Possible future violent events in the active hydrothermal, magmatic, and tectonic system of Yellowstone National Park pose potential hazards to park visitors and infrastructure. Most of the national park and vicinity are sparsely populated, but significant numbers of people as well as park resources could nevertheless be at risk from these hazards. Depending on the nature and magnitude of a particular hazardous event and the particular time and season when it might occur, 70,000 to more than 100,000 persons could be affected; the most violent events could affect a broader region or even continent-wide areas. This assessment of such hazards is presented both as a guide for future activities of the Yellowstone Volcano Observatory (YVO) and to aid appropriate response planning by the National Park Service and surrounding agencies and communities. Although the assessment is presented here in some technical detail, this summary is intended to be understandable to non-scientists. The principal conclusions also will be made available in other forms, more accessible to general readers.The Yellowstone Plateau was built by one of Earth's largest young volcanic systems, having episodically erupted great volumes of both lava and explosively ejected pumiceous ash for more than 2 million years. These eruptive materials are products of two compositional types of subsurface magma: basaltic magma is relatively fluid and, in this setting, generally produces small to moderate volumes of lava in relatively brief eruptions; rhyolitic magma is more viscous and either can erupt effusively to produce small to large volumes of lava or explosively to produce coarse pumice and finer ash. The three largest Yellowstone eruptions produced blanketing deposits of rhyolitic ash so hot that they welded into sheets of dense rock covering large areas, extending beyond the national park. Each of them also produced a rain of ash that spread over much of western and central North America and beyond; these ash deposits are greater than 2 m thick near their eruptive sources and as much as a meter thick in Surrounding areas. Each of these three eruptions produced a caldera, or deep crater-like depression, tens of kilometers wide, formed by collapse of the ground surface into a partly emptied subterranean magma chamber. The latest of these three great eruptions formed the Yellowstone caldera. Renewed rhyolitic magma influx beneath the Yellowstone caldera in central Yellowstone National Park uplifted parts of the caldera floor and produced voluminous intracaldera lavas, the youngest of which extruded in a series of eruptive episodes about 164,000, 152,000, 114,000, 102,000, and 72,000 years ago. During the same span of time, generally smaller flows of both basalt and rhyolite have erupted in several areas outside the Yellowstone caldera: (1) northwest of the caldera, (2) near the southern boundary of the park, (3) in the basin of Island Park, west of Yellowstone National Park, and (4) especially in the southern part of a corridor between Norris Geyser Basin and Mammoth Hot Springs.Disruption of the Earth's surface by faulting and regional uplift characterize the geologic framework of Yellowstone Plateau volcanism. Some of the regional faults that bound the mountain ranges around the Yellowstone Plateau are capable of producing large-magnitude (M>7) earthquakes. In contrast, faults within the caldera are mainly small, produce smaller-magnitude (M 6.8), relatively shallow earthquakes, and reflect strains in the Earth's crust associated with magmatic intrusion and hydrothermal activity. Swarms of generally small earthquakes occurring within localized areas over restricted periods of time characterize much of the earthquake activity within and adjacent to the Yellowstone caldera. This seismicity is monitored by a network of seismographs within and adjacent to the park and is recorded and processed nearly in real time at the University of Utah as part of the YVO program and archived as a contribution to the U.S. Geological Survey (USGS) Advanced National Seismic System database.Leveling surveys, satellite-based measurements, and geologic studies of former shorelines of Yellowstone Lake all show that the entire area of the Yellowstone caldera and a seismically active zone to the northwest undergo episodes of ground uplift and subsidence, sometimes encompassing the entire caldera and sometimes in more local and complex patterns of both uplift and subsidence. Such deformation in the Yellowstone region is monitored by YVO mainly through a network of continuously recorded Global Positioning System (GPS) receivers recorded at the University of Utah. The GPS data are incorporated as part of the National Science Foundation's EarthScope Plate Boundary Observatory, archived at UNAVCO and available through the YVO website (http://voIcanoes.usgs.gov/yvo/index.html).The active hydrothermal system of Yellowstone National Park is one of the largest on Earth. Although accidents involving hot water injure Yellowstone visitors from time to time, conformance with normal Park Service procedures and regulations would ordinarily be sufficient to prevent most of them. By contrast, a commonly recurring, more acute hazard at Yellowstone is the explosive ejection of steam, water, and rock with no associated volcanism. These hydrothermal explosions are caused by hot subsurface waters that flash to steam, breaking the overlying rocks that confine them and ejecting the debris to form a crater. It is generally not clear just what triggers these events, but possible triggers include strong local earthquakes, seasonal or long-term declines in ground water levels, and changes in the underground distribution of heat. Many hydrothermal explosions have few if any premonitory indications.At least 26 hydrothermal explosions have been documented in the 126-year historic record of the national park, and others undoubtedly escaped observation. Since the Yellowstone Plateau was last glaciated, ending about 16,000 years ago, at least 18 large hydrothermal explosions have formed craters wider than 100 m. Conservatively, at least one rock-hurling explosion every two years is estimated to occur at Yellowstone, but because most of these events are small and usually Occur when few visitors are present, the likelihood of harm to park visitors is relatively small. The average recurrence of an explosion that could cause personal injury is probably between 10 and 100 years. Average recurrence time of an explosion large enough to produce a 100-m-diameter crater is probably about 200 years, but such an event could expel rocks and other hot debris more than 2 km from the explosion site. Most hydrothermal-explosion craters at Yellowstone are in the Firehole River geyser basins, beneath or around Yellowstone Lake, and in the southern part of the Norris- Mammoth Corridor.In addition to hydrothermal explosions, toxic gases-mainly carbon dioxide and hydro-en sulfide-pose hazards. Concentrations of these gases in the atmosphere are generally low at Yellowstone, but they can build Lip in confined areas such as valleys, caves, and tunnels, especially during windless conditions. Most areas with toxic-gas hazards can be kept off-limits to people, but gas emissions should continue to be monitored.No volcanic eruption has Occurred in Yellowstone National Park or vicinity in the last 70,000 years or more. Nevertheless, several types of volcanic eruption hazards are possible in the future.Basaltic lavas have erupted around the margins of the Yellowstone Plateau volcanic field throughout its evolution. These relatively low-viscosity lavas generally erupt rapidly, most eruptions lasting no more than a few weeks to a few months though the largest flow fields may accumulate in multiple eruptions lasting months to years. The average period between basaltic eruptions in the Yellowstone region since formation of the Yellowstone caldera has been about 16,000 years. The most likely location of a future basaltic eruption is within the basin of Island Park, west of Yellowstone, but basalts could erupt anywhere in a 40-km-wide band around the caldera. Future basaltic eruptions could cover several square kilometers with lava up to tens of meters thick. Basaltic ash and cinders also might blanket hundreds of square kilometers to depths of a few meters to a few centimeters, and if a vent emerged beneath water or saturated ground, more explosive eruptions could cause significant destruction, such as blasting down trees or structures.Large rhyolitic lava flows, many having volumes greater than 10 km(3), have erupted within the Yellowstone caldera during the past 170,000 years. Initially these larger eruptions were preceded by explosively ejected pumice and ash. In a similar future eruption, ejecta could bury broad areas, locally to many meters. Subsequent lava extrusion could last for years, covering areas as great as 350-400 km2 to thicknesses of tens or hundreds of meters and volumes of 5 to more than 50 km3. Because such voluminous rhyolitic lava eruptions have not been observed anywhere in historical time, it is uncertain how long such an event might continue; extrusion might be orders of magnitude faster than for smaller flows. The probability of a future large intracaldera rhyolitic eruption is difficult to estimate. Available data suggest a highly episodic behavior of past eruptions of this sort, periods of a few thousand years characterized by numerous eruptions being separated by longer intervals of about 12,000 to 38,000 years without eruption. One statistical measure of eruption probabilities based on this episodic behavior suggests an average recurrence of 20,000 years. The fact that no such eruption has occurred for more than 70,000 years may mean that insufficient eruptible magma remains beneath the Yellowstone caldera to produce another largevolume lava flow.Small rhyolitic lava flows postdating the Yellowstone caldera have erupted mainly north of the caldera, but one such flow also lies near the South Entrance to the park. Two distinct types of primary hazards might be associated with small rhyolitic eruptions at Yellowstone. Just as for larger rhyolitic lava eruptions, initial venting almost certainly would explosively eject rhyolitic pumice; the coarser fragments would fall back close to the vent, but finer pumiceous ash would enter the atmosphere and fall downwind for many kilometers. Structures, power lines, etc. could be damaged by ash loading, especially if eruption were accompanied by heavy rain. The initial explosive eruptions could last a few hours to several weeks and be followed by viscous extrusion of rhyolitic lava, covering several square kilometers to tens of meters thick; lava could continue to extrude for many months or even years. Viscous rhyolitic lava would advance much more slowly than a basaltic flow; most affected facilities could be safely evacuated and perhaps relocated. The average recurrence period of small extracaldera rhyolitic eruptions in the Yellowstone Plateau volcanic field is about 50,000 years.In addition to the primary hazards posed by any future eruption of basalt or a small or large rhyolitic lava eruption, important possible secondary consequences include wildland fires, debris flows, and floods triggered by the displacement of surface drainages by lava.Systematic seismic, deformation, and hydrothermal monitoring by YVO is likely to provide indicators of any impending volcanic eruptive activity in Yellowstone National Park. Premonitory events detected by such monitoring might include multiple shallow earthquake swarms of increasing frequency and intensity, the ground vibrations called volcanic tremor, localized uplift of the surface, ground cracks, and anomalous gas emissions.Of all the possible hazards from a future volcanic eruption in the Yellowstone region, by far the least likely would be another explosive caldera-forming eruption of great volumes of rhyolitic ash. Abundant evidence indicates that hot magma continues to exist beneath Yellowstone, but it is uncertain how much of it remains liquid, how well the liquid is interconnected, and thus how much remains eruptible. Any eruption of sufficient volume to form a new caldera probably would occur only from within the present Yellowstone caldera, and the history of postcaldera rhyolitic eruptions strongly suggests that the subcaldera magma chamber is now a largely crystallized mush. The probability of another major caldera-forming Yellowstone eruption, in the absence of strong premonitory indications of major magmatic intrusion and degassing beneath a large area of the caldera, can be considered to be below the threshold of useful calculation.