Before I start on the subject of this post, we would like to thank everyone for their input and support of VC recently! It is good to see so many of our contributors, old and new, and some of the input is truly staggering in its quality. Thank you! Also, we will let the the Scientific Project post run for a while longer before we follow it up, so please, there is still time to search and post your contributions! The more, the better! And now onto today’s post which is speculative:
In his latest installment on Mt Cameroon, Carl remarks that “The ensuing volcanism is highly programmatic and follows a pattern where the volcanoes are born through large scale basalt eruptions creating layers between 50 and 600 meters thick. After that comes a period of trachytic lava with minor rhyolitic ignimbrites, after that comes a large caldera event… …There is no good explanation to why the basaltic eruptions during a fairly short time switch to highly explosive volcanism.”
The same might be said of volcanism everywhere. How come there is such a wide variety with the eruptions of some volcanoes being so very explosive, the lavas they erupt being of such greatly varied composition that one, Ol Doinyo Lengai, even erupts carbonatites? After all, isn’t all volcanism driven by basalt from the Earth’s mantle and basalt is basalt is basalt, remarkably uniform in its chemistry and temperature as it leaves the mantle for its journey towards the surface?
One answer that is given is that if magma sits long enough in a magma chamber, it will begin to cool and as it does so, minerals begin to drop out of the solution as their solidification/crystallisation point is reached. This changes the composition of what remains from mafic to progressively acidic as more and more minerals crystallise out and the remaining solution moves from basaltic through andesitic then dacitic until what remains is rhyolitic. But this takes so many thousands, if not tens or even hundreds of thousands, of years for the very large bodies of magma required to explain some of the larger eruptions known. It does not explain how some volcanoes very quickly turn from mainly effusive or strombolian basaltic eruptions to explosive eruptions of large quantities of evolved magmas. And it does not explain how the thrachytes, phonolites or various other varieties of evolved magmas, not to mention carbonatites, are produced. The answer must be that the chemical composition of the crust plays a major role in the formation of evolved magmas, one not yet fully recognised.
Some years ago when I was doing research for the Roccamonfina article, I came across an interesting piece of information. Roccamonfina is a large, dormant or extinct stratovolcano with an associated caldera about 6 x 7 km wide located about 60 km NNW of Naples. During the major part of Roccamonfina’s “life”, it erupted magmas rich in the feldspathoid mineral leucite. Only towards the end did they become leucite-poor. In one of the four then-available papers on this volcano, there was some pertinent information on the underlying geology. Because Springer Publishing have now appropriated the rights and demand $39.95 / €34.95 / £29.95 for each of them – an unmitigated curse on that greedy, moneygrubbing obstacle to scientific progress – they are no longer available to us, nor can I point out exactly where I obtained my information, a basic tenet of science.
The salient information was that underlying the the top 10 km of crust, there is an equally thick layer of sediments underlying all the Campanian region of Italy. Interestingly, in the vicinity of the Roccamonfina volcano that layer has been depleted and is no more than 5 km thick. Another aspect of Roman volcanism, Pliocene to recent, is that it is relatively rich in potassium.
Another interesting observation is that so many Arc volcanoes, that is volcanoes overlying a subduction zone, are characterised as bimodal. They mainly erupt basalt or basaltic andesite and even if not all follow the same pattern slavishly, as an eruption progresses, the erupted material becomes more basaltic in composition. Typically, their eruptions are in the VEI 2 to low VEI 3 region, but occasionally they have much larger ones, often erupting more evolved magmas such as dacite. Eventually, most of them seem to have caldera-forming VEI 6 to VEI 7, ingimbrite eruptions of dacitic or even rhyolitic magmas. Now why is this?
Let us start at great depth, at the Benioff Zone some 150 km or more down where the subducting plate is reabsorbed! Here, all water carried down with the kilometres or even tens of kilometres thick sediment layers “evaporates”. In effect, the subducting slab is being dehydrated and the superheated water begins to rise upwards. As it does so, it enters into solution with the basalt of the mantle. Not only does it rise upwards being of lesser density due to the water content. At such temperatures and pressures, water acts as a flux, a catalyst if you will, that greatly enhances the speed of dissolution of already formed minerals back to their basic constituents.
As it rises, it will eventually rise above the mantle proper into the Astenosphere, the very top layer of the Upper Mantle, which is semi-solid. As the ~1,400 degrees C hot “glob of juvenile magma” eats its way through the Astenosphere, it also reheats and dissolves the astenospheric minerals. As they are basaltic in nature, more basaltic magma is formed but the temperature is somewhat lowered. The next resistance to its progress is the underside of a continental craton. This usually consists of old oceanic crust, again basaltic, so yet more basalt is formed by remobilising and the temperature again drops slightly as it eats its way through the approximately 1,100 to 700C hot old oceanic crust.
Once through the old oceanic crust, the juvenile magma quickly finds a path through the overlaying sedimentary layer and the topmost layer containing the most recent deposits. it does so quickly due to two factors, the chemical composition of the overlying layers and that being of progressively lower temperature as we approach the surface, they become progressively more brittle as well. Eruptions at this stage are basaltic, very large and usually form a broad shield that can be several tens to hundred kilometers in extent. Initially, because of the resistance it encountered, there was a lot of pressure built up which is why so much material was erupted initially. Once over, the conduit through the uppermost layer quickly solidifies which means the blob has to begin to build up more pressure for a new cycle to begin. It is now the fun begins.
The sedimentary layer is composed of weathered minerals, primarily quartz and feldspar clays with a large amount of water intermixed. At depths of several kilometers, pressure and temperatures are such that it metamorphoses into sandstone – there is a slight surface remelting which fuses the individual grains together. (Yes Dr Behncke, I remember. Etna is known to have erupted sandstone!) Left long enough, deep enough so that it is hot enough, this will eventually metamorphose into gneisses. But if you emplace a body of basaltic magma, usually well in excess of 1,000 degrees Celcius, plus the abundant presence of water acting as a flux, this quickly remobilises – melts, dissolves – the sedimentary minerals. And quartz and feldspar are the building blocks of evolved magmas such as dacite and rhyolite.
At first, they enter solution with the basaltic magma left behind. Because quartz (rho 2.65) and feldspar (rho 2.55 – 2.76) are lighter than basalt (rho 2.8 – 3.0 with the higher value for juvenile material), the resulting basaltic-andesitic mixture is lighter and thus rests on top. If there is an eruption now, the lightest magmas will thus erupt initially and once they have cleared the vent, what follows becomes progressively more basaltic in nature. This is evident from a plethora of volcanic eruptions. At this stage, an andesitic stratovolcanic edifice is built over the initial shield.
But with each eruption, the sedimentary layer is progressively being depleted which leaves room for more juvenile magma to enter. The larger the “cavity” becomes, the greater – exponentially greater – the surface area becomes and the larger the surface area, the more material is remobilised. A self-perpetuating magma chamber has been born, one that depending on the chemical composition of the sedimentary layer produces different magmas. The greater the content of feldspars in relation to that of quartz, the more trachytic the magma produced. At one point, this magma reservoir will have grown so large that it produces large quantities of evolved magmas, geologically speaking, quickly. There is too much of the stuff produced for it to mix with the basalt to form andesites. In time, you will have very large dacitic to rhyolitic ignimbrite forming eruptions.
As the stratovolcanic edifice grows, more and more of the sedimentary layer is depleted and the top crust layer begins to sag under the weight. Because of the sag and because topside, rocks are not ductible at all, concentric cracks begin to appear at the perimeter. An eruption now will be so great that the top collapses, pushing out even more magma. You have a caldera-forming ignimbrite eruption. After such an eruption, immediately subsequent eruptions will be relatively low on sedimentary contributed quartz and feldspars such as observed at Roccamonfina. Then one of two things will happen. If there remains enough sedimentary material, the cycle will begin anew. A new stratovolcanic edifice or dome complex will be built centrally followed by further ignimbrite forming eruptions. But if the sedimentary layer has been exhausted, the central volcano will seemingly become extinct. The self-sustaining chemical reaction will move to where there is sufficient available sedimentary deposits, at the edges of the old magma reservoir. Around the edges of the surface caldera, new stratovolcanic edifices will be built or in some cases, to the side of the caldera.
This, to my mind, is why Iceland and Hawaii, poor in sedimentary deposits, have so little explosive volcanism. This is why areas around the Ring of Fire such as Italy, Indonesia and the Philippines, which posses rich sedimentary deposits, have these very large andesitic stratovolcanoes and the associated huge, caldera-forming ignimbrite eruptions. This is also why the Taupo Volcanic zone has been able to produce no less than six very large eruptions, two of which were VEI 8, in the past 280,000 years and I am pretty certain that if a detailed analysis of the bedrock underlying Ol Doinya Lengai was undertaken, a deep layer of carbonatite crust would be found.