Vol Lecture Notes 5

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Unformatted text preview: e s p r im a r ily fr o m a s h Both from cooling and solidifying H e a tin g e ffic ie n c y is p r o p o r tio n a l to m a s s flu x . H e a tin g e ffic ie n c y is a ls o r e la te d to g r a in s iz e : s m a ll g r a in s g iv e u p h e a t m o r e r a p id ly th a n la r g e ones. Key Factors Controlling Stability G a s c o n te n t E je c tio n v e lo c ity Depends on gas content M a s s F lu x Depends on velocity, but also vent radius (recall that velocity deep in system depends on square of radius because of viscous effects) u= g( !cr " !r )r 2 8# Mass flux therefore depends on 4th power of radius V e n t R a d iu s Positive effect on mass flux, negative on entrainment 11 Two Scenarios for Collapse S c e n a r io 1 : Vent widens by erosion to the point where column becomes unstable (can’t entrain enough air) S c e n a r io 2 : Water content of magma drops, decreasing eruption velocity at constant radius Case History of Eruptive Column Collapse: Vesuvius, 79 AD S o u rc e s : Pliny the Younger’s Letter to the historian Tacitus Pliny Geological field work & archeological excavation 12 Background Vesuvius had been dormant for centuries n o t r e c o g n iz e d a s a v o lc a n o b y th e R o m a n s ( a lth o u g h E tn a w a s ) S tr a b o h a s n o tic e d s im ila r itie s o f r o c k s to E tn a Large, damaging earthquake in 62 AD O c c a s io n a l s u b s e q u e n t s h o c k s o v e r n e x t 1 6 y e a rs . 13 Pliny’s Account Pliny, in Misenum, observes large cloud over Vesuvius on the afternoon of August 24 Pliny the Elder, an admiral, organizes rescue effort; can land no nearer than Stabiae Ash and fumes at Stabiae that night, Pliny the Elder dies (heart attack ?) trying to escape Pliny’s Account Morning of August 25, ash blocks out Sun in Misenum (as dark as a s...
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This document was uploaded on 03/04/2014 for the course EAS 458 at Cornell University (Engineering School).

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