Sibley rock display: vesicular basalt lava

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Sibley's rock collection: vesicular basalt lava

Vesicular basalt lava:
vesicles formed by escaping gas.

This rock is displayed in “Sibley's Geological 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.