File:West Yellowstone Flow (Upper Pleistocene ~111-114 ka; Firehole Canyon, Yellowstone, Wyoming, USA).jpg

English: Rhyolite in the Pleistocene of Wyoming, USA.

Rhyolite is a felsic, aphanitic, extrusive igneous rock - it's one of many specific types of lava produced by volcanoes. Rhyolite is generally light-colored and finely-crystalline. When molten, it almost always has a high viscosity - it is thick and sticky, almost like toothpaste. Sometimes, rhyolitic lava cools into glass (obsidian) or glass with scattered crystals (vitrophyre). Some explosive volcanic eruptions consist of rhyolite ash, which is finely-pulverized rock, or rhyolitic pumice, which is light-weight and frothy-textured, with numerous tiny holes that were originally gas bubbles.

Seen here is a Pleistocene-aged, rhyolite lava flow in Yellowstone, Wyoming. Yellowstone is a hotspot volcano that has occasional enormous explosive ash eruptions. After the last one, about 640,000 years ago, smaller-scale volcanism has occurred at Yellowstone - some involving dark-colored basaltic lava flows and some involving light-colored rhyolitic lava flows. This example is the West Yellowstone Flow, which consists of rhyolites dating to about 111 to 114 thousand years ago.

Stratigraphy: West Yellowstone Flow, Central Plateau Member, Plateau Rhyolite, lower Upper Pleistocene ~111-114 ka

Locality: outcrop next to Firehole Falls, Firehole Canyon, Yellowstone, northwestern Wyoming, USA