If you think back to the fall, I asked Eruptions readers to submit questions for Dr. Shan de Silva (right) from Oregon State University, who has been in the news for research at the inflating Uturuncu volcano (below) in Bolivia. He is also the resident volcanologist at Volcano World, a website I know many of us use regularly. Well, the answers are here! I offer a big "thank you" to Dr. de Silva for taking the time to tackle some of the great questions you all submitted.
Pieter: Is it true that certain volcanic areas go through 'caldera-forming phases'? A certain period of time in which volcanic activity is high and the caldera-forming eruptions are more common. For example, most of the calderas in the Rift Valley in Africa were formed during the late-Pleistocene, but ever since the Menengai eruption there hasn't been a caldera forming eruption that I know of.
Are so called 'supervolcanoes' regular strato/shield volcanoes before their first major eruption?Dr. de Silva: These are great questions that deal with something very close to my heart which is the critical importance of heat delivery to a volcanic system. I am going to take these two questions together as they deal with similar issues. The first issue is that not all calderas are the same. I am going to deal with andesitic to rhyolitic calderas (high silica content) that are formed through explosive volcanic processes and not basaltic calderas like Kilauea that are drainage calderas. Stratovolcanoes or composite cones (e.g. Crater Lake, Vesuvius) and shield volcanoes (Menengai, Newberry) tend to form calderas through collapse of the upper parts of their edifice and are typically quite small <10km in diameter. Larger 20 – 80 km calderas (Valles, Galan, Long Valley, Yellowstone, Toba, etc. getting into supervolcano territory here), form when the entire crust above the chamber collapses to form a depression. Any pre-existing composite cones or shield volcanoes would be swallowed up by this process.
Any individual volcano or volcanic system (temporally and spatially linked magma system) can wax and wane as a function of magma supply which is ultimately a function of the heat being delivered to the system. So small systems like Crater Lake or Vesuvius generally sputter along but then could have an explosive eruption that forms a caldera. Supervolcanoes are cyclical as well. The key is the rate that heat and magma are delivered to the shallow magma chamber from which eruptions initiate. If heat delivery is too slow then large magma batches can’t form because they would solidify after some time, but if heat is delivered rapidly then the magma stored in a chamber can remain “viable” (or eruptible) and grow – this is called the incubation rate. If the incubation rate is low (high heat flux) then each natch of magma does not have time to solidify before the next batch of magma arrives, keeping the system viable. A model that Patricia Gregg here at OSU is developing also shows that this incubation rate is important in keeping the rocks around the magma chamber warm thus allowing chambers to expland without breaking the rocks. So in both large and small systems formation of a viable batch of magma is critical and this is controlled by the rate of heat input (which is through magma delivery). So a caldera forming eruption could be seen as the final signal of increased heat (and magma) input into a volcanic system. This is cyclical. In answer to your second question, there is a clear difference between composite cones and supervolcanoes in terms of the size of their eruptions. The largest eruptions from the former are <100 km^3 while supervolcanoes are >1000 km^3. Supervolcanic eruptions occur less frequently and their cycle times are longer – so maybe supervolcanoes are simply composite cone magma systems that simply grow bigger over time. This is not the case because, again the rate of heat delivery rears its ugly head. To grow a supervolcanic chamber requires a much higher (2 to 3 order of magnitude higher) rate of heat delivery to keep the growing volume of magma eruptible. An analogy would be how streams remain flowing in winter….small streams tend to freeze up, while faster flowing larger streams remain unfrozen – the flow rate or discharge is the key. I have dealt with where the heat comes from and why and how the heat flux might vary – that is another story (see later answer to George B). I am talking primarily about what needs to happen in the upper 10km of the crust. So after that long winded explanation, getting back to your original question, yes, silicic and intermediate volcanoes can go through cycles of caldera forming activity if they are subject to changes in the rate that heat is delivered to the magma storage region. If not they are in what is known as a steady-state where they sputter along with small effusive and explosive eruptions. From my perspective supervolcanoes and composite cones or shields are different systems so they are generally unrelated. If of course the heat flux in a volcanic region changed by orders of magnitude you could technically switch from composite cones to supervolcanic systems, but this requires a major change in the plate geometry or crustal structure (see my later answer to George B). If you had to put your money on any area, where would you guess for the next 'super-eruption' to occur? Money and intuition are never a good mix (at least in my case). If I were to speculate I would say that barring any major arrangement of plates any future supervolcanic eruption is most likely to happen where there has already been supervolcanism in the last 5 Ma. Next most likely is where there is active volcanism today, but only if there was some major change in the heat flux – that is why places Uturuncu are interesting. It is a composite cone sitting in the middle of all these supervolcanoes, and the deformation there suggests supervolcanic rates of magma/heat input. Why do the Andes interest you so much compared to other volcanic areas? The Central Andean plateau is one of the most mindblowing landscapes on the planet. You feel like you are on another planet (we use it as analog for Mars) and everywhere you look are volcanoes and volcanic deposits! Millions of years of volcanic history are beautifully preserved waiting to be admired and deciphered. It is a high altitude desert and logistically quite challenging. It is relatively poorly studied, so it is something of an open book, few other groups work there, which has allowed us to really pick and choose some great projects out there. Every place you go to triggers enough questions to fill a lifetime of work for a volcanologist. It is rare to have such a wonderful natural laboratory available.
