The Yellowstone volcano is particularly dangerous because its magma is of the thick continental variety, as opposed to volcanoes located in the sea or continental edges. Being thicker, it traps the various gases, causing them to build to catastrophically violent levels. Volcanoes like those on Hawaii, on the other hand, consist of highly fluid “runny” magma. Those on continental margins, like Mt St Helens, are a combination of the two because both magma types are in the mix.
https://volcanoes.usgs.gov/volcanoes/yellowstone/yellowstone_hazard_44.html
LOL. The USGS is hedging its bets on the type of lava...