The stratigraphy and nature of the stratocone of Mt. Cemara Lawang in the Bromo-Tengger caldera East Java, Indonesia

Abstract

The stratocone of Mt. Cemara Lawang is part of the Mt. Bromo - Tengger volcano complex, East Java, Indonesia. Mt. Cemara Lawang is composed of a sequence of deposits, comprising the big feldspar lava, andesite lava, phreatomagmatic ash fall, phreatic, basalt lava, basaltic andesite lava, andesite lava, alternating layers of pyroclastic fall and flow and the brown ash fall deposits from the oldest to the youngest, respectively. This sequence can be grouped into lower group which was deposited between ca 144,000 to 135,000 years ago and an upper group between ca 100,000 to 33,000 years ago. Mt. Bromo - Tengger rocks, including Mt. Cemara Lawang rocks are medium-K basalt to medium to high-K andesite with one dyke being low-K tholeiite. Nearly all lava flows are porphyritic or glomeroporphyritic with plagioclase, pyroxene, ± olivine and opaque minerals in pilotaxitic groundmass of microlites of plagioclase, pyroxene, opaque mineral, glass, ± olivine. The basalt lavas and juvenile pyroclastics of Mt. Cemara Lawang often contain andesite lava and other igneous rock inclusions. Petrography and rock chemistry indicate fractional crystallization, magma mixing and slight contamination processes have influenced the basaltic parental magma of Mt. Bromo - Tengger. Physiography of Mt. Bromo - Tengger can be interpreted as two calderas, the Ngadisari caldera (ca 152,000 years old) and the Sand Sea caldera (ca 33,000 years old) but only one ignimbrite deposit (the Sukapura ignimbrite) is associated with the formation of these calderas. It is assumed to be the Ngadisari caldera product. Formation of the Sand Sea caldera is still unresolved. The evolution of Mt. Bromo - Tengger is divided into five periods : the first period is formation of a single giant volcano as high as 4,000 meters above sea level with crater lake and parasitic cone of Mt. Ijo in the western flank. The second period commenced with the formation of the the Sapikerep valley and the Ngadisari caldera with associated Sukapura ignimbrite (ca 152,000 years). The third period produced the big feldspar lava, andesite lava and the phreatomagmatic ash fall deposits. The fourth period was destruction of the western caldera wall of the Ngadisari caldera which later yielded the Argowulan crater and was followed by movement of the central eruption point to the west about 1.5 km from Mt Cemara Lawang. This period was marked by filling of the Argowulan crater by basalt, basaltic andesite and andesite lava flows, alternating layers of pyroclastic fall and flow and the brown ash fall deposits. The fifth period was the second paroxysm which destroyed the western part of the Argowulan crater to produce the Sand Sea caldera. The young volcanoes within the Sand Sea caldera are basalt scoria cones. Mt. Bromo is the youngest scoria cone and is still active today.