How do Ignimbrites/Pumice flows form?
-Talked about convective region of Plinian column – could only rise if less dense than surrounding atmosphere. -Once Plinian column loses that density advantage, will collapse into pyroclastic flows. Initial Plinian phase will be followed by pyroclastic flow. -How do you lose the density advantage? Increase the mass eruption rate (mass/time) by too much Mass eruption rate depens on: • %volatiles (affects eruption velocity) • vent radius (allows more material through) Bigger each of those, taller the column (until a certain point) 1. Why would volatile content decrease? Progressively tapping deeper part of magma chamber: Click on this image for expanded view 2. How does vent radius change as eruption proceeds? Vent erodes with time and eruption column gets taller. Eventually too much mass comes out and the column collapses. Click on this image for expanded view At some point, increased mass eruption rate can no longer sustain Plinian column. So much pyroclastic material comes through th
