Rhyolite is a very dense, durable, crystalline form of igneous bedrock. This deposit is a 1.3-billion year old volcanic caldera in St. Francois Mountains - among North America's most ancient rocks. This very hard rock is favored for cobblestones, roadbeds, curbstone, and other high-wear surfaces.
GEOLOGY: Rhyolite, an igneous, silica-rich (typical > 69%), volcanic rock, is an extruded equivalent to granite, and thus, may resemble granite. With high silica, low iron & low magnesium content, Rhyolite melts into highly viscous lava, also as breccia, volcanic plugs and dikes. Rhyolite that cools quickly grows crystals of natural glass called "obsidian." Slow cooling forms microscopic crystals to yield fine textures, foliations, spherics, nodules, and larger aggregates. Eruption of this igneous rock is rare; only three Rhyolite eruptions were recorded since 1900: St. Andrew Strait Volcano, Papua New Guinea; Novarupta Volcano, Alaska; Chaiten Volcano, Southern Chile.