Sibley rock display: lapilli agglomerate

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Sibley's rock collection:lapilli agglomerate

Lapilli agglomerate:
spatter from vent, agglutinated while still hot.

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).

Agglomerate rock is composed of rock fragments in a matrix of volcanic ash. Agglomerates typically occur near or in volcanic vents, where they formed from hot ejected magma. The type of magma can be specified by analyzing the agglomerate composition in detail.

Lapilli are small pyroclasts: rock fragments, smaller than 64 mm in diameter, that formed from molten or partially molten lava droplets thrown into space during a volcanic eruption. A lapilli agglomerate consists of such small pyroclastic rocks bound together while they were still hot and viscous.