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
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
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
phreatic rather than phreatomagmatic, and all the blocks that
were thrown out were solid pieces of pre-existing lava.