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.
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. 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.