Monday, October 24, 2011

Mt. Hood Quakes

There are some ticking time bombs in the cascade range we truly wish would never awaken. From Shasta, to Hood, to Ranier, all of these could easily dwarf the eruption of Mt. St Helens given the right circumstances. Indeed in the past couple of weeks, some quakes have come mighty close, and even occurred under these sleeping giants.

Mt. Hood is one of the most historically active volcanoes in the Cascades, last having erupted in the 19th century. Hood is especially dangerous (and tricky to monitor) as are a lot of the cascades volcanoes due to its age and structural weakness. Being a very high volcano, ice and the placement/contraction of glaciers changes the chemical structure of the lavas over time, creating a sort of "lava rot". Oxidization of the adesite/dacite composition leads to occasional avalanches and collapses, similar to what was witnessed at St Helens in 1980, when structural weakness combined with doming of the NNW face combined to create a catastrophic failure of the volcanoes structure.

Hood has had several very minor eruptions as its most recent activity, but has otherwise been relatively quiet save for some recent earthquake activity. On Oct 20, two shallow 1.2-1.3 magnitude quakes occurred on its NNW face. The reason cascades volcanoes are tricky to monitor, is that sometimes these quakes are not tectonic or magmatic at all, rather they can be anything from avalanches, to snow/ice movement, to a simple release of deep steam. In this case, the quakes appear to be magmatic in nature, purely because of the location and depth... but I am NOT a seismologist so do not take my word for it.

Earlier last week, large quakes (mag 4.0 and less) occurred near Mount Rainier to the West of the summit. This does in fact appear to be magmatic, but again, a scientist should weigh in on this. Typically however, magma movement goes in cycles that generate many small harmonic tremors, which have not occurred at Rainier, so this could be purely tectonic too. Time will tell. Rainier is probably the most dangerous volcano in the Western USA, so it behooves us all to stay alert to its activities. Estimates of an eruption, pyroclastic flow, or lahar say that if this occurs at Rainier, people will not have enough time to get out of the way and hundreds of thousands (if not millions) of lives would be at risk. So scientists monitor its seismicity in order to even attempt to predict an eruption.

Some volcanoes give us warnings, as Nevado del Ruiz in Columbia (and unfortunately for the Columbians, these warnings are sometimes ignored... a mistake they won't be repeating soon!). In 1985, scientists detected volcanic tremor and a type of quake that had not been previously identified, called a "Long Period Event" or LPE. This directly preceded a catastrophic eruption that killed many, and displaced hundreds. Some scientists were on the mountain at the time of the eruption and ended up losing their lives.

Eruption prediction is probably never going to be even as close as predicting the weather, especially in our lifetimes. But every volcano is different, and some are more obvious than others. Some however, give very little warning before eruption, as was the case in Chile with Chaiten volcano, which had not erupted for over 8,000 years. Nabro volcano in Eritrea erupted a couple months ago, never having shown any historical activity whatsoever. These eruptions were preceded by large quakes (mag 5.0 and greater), and erupted only hours later.

Keeping track of earthquakes as they relate to volcanoes is a fun hobby, and who knows? It might be the hobby that ends up saving lives. Stay tuned!

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