Sibley rock display: massive basalt lava

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Sibley's rock collection: massive basalt lava

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.