Lava Flows and Associated Hazards at Yellowstone
The most likely type of volcanic eruption at Yellowstone would produce lava flows of either rhyolite or basalt. These would be significant and produce flows with volumes greater than 1 km3, but all of these would most certainly remain within the boundary of Yellowstone National Park.
Since Yellowstone's last caldera-forming eruption 640,000 years ago, about 30 eruptions of rhyolitic lava flows have nearly filled the Yellowstone Caldera. Other flows of rhyolite and basalt (a more fluid variety of lava) also have been extruded outside the caldera.
Each day, visitors to the park drive and hike across the lavas that fill the caldera, most of which were erupted since 160,000 years ago, some as recently as about 70,000 years ago. These extensive rhyolite lavas are very large and thick, and some cover as much as 340 km2 (130 mi2), twice the area of Washington, D.C. During eruption, these flows oozed slowly over the surface, moving at most a few hundred feet per day for several months to several years, destroying everything in their paths. An eruption of lava could cause widespread havoc in the park, including fires and the loss of roads and facilities, but more distant areas would probably remain largely unaffected.
If rhyolitic lava flows do erupt, they 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.