subduction zone (the Java Trench), which lies offshore and parallel to the southern coast of Java Island. The presence of very young volcanoes throughout much of Indonesia results from the ascension of melted crustal material from the subducted lithospheric plate.
The Island of Java has numerous volcanic centers along its E-W axis which is located near the boundary of two major lithospheric plates, the Indian Ocean-Australian plat on the south and the Eurasian plate to the north (Figure 1). Collision of these two plates has resulted in the formation of a large subduction zone (the Java Trench), which lies offshore and parallel to the southern coast of Java Island. The presence of very young volcanoes throughout much of Indonesia results from the ascension of melted crustal material from the subducted lithospheric plate.The mountain range in central Java has an east-west trend, extending from Gunung Slamet on the west to Gunung Ungaran on the east. These volcanic centers are considered as high in their geothermal prospects. The Dieng mountain complex is located between the two mountain ranges and is part of this E-W trend with its highest top at Gunung Prau (2555 masl). Another major structural feature is defined by the NW-SE volcanic alignments of Gunungs Mangunan-Dieng (NW), Sundoro, Sumbing and Merbabu-Merapi (SE).
The Dieng mountain complex is composed of Quaternary volcanic rocks aligned similarly as to the regional structural features. The E-W trend extends from Gunung Butak on the west to Gunung Prau on the east. The peaks of these mountain complex rise to elevations between 2200 to 2555 masl. The Dieng geothermal field lies partially within the Dieng mountain complex with moderate topographic relief surrounded by the NW-SE trending alignment of very young volcanic centers from Gunung Sipandu to Pakuwaja and by the older Gunung Prau to Patakbanteng volcanic centers on the NE side. The thermal features also show similar structural trend. Elevations of the mountains within the Dieng field ranges from 1900 to 2100 masl.
A number of scientists have analyzed the volcanic history and structure of the Dieng area. Their findings and views are summarized by Delarue et al. (1977). The surface rocks in the project area is covered by Quaternary andesite lava flows and pyroclastic units.
Figure 2 is a simplified geological map of the project area taken from Sukhyar et al. (1986). The Dieng mountain complex appears to have attained its present shape as results of sequence of events. The oldest rocks mapped in the area are pyroxene andesite and basaltic andesite lavas and outcrop surrounding the Geothermal field. These great quantities of magma were erupted around between 2.5 to 3.6 million years (my) before the present time (bp).
This was followed by huge caldera collapse within the last 1 m.y.b.p., including all parts of the Dieng field, with Sileri at the north and Pakuwaja-Sroja at the southeast. Part of the caldera wall is still clearly exposed today as the steep SW escarpment of G. Prau and G. Patakbanteng.
Post-caldera events were marked by eruptions of andesite to dacite compositions of lava flows, domes and pyroclastic flows and falls during the time period from 0.5 to about 0.07 m.y.b.p. The volcanic eruptions were also accompanied by deposition of lacustrine and alluvial sediments within the caldera. The SE volcanic centers (Pakuwaja area) have produced mainly biotite andesite and few pyroxene andesite lava flows. The youngest volcanic rocks are from this area. The rest of the area is covered predominately by pyroclastic material and few pyroxene andesite lava flows located at G. Sipandu and G. Pangonan.
The Dieng mountain complex is composed of Quaternary volcanic rocks aligned similarly as to the regional structural features. The E-W trend extends from Gunung Butak on the west to Gunung Prau on the east. The peaks of these mountain complex rise to elevations between 2200 to 2555 masl. The Dieng geothermal field lies partially within the Dieng mountain complex with moderate topographic relief surrounded by the NW-SE trending alignment of very young volcanic centers from Gunung Sipandu to Pakuwaja and by the older Gunung Prau to Patakbanteng volcanic centers on the NE side. The thermal features also show similar structural trend. Elevations of the mountains within the Dieng field ranges from 1900 to 2100 masl.
A number of scientists have analyzed the volcanic history and structure of the Dieng area. Their findings and views are summarized by Delarue et al. (1977). The surface rocks in the project area is covered by Quaternary andesite lava flows and pyroclastic units.
Figure 2 is a simplified geological map of the project area taken from Sukhyar et al. (1986). The Dieng mountain complex appears to have attained its present shape as results of sequence of events. The oldest rocks mapped in the area are pyroxene andesite and basaltic andesite lavas and outcrop surrounding the Geothermal field. These great quantities of magma were erupted around between 2.5 to 3.6 million years (my) before the present time (bp).This was followed by huge caldera collapse within the last 1 m.y.b.p., including all parts of the Dieng field, with Sileri at the north and Pakuwaja-Sroja at the southeast. Part of the caldera wall is still clearly exposed today as the steep SW escarpment of G. Prau and G. Patakbanteng.
Post-caldera events were marked by eruptions of andesite to dacite compositions of lava flows, domes and pyroclastic flows and falls during the time period from 0.5 to about 0.07 m.y.b.p. The volcanic eruptions were also accompanied by deposition of lacustrine and alluvial sediments within the caldera. The SE volcanic centers (Pakuwaja area) have produced mainly biotite andesite and few pyroxene andesite lava flows. The youngest volcanic rocks are from this area. The rest of the area is covered predominately by pyroclastic material and few pyroxene andesite lava flows located at G. Sipandu and G. Pangonan.
After the last eruption, erosion of the caldera wall and the soft pyroclastic and sedimentary deposits continued within the caldera to the present day. Some group considered the Dieng Mountain complex as the remnants of a large caldera structure. Gunawan (1968) and others, however disagree with the caldera theory and suggests that the escarpment exposed at G. Prau and G. Patakbanteng to be an older eroded volcano. The SE-trending young volcanic centers are also considered to be genetically related to a major fault zone.
Historic eruptions have been reported in the Pakuwaja area in 1826 and 1847. But it has not been confirmed whether the eruptions were from Pakuwaja or nearby volcanic centers. A number of historic phreatic or hydrothermal eruptions have occurred in Sikidang and Sileri areas. Similar activities have also been reported in the Candradimuka area, NW of Dieng project area.
The nearest exposure of older sedimentary rocks are in the ? western part of the Dieng field, near Rataamba. The sedimentary outcrop are Pliocene limestone underlain by marine clays.
The field has three main active hydrothermal surface thermal features, Pakuwaja, Sikidang and Sileri areas. The features are active fumaroles, mud pools, steaming ground and are surrounded by extensively altered ground. Thermal features also include neutral pH chloride hot springs located below 1700 masl. The Sikidang and Sileri thermal features cover significant steaming and extensively altered grounds. The Bitingan and Siglagah hot springs have temperatures ranging from 45 to 58°C and neutral pH. Pulosari hot spring (48°C) and the Jojogan warm spring (25°C) are located about 3.7 km WSW and 2 km NE of Sikidang fumaroles. The field has several small lakes of volcanic or hydrothermal origin. The presence of active thermal features and very young volcanism made the field one of the attractive target for commercial development.
Historic eruptions have been reported in the Pakuwaja area in 1826 and 1847. But it has not been confirmed whether the eruptions were from Pakuwaja or nearby volcanic centers. A number of historic phreatic or hydrothermal eruptions have occurred in Sikidang and Sileri areas. Similar activities have also been reported in the Candradimuka area, NW of Dieng project area.
The nearest exposure of older sedimentary rocks are in the ? western part of the Dieng field, near Rataamba. The sedimentary outcrop are Pliocene limestone underlain by marine clays.
The field has three main active hydrothermal surface thermal features, Pakuwaja, Sikidang and Sileri areas. The features are active fumaroles, mud pools, steaming ground and are surrounded by extensively altered ground. Thermal features also include neutral pH chloride hot springs located below 1700 masl. The Sikidang and Sileri thermal features cover significant steaming and extensively altered grounds. The Bitingan and Siglagah hot springs have temperatures ranging from 45 to 58°C and neutral pH. Pulosari hot spring (48°C) and the Jojogan warm spring (25°C) are located about 3.7 km WSW and 2 km NE of Sikidang fumaroles. The field has several small lakes of volcanic or hydrothermal origin. The presence of active thermal features and very young volcanism made the field one of the attractive target for commercial development.
