When external water is involved in a basaltic eruption, the eruption and vent constructs are markedly different, and such an eruption would be one to avoid experiencing close up. The water is usually ground water, but it can also be shallow ocean or lakes. These phreatomagmatic eruptions have been most closely studied at Surtsey, Iceland (Thorarinsson 1967). The interaction of erupted lava and water is explosive, and therefore the vent constructs are very different from those produced by "dry" eruptions. During a phreatomagmatic eruption there are thousands of shallow, closely-spaced, steam explosions that fragment the lava into ash-sized particles. Because the explosions are so shallow, much of the explosive energy is directed sub-horizontally. These sub-horizontal blasts are called base surges and each deposits a single layer of ash. The resulting vent construct from such an eruption is more like a ring (much wider compared to its height) than a cone, and consists of the thousands of individual layers. The fine-grained phreatomagmatic ash weathers into tuff, and the constructs are called tuff rings or tuff cones. The best examples in Hawai'i are those of the Honolulu volcanic series, however, there is one tuff ring on (Kapoho cone) and one on Mauna Loa (Pu'u Mahana). Both of the big island examples formed near the coastline where the water table is within a few meters of the surface.

The very explosive eruption of Kilauea in 1790 was caused in part by groundwater interaction but was apparently so explosive that no tuff ring was formed; all the ash was dispersed widely. In 1924, ground water also came in contact with hot rocks in the shallow summit region, but this time there was no juvenile magma involved. Technically these 1924 steam eruptions were therefore phreatic rather than phreatomagmatic, and all the blocks that were thrown out were solid pieces of pre-existing lava.


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