We walked the usual lava trail - as with last year we walked to the middle of the 1990s lava flow and back again rather than a full transect. Arenal was fully visible once we were on the lava flow. The new active lava flow which the Smithsonian and OVSICORI report has been active since January was visible as a dark area above the youngest scree. Most of the boulders were falling from this flow as it slowly inched forward. Often they would be hard to see on the fresh scree, but would kick up clouds of dust as they hit the older (darker) scree. It seemed like an unusually large number were making it to the vegetation.
Early on we had a number of small collapses at the front of the flow which i guess you could call pyroclastic flows but they were tiny and did not last long.
Our timing was spot on and we did not have to wait long before our first proper pyroclastic flow. In the case of Arenal, these are produced by the lava flow front becoming unstable. The lava flow slowly inches forward. Boulders will often fall, but sometimes the gradient of the front becomes too much and it collapses under gravity. This releases the pressure of the hot lava inside the flow which depressurises explosively. Gases in the lava form bubbles (or are already bubbles) which expand quickly with the depressurisation and drive the explosion.
The result is an avalanche consisting of hot air,gas,steam, and rock that can be very hot (1000C has been recorded) and travel extremely fast (speeds over 100mph are typical). Infamous pyroclastic flows destroyed St Pierre on Martinique in 1902, and Pompeii in 79AD.
Arenal's flows are much smaller and are readily channeled by valleys and gorges. We were not in any danger from these particular ones, and the National Park is generally quick to close when the risk is considered too high.
Here's the first one:
This was the first active pyroclastic flow that I have seen. I have seen the products of pyroclastic flows dozens if not hundreds of times. Indeed my undergraduate dissertation covered a part of Wales that included a number of rhyolite tuffs (high silica pyroclastic flow / ash deposits). However, those were about 450 million years old; and this was the first time I saw a 'live' one.
Shortly after the first one, we started to get a larger flow which lasted much longer:
Note the dust kicked up by boulders bouncing ahead of the flow.
The flow itself is at the base where it is well defined. The billowing clouds above this are where air gets entrained and heated up. The result is a buoyant but dilute dust cloud which lifts up from the flow. The plume from this flow reached a height much higher than the volcano and would have been visible from the cabins (although the flow itself was on the far side from the cabins).
Of course this was also the chance to pose for a photo or two:
As we headed back there were further flows. As we were leaving the park, a much darker 'cloud' from a flow could be seen between the trees and the growing cloud. This would appear to have had much more volcanic dust in it. The park was also beginning to close and had already stopped new visitors from entering.