Hyaloclastite

It commonly has the appearance of angular flat fragments sized between a millimeter to few centimeters.

The fragmentation occurs by the force of the volcanic explosion, or by thermal shock and spallation during rapid cooling.

It is transparent and pure, lacking the iron oxide crystals dispersed in the more commonly occurring tachylite.

Fragments of these glasses are usually surrounded by a yellow waxy layer of palagonite, formed by reaction of sideromelane with water.

Hyaloclastite ridges, formed by subglacial eruptions during the last glacial period, are a prominent landscape feature of Iceland and the Canadian province of British Columbia.

Hyaloclastite between pillows of lava in Montana
Pahoehoe lava enters the Pacific at Hawaii Volcanoes National Park , the Big Island of Hawaii