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.
