Explosive eruption columns and pyroclastic flows

Only one thing can make a huge mountain more awesome – its partial or complete self-destruction.
 

 
Mount St. Helens famously produced that lateral blast in May 1980.

It also had five smaller eruptions later in the year that, like many other explosive eruptions, sent massive amounts of pulverized rock, steam and gas straight up in a tall column.
 

POW!

POW! Sarychev Peak in the Kuril Islands punches the sky in June 2009. (NASA)


 
The top of such an eruption column starts spreading out laterally when it gets far enough above the vent and soon gravity causes ash particles to rain out of the cloud onto nearby regions.

Dr. Erik Klemetti explains how this works in his post about this spectacular blast during the ongoing eruption at Sinabung volcano in Indonesia:
 


 
Pyroclastic Flows

Lower parts of the eruption column can collapse while the rest gets blown sky-high. For that reason, or because of dome collapse and/or some other factor (every eruption is unique), pyroclastic flows – which are basically fast, fiery, gray death clouds – will also pour out of the vent in any direction.

You can see the flows labeled in the top image (Sarychev Peak), and if you look closely, you’ll also see them at the base of the huge column in the Sinabung video.

Pyroclastic flows are always bad news, but they’re especially devastating when they contain a lot of gas, which powers them over topographical boundaries like hills or the sea. Such surges have killed many people over the ages, sometimes tens of thousands at once.

The largest known dome collapse/surge event in historical time reportedly happened in 2003 on the island of Montserrat in the Caribbean. Fortunately, the hardest hit area had already been evacuated, so no one was killed in this event (19 people did die there in an earlier partial dome collapse back in 1997).

While the total volume of volcanic material was less than 1 cubic km, when the dome on Montserrat’s erupting Soufriere Hills volcano collapsed on July 13, 2003, it blew an ash column 15 km/9.3 miles high and created a shock wave as well as…

…[m]assive PFs [pyroclastic flows that] swept into the sea…This resulted in tsunamis reaching heights of several meters and running inland up to 15m a.s.l. in places (Herd et al., 2005) and a powerful hydrovolcanic explosion resulting in a base surge which flowed inland and along the coast as far as Spanish Point (Edmonds et al., 2006. J. Volc. Geotherm. Res. 153, p.313-330). Dense lithic fragments fell over the whole island and heavy ash-fall occurred, with 15cm of ash being deposited in Old Towne, several km to the NW (source).

That’s why, in Montserrat, they call a pyroclastic surge (see it overtopping the ridges?) like this one in 2009 “small”:
 


 

That’s a science video and has no soundtrack, so I would suggest this as background music. It fits well.

Etna

Not every volcanic eruption with massive eruption columns is disastrous, however. Consider Mount Etna, which put on quite a show back in late October this year (raw video is by volcanologist Boris Behncke – click “YouTube” for more information):
 


 
Believe it or not, there is also a third lava flow ongoing from the saddle in between those two columns. The video ends with more lava fountaining and some incredible Strombolian activity that can be heard as well as seen.

Yes, Mount Etna is a drama queen.

MORE INFORMATION

Etna, Sicily:

Soufriere Hills, Montserrat:

Sinabung, Indonesia:

Sarychev Peak, Russia: Note: There’s no seismic network or webcam set up in the Kuril Islands.

(Besides the above film, we also visited Mount St. Helens on Sunday this past August.)



Categories: Sunday morning volcano, volcanoes

Tags: , , , ,

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