Close-up photo of hialoclastite in Iceland. Cell phone scale. Hialoclastite seen in a petrographic microscope.
Hyaloclastitis is a vitreous (similar to an obsidian but not as brightly colored) volcanic rock that is formed in eruptions underwater. It is formed by vitreous fragments embedded in a matrix of the same origin and the same material but of finer size. The lava, when in contact with the fluid, cools very quickly and generates steam, which causes explosions (very significant increases in fluid volume) and fragments (breaks) the solidified material or separates (divides) the lava. The interior of the larger fragment fragments may show a cooling pattern inward, which favors rock degradation.
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