How Volcanoes Work

 Image Described in Caption
 Hyaloclastite -- The dark, lapilli-sized fragments are composed of siderlomelane (basalt glass), whereas the yellowish matrix is palagonite. The glassy basalt fragments formed by chill shattering of lava as it entered water, and the palagonite formed from hydration and altering of the basalt glass. The ~15 million year old deposit is interbedded with Columbia River basalt flows near Lewiston, Idaho. Photo by Vic Camp