What you are looking at is a pyroclastic flow, aka a superheated current of volcanic gasses,
ash, lava, and other eruptive products descending the southwestern flank of the Mount Merapi
volcano in Indonesia.
Moving at a speed of 103.6 miles per hour or 166.7 kilometers per hour, this pyroclastic
flow traveled a distance of more than a kilometer in only approximately 20 seconds.
Everything caught in this flow which was luckily only small amounts of vegetation was burnt
to a crisp, being incinerated by a temperature of more than 500 degrees Celsius or 932 degrees
Fahrenheit.
This was merely one of more than 4 dozen pyroclastic flows which raced down the slopes of the highly
active Merapi volcano in the last 2 days.
This sequence of events began around noon on March 11th 2023, when a large section of
Mount Merapi's southwest dome became increasingly unstable.
For context, Mount Merapi had been continuously erupting since January of 2021`, with the
eruptive process growing two lava domes; one in its central crater breached to the south
and one high up on its southwestern section.
However, as you can see by the precarious position this dome is in, and the slope of
the relevant flank of Merapi, the dome is highly prone to structural collapse.
This can either occur via a small amount of material or a large percentage of the dome,
with the last major collapse occurring between July and August of 2020.
Thus, it did not come as a surprise when a little less than two thirds of the existing
southwest lava dome collapsed during a single eruptive event on March 11th.
Since the lava dome had been acting as a plug above an active magma conduit, when it slid
off of the conduit all of the gas and pressure which had been building within it escaped
in what is termed a “Merapi” type vulcanian eruption.
This gas generated a pyroclastic flow which traveled up to 3750 meters distant along the
Bebeng drainage and 3550 meters distant along the Kasak drainage.
Luckily, much of the pyroclastic flows which occurred during that sequence were funneled
into a series of river drainage channels descending the volcano with only minimal sections of
the superheated current being directed above the ravines.
This caused a large plume of ash to rise to a height of 20,000 feet or 6 kilometers which
subsequently drifted to the north, west, and northwest, raining down ash up to 100 kilometers
distant, leaving behind a dusting of material.
While spectacular, no one was severely harmed by this eruption as the pyroclastic flows
did not travel into areas with a permanent population.
This makes the eruptive sequence which was the largest to occur at Merapi in more than
6 months somewhat fortunate, as the island of Java as a whole has more people who live
on it than the entirety of Russia.
Over the next 24 hours, this was followed by several dozen much smaller collapses which
also generated pyroclastic flows although these did not travel as far as the first one
did.
In my opinion, during the next several days to weeks, more pyroclastic flows are likely
to occur at Merapi but they are unlikely to be of a similar magnitude to the longest pyroclastic
flow on March 11th as it was a one off event and not enough of the southwest lava dome
now exists to repeat this process on the same scale, although smaller collapses could still
occur.
I want to make a correction to several media reports not my myself about Merapi's eruption.
For example, several sources claimed that the pyroclastic flows traveled up to 7 kilometers
distant.
This is not accurate as they only travelled up to 3.75 kilometers with the 7 kilometer
figure originating from the current maximum distance from Merapi's summit that the active
exclusion zone goes out to as of March 11th 2023.
These same news articles also reported that Merapi produced a more than 1 kilometer long
lava flow which is also incorrect.
Rather, since two thirds of its southwest lava dome had collapsed, new molten rock pushed
upwards towards the surface and began growing a new viscous lava dome.
However, since this new dome was emplaced on a steep slope, small fragments of it fell
off and rolled down the mountain making it appear as a lava flow when in actuality it
was a lava avalanche, somewhat like rolling hot coals down a hill.
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