File:Mahogany obsidian (Nez Perce Creek Flow, Middle Pleistocene, about 152 ka; Firehole Canyon roadcut, Yellowstone, Wyoming, USA) 5 (20670391356).jpg

Rhyolitic mahogany obsidian in the Pleistocene of Wyoming, USA.

This 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 exposure 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 (seen here), and rhyolite breccias having obsidian clasts.

The mottled black and dark reddish-brown material shown above is rhyolitic obsidian (a.k.a. "glassy rhyolite" or "rhyolite glass"). Obsidian is a glassy-textured, felsic to intermediate, extrusive igneous rock. It can be rhyolitic or rhyodacitic or dacitic or andesitic in composition. Most obsidian is black in color, but sometimes can be a rich reddish-brown color. The latter is a variety called mahogany obsidian.

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