The outpouring of lava
The final violent eruption 600,000 years ago, although releasing much of the explosive energy of the gases contained in the magma, did not quell all potential volcanic activity in the twin chambers. Molten rock again rose in both of them, and in a few hundreds or thousands of years the overlying caldera floor was domed over the two chambers. One of these prominent domes lies near Old Faithful and the other east of Hayden Valley (figs. [22] and [23]D). Soon, too, the magma found its way upward through the wide ring fracture zones encircling the caldera. Pouring out rather quietly from many openings ([fig. 23]D), the lavas flooded the caldera floor and began to fill the still-smoldering pit. The first lavas appeared soon after the collapse 600,000 years ago, and the latest ones only 60,000-75,000 years ago. The flows were confined chiefly to the caldera proper, but here and there they spilled out across the rim, particularly toward the southwestern part of the Park ([fig. 28]). Some flows also erupted along fractures outside the caldera, the most prominent flow being the very famous one at Obsidian Cliff ([fig. 29]).
YELLOWSTONE LAKE. View southeast across Yellowstone Lake toward the western foothills and crest of the Absaroka Range. The Absaroka Range is an erosional remnant of a vast pile of volcanic lavas and breccias (Absaroka volcanic rocks) that once covered all of Yellowstone; the lake occupies part of the Yellowstone caldera. (Fig. 27)
The chief rock type in the lava flows is rhyolite, similar in composition to the welded tufts erupted earlier but different in other major characteristics. The rock, for example, shows much contorted layering as evidence of having flowed as a thick liquid across the ground ([fig. 30]). A coarse brecciated texture is also a common feature, well shown by lavas along the Firehole Canyon drive ([fig. 31]). Locally, some parts of the flows cooled so rapidly that few crystals formed, and the lava solidified mainly into a natural glass ([fig. 32]).
RADAR IMAGE of a part of southwestern Yellowstone National Park. The lobate landforms are the edges of a lava flow of the Plateau Rhyolite that forms the Pitchstone Plateau ([fig. 1]). The low concentric ridges that parallel the toe of the flow are pressure ridges produced by the wrinkling of the nearly solidified crust of lava along the edge of the flow. (Image courtesy of National Aeronautics and Space Administration.) (Fig. 28)
OBSIDIAN CLIFF, Jim Bridger’s famous “mountain of glass.” The rock is rhyolite lava which contains a high proportion of obsidian, a kind of black volcanic glass. Note columnar jointing along the sides of the cliff, similar to that shown by the basalt flows at Tower ([fig. 33]). The cliff is approximately 200 feet high. (Fig. 29)
