Formation of Mount Loyang: Easternmost Scoria Cone of Slamet Stratovolcano, Central Java, Indonesia

Scoria cones commonly occur in the lower flank of stratovolcano close to the populated area. This close spatial relationship increases hazardous risk of direct impact of volcanic activities to the people. Therefore, it is necessary to gain more understanding to the formation of scoria cone to better assess the volcanic hazards. We studied Mount Loyang, the furthest scoria cone of Slamet volcano, approximately 14 km from the recent active crater. Quarrying to this scoria cone enables us to observe the interior structure towards the surface as represented by its volcanic rocks sequence. The sequences include 61 layers of voluminous ash and lapilli deposits resembling simultaneous scoria cone formation which attributed with lava effusion on the lower part of the cone. Chemical compositions were gained through ICP-MS-AES analysis including major and 13 trace elements for 17 representative samples yields typical basaltic compositions of SiO2 46.1 to 50.7 wt.% and MgO 5.9 to 7.5 wt.%. Median grain size (Md Phi) and sortation coefficient (delta Phi) of -0.4 to -1.85 and 1.025 to 2.35, respectively, suggested for Hawaiian to Strombolian eruption type. In detailed, initial stage composed of bomb rich (size range 10 to 30 cm) embedded in ash materials (Phi scale -2) typical of opening vent stage. Average grain size shows general coarsening upward trend (Phi scale -3 to -5) and suggest for increasing explosive eruptive style. In the late stage, pyroclastic surge deposit typical of phreatomagmatic eruptive style started occur implying for the transition from dry to wet environment. The external water influence might triggered more explosive eruptive style once there is new magma ascend to the surface. We suggest that such eruptive style through a scoria cone need to be integrated into the hazard assessment of a potentially active volcanic area such as that in Slamet volcano.