The Lamongan volcanic field, East Java, Indonesia: physical volcanology, historic activity and hazards

Abstract

The Lamongan volcanic field (LVF) in East Java, Indonesia, consists of up to 61 basaltic cinder or spatter cones, a minimum of 29 prehistoric maars, and a central compound complex comprising three main vents including the historically active Lamongan volcano. The field lies on the Sunda volcanic arc and abuts onto the currently active Tengger caldera–Semeru complex to the west. Between 1799 and 1898, Lamongan was persistently active and discharged up to 15 lava flows of basaltic to basaltic andesite composition, which is probably more than what was erupted by any other Indonesian volcano in that period. Several seismic crises have been recorded in the LVF since Lamongan's last eruption. There are no existing records of cinder cone or maar-forming eruptions in the area. Despite the possible hazards posed by future activity in the LVF, the history and structure of the field have been subject to little scrutiny. This contribution presents the results of a detailed morphometric and volumetric investigation of prehistoric vents and historic lava flows in the LVF, using field data, historic records, satellite imagery and modelling of erupted volumes. A typical LVF maar is characterised as a crater with a diameter of 450 m; generated by an eruption with a model ejecta volume of 0.016 km 3 dense rock equivalent (DRE) and a model juvenile component volume of 0.0096 km 3 DRE. Diameter/depth ratios of some maars display a morphological freshness comparable to recently formed phreatomagmatic eruption craters elsewhere. The median LVF cinder/spatter cone has a basal diameter of 600 m, a height of 75 m, a volume of 0.009 km 3 and a model total erupted volume of 0.014 km 3. Variation in cone basal diameter to height ratios indicates that the age of some prehistoric cones may not exceed a few centuries, and morphological classification identifies the youngest prehistoric vents on the northern flanks of the central edifice. Historic activity from vents on the western flanks of Lamongan produced at least ∼0.05 km 3 of basaltic to basaltic andesite lava. Resurfacing rates at Lamongan in the 19th Century imply an age of ∼900–2100 years for the surface of the volcano. The steady-state eruptive rate from 1843–1898 is evaluated as 0.025 m 3 s −1, which is low in comparison to estimates from many presently active systems. Using eruptive rate and volume data, the age of the LVF is estimated as between 13 and 40 ka, placing it among the youngest dated Indonesian volcanoes. Complex regional tectonics in conjunction with magmatic pressure have probably been the major influences on the distribution of volcanism in the LVF. The occurrence of maar-forming eruptions may have been controlled by a porous substrate, possibly a deposit from the Holocene Iyang–Argapura complex to the east. Volcanic hazards from future eruptions in the LVF include base surges, ash flows, ashfall and ejected blocks associated with phreatomagmatic activity, and ashfall and ejecta from magmatic eruptions.