A Brief History of Irosin   Geologic history According to geologists who conducted studies and drilled in this area, Irosin was part of a giant volcano more than 40,000 years ago. These geologists have offered three geologic periods in the history of this place: the pre-caldera period, the caldera period and the post-caldera period. The pre-caldera period is described as a sustained activity with intermittent eruptions. About 36,000 years ago, during what is now called the caldera period, a major eruption of this volcano resulted in the construction of a rim which is now called Irosin Caldera. Along the walls of this 11-kilometer-wide Irosin Caldera, several cones and lava domes continue reading : History