For a long time, the boilers were attributed to explosions, but nowadays it is necessary to distinguish at least two sources in the formation of boilers. Smaller boilers, with a few hundred meters in diameter, are surrounded by walls slag mixed with abundant rocks of the region, whose presence demonstrates that it is a blast. The larger boilers with one or more kilometers in diameter, are due to afundimentos or rebates which are explained as follows:
1. The first explosions expel a portion of the melt, reducing therefore the level of the chimney, while building the volcanic apparatus.
2. The crater lips start to crack it in concentric circles, and then the blocks separated by cracks due to the lack of underground support, are slaughtered, the magma chamber. Thus is formed a fake explosion crater, which is in fact a crash of the crater, rebate or subsidence. Subsequent eruptions can produce new cones inside the crater. Subsidence craters are volcanoes characterized by plenty of issuance of lava or ash. Large Hawaiian craters are this tipo.O boiler name originated in the Azores and the Canary Islands. In the Azores there are good examples of boilers, some occupied by ponds, one of the most known the of the seven cities on the island of São Miguel.