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Tens of rhyolite lava flows were erupted one after another in central Yellowstone. Canyon Village is built on one. Elephant Back Mountain, west of Lake Hotel, is another. Several flows make up the plateau between Canyon Village and Norris, and several more bound the western margins of Yellowstone Lake. Flows enclose Lewis and Shoshone lakes; they form the wooded boundaries of the geyser basins. Many streams follow seams between flows of different ages. 

Lava flows can be readily dated. They contain various radioactive elements which decay to form daughter products. By measuring the relative amounts of parent material and daughter products and knowing the rate of change from parent to daughter, a geochronologist has a radioactive clock for dating the ages of flows. Analysis, though, is not simple and geologic dates are usually followed by a fudge factor such as +/- 6,000 years. Between the Washburn Range and the Red Mountains, lava flows range in age from about 500,000 years to 100,000 years. They are much younger than the 50 million-year-old Absaroka volcanics. 

To summarize, the Washburn Range is made of debris flows preserved in the north flank of an old dissected volcano. This volcano and the Red Mountains, about 37 miles to the south, are joined by an arc of Absaroka volcanic mountains east of Yellowstone Lake. They form, in aggregate, a sort of geologic horseshoe open to the southwest. Rocks forming the horseshoe are at least 50 million years old. Cradled within the horseshoe are half a million years old or younger. Both the large difference in age and fundamental, chemical composition show older and younger volcanic rocks are unrelated, though they to occupy common ground. 

Early students of Yellowstone geology failed to recognize the age break between the Absaroka volcanic breccias and the much younger lava flows of the Yellowstone Plateau. They believed that a continuum of volcanic activity linked the Absaroka voicanics and the lava flows. 

This comfortable scenario was shattered by a Harvard graduate student, Francis R. Boyd, who chose Yellowstone for his thesis project. Boyd did his field work in the 1950s. During his studies he saw that some of the so-called lava flows were something quite different-they were welded tuffs. Welded tuffs are products of explosive volcanism. Siliceous lavas charged with dissolved gas literally explode out of volcanoes as mobile froths flowing rapidly across surrounding landscapes. When such ash flows settle, they quickly begin to compact and if the ash retains enough heat to re-fuse, the rock becomes a welded tuff. Even after compaction, the individual shards are visible under a microscope or even to the naked eye, although they may be severely contorted by flowage and compaction. 

Before Boy&s time geologists were only beginning to recognize welded tuffs and their distinctive qualities. The significance of his work, published in 1961, was that a previously unrecognized volcanic event in Yellowstone had produced violent explosions and staggering volumes of volcanic ash, later consolidated into welded tuffs. He demonstrated that these tuffs covered thousands of square miles of Grand Teton and Yellowstone and that they rimmed a large tectonic basin in Yellowstone that contain even younger lava flows. 

The explosive volcanic events that produced these tuffs were unbelievably large and violent-many times greater than the 1981 eruption of Mount St. Helens. They destroyed the southern half of the Washburn volcano and whatever mountains existed between Mt. Washburn and the Red Mountains. Geologists have identified streaks and thin layers of Yellowstone volcanic ash from as far away as California, Saskatchewan, Iowa, and the Gulf of Mexico. Volumes of ash blasted into the stratosphere circulated around the globe and must have altered the weather worldwide.

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