Spherulitic obsidian in obsidian clast breccia in the Pleistocene of Wyoming, USA. (centimeter scale)

This remarkable outcrop is part of a rhyolite lava flow that erupted in the Yellowstone Caldera during the last Ice Age (Pleistocene). Yellowstone itself is an enormous hotspot volcano - it has semi-regular, giant ash eruptions every 600,000 to 650,000 years or so. After Yellowstone's last mega-eruption and caldera collapse (at ~640 ka), relatively minor volcanic activity occurred in the caldera. Most of this consisted of rhyolite lava flow volcanism, but some basalt lava flow volcanism also took place.

The outcrop shown above is part of the Nez Perce Creek Flow (Middle Pleistocene, ~152 ka) at its westernmost margin. This lava flow is well exposed in a series of roadcuts along Firehole Canyon Drive in Yellowstone National Park. Rocks there include gray rhyolites, finely-fractured obsidian, and rhyolite breccias having obsidian clasts (seen here).

The gray material is rhyolite & the dark fragments are small to moderately large clasts consisting of felsic porphyritic obsidian (= glassy rhyolite) that have small whitish crystals (feldspar and other minerals) plus small to large spherulites. Spherulites are subspheroidal to somewhat irregularly shaped masses in some volcanic rocks. They typically have conspicuous, outward-radiating textures.

Stratigraphy: Nez Perce Creek Flow, Central Plateau Member, Plateau Rhyolite, upper Middle Pleistocene, ~152 ka (148-160 ka)