In contrast to the 1790 eruption, the 1924 explosions were caused entirely by steam, not poisonous volcanic gases. This, and the fact that all the rocks thrown up by the explosions were solid, not molten, indicates that the explosions were not magmatic, but were phreatic, caused by flash-heated groundwater. It is thought that the sudden sinking of the summit, caused by rapid withdrawal of magma down the rift zone, caused the rocks to crack, allowing groundwater to come into contact with the hot, but solid, rocks in the deeper parts of the summit crater. Instantaneous boiling of the water built up pressure causing more fracturing and explosions as the volcanic rocks broke apart and were hurled into the air. All rights reserved.
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