Sibley rock display: massive basalt lava

Sibley's rock treasures

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

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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massive basalt lava:
with thin buff-colored weathering rind.

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. Basalt rock exposed to air is affected by chemical weathering, which results into a chemically modified outer layer of the rock, called weathering rind.