Preliminary study on stratigraphy, petrology-geochemistry, eruption styles, and geomorphology of Merbabu volcano, Central Java, Indonesia: Implication for the volcanological hazards of an infrequently active volcano

Abstract

This is the first study that coupled the detailed stratigraphic information with petrography and whole-rock geochemistry data on the Merbabu volcano (Central Java, Indonesia). A total of 22 pyroclastic layers originating from magmatic and phreatomagmatic eruptions were identified; among these, the massive orange lapilli (mLo) layer occurs as the key layer due to its widely dispersed characteristic and possibly originates from a VEI 4-scale eruption. Basaltic andesite pumice was identified as the main juvenile phase in pyroclastic deposits and is characteristically amphibole-rich. Five lava flows and three lava domes were identified. All lavas are porphyritic and pyroxene-rich; however, the composition of lava flows varies from basalt to andesite, while lava domes are exclusive to andesite. We also distinguished three layers of the lahar deposit; two represent the debris flow type, and one represents the hyper-concentrated flow type. Interestingly, all pumices typically have a high Zr/Nb value, while all lavas are characterized by a low Zr/Nb value. This evidence, coupled with the mineralogical differences between pumice and lava (amphibole-rich for pumice and pyroxene-rich for lava) strongly suggests the presence of two magma reservoirs beneath the volcano. All of these deposits successfully construct the present volcano landforms, which are further divided into upper cones, middle cones, and lower cones. Pyroclastic deposits and lava primarily constructed the upper and middle cones, while lahars and some pyroclastics built the lower cones. Landforms associated with lava typically have small drainage density values (3.0 – 3.4 km/km²); whereas, landforms associated with pyroclastic and lahar deposits have a characteristically high drainage density value (4.5 – 6.4 km/km²). This evidence suggests that the difference in material types strongly controls the erosion intensity. Moreover, the occurrence of three horseshoe escarpment landforms implies that Merbabu can produce explosive flank collapse eruptions.