It's been speculated that the island was formed relatively recently, in terms of the age of the earth, anyway. Apparently, as the nearby continental shelf in the area (now located near Montserrat, and the cause of the volcanic activity on that island) moved through the area, it forced a large mass of rock to the surface, creating the islands of the Lesser and Greater Antilles. Some of this activity also unleashed molten rock, which explains some of the volcanic formations you might see on Bonaire. However, since the continental shelf has passed us by, there's no real threat of volcanic activity on Bonaire these days.