Magma chamber stratification of the 1815 Tambora caldera-forming eruption

Abstract

The eruption of the Tambora volcano in 1815 was initiated by two precursory Plinian falls and formed two generations of pyroclastic density current (PDC) deposits. In this study, we found slight changes in phenocrysts (modal mineralogy, content, and size), bulk-rock and feldspar microlite chemical compositions, and bubble and microlite number densities through the stratigraphic position. Plinian fall units are characterized by a lower phenocryst abundance (avg. of 5.1%), smaller phenocryst size (avg. of 0.06 mm2), and higher silica content (bulk-rock, 58–58.5 wt.%). The PDC deposits are characterized by a relatively higher crystal abundance (avg. of 12.1%), larger phenocryst sizes (avg. of 0.13 mm2), and lower silica content (bulk-rock, 56.7–57.9 wt.%). Therefore, the deposit stratigraphy and analyses suggest that phenocryst stratification in the magma chamber was established prior to the 1815 eruption and was thus responsible for yielding a slight contrast in bulk compositions. Feldspar microlite moves toward slightly more albitic compositions from Plinian falls to the PDCs, suggesting a slight decrease in the initial melt temperature from the upper to the lower magma chamber portion. Because the Plinian eruptions extracted the hottest magma, the degree of supercooling became low, and consequently yielded microlite-poor juveniles. By contrast, the PDCs experienced a larger degree of supercooling because the temperature was relatively low, thus yielding microlite-rich juveniles. Moreover, such temperature stratification coupled with the evidence of homogeneous melt composition (58.5–58.9 wt.% SiO2) and the minor evidence of crystal mush (at most 27%) might suggest that the Tambora case is still in the early stage of magmatic evolution under cooling from the surrounding rocks.