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University of Missouri Department of Geological Sciences
Rock type: Welded TuffLocation: Bell Mountain, MO
Bell Mountain is near Taum Sauk Mountain, and both rhyolites are “welded tuff”, formed when hot
volcanic ash (which is actually tiny shards of volcanic glass) are compressed and weld together at
high temperatures. These rocks are typically the products of pyroclastic flows, hot mixtures are
ash and gas that are as dense as water (~1000 kgm-3), hot enough to melt lead (~350˚C or
660˚F but can be much hotter, more than 1000˚C or 1830˚F), and that can travel at speeds in
excess of 100 ms-1, or 220 mph. This particular unit contains a layer rich in lithophysae, or
“thunder eggs”, which form when crystals grow from trapped bubbles of hot volcanic fluid.
Thunder eggs are the state rock of Oregon.
Photo by Alan Whittington, MU Geology
MU Geology Department