▁ 8 . 7 6 – 2 6 . 1 6 ▁× ▁ 1 0 1 7 ▁jou le ▁for ▁Pl in ian ▁er u ption ▁and ▁keeping ▁er u ption ▁column . ▁ ▁Ign im br ite ▁stages ▁ ▁In ▁many ▁sections , ▁a ▁large ▁grey ▁ground - sur ge ▁under ▁an ▁ign im br ite ▁sheet , ▁which ▁might ▁be ▁from ▁the ▁front ▁part ▁of ▁py ro cl astic ▁flow , ▁and ▁the ▁unw eld ed ▁ign im br ite ▁always ▁under lie ▁a ▁large ▁ash - cloud ▁sur ge . ▁Ign im br ite ▁was ▁depos ited ▁within ▁a ▁ 4 0   km ▁radius ▁of ▁the ▁cal der a , ▁at ▁an ▁average ▁thick ness ▁of ▁ 7 . 4 7 ▁m . ▁In ▁many ▁valle ys , ▁the ▁thick ness ▁of ▁ign im br ite ▁may ▁be ▁ 7 0 – 8 0 ▁m . ▁The ▁Ch ang ba ish an ▁ign im br ite ▁has ▁a ▁low ▁aspect ▁ratio ▁of ▁ 1 . 8 7 ▁× ▁ 1 0 − 4 . ▁The ▁speed ▁of ▁the ▁initial ▁py ro cl astic ▁flow ▁might ▁have ▁been ▁ 1 7 0   m / s ▁( 6 1 0   km / h ), ▁and ▁ 5 0   m / s ▁( 1 8 0   km / h ) ▁at ▁ 5 0   km ▁away ▁from ▁the ▁cal der a . ▁ ▁D uration ▁ ▁The ▁vent ▁radius ▁and ▁water ▁content ▁of ▁mag ma ▁indicate ▁that