The latest article really did invite a sequel as the short time at my disposal and the dearth of publicly available material led to it not being on as solid scientific foundation as we like them to be. This will be set right now. Early this week there was a letter in my mail, physical as opposed to electronic, which upon opening revealed an absolute gem of paper on the TMVB. Unfortunately like so much these days, the full version of which I received a print-out is hidden behind a paywall.
A good starting point for a closer view on the tectonics of Mexico and Central America is the Chixchlub impact 65 million years ago. Although some may not credit this with much of an impact on the geology of today, it is well to remember that a simultaneous and similarly-sized impact in the Indian Ocean is credited with causing the Deccan Traps Large Igneous Province eruption and that at 250 MY, a greater impact variously believed to be hidden in the Indian Ocean near Reunion or under the Antartic Ice Sheet caused Gondwanaland to split asunder and separated what now are Africa, South America, Antarctica and Australia from each other. An impactor 10 – 15 km in diameter travelling at ~25 km a second carries such an enormous amount of kinetic energy that it makes a mockery of the efforts of even the thickest of cratons to resist it. Whichever plate was hit would have been punched through completely and at the very least fractured if not broken up into fragments.
Today, the Trans-Mexico Volcanic Belt, TMVB, the southernmost portion of the North America Plate, overlies the Riviera Plate to the west and the Cocos Plate to the south-south west. This portion itself is made up of at least seven terranes, smaller pieces of continental crust welded together. The paper begins by describing the mainly rhyolitic volcanism during the period between approximately 40 – 34 MY BP. In the NDVP article, I mentioned the Tilzapotla Caldera located about 100 km south of Mexico City but this paper makes it clear that it occurred in the “Guerrero-Morelos platform” terrane and has nothing to do with the TMVB, except bordering to it.
About seven million years ago, the Trans-Mexico Volcanic belt began to form when the underlying and subducting Cocos Plate detached from the bottom of the Mexico City craton at the northern part of its main thickness. Before this, the Benizoff Zone of dehydration which drives Arc volcanism was located just to the north of the TMVB and is the cause for the arc of ancient volcanic features that runs along the northern edge of today’s TMVB. Incidentally, almost all identified calderas in the TMVB lie near this old volcanic arc, just to the south of it. Also, they are clustered in the eastern half, above the Mexico City craton and not the western Guadalajara one.
After detaching about 7 MY, the subducting slab began to bend downwards which resulted in the creation of the Mexico City and Toluca basins as well as the volcanism of the TMVB. The angle at which the subdacting plate disappears is ~20 degrees from the perpendicular, which could explain how it was energetic enough to burn through the thickest part of the continental craton. Ferrari et al claim that the dehydration zone “lies at a depth of 40 – 100 km”. This is unclear as it would place it inside the Mexico City craton. Most likely, they meant that it lies 40 – 100 km below the craton, something which would place it at a depth of about 90 – 150 km below the surface which would be consistent with findings elsewhere.
There you have it. This is “our Beast”, an unusually steeply inclined subduction zone which concentrates the energy until it was sufficient to burn through a continental craton up to 55 km thick. It is not unique, there are places such as Etna where a thin slice of the subducting slab has been bent backwards but here in Mexico, it seems to be uniformly over a large area.
A final note on the Chichinautzin Volcanic Field just south of Mexico City. Piecing together the information from several papers reveals an unusual feature or sequence of events. The latest eruption in the CVF was from the 340 AD eruption of the Xitle scoria cone. Not only do the lava flows from this eruption reach far inside modern Mexico City, they also partially buried the highest peak of the CVF; Volcán Ajusco. The paper on the Xitle eruption calls Volcán Ajusco “a lava-dome complex” whereas others unequivocally identify it as a stratovolcano. If the latter is true, we have the highly unusual situation of a volcanic field being in the process of burying a stratovolcano. The CVF does indeed merit further studies as does the entire TMVB, something Ferrari et al point out in their “Concluding Remarks” – “Several aspects of the TMVB however are still unclear and constitute opportunities for future research.” Their paper is definitely very rewarding for those who manage to get hold of it.
Although one can never take the behaviour of one volcano as proof that another will behave in exactly the same manner, the similarities do lead to some conclusions. First of all, make note of how the relative small eruption plume from the crater is dwarfed by what happens once the hot magma hits glacier snow and picks up energy. This is a phenomenon also seen occasionally at Etna. It identifies the eruption type as Strombolian and the magma as most likely basaltic-andesitic in composition.
As we have magma erupting, and has been for the best part of a week, the vent is now open and the likelihood of a large explosive eruption has diminished. Although a larger eruption cannot be entirely ruled out, it seems likely that Cotopaxi will continue in this vein for some time to come and the longer it does so, the better it is. This gives the glaciers time to melt slowly and in small increments, something that would greatly reduce the risk for devastating lahars. Caveat: With volcanoes, you can never take anything for granted. If you are a local reading this or someone planning a visit in the near future, remember! This is an amateur blog. For accurate information and updates, please visit the Instituto Geofisico website at
Yesterday, the JMA confirmed that the Showa Crater has been blocked up and that as a result of this, pressure is increasing according to Masato Iguchi (Director of the Kyoto University’s Sakurajima Volcano Research Center). With predictions for a large eruption floating about the Internet, a few facts might be well to keep in mind. The total amount of uplift this year is 16 cm, a figure that should always be doubled as magma pushes down as well. Uplift is also clearly limited to the volcanic island itself. When Eyjafjallajökull erupted in 2010, the maximum uplift inferred was about 75 cm and the area uplifted had a radius of some 20-25 km, vastly greater than the current crisis at Sakurajima. Yet for all that, Eyjafjallajökull erupted no more than ~0.140 cubic km over a couple of months.
However, Sakurajima is located at the centre of the Aira Caldera and there has been steady uplift elsewhere for years. I have not seen any news that this uplift too has increased, which could mean that the caldera was becoming active or “restless”. Nor has there been any news that there has been a sudden drop, something which could indicate that magma from other parts of the caldera are being fed into Sakurajima. The evidence this far points to the crisis involving Sakurajima volcano and the Showa crater. The amount of uplift would indicate a possible explosive VEI 2 or possibly low VEI 3, something that would be very serious but probably not devastatingly catastrophic. However, with the Showa crater blocked, magma could force its way to the surface through another vent which could be the reason behind the JMA report on August 15th of an increase in the frequency of volcanic earthquakes detected under Minami-Dake Crater accompanied by rapid inflation which was what prompted the JMA to raise the status to Alert Level 4.
Finally, a couple of personal notes. For reasons private and undisclosed, Carl has been unable to take an active part for the past couple of months. Also, there are some issues relating to my personal health which unfortunately mean that I cannot pick up the slack even if I did take over one of Carl’s NDVP posts. This means that new posts will be “as when” and not the regular twice a week we had initially had aimed for. But there is always room for Guest posts and should you feel so inclined, please do not hesitate to contact us via our Gmail account!
Luca Ferrari, Teresa Orozco-Esquivel, Vlad Manea, Marina Manea, “The dynamic history of the Trans-Mexican Volcanic Belt and the Mexico subduction zone”, Tectonophysics 522-3 (2012)