Phreatomagmatic eruption
A phreatomagmatic eruption is a special type of volcanic eruption during which magma or lava comes into contact with groundwater or surface water. This leads to the formation of steam capable of rapid expansion and hence to fragment magma as well as surrounding rocks.
This type of volcanic eruption is usually accompanied by a cloud of fine-grained volcanic material. The ejected material consists of both a piece of solidified magma and fragments of rocks located nearby the volcanic eruption. The ejected material is usually deposited in a number of layers (there can be up to several thousand), which may differ in color or grain size. The formation of layers is caused by the fact that phreatomagmatic eruptions usually take place in a series of rapidly successive explosions.
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Photo: Volcanic eruption of Mt Matavanu on Savai’i island in Samoa, 1905, photo by Thomas Andrew