Ash and tephra fall are the most widespread volcanic hazard. Even lava-flow eruptions could include explosive phases that might produce significant volumes of volcanic ash and pumice. The least-likely scenario is another caldera-forming eruption, in which case much of the United States, southern Canada, and northern Mexico would experience some ashfall.
The most likely type of volcanic eruption at Yellowstone would produce lava flows of either rhyolite or basalt; rhyolitic lava eruptions could also include explosive phases that might produce significant volumes of volcanic ash and pumice. Such eruptions could range in size from smaller than the 1980 eruption of Mt. St. Helens through much larger than the 1991 Mount Pinatubo eruption.
The least likely but worst-case volcanic eruption at Yellowstone would be another explosive caldera-forming eruption such as those that occurred 2.1 million, 1.3 million, and 640,000 years ago. Such an eruption would produce ash columns that exceed 10 km (6 mi) and cover much of the United States with some ash. Once entering the stratosphere (higher than about 10 km or 6 mi), the ash particles would circle the globe and, in combination with the sulfur dioxide emitted during an eruption, could cause global temperatures to drop. However, the probability of such an eruption in any given century or millennium is exceedingly low — much lower than a hydrothermal explosion or lava flow eruption.