Mount Tambora formed over many centuries as a stratovolcano, also known as a composite volcano. A stratovolcano contains alternating layers of lava flow and ash to build its conical shape. The lava flows harden into rock that is resistant to erosion. In 1815, Mount Tambora erupted after several centuries of inactivity and lost 1500 meters in elevation.
Mount Tambora is located on Sumbawa, an island in Indonesia. Ocean crust flanks the island to the north and south. Subduction zones, created by tectonic plate boundaries, allowed lava to build the mountain to its greatest height, estimated at 4,300 meters. At that point, it was one of the tallest peaks in Indonesia. Scientists believe the lava inside the mountain's magma chamber then drained. Over several centuries, lava refilled the chamber, with volcanic activity reaching its peak in April 1815.
By using radiocarbon dating, scientists have determined Mount Tambora erupted three times in history prior to 1815. Each of these eruptions was a central vent eruption, and all produced lava flow except the last. In the 1815 eruption, pumice stones up to 8 inches in diameter started to fall to the earth an hour after the eruption, followed by ash. Lava flows cascaded down the mountain in all directions, forming yet another layer of this active volcano.