Just like animals, volcanoes go through a life cycle of birth, growth, decline and finally death; they become extinct. In this installment, we will examine such a cycle of a fictitious volcanic complex and introduce another bucketful of terminology to the reader.
In the previous installment, we said that there were two main sources of volcanism, tectonic driven or hotspot driven. In order to simplify things, we will examine the life cycle of an Arc Volcano, one where the deep source of magma is the re-melting of a subducted continental slab. Some tens of millions of years ago, two continental plates collided and one, usually the “heavier” one, was subducted. A few million years ago, the edge of the slab had reached a depth of at least 50 km where it began to melt, dissolving the minerals into their constituent molecules and forming a glob of magma that began to rise through the semi-crystalline Upper Mantle or Astenosphere.
When it encountered the lower edge of the overlying continental plate, two things happened. The first was that it began to lift the overlying plate, inducing stresses in it that made it fracture. These small and very deep earthquakes as the plate began to fracture, together with the uplift were the first signs of a new area of volcanism. As more and more magma from the glob made its way upwards, the uplift increased. There were more fractures and fissures through which the juvenile magma could begin to make its way through, towards the surface.
The second was that being in excess of 1,400°C, the juvenile magma began to melt the overlying plate, assimilating a tiny portion of its material. Finally it reached a point at about 10 km depth where the overlying rock was no longer slightly ductile but hard. Here the magma began to collect in a body that was to become the future volcano’s deep magma chamber. As it collected over tens of thousands of years, about 10 million cubic metres or 0.01 cubic km of it, with every new spurt or magmatic intrusion, it pushed the overlying rock upwards and that below downwards. Thus we must always remember when we read about uplift due to magmatic intrusion that the actual size of the intrusion is always twice that of the measured uplift!
Finally, enough juvenile magma had collected for it to cause such stress to the overlying rock that it too began to crack accompanied by great swarms of earthquake, tens of thousands of them. Then at about 5 km depth, the magmatic dike extending down to the deep magma chamber, stalled. Again, magma began to collect in a body, this time much smaller. But as time went on, it began to enlarge through re-melting of the surrounding bedrock and the addition of more magma from further intrusions. Also, it began to cool towards a point where minerals started to form. The magma began to evolve. Had no more magma been intruded from the mantle, over the next tens to hundreds of thousands of years, the intruded magma of the cryptodome would have cooled, mineralised and formed a pluton. This process is referred to as Plutonic mineralisation.
As the millennia passed, the cryptodome that was destined to become the volcano’s upper magma chamber enlarged and heated more and more of the surrounding bedrock, making it ductile, until the remaining, brittle crust could no longer withstand the pressures from below. It began to fail and accompanied by another huge swarm of earthquakes, the now slightly evolved, but still mainly basaltic magma made its way to the surface in just a few days or weeks. For the very first time, the volcano erupted.
Because there had been so much magma collected before the pressure and stress induced made the overlying rock yield, the initial eruption was large and lasted a long time. With repose times between subsequent eruptions being measured in tens to maybe hundreds of years, the initial eruption phase lasted several thousand years, until the pressure difference built up from the point in time where the juvenile magma first began to form and collect in the deep magma reservoir had reached an equilibrium. Because the magma was basaltic, it flowed out across the overlying land to for a vast shield covering many hundreds of square kilometres with a few hundred cubic kilometres of magma that once it had erupted is referred to as lava, which solidified as blocky, glassy dark rocks of basaltic obsidian.
In its first incarnation, our volcano was a Shield volcano. While it may have covered an area roughly 15 km in radius, some 700 square kilometres, with in excess of 200 cubic kilometres of lava, because of the “runny” nature of basalt, the average inclination was only little more than a couple of degrees and the peak rose no more than 650 metres above the surrounding landscape, what is referred to as a mountain’s prominence.
It now entered a long period of dormancy, occasionally broken by additional small eruptions of no more than a few hundredths of a cubic kilometre at a time. Also, because of the magma reservoirs being depleted with each further eruption, there was some collapse of the central parts which led to an enlargement of the summit crater to a diameter of a few kilometres. This caused the shield volcano to lose height until its prominence was no more than 575 metres. Because of its size and low gradient, you could stand on it and not know which direction was up and which was down. Indeed, if not seen from a great distance, it would be hard to know it was even there once vegetation had begun to cover it.
Then just over half a million years ago, a new glob of juvenile or primitive magma from the melting, subducted continental slab began to arrive. Again, magma began to collect in the now evolved magmas of the deep magma chamber. The old magma was remobilised and assimilated each subsequent pulse of juvenile magma from the mantle. The resulting magma mix was still basaltic in composition, but leaned towards basaltic-andesitic. This process was repeated when magma from the deep magma chamber made its way to the upper magma chamber, but because this was smaller, fractionation and mineralisation had progressed much further. Here, the resulting mix was basaltic-andesitic to andesitic. It was also rich in volcanic gasses, water, both from the original magmatic intrusions and from water contained in the bedrock that had made its way from the surface over uncounted millennia. The resulting mix was explosive.
Again accompanied by great earthquake swarms, not so great in number perhaps, but greater in size due the much higher pressures involved this time, the overlying structure yielded, opening a path to the surface a few kilometres from the original crater. The first, mainly andesitic eruption was explosive and sent a sub-Plinian column of ash 14 km into the sky. As the column collapsed, pyroclastic flows swept down the gentle slopes of the former shield volcano, destroying everything in its path and deposited a layer of ash many tens of metres thick near the vent and few centimetres thick as far away as ten kilometres.
Because the upper magma chamber was not large, the initial eruption was only a VEI 4 (Volcano Explosivity Index) and erupted a total of ash and lava of just under ½ cubic km DRE (Dense Rock Equivalent). Subsequent eruptions in this period of activity that lasted almost ten thousand years were mostly smaller and added layer upon layer, or strata upon strata, of pyroclastic deposits and lava flows, always much thicker the closer you got to the central vent. Our volcano had now become a symmetrical Stratovolcano with a base radius of about five kilometres and had a prominence of just over 2,500 metres. It was now easily visible and recognisable as a typical volcano. After what seemed the final eruption, the vent had been “plugged” by gas-poor magma that did not erupt but formed a solid plug or Lava Dome, effectively sealing off the vent.
After a short repose of several millennia, our volcano finally had its first very large eruption. With each preceding eruption having enlarged the upper magma chamber, it spanned from four to six km depth and extended laterally roughly two kilometres in every direction. It now contained no less than 13 cubic kilometres of magma while the deep magma chamber may have held as much as ten times more. Of the 13 km3, somewhere around 40% was eruptible. Over the centuries, all seemed calm and serene on the surface. Natural erosion plus a cycle of glaciation and deglaciation had reduced the prominence from 2½ km to just over two. The volcano was now covered with a slowly receding set of glaciers and our distant hominid ancestors must have thought it beautiful, the weather patterns induced – in particular the lenticular clouds – must have filled them with awe. While below, the pressure gradually increased, unnoticed…
Then one day, a critical point was reached and our volcano burst back into life over a period of a few months. First, there had been a series of small, phreatic explosions as ground water inside the volcano became superheated and exploded near the summit, sending pulverised, old rock and ash up to a kilometre or higher above the summit. Meltwater from the glaciers cascaded down the slopes carrying old deposits with it, a mixture of ash and rock turned to clays by volcanic gases, sulphur dioxide which together with water had turned into sulphuric acid which dissolved the rocks. As they reached the valleys below, these thick, muddy clays with the consistency of wet cement poured across the valleys below as lahars. Because these were only precursor explosions – the volcano clearing its throat – the lahars were modest in size, yet deadly to anything caught in their path.
Then the main eruption. Suddenly the summit could no longer resist the enormous pressures from below. It disappeared over 25 km high into the stratosphere in a huge, Plinian explosion, shattered into tiny fragments and was accompanied by vast quantities of magma. The gas contained in the magma formed microscopic bubbles. As these droplets cooled quickly, they burst into incredibly sharp shards of volcanic glass that turned into cement when inhaled, a cement made of volcanic ashes and bodily fluids that slowly choked its victims as far away as fifty kilometres or more downwind from the eruption.
The initial phase of the main eruption lasted only about an hour and a half before the eruption column began to collapse, sending huge flows of clouds that glowed at night, nuees ardentes or pyroclastic flows, miles from the erupting volcano. Then, only some 12 hours into the eruption, the volcano began to collapse into the emptying magma chamber as the weight of the mountain could no longer be supported. This caused further Vulcanian eruptions accompanied by even more devastating base surges of pyroclastic products. Any animal caught near the volcano was instantly cooked by the 7-800°C hot flows. Even ten kilometres away, temperatures of well over 300°C were enough to kill anything caught by them.
When it was all over after less than 48 hours, the volcano had lost almost a kilometre in height. Where the summit and the 400-m wide summit crater had been, now gaped a huge summit caldera just over three km in diameter and almost 500 m deep at its lowest point. The upper magma chamber had been all but destroyed after erupting no less than 4½ cubic kilometres of magma. The area around the volcano was utterly devastated up to between 10 and 25 kilometres by pyroclastic flows and in the years immediately following, lahars caused by annual rainfall. In the following decades, there were a few minor eruptions that began to fill in the summit caldera and form a new central cone. This was not due a new influx of magma from the mantle but the result of readjustment of pressures from surface to deep magma chamber. Also, but not immediately visible, the subsidence had caused radial fractures extending several kilometres in every direction from the former summit and also a series of tangential fractures, ring fractures, about 6 km in diameter, centred on the old summit and roughly following the outline of the now collapsed upper magma chamber.
Our volcano now went into a long period of repose that lasted almost a quarter of a million years as the last dregs of the blob of magma from the subducting plate had been expended. Certainly, there were a few very minor episodes of unrest, but to all intents and purposes, the volcano appeared to be extinct. Then 230,000 years ago, a new cycle of eruptions began. Initially, magma from the deep reservoir made its way along the cracks left by the previous cycle and erupted on the flanks of the old volcano. This resulted in a series of monogenetic cones that eroded away over the subsequent millennia. But there were also two subsidiary stratovolcanic cones built a few kilometres away from the former main vent. Our Shield volcano that had turned Stratovolcano was now also the Central volcano of a volcanic complex.
With the damage from the end of the previous cycle “repaired”, the fissures sealed by fresh magma, the central volcano began to fill in the summit caldera and build a new edifice upon the old one. As it steadily gained in elevation, the former subsidiary stratovolcanic cones were swallowed by the rapidly growing main edifice, becoming first side vents where there were a few very minor eruptions before they disappeared forever. This cycle lasted some 50,000 years and when it ended, our volcano had a prominence of almost 3½ kilometres and the radius of the edifice was close to ten. Then there was another long period of dormancy.
Only some 15,000 years ago, a new cycle began. Broadly, it followed the pattern of the previous cycle even if the final, cataclysmic eruption was many magnitudes greater. This time, there was a series of very large eruptions that emptied the by now much enlarged upper magma chamber to the extent that almost the entire edifice disappeared in the ensuing collapse when the roof no longer could bear the weight of the huge edifice. Where it had stood, there was now a large caldera, some 11 x 9½ km in diameter and well over 2½ km deep that slowly filled with water over the following millennia.
Wither now our volcano? Who can tell! Perhaps, it has had its final major eruption before slowly meandering towards extinction. Perhaps another cycle of activity will begin 50,000 or 250,000 years from now. Although some may think they recognise the volcano of our story as Mount Mazama, the precursor of Crater Lake, it is not so. Our volcano is completely fictitious although there are many real-world volcanoes with similar histories. Hope you have enjoyed our little tale!