7 comments on “Poás

  1. Great post, Granyia! Now, a question. What is the ‘dome’ always mentioned in Poas’ crater? It’s obviously not a ‘dome’ in the sense of say St Helens or Sinabung, but most sources I’ve seen are a bit vague about what it actually IS. It sounds like what they call a cryptodome; ie a shallow intrusion that has forced the crater floor upwards without breaking through to the surface, but if that’s the case why not say so? And if that’s NOT the situation, what is it?


    • Thanks Michael! This is a question that has bothered me a lot when I collected the material, so I decided not to mention it at all in the post. 🙂 Indeed, reports from OVISCORI-UNA, as well as some scienfic papers call it a cryptodome. Other papers consistently use the term dome. GVP says: “On the S edge of the lake is a dome (that OVSICORI-UNA sometimes in their reports refers to as a ‘cryptodome’), …”. It seems to me that we have here a conflict of opinion among scientists. In the Spanish caption of one picture from 2011, the “El Domo” is set in quotation marks while other features are not.

      This confusion might derive from the question exactly when does a cryptodome become a dome? I did not find any recent papers on the subject, nor a publicly accessible version of “Magma chamber below Poas volcano, Costa Rica” (Thorpe et al, 1981). Based on gravimetric measurements, Rymer and Brown (1989) suggested that a small shallow magma chamber is located within 500 m of the crater floor. – Temperatures on the dome vary in cycles between 200-400°C and ~900°C. My thoughts are: As the reservoir is that shallow, there might not be much room for rising magma to produce a cryptodome. If there were a cryptodome, the temperatures would be more consistent. If it reaches temperatures of almost 1000°, magma should be so near to the surface (at times) that it would be hard to tell whether any covering material left should be called “piping hot rock” or “lava”. If lava – we have a dome. But, caveat, I am just a volcanoholic…

      Actually, I have found so much interesting material on Poas that I was thinking to do a part 2, covering the more scientific aspects of the volcano. But for that, I have to understand first what I’m writing about 😉 … maybe I will put the Dome question on Eriks blog.

      (Photo: Carlos Ramirez)


      • I have this mental picture…a small pod of magma sitting under the crater for decades (centuries?) slowly cooling and degassing; not so much a melt as a mass of red-hot glass,. Add the effect of acid gases and hydrothermal action (lot of groundwater because of the heavy rainfall) and you have a situation where there is no sharp boundary between ‘magma chamber’ and existing structure, but a gradual transition from magma, to solidified magma, to solidified magma with increasing degrees of alteration, to the acid-marinated sulfurous mud of the crater floor. If some of this semi-moribund magma is squeezed upwards, there will be a bulge on the crater floor… but is it a dome by definition?

        On the temperatures: I wonder if the time of year has an effect? At Momotombo it’s been noted that the crater fumaroles vary in temperature by several hundred degrees between the wet and dry seasons (dry season temperatures are higher)


  2. Omg, wherever you look, it’s all flooding, fires, storms, landslides and so many people dead, injured, lost their homes etc.; you really don’t want to read any more Job’s news! I hope our beloved volcanoes keep at bay for the time being!


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