Angle of repose

23 March 2009

Our encounters with the current life of Volcan Arenal have had three components -- two visual and one audible. I only have photographic evidence of one, however.

All of the activity seems to be related to the ejection of, or the breaking loose of, large chunks of hot, solid lava. These tumble down the mountainside raising clouds of dust where they hit. A single event will last for 5-10 seconds of the tumbling followed by maybe 15-30 seconds of visible remnants while the clouds of dust dissipate. These seem to occur for awhile and then pause for a period. When they are occurring, there might be an event every 10-20 minutes. It seems that the quiet periods can last for many hours, but I don't think a day has gone by in which we haven't noticed at least a few of these events. The photo at top illustrates one of these events from our walk today on the grounds of the Arenal Observatory Lodge, where we have moved for our last 2 nights here. One can see remnants of the dust cloud coming down the flank of the mountain beneath the cloud layer. The pitch here clearly exceeds the angle of repose.



The visual evidence (clouds of dust from points that migrate down the volcano's side) is accompanied by sound that is something like distant thunder. A low rumbling, it seems to occur not so much from the material moving down the mountain but rather in connection with the release event.

The second visual evidence comes only at night and only when the clouds lift sufficiently as they did during our dinner last night. Then, we could see moments of glowing red lava at the top of the mountain and faint red streams of sparks traveling down its side. We think that these nighttime displays are of the same type of event as the daytime ones.

Adjacent to Volcan Arenal is an old dormant volcano called Cerro Chato. We're thinking of climbing it tomorrow. There is reputed to be a nice lake at the top, plus good views of Volcan Arenal, Arenal Lake, and the surrounding countryside. The middle photo depicts the saddle joining Arenal and Chato. I find the shape quite elegant. The negative space could almost pass for an inverted cone volcano. The final shot, below, was taken a few days ago from the road near La Fortuna.