Fig. 4

Example for the transition of a pulse from gas thrust to convective buoyancy: a 1.7 s after occurrence, the pulse enters the transitional stage and subsequently starts to show clear signs of radial expansion. This moment coincides with a change in the vertical velocity of the pulse front (see Fig. 2d). In the case presented, the maximum pulse width d _trans(t _I) (red arrow) is 60 m. Its location determines the lower margin of the transitional level. Both boundaries of this region are indicated by blue lines. b The upper margin of the transitional zone is defined by the tip of the pulse when d _trans(t) shows a maximum, here at t _II = 3.9 s. This particular pulse is characterized by a relatively low transitional uplift velocity v _trans of 7.2 m/s. c At t _III = 7.5 s, d _trans(t _III) has decreased to d _trans(t _I)/e^0.5, i.e., 36 m. With a resulting τ of 5.8 s, the resulting mass flux of solids ranged, according to Eqs. (13) and (14), between 0.9 and 1.5 · 10⁴ kg/s, if a time base of t _pulse = 4.2 s is used. This number gives the mass eruption rate for the hypothetical case when all pulses during the eruption would have been identical with the presented one. The mass of tephra transported by this type 2 pulse ranged between 4.0 and 6.3 · 10⁴ kg