<P> 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 60 km (37 mi). The volcano frequently erupted lava, which descended over steep slopes . Tambora has produced trachybasalt and trachyandesite rocks which are moderately rich in potassium . The volcanites contain phenocrysts of apatite, biotite, clinopyroxene, leucite, magnetite, olivine and plagioclase, although the exact composition of the phenocrysts varies between different rock types . 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 . </P> <P> Since the 1815 eruption, the lowermost portion contains deposits of interlayered sequences of lava and pyroclastic materials . Approximately 40% of the layers are represented in the 1--4 m - thick (3.3--13.1 ft) lava flows . Thick scoria beds were produced by the fragmentation of lava flows . Within the upper section, the lava is interbedded with scoria, tuffs, pyroclastic flows and pyroclastic falls . Tambora has at least 20 parasitic cones and lava domes, including Doro Afi Toi, Kadiendi Nae, Molo and Tahe . The main product of these parasitic vents is basaltic lava flows . </P> <P> Radiocarbon dating has established that Mount Tambora had erupted three times during the current Holocene Epoch before the 1815 eruption, but the magnitudes of these eruptions are unknown . Their estimated dates are 3910 BC ± 200 years, 3050 BC and 740 AD ± 150 years . An earlier caldera was filled with lava flows starting from 43,000 years BP; two pyroclastic eruptions occurred later and formed the Black Sands and Brown Tuff formations, the last of which was emplaced between about 3890 BC and 800 AD . </P> <P> In 1812, Mount Tambora became highly active, with its maximum eruptive intensity occurring in April 1815 . The magnitude was 7 on the Volcanic Explosivity Index (VEI) scale, with a total tephra ejecta volume of 1.6 × 10 cubic metres . Its eruptive characteristics included central vent and explosive eruptions, pyroclastic flows, tsunamis and caldera collapse . This eruption had a long - term effect on global climate . Volcanic activity ceased on 15 July 1815 . Activity resumed in August 1819--a small eruption with "flames" and rumbling aftershocks, and was considered to be part of the 1815 eruption . This eruption was recorded at 2 on the VEI scale . </P>

When was the first time mount tambora erupted