Table 2. Duration of Supersonic Flow at the Vent and the Formation Time of the Mach Disk Shock for the Systems Considered in This Studya


System	Pr(0)/Pa	tde	tMDS

Laboratory nitrogen jet	150	19 ms	36 mus
Laboratory nitrogen jet	40	12 ms	36 mus
Laboratory helium jet	150	7 ms	-
Laboratory helium jet	40	4 ms	-
vulcanian	100	65 s	~1 s
vulcanian (steam)	100	7 s	-
Mount St. Helens	100	84 s	12 s
Mount St. Helens	150	93 s	12 s
Mount St. Helens	250	104 s	12 s

aHere tde is the duration of supersonic flow at the vent and tMDS is the formation time of the Mach disk shock. Data for laboratory reservoirs with Vr/An = 1.0 mm are shown.