Lamongan Volcano is a small volcano situated between Bromo-Tengger and Iyang volcanic ranges in
East Java. This volcano is supposed to be Plio-Pleistocene in age and it is characterized by the occurrence
of maar-lakes in its surroundings.
Petrographic and geochemical (major and trace elements) study on data from this volcano indicate the
lithology of basalt, basanite, basaltic-andesite, trachyandesite, absarokite and shosonite, having magmatic
affinity in transition from alkaline to calc-alkaline. Based on MgO and TiO2 contents the volcanic
products can be subdivided into 4 volcanic groups. Group-1 (TiO2>1.3%; MgO >6%) and Group-2
(TiO2>1.3%; MgO<6%) indicate gradational modification in chemical compositions from continental to
subduction system. Variations on Ba vs K/Rb contents indicate the contribution of lower crust in forming
magma of Group-1. This process tends to be influenced by continental magmatic system. These volcanic
groups are also characterized by the presence of silicate undersaturated rocks (basanite and trachyandesite),
as well as petrographical textures representing the magma mixing process. Group-3
(TiO2<1.3%; MgO >6%) and Group-4 (TiO2<1.3%; MgO >6%) indicate characters of subduction related
magmatism with variations in MgO contents.
Lacking of age data in volcanic products of Lamongan Volcano, lead to the interpretation of volcanic
successive as follows : Magmatism of continental type was formed in the area and resulting magma
chambers filled with magma of continental characters. The area, then, was affected by subduction activity
producing magmas of subduction characters. As the latters were introduced to ‘the already formed’
magma chambers, they might cause mixing of two kinds of magmas, and the volcanic products would
present a gradation of mixing as represented by Volcanic Group-1 and 2. As the subduction continues,
the following magmatic characters tend to be more of subduction-related system (Volcanic Group-3 and
4) rather than the continental system.
In point of view of the East Java geodynamic system, the basement of Lamongan Volcano may represent
a micro-continent coming from somewhere in south-southeast of Sundaland, and it has already had
volcanism of continental character. As the micro-continent approached and embedded in the Sundaland, it
was then affected by north trending subduction to give 4 groups of volcanic products.
By:
Jhonny1, Bambang Priadi1, and Rosgandika Mulyana21Department of Geology, Bandung Institute of Technology
2Volcanological Survey of Indonesia