Here's a decent shot of the impressive blue bubble which signals the eruption is about to happen.  Water at a depth of 23 meters is around 120°C, but cannot boil because of the weight of the water pushing down on it from above. When this water is forced up to around 16 meters, some of the water may be above boiling point, and this sets off the chain reaction we see. The pressure decrease allows more water to boil and flash boil into steam, which drives the unboiled water further up the conduit. As this happens closer and closer to the surface, with increasing velocity, the water and steam is forced out, and it is this mixture of water & steam that forms the eruption.