This rock is displayed in
Treasures” collection on the roofed patio of the visitor
center in the Robert Sibley Volcanic Regional
Preserve. The above caption is the same that is given on-site
(June 11, 2012).
Basalt is a fine-grained, igneous rock that forms during lava flow. Basalt is mainly composed of plagioclase feldspar and pyroxene and olivine minerals.
Vesicular lava contains dissolved gases such as carbon dioxide and water vapor. During cooling and decrease of pressure, degassing occurs. This process leaves bubbles or vesicles behind. Some gas may also be trapped. The result is a vesicular rock with an overall low density; such, that it usually will float on water.