Sembalun Volcanic Complex is remnant of quarternary volcanic activity which belongs to old volcanic complex estimated to be 0.45 million years old (A. Nasution, 2010). Sembalun Valley is remnant of volcano that underwent surface change process such as erosion, corrotion and denudation. These changes resulted in the formation of rough and steep surface with the elevation between 550 – 2250 m amsl. This old volcano has a caldera with a width of over 1 km2 estimated to be an end process of destruction.
The morphology of the caldera base has the form of wide flatland in the elevation above 1000 m amsl and is a fertile piece of land. The formation of geothermal system in Sembalun area is closely related to the formation of the Quarter-aged Sembalun volcano. Morphological feature of Pusuk Mountain in Sembalun’s caldera reflects a steep escarpment. Result from withdrawal topographic and image lineament also indicates a fault stretching close to southwest – northeast. Sembalun’s caldera wall covers Sembalun Bumbung Village in the form of a horseshoe which peak consists of Telaga Mt., Pusuk Mt., Tanakiabang Mt., Nangi Mt., and Banjer Mt. Opening up north while making Sembalun Lawang village as the caldera floor.
Besides, there are breccias and blackish lava solidified and faulted in the direction almost northwest – southeast in Greggengan River. These rocks are product of pre Sembalun’s caldera.
Also situated in this complex are black – reddish andesite lava, porphyritic texture, hard, vesicular and the presence of smooth flow structure (similar to laminating layer) from its black mineral. Lentih vesicular lava is gray – bright gray andesite lava, porphyritic, vesicular in some places and composed of plagioclase mineral, pyroxene, and hornblenda. There is shear joint as an indication of a tectonic activity.