Table 9.2 Eruptive History of Mount St. Helens, Compiled and simplified from St.

ID: 108059 • Letter: T

Question

Table 9.2 Eruptive History of Mount St. Helens, Compiled and simplified from St. Helens Eruptive History-Global Volcanism Program 1201-05-&volpage; erupt October 2004 lanuary 2008 VEI: 2- Lava Volume: 9.3 x 10' m' B Explosive eruption and phreat explosion (s) from central vent and flank vent ic dome extrusion and spine extrusion Mudflow(s) (lahars) November 1990-February 1991 VEI: 3 Explosive eruption and Phreatic explosion(s from central vent Pyroclastic flow(s) Mudflow(s) (lahars) December 1989-lanuary 1990 VEI: 2 Explosive eruption and Phreatic explosion(s) from central vent March 1980-October 19e6 VEI: 5 Lava Volume: 7.4 x 10' m Tephra Volume: 1.2 x 10' m' Explosive eruption and Phreatic explosion(s) from central vent and flank vent B Pyroclastic flow(s): Lava dome extrusion spine extrusion Mudflow(s) (lahars) Debris avalanche( 1847-1857 Intermittent episode of explosive eruption from flank vent Lava dome extrusion (1847 only) VEI: 3 Tephra Volume: x 10' m' Explosive eruption from flank vent Pyroclastic flow(s) Lava dome extrusion March 1835 Explosive eruption from flank vent Lava flow (s); Lava dome extrusion August 1831 Explosive eruption from flank vent Lava flow(s Lava dome extrusion January-March 1800 VEI: 5 Tephra volume: 1.5 x 10 m Explosive eruption from flank vent Lava flow (s); Mudflow(s) (ahars) 1610 t 40 years VEI not available Explosive eruption from central vent Pyroclastic flow (s) Mudflow(s) (ahars); Lava dome extrusion 1525 t 25 years VEI not available Explosive eruption from central vent Pyroclastic flow(s), Mudflow(s) (lahars); Lava dome extrusion 482 VEI: 5 Tephra Volume: 1.5 x 10' m'

Explanation / Answer

(a) With the values we have in hand,

V = 1/3 pi R2h

= 1/3 *(22/7*14)km3 = 44 km3.

(b) For volcanic ash in the atmosphere, it is difficult to use a universal detection because the ash particle radius in the cloud usually varies from 1 to 15 meu m and the chemical properties may vary from one volcano to another.

Value

Total removal (g/d)

5.1*1013

Sedimentation %

80.0

Wet deposition %

14.6

Dry deposition %

5.4

(c) Eruptions rated at VEI 1 produce between 0.0001 and 0.001 cubic kilometers of ejecta. Above VEI 1, the scale becomes logarithmic, meaning that each step in the scale represents a 10X increase in the amount of material ejected. VEI 2 eruptions produce between 0.001 and 0.01 cubic kilometers of ejecta. VEI 3 eruptions produce between 0.01 and 0.1 cubic kilometers of ejecta. With each step in the scale representing an explosivity increase of 10X, a VEI 5 is roughly ten times more explosive than a VEI 4. Two steps of the scale is an increase of 100X in explosivity.

Here VEI 6 is roughly 100 times more explosive than a VEI 4.

(d) Ejecta is easily eroded and often covered by younger materials. In these situations, "best estimates" must be made. When assigning a VEI number is difficult, a question mark is often added to the number to indicate uncertainty. Accurate estimates become more difficult when the eruption occurs in a remote area and very difficult when the eruption occurs on an island that is a great distance from other islands or land masses. The largest eruption is 2100 ac = - 300 years.