A high volcanic cone with a single central vent formed before the 1815 eruption, which follows a stratovolcano shape. The diameter at the base is . The volcano frequently erupted lava, which descended over steep slopes. Tambora has produced trachybasalt and trachyandesite rocks which are rich in potassium. The volcanics contain phenocrysts of apatite, biotite, clinopyroxene, leucite, magnetite, olivine and plagioclase, with the exact composition of the phenocrysts varying between different rock types. Orthopyroxene is absent in the trachyandesites of Tambora. Olivine is most present in the rocks with less than 53 percent SiO2, while it is absent in the more silica-rich volcanics, characterised by the presence of biotite phenocrysts. The mafic series also contain titanium magnetite and the trachybasalts are dominated by anorthosite-rich plagioclase. Rubidium, strontium and phosphorus pentoxide are especially rich in the lavas from Tambora, more than the comparable ones from Mount Rinjani. The lavas of Tambora are slightly enriched in zircon compared to those of Rinjani.