One difference between wide & narrow craters is the presence of this to mix with magma & cause big steam explosions
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Large hydrothermal (steam)-explosion craters formed in the past 14,000 ... An 
eruption of lava could cause widespread havoc in the park, including... [DOC] [TLE] Volcanoes [text] - U.S. Geological Survey [PAR] Dec 20, 1999 ... Its surface and crater were green and covered with vegetation, so the ...