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Surtseyan Tuff
Volcanic pyroclastic rock consolidated, of thin grain size, this is, formed by ash and lapilli.
Surtseyan tuff (hyaloclastite tuff) is a pyroclastic volcanic rock, that results from submarine explosive eruptions, generally yellowish or brownish colour, and consolidated with fine-grained (ash or lapilli), sharp stratification and interspersed rock fragments from the host rocks also projected during the eruption.
Curiosity: was the rock most used in the Azores in the construction of fortresses.