Volcanism at Mexico’s Popocatépetl is highly punctuated, especially during its current level of activity where domes of lava grow in the summit crater. These domes occasionally collapse or are destroyed by explosions that can lessen the pressure on the magma beneath to create an even larger explosion. This is akin to popping the top off a shaken bottle of soda — the dissolved bubbles come out of solution rapidly as the pressure is released and you get an explosion of soda.
Today, Popocatépetl had one of those explosions, and thanks to the beautiful weather in Mexico and some nice placement of webcams surrounding the volcano, the explosion was caught on some pretty amazing webcam footage compiled by webcamsdemexico (see above). The video is short, only 30 second long, but after the first few seconds of calm, the explosion occurs, sending a dark grey plume into the atmosphere above the volcano. Now, these explosions come with a lot of force, and you can see after the initial explosion is how the clouds of water vapor around Popocatepetl shudder as the explosion front moves past. Then quickly, the upper flanks of the volcano turn grey from the rapid raining out of ash and volcanic debris (tephra). It is a little surprising how little the clouds actually care that the explosion just occurred at first, but as the explosion continues in this sped up video (UPDATE: the rough estimate made by a commenter was ~24x, so this sequence might have lasted 20 minutes — but it does really show that shockwave clearly), the clouds do begin to show more disruption from the hot ash and volcanic gases being released during the explosion. You can also notice how the plume reaches neutral buoyancy not too far above the volcano (bigger the explosion, the higher it can reach before this happens) as the plume begins to spread laterally (to the right in this video) into that classic shape. My guess is the plume was a few kilometers tall by the time the video ends.
You can see how pulsatory the eruption is as well, with the dark plume churning like steam from a steam engine. This might be due to new magma rising in the conduit, feeding the eruption as it continues. However, even with all this fury, the volcano went back to looking idyllic with only some minor puffs of ash within two hours after the explosion (see below) and only the grey ash on the slopes to show for the seemingly giant explosion. Even as impressive as that explosion seems, these ash and tephra deposits usually are wiped clean out of much of the geologic record by rains as they are only a few centimeters thick near the volcano and millimeters thick further away.
Remember, you can check out the activity at Popocatépetl on the CENAPRED webcams, including this view from the Altzomoni Tlamacas. The alert status remains at Yellow Phase 2 as these explosions continue to occur and seismicity rolls on.
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