Hydrothermal megaplumes are huge volumes of anomalously warm water that are located up to 1,000 metres above the sea floor and appear to be generated at mid-ocean ridges. Since their discovery in 1986, there has been considerable debate concerning their origin, A theoretical model is used to argue that the cooling of pillow basalts, which are erupted at similar to 1,200 degrees C into sea water and are the most common form of submarine volcanic activity, is responsible for the megaplume formation.