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Normal volcanic calderas form when the magma chamber containing molten rock beneath a volcano empties, thus causing the ground above to collapse, no longer supported below by the pressure of the magma. That even very large calderas were related to volcanic phenomena as well as their shape was clear from what surrounded them.
Enormous deposits of tuff, even more than 400 meters thick, clearly showed that they were not a succession of materials deposited by multiple eruptive phases, but had been formed during a single volcanic episode. The characteristics of supervolcano eruptions have been delineated by studying the zircon crystals and their age, which have two characteristics: they are very recent, with an age slightly earlier than that of the eruptions that formed the tuffs that contain them, and show a composition isotope of oxygen typical of the Earth's surface and not of the depths from which these magmas come.
A fundamental contribution in the study of supervolcanoes was given by the characteristics of the zircons: it was discovered that the magma, in addition to a deep component, is also formed by lavas which are the result of the melting of the crust caused by the mantle magmas, due to the high amount of heat they transported.
What happens inside a supervolcano
Therefore the zircons have inherited the oxygen isotopic composition of the crustal rocks, in which the contribution of rainwater percolating into the crust is evident. The phenomenon is not limited to supervolcanoes: it is not difficult to see a very hot basaltic magma cause partial or total melting of the adjacent or overlying portion of crust.
The result is both a mixture between the two magmas and volcanic provinces with a marked bimodality of the magmas, one deep and one crustal. Here the difference is above all quantitative. As regards the dating of the zircons, it is clearly seen that there is the whole range of ages between the arrival under the crust of the basaltic magma and the explosion that formed the caldera.
The eruption of a supervolcano is a fairly simple process: a huge bubble of magma reaches the surface and melts part of the underlying crust. Above the magma the pressure swells the overlying crust on which fractures consequently form, especially along a ring that corresponds to the external part of the bulge.
The fractures reach the magma chamber and trigger the ascent of magmas and various eruptive centers are formed along this ring. The fractures increase in number and size until the crust inside them becomes a cylinder isolated from the rest of the crust that surrounds it.
At this point, obviously, the cylinder cannot stay in place and therefore collapses. The collapse causes the instantaneous emptying of the magma chamber, with the emission of the typical enormous quantity of tuffs, ignimbrites and so on.
It should be noted that even in normal-sized calderas such as that of Rabaul in New Guinea, it happens that adventitious craters form exactly on the edge of the caldera which is also a band in which seismicity is concentrated. An example of this is the Long Valley caldera.
