This rock is displayed in
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.