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.