Sibley rock display: vesicular basalt lava

Sibley's rock treasures

Autoclastic basaltic breccia Basaltic breccia from flow Basaltic breccia/limestone Conglomerate/Orinda Formation Cretaceous sandstone Lapilli agglomerate Massive basalt lava Rhyolite tuff Ripple-marked pebbly sandstone

Sibley Volcanic Preserve

East Bay Regional Park District Bay Area Hiker Exploring the Preserve

Lassen Volcanic rocks

Lassen Volcanic National Park featuring rocks in the Loomis Museum and along the Devastated Area Trail: banded pumice • black dacite • light dacite pumice • quartz-pyrite pseudomorph • red dacite • sulfurous andesite

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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.