Mount Tambora eruption. Cut-away computer illustration showing the internal structure of Mount Tambora, an active stratovolcano on the island of Sumbawa, Indonesia. Over the course of several decades spanning the late 18th and early 19th centuries, a large magma chamber (centre) built up inside the mountain. This volcanic activity reached a historic climax in the enormous eruption of 10th April 1815. This eruption reached a volcanic explosivity index (VEI) of 7, the only unambiguously confirmed VEI-7 eruption since the Lake Taupo eruption in about AD 180. This made it the largest volcanic eruption in recorded history.