Kikai Volcano in Japan is actually a caldera where four volcanoes are attached. Of the four, it is only Mount Io that erupts continuously. However these are considered small ones only. The eruption that happened in 4350 BC was so strong that it created a caldera when magma overflowed. The explosion was a good 7 on the VEI scale. For this reason alone, volcanologists are wary that this might happen again.