Eruptions

All quiet on the Cascade front

The only volcanic arc in the lower 48 states continues to be pretty darn quiet according to the USGS. The Cascade range that spans from Lassen Peak in the south to Mt. Baker in the north (and maybe Mt. Garibaldi in Canada) has been remarkably quiet in terms of eruptions for the last century. In fact, only Mt. Saint Helens and Lassen Peak have erupted since 1900 – and most of the volcano have shown very few signs of even coming close to eruption beyond minor seismic swarms, steaming or land deformation. This is in contrast to the reports from the 1800s that suggest that Hood, Shasta, St. Helens, Baker, South Sister(?) and Rainier all experienced at least minor ash emissions. Now, these historical accounts are notoriously sketchy, but few people would argue that compared to other arcs such as the Aleutians or Kamchatka, the Cascades are a mighty quiet arc. The reasons for this are anybody’s guess at this point, with suggestions varying from the migration of the Mendocino Triple Junction to the shallow dip of the Juan de Fuca plate to random chance. It is definitely a fascinating question to consider, especially with the potential hazards a “reawakening” of the Cascade Arc might have on the Western U.S.

Comments

  1. #1 doug
    October 17, 2008

    Maybe they are waiting for a major subduction zone ‘unsticking” event to release the compression on their basement plumbing

  2. #2 damon hynes
    October 18, 2008

    And how long would that take to propagate upwards? This is a vast oversimplification, but if the 1964 Aleutian earthquake resulted in this summer’s three eruptions, then maybe in ~40 years Toba will erupt as a result of the Sumatra Christmas earthquake?

    Still a lot to learn about magma paths, repose times, etc.

  3. #3 bruce
    October 19, 2008

    Crikey the idea of Toba unsticking is somewhat frightening! Be damn interesting though.. how far apart do you think fault movement can be from a vent to still have an impact? Toba is about 200 km from the active fault at the surface tho I have no idea how steeply the Australian plate dives underneath Sumatra. And how rapidly can new melt rise to feed a magma chamber?

  4. #4 Bruce
    October 20, 2008

    come to think of it, I realize the Australian plate slipped horizontally in relation to the Asian plate (I remember a number of something like 20 m in places) but does this actually mean the whole slab shifted under the fault boundary? (that would be a pretty humungous earthquake, wouldn’t it?).. and at what depth does a descending slab start to generate new melt? Wouldn’t the more likely mechanism be that the earthquake opened up fractures that faciliated the rise of existing melt? In this case the repose time between earthquake and eruption would depend a lot on how much melt there is and where it was along with the whole dynamics of local faulting and geology.

  5. #5 Erik Klemetti
    October 20, 2008

    I’ve been pondering these comments have this is what I have come up with at this point:

    - The question of “how long does it take for magma to rise from zone of generation in the mantle wedge (in subduction zones like the Cascades or Indonesia) to the surface is an area of debate. Some people (see Turner et al., 2003) want it to be fast – days to weeks while most folks would say it could be months to years for the magma to rise from generation to the surface.
    - Re: the “sticking/unsticking” of the plates, this should have little effect on magma rising to the surface from depth. This is because magma isn’t really “stuck” down there waiting to be disturbed. And unsticking shouldn’t generate new melt as it is the dehydration of the plate and the convection of the mantle that is really going to dictate that – and even if the “unsticking” bring new, wet slab into contact with hot mantle, the timescales for that are not well known at well, but likely not quick enough to generate a lot of melt that we would see in our lifetime.
    - Earthquakes (tectonic) most likely would not have an effect on magma rising from depth, but instead potentially promote magma that is already in the crust to move. However, what that means in terms of promoting eruption is unclear.

  6. #6 doug
    October 21, 2008

    thank you all for the interesting comments on my posting. I was reflecting on some recent reports that though magma rises continuously, magma chambers (or holding zones) recharge sequentially with some sort of valving process operating between zones at different depths. So maybe the compression forces on the north american plate, if relieved via the forecasted subduction zone great quake, could in effect open some valves. Could this be seen by comparing geologic records of great quakes and cascade eruptive history?

  7. #7 bruce
    October 22, 2008

    THanks Erik for your reply to our questions. It’s certainly an interesting discussion! I’ve got so many questions I don’t know where to start. I hope nobody minds me asking them because no doubt some of them (at least!) are pretty basic, so here goes:
    1. What would this valve be? Assuming there is a conduit between holding zones.. how long would it take for magma in the conduit to cool off and solidify? i.e wouldn’t there have to be a continuous flow of heat (if not magma) to keep it open? or does the valve take the nature of a local fault slipping and allowing magma to rise?
    2. What actually stops a magma chamber from melting the rocks above it and rising inexorably to the surface? Is it the melting points of those rocks? the heat potential of the chamber? (I read that structures like the Kimberly Dike (sp?) represent such vertical movement. If it works at that scale why not on a much larger scale like Toba or the Plegian Fields?)