Hyaloclastite - John Seach

Hyaloclastite is a hydrated tuff-like rock formed when lava flows into, or beneath water.
Hyaloclastite forms by granulation of the lava front due to quenching.
It is composed of angular, flat fragments 1 mm to a few cm across.
 

Hyaloclastite
www.volcanolive.com
Copyright John Seach