The magma involved in the 1815 eruption originated in the mantle and was further modified by melts derived from subducted sediments, fluids derived from the subducted crust and crystallization processes in magma chambers. 87Sr86Sr ratios of Mount Tambora are similar to those of Mount Rinjani, but lower than those measured at Sangeang Api. Potassium levels of Tambora volcanics exceed 3 weight percent, placing them in the shoshonite range for alkaline series.