In 1864, three people set out on the journey of a life time. Starting at Snaefellsjokull, Hans, Axel and Otto descended into the crater, found openings to the depth and started on a journey to the centre of the Earth. At least, so Jules Verne told us in his book Voyage au centre de la Terre. They descend to over 60 km depth where they find a lost world, one that has inspired many stories. Even the Jurassic Park series can be traced back to Verne’s imagination. A climb back up of 60 km seems rather daunting, worthy of a book by itself. In the end, the party is carried back up to the surface pushed up by a fountain within a volcanic chimney, lucky for them a fountain of water rather than magma.

The book is among the earliest ‘science fiction‘ rather than ‘fantasy‘, in spite of the ‘fantastic’ content. Verne tends to stay close to physics as it was known at the time. Where the story deviates from that, Professor Otto carefully explains the difference and the possible reasons. But the journey back has been overlooked in the discussions of the physics of the ‘Verne-verse‘. What can cause the fluid to rise 60 km up, with enough force to carry three people?

And what if the fluid had been magma rather than water, as it should have been inside a volcano? There are obvious reasons why Verne avoided magma. (It would have been a very different story had Jesper written it.) But magma does exactly what Verne wanted. It does travel upward by that much, and manages to do so through solid rock. How does magma get around the laws of physics? Does it live inside its own ‘magma-verse‘? It is time for the definitive discussion of the physics of rising magma.

Fear not. This is not that discussion. But it is the bit of the story Verne never wrote. It is about the journey of magma from its formation to its closure. This is a story about pressure – and about a fight against gravity.

Physics for beginners

Physics knows four fundamental forces. These are named the strong force, the weak force (physics is not known for literary eloquency), the electromagnetic force and gravity. The first two you are unlikely to meet in real life as they only show up at the smallest scales, atomic nucleus or smaller, although the universe, Verne-verse or magma-verse could not exist without. The electromagnetic force is experienced occasionally by us, such as when using a compass or being hit by lightning (or both, if you are unlucky – a compass cannot be trusted after being hit by lightning. Ask Captain Ahab). But it is omnipresent: much of what this force does is hidden from view. And finally, gravity is everywhere; it is the force we experience most often. But it is not a ‘may the .. be with you’ kind of force. In physics, gravity always wins. As it is written, ‘the greatest of these is charity gravity’.

“Science, my boy, is made up of mistakes, but they are mistakes which it is useful to make, because they lead little by little to the truth.”
― Jules Verne, Journey to the Center of the Earth

Volcanoes seem to be impervious to gravity. While gravity pulls everything down, volcanoes go up. Liquid rock, not a substance known for its lightness of being, spouts out of a crater in total disregard of gravity. Volcanoes rise and grow, in the case of Mauna Loa to a staggering 9 km above the seafloor on which it sits. Normal mountains require plate collisions in order to make crumple zones, so that as much matter goes down as up. Below the Himalayas are the anti-Himalayas, the deep roots going the other way. (I now imagine an upside-down world where magma-beings walk among these inverted mountains. Where is Verne when you need him!) Volcanoes don’t need this. They do it all by themselves. How is that possible? How does magma manage gravity?

Beyond physics

There are many other forces which physics does not recognize as ‘fundamental’ but which still affect our lives. Pressure is an example. We know about the pressure of expectations and about peer pressure. Pressure in itself has no direction: it pushes equally from all directions and therefore has no other effect than making us feel ‘under pressure’. But the pressure may be different elsewhere. For instance, there may be a lot of people trying to get to the bar and get a drink, while there are fewer people further back. Now there is a force pushing you away from the bar – hopefully with a drink in hand. (Hint: if you see a beer called ‘magma’, go for it.) This is a general principle: if there is lower pressure in some direction nearby, we feel a force pushing us in that direction. This is very notable with the pressure of expectation: we easily move to a place where expectations are lower and feel reluctance to move to a more demanding environment. When there is no such direction, pressure can become unbearable.

Pressure is opposed by another force we are all familiar with: inertia, or resistance to change. It turns people into immovable objects. It can be good or bad. Some people thrive under pressure, some just cope, and some wilt. Each are kept in their place by inertia. But against gravity, such resistance seems futile: gravity is an irresistible force. Unless you are a mountain goat, that is: these are the only mammals known to be resistant to gravity. Everyone else should pay attention. As the UK found out, if you run the economy off a cliff, don’t expect an easy ride back up.

Back to physics

It is a quirk of physics that inertia goes with mass in the same way that the force of gravity does. No other force has this property. Because of this, even though the force of gravity on an object ten times as heavy is ten times as great, the effect is identical: the two objects move exactly in the same way. Hence, the famous experiment showing a bowling ball and a feather falling at exactly the same rate. Gravity is a funny beast.

If you want to do this experiment yourself but can’t get hold of a large vacuum chamber, try to find two bowling balls of the same size but different weights and drop them from a suitable height. A risk assessment should be undertaken first, and anyone in the way should be forcefully removed. The two balls will fall at the same rate.

But some things seem impervious to gravity, and manage to hold their own. The world around us manages it: neither the solid ground nor the liquid sea fall in on themselves. Why is that? The saving grace is that gravity is amazingly weak. If you stand rather close to another person in the queue for the bar, you may feel many forces between you, some repulsive, some (perhaps) attractive, but gravity is not one of them. Atoms are kept apart by electrical forces, and those forces are far stronger than gravity. So materials can withstand gravity even from something as large as the entire Earth. But with great difficulty, and with limits, as you will find out when trying to build a mountain higher than Everest.

Our atmosphere also manages to stay up against gravity. Air is easily compressible (try to squeeze a balloon filled with air and one filled with water), which raises the question, why doesn’t all air collect at the lowest point? It doesn’t happen because of air pressure. As the air falls down under gravity, it becomes denser and therefore has higher pressure. This is something we are all familiar with. Climbers on Everest (ok, not everyone may have done that) need extra oxygen. Even at a height of a few hundred meters, the air pressure is notably different. As mentioned above, a pressure difference creates a force towards the region of lower pressure. A stable situation forms where the force from the pressure difference exactly balances that of gravity. Hence the fact that we have a usable atmosphere, but that the air pressure declines when going up.

Finally, floating balances gravity. If your density is less than that of water (true for people), than being submerged causes an upward force: you end up floating on the surface with about 90% of your body submerged. Try the same thing with a brick and it won’t work so well. Ice has a lower density than liquid water, so it collects at the surface. Cold water sinks, as the density of water gets a bit higher when it cools -so the bottom of the ocean is very cold,

Now we are ready to attack the story of the magic physics of volcanoes.

Magmarising

The crust and the upper mantle of the Earth form the lifeblood of volcanoes. They contain three distinct regions. The upper mantle consists of a mixture of silicates (45%), iron oxides and magnesium oxides, with a density of 3300 kg m^-3. This density increases with depth because of the weight above it! The crust above it has two different types. Oceanic crust is like the upper mantle but with a bit higher silicate fraction (50%) and a bit lower density (2900 kg m^-3). The lower density is in part because it has less weight to carry. Continental crust has a higher silicate fraction (60%) and a lower density of 2600 kg m^-3.

The crust and upper mantle can also be divided into the lithosphere and asthenosphere. The lithosphere is the rigid part of the crust, while the asthenosphere lies deeper and is more ductile (lower viscosity): the rocks can deform a bit. Below the asthenosphere lies a more rigid part of the upper mantle.

Deeper rock has higher density. So it won’t easily come to the surface: gravity stops it. Volcanoes, which are in the business of bringing rock up from the deep, seem to be fighting a losing battle. But there is a loophole, and volcanoes use it.

To melt or not to melt

A substance has a melting temperature: the temperature at which it ceases to be a solid. This melting temperature changes with pressure: at higher pressure, a higher temperature is needed for melting to occur. As we go deeper into the crust and mantle, both the temperature and pressure increase. A rock will melt if at some depth, the local temperature is above the melting temperature for that pressure.

The relation between depth and temperature is shown in the figure above. As an aside, note that the temperature in such plots is often given as the potential temperature: this is the temperature the rock would have without the pressure it is under. Pressure increases the density by making the rock contract (even rocks do this, a little, if the pressure is high enough). The contraction heats the rock a bit. You can try this with a bicycle pump: as you compress the air in the pump, the pump becomes warmer. In contrast, air escaping from a tyre feels cold. This is also the reason why sea-level air is warmer than that high in the mountains. If you would bring the rock to the surface without adding or removing heat, it would arrive slightly expanded and with its potential temperature. The potential temperature is used just to confuse you and I won’t mention it, or at least, will ignore this distinction.

After this diversion, back to the story. The figure has a black line and a dashed red line. The black line is the actual temperature, also called the geotherm. The dashed red line is the solidus: the temperature at which the rock begins to melt. The solidus temperature is different for each depth.

The red line becomes very close to the geothermal (the actual temperature) at a depth of some 150 km. The red line may be just above or just below it: it varies a bit in different places on Earth. In the first case you get slightly ductile rock; in the second case you get some melt. The close approach of the two lines is an accident of nature: it may be different on different planets. It is not an accident that this location is in the middle of the asthenosphere. The rocks in the asthenosphere are more ductile precisely because their temperature is close to the solidus.

Why is it called the solidus rather than the melting temperature? That is because there is a bit of funny magic physics about melting rock. Whilst water is either frozen or liquid, rock can be partially molten. If the solidus is at 1500 °C, then at 1550 °C the rock may be 20% molten, at 1600 °C 40%, at 1800 °C 60% and only at 2050 °C is it fully molten. The temperature at which it is fully liquid is called (hold it..) the liquidus.

Because the solidus and the actual temperature are so close together, you may only get a few per cent melt. If you want to do better, there are some ways to increase it.

The first way is to increase the temperature: insert a hot spot or even a plume, the local temperature goes up by perhaps 50 C, and the melt fraction increases to 20%. Bingo, job done.

The second way is to reduce the solidus temperature. This can be done by adding water to the rock. This happens above a subduction zone: the subduction brings wet rock down to the mantle, the water comes out and wets the mantle wedge above. That wedge now has a lower solidus. This occurs at a depth of around 150 km, where the temperature is already close to the solidus, so a small change can push the rock into the melt zone. Don’t expect too much: a small percentage of the rock may melt. It doesn’t work in the subducted slab itself because this slab is too cold.

How does this work in practice? Let’s first look at a typical piece of mantle, away from any rifts, hot spots, plumes or subduction. This gives the following profile for the geotherm and the solidus:

As shown by the heading in the figure, these images come from the Geological Society (“Serving science, profession and VC”). In this typical region away from plate boundaries, as indicated by the arrow on the left, the geotherm is well below the solidus, and therefore the rock is too cold to melt. There is no magma.

Now let’s look at other cases. At a mid-oceanic spreading rift, mantle material rises up. It is hot to begin with, and although it cools a little while rising, it stays hotter than rocks here would normally be. In the figure below, this situation is shown in is the second panel. The geotherm is now much steeper, and it stays above the solidus for a range of fairly shallow depths, here between 10 and 50 km. The reddish section shows where magma is generated. The melt fraction remains low, at typically 2%-4%.

The third panel shows a plume in action which brings up hot material up from the low mantle. But because the crust itself is not splitting apart, the plume hits a brick and can’t rise further. A plume head develops, much wider than the plume itself, where the warm material collects. Magma is now generated at depths of 100 to 150 km. This is the situation in Hawai’i. Iceland is a bit more complicated, as it is both a spreading rift and a (mild) hot spot. So it is intermediate between this and the previous case.

The fourth panel depicts a subduction zone. In the well-watered mantle wedge, the solidus has shifted to lower temperatures, creating a deep red-coloured region. Magma is now generated at depths of 100 to 200 km.

So now we have situations where some rock 150 km deep has partially melted. What happens next is that it finds a way to circumvent gravity. This is the Theory of Magma Quantum Gravity.

Falling up

The rock now contains a small fraction of liquid. Everything is under a very high pressure, having 150 km of rock weighing down on it. The rock with its extreme inertia just has to grin and bear it, but the liquid does not like pressure. It is both incompressible and deformable. Think squeezed sponge: the rock gets squeezed and the liquid is squeezed out of the pores. The liquid begins to form drops.

Molten rock droplets (proto-magma) have a bit lower density than the surrounding solid rock. The pressure now acts as an upward force, pushing the drops upward. This is rather difficult, as the solid mantle rock is in the way. The liquid drops are trying to move something with the inertia of a mountain 150 km tall. That is obviously not the way upward. It is worse than making your boss change their mind. The drops instead move from pore to pore, trying out how permeable the rock is. The fact that the rock is partially molten makes it a bit permeable, luckily. As one drop is squeezed out of a pore, another drop from below replaces it. Let the rise begin.

But not too fast. A typical melt velocity through the asthenosphere is 100 meter per year. To rise 100 kilometers, to the bottom of the rigid lithosphere, may take 1000 years. During that rise, it may have to content with a lower temperature and pressure, while trying to stay above the solidus. Luckily, solid rock is an excellent insulator and the rising melt brings its heat with it. The melt fraction may even increase, under the right conditions. But by and large, making magma is not so much like an air fryer and more like a slow cooker.

At some point, the ascent becomes more difficult. The rock above is now well below the temperature of the solidus, and becomes rigid and not particularly permeable. Force is needed to break through. It is time to catch breath, after the slow dash up. The melt may accumulate at this depth, forming liquid magma chambers, waiting for the right opportunity. The moho is a common location for such stalled magma. Over time, the liquid can cool a bit, causing the magma chambers to become a ‘mush’, a liquid with a lot of crystals where the liquid fraction may become as low as 10-20%. If it fails to break through, then the magma may even solidify here. In such cases, magma’s journey may end here, Otto didn’t make it back out of the Verne-verse and new crust is plastered onto the existing one.

“Is the Master out of his mind?’ she asked me.
I nodded.
‘And he’s taking you with him?’
I nodded again.
‘Where?’ she asked.
I pointed towards the centre of the earth.
‘Into the cellar?’ exclaimed the old servant.
‘No,’ I said, ‘farther down than that.”
― Jules Verne, Journey to the Center of the Earth

CO2 wind

To rise further is indeed difficult. The rock above still weighs more than a mountain and won’t just move aside: to rise requires breaking rock. Help is needed. Often, this help involves volatiles.

The volatiles in the magma include CO₂, water and SO₂. The most important of these is CO₂. The volatiles account for a few percent by weight of the magma, and initially remain safely dissolved in the magma where they are not much use to anyone. But as the magma rises and pressure decreases, less volatiles are allowed in the liquid and the magma may become saturated. The magma behaves like a fizzy (carbonated) beer and CO₂ especially can now form gas bubbles. When dissolved, CO₂ is part of the liquid and has the density of the liquid – liquid rock. The gas bubbles have a lot lower density: adding the bubbles makes the magma much more buoyant. An added bonus of the CO₂ dissolution is that this process generates heat and so the magma reheats a bit. Water does not do this.

The magma is now driven up against the will of gravity by the bubbles. It is blown by an underground wind of CO₂.

This process of wind-driven magma ascent can happen even at great depth if enough CO₂ is available. The main example of this is kimberlite, a kind of eruption we have never seen ourselves – but we can hope.

The formation of the mush can cause CO₂ to reach saturation. As more of the melt crystallizes, the CO₂ remains in the liquid and reaches higher and higher levels. The liquid becomes saturated, the gas comes out of the solution and forms bubbles against the confining cap rock. Boy, is that rock in trouble. The gas expands as it comes out of solution but there is no room for expansion. Pressure goes through the roof – literally in this case as the cap rock gives way. Initially, the gas also needs to break through the crystal mush but that is peanuts in comparison to the rock. The gas bubbles break a path through the rock and allow the magma to restart the ascend. The crystal mush may be carried with or may remain behind.

The wind often makes use of existing planes of weakness. Luckily, these are not uncommon. They may be inclined faults or there may be horizontal layers of different compositions. In the latter case, the magma forces the layers apart and forms a sill between them. Sills seem to be the most common type of magma chambers. Underneath volcanic regions, there may be a whole series of such sills at different depths waiting for their chance. The high pressure of the forming bubbles fractures the rock and causes microseismicity, and for the first time the rising magma is detected by our instruments. This is happening at the moment at Snaefellsnes, in a twist to Verne’s story.

The process can repeat multiple times, leading to the formation of magma sills at a number of depths. A magma batch may bypass intermediate sills and go straight to a less deep one. That happened, for instance, at Bardarbunga a few years before the 2014 eruption when magma rose from 22 km to to 7 km, bypassing the lower magma chambers between 8 and 12 km.

So, passing a bit of wind clears the way. (It may work in the crush for beer at the bar as well – I haven’t tried.) Some of the microseismicity we detect in various volcanic regions is from this process. Often it does not lead to an eruption. But sometimes it does, with a final push to the surface.

Anti-gravity: the final ascend

At the end of this process, the magma sits a few kilometer deep. So far we have had a successful intrusion with magma rising from 150 km to near the surface. But unless the last few kilometers are also succeeded, it counts as a failure. What to do? The CO₂ wind has had it last breath and gravity refuses to go away.

As gas continues to come out of the magma (CO₂, H₂O, SO₂), the magma develops froth and the frothy magma rises, creating a conduit if necessary. During the rise, more gas bubbles form and the density of the magma decreases fast. This accelerates the magma.

The conduit that forms (Otto’s chimney from the deep) has constant size with depth. The flow rate of the magma, in kg/sec, must be the same at all depths, otherwise more magma would enter than leave. or the opposite. As more bubbles form, the density keeps going down. The volume increases but the size of the conduit stays the same. The only way to keep the flow rate constant is by increasing the speed. At constant width of the chimney, the flow rate is equal to the density times the velocity. If the density goes down, the velocity must go up. And so it goes.

The gas bubbles become larger and congegrate into slugs. This happens within hundreds or even tens of meters of the surface. These drive the magma at ever higher speed. Because of this acceleration, when the magma reaches the surface, it comes out in a tall fountain, typically 10 to 100 meter high. The lava may come out of the conduit with speeds of 10 to 100 meters per second. It is a long way from the 100 meter per year at the start of the journey.

The tallest recorded fountains are over 2 km high! The height is determined by the gas content, the size of the opening and the spread angle. The height reduces if the magma becomes degassed. It also decreases if the conduit widens close to the surface. This happen, for instance, if the magma overflows the conduit. The magma fills the crater and forms a lava pond. In effect, this pond acts as a much wider part of the conduit. The flow speed slows down dramatically in the pond, and the fountains become smothered.

The process does more than produce fountains. The forces opposing gravity are so strong that magma is pushed far beyond sea level. It builds mountains, typically 2-3 km tall but in some cases (Hawai’i) much higher.

It matters whether the eruption is on oceanic or continental crust. Basaltic magma has a density that is a little lower than that of oceanic crust. It can erupt easier on the latter, especially for effusive eruptions. This is the case in Iceland, which in spite of appearances consists of oceanic crust. Gas content helps, and the eruption may be much faster when it has a high gas content. As gas content reduces the eruption becomes calmer and eruption rates become lower. This is the normal process in Iceland: a slow start if old magma needs to be pushed out, a fast phase of fresh, gassy magma and a tapering off as the gas reduces, until a sudden end when the density of the magma is insufficient to overcome gravity and inertia.

On continental crust, density is a much bigger problem. Basalt has a higher density so needs help. This can be a very high temperature (mantle plume), a high gas content or a thorough wetting. The latter reduces the solidus to a much lower temperature and allows basalt to remain liquid in the cool crust. If the magma consists of molten crust, then its density is also less and the problem goes away: silicic volcanoes have it much easier than basaltic ones.

Down ..

Once the magma has completed in journey and the three intrepid travelers are back on Earth, gravity again reigns supreme. When magma turns to lava, it can only flow downhill. But not necessarily all the way to the lowest point. Thermodynamics kicks in to help.

Lava exposed to the air loses the insulation of the solid rock, and is free to radiate its heat away. And so it does, and it quickly cools to levels where the surface of the lava solidifies. This slows down the flow dramatically. Eventually, the flow stalls and the lava turns back to solid rock.

The distance traveled can range anywhere from hundreds of meters for viscous, cooling lava to tens of kilometers for the hottest, lowest viscosity stuff. On Venus, they can run for thousands of kilometers, aided by the heat of the atmosphere which slows down the cooling dramatically. If the lava runs over long distances, then it builds mountains of wide shields. If it stops quickly, it builds steep cones. The more viscous the lava is, the steeper the cones it can built.

The most viscous stuff of all are the cinders, lava that solidifies in the air. Cinder cones can especially steep, and, speaking from experience, the loose stones are a nightmare to climb. Ashy falls are not as steep but still not easy to walk on: even the path up Vesuvius can be too hard for some. Frozen lava flows are less steep.

But in all cases, a mountain gets built. The beautiful cones of Mount Fuji or of Mount St Helens (it was much prettier before it blew up) are signs of defiance against gravity, of the power of inertia.

.. and out

But mountains don’t last. Gravity has a way of sneaking up on you. Gravity has powerful allies. One is weather, the power of falling rain and of blowing wind which can take down any mountain over time. It can go fast in the tropics and slow in deserts, but the outcome is the same. Coastal erosion can be particularly fast. Many of our landscapes were formed by the glaciers of the ice age: the world looked very different a million years ago, before the ice came. Erosion creates, but the creation comes out of destruction. Erosion increases entropy.

The other ally of gravity is the asthenosphere, that ductile layer underlying all volcanoes. As the volcano grows heavier, the asthenosphere slowly -very slowly- adjusts by flowing sideways. The mountain now begins to sink. Even Mauna Loa, giant of giants, is sinking by 4 cm per year, and is desperately trying to hold its own by expelling lava at the same rate. Eventually, gravity will win. The skeletal remains of older Mauna Loas litter the ocean floor between Hawai’i and Siberia, sunk to kilometers below sea level. Here they find their own ally: the ocean itself helps carry the weight and stops the rain from reaching the remnants. But even that doesn’t last: the ocean floor carries the mountains to a final fate in a subduction zone. Gravity will, in the end, return the magma to the mantle.

There is nothing more powerful than this attraction towards an abyss.”
― Jules Verne, Journey to the Center of the Earth

There is an exception to this. Continental crust does not subduct, and lava coming to the surface here can survive much longer. Still, erosion comes to us all and even in the driest regions, eventually the thickest lava shield will be gone. It can take a long time: it may go down by 1 cm per 1000 years, or 1 km per 100 million years. Siberia still has large fragments of the flood basalt that covered it 250 million years ago. But if you are looking for one of a billion years ago or older, you may have left it too late.

Explosions

But the surface is not the summit of the magma’s journey. It now may take to the air.

The eruptions described above are effusive: magma rises to the surface through an open conduit. But not all eruptions are like that. Sometimes, the battle against gravity is a high pressure event fought with explosives. These are the eruptions most likely to go above the cal of duty and affect the atmosphere.

We have mentioned how the liberated gas moves within the magma. But sometimes, the inertia of the magma becomes an impossible obstacle. This happens when the magma is cool and viscous. Gas comes out of the solution as the magma chamber slowly crystallizes, but the bubbles have nowhere to go. The pressure goes up and up but the gas find no escape from the magma. This typically happens in shallow magma chambers, and because the magma is viscous, it is most likely to happen under steep volcanoes.

Thus situation can lead to sudden, instant failure. It may start with an earthquake which creates a weakness in the mountain. This reduces the pressure a bit, but the lower pressure just causes more gas to come out of the solution, instead increasing the pressure further. The process can rapidly cascade into catastrophe. This is the main cause of the largest volcanic explosions. They are more common in evolved continental volcanoes where the magma is viscous, but can also occur in subduction volcanoes where the magma is saturated with water, or, unwisely, gains access to water, as happened in Hunga Tonga and Krakatoa.

Such explosions can throw out lava bombs to impressive distances. Reportedly, Hekla in 1947 ejected them as far as 30 km. This required an ejection speed of 500 m/s, reaching as high as 6 km – aircraft in danger! But mainly, these eruptions just blow mountains to smithereens.

So the battle against gravity ends with an explosive fight. But it is not wise for a volcano to blow up the mountain it itself painstakingly built. In the end, they themselves become gravity’s ally. The volcanic world is littered with mountains that are no longer there. Those volcanoes would make strong candidates for a Darwin award.

Up in the air

But this brings us to the final battle with gravity, where the magma uses the atmosphere as an ally. Lava fountains can reach 2 kilometers and lava bombs reach 6 kilometers height. But volcanic ejecta can do much better. Eruption plumes can reach the stratosphere and in the unique case of Hunga Tonga, the mesosphere. How do they do it?

We have seen that air pressure keeps the atmosphere up. Volcanoes have found a way to make it work for them. It uses not the explosive power of the eruption, but its heat.

The eruption heats the air, both by the explosion itself and from the hit interior it exposes. Hot air is overpressured, and therefore rises. As it rises, the hot air expands and cools, and this eventually brings it back into equilibrium. Up to that point, the hot air will continue to be hotter than the surrounding air and therefore continue to rise.

The rising air can reach high in the troposphere, and for larger eruption, even enter the stratosphere. The stratosphere has an inverted temperature profile: here the air temperature goes up again with altitude. This eventually suppresses the rising convective column, at height between 15 and 30 km. The rising column carries with it some tephra and lots of sulphur. These form sulphates, aerosols which turn the skies white and opaque, and once in the stratosphere, they stay there and are circulated around the world. We have seen this a little after Pinatubo but have little remembrance of the much larger events of 1815 or 1257. Those were dark times, and they could come again.

The sulphates don’t last forever in the air. Over time they slowly drift down. It can take several years for it to disappear, and it ends with the sulphates being distributed around the world. But for a while, the sulphates form the pinnacle of the voyage of the magma, from 150 km down to 15 km up. It is a battle against gravity, using pressure to its advantage, in a journey from solid rock to air.

Winners

The ultimate winners against gravity are the continents. They lie some 6 kilometers above most of the ocean floor, because of their low density compared to the oceanic crust. That low density comes from an overabundance of magmatic silicates. We float on granite.

The original cause of this lies in magma. It starts in a cooling magma chamber at the bottom of the crust, at the moho. Over time it cools and the minerals with the higher melting temperatures become solid. Eventually, all but the silicates solidify. The remaining liquid is now less dense and more buoyant. But is it also sluggish, viscous. It pushes against the basement rock, forming large intrusions of granite called plutons. They stay deep in the crust. The process can repeat underneath mountain ranges where some of the continental can melt.

Continents once formed from series of such intrusions. Basalt does find its way up as well, either from greater heat or from greater water content: wet basalt also can have a rather low melting temperature. But it is the silicate intrusions that create the high continents. Without them, the entire world would be under water.

In many places these granite intrusions have come to the surface, after tens of kilometers of overlying crust has eroded away: Paarl mountain in Cape Town, the moors of Cornwall – do add your favourite places!

Erosion leaves the continents unharmed. Remove the top of a continent, and the rest just floats a bit further up to compensate. It is just like taking the top of an iceberg: you’ll find the iceberg is still there while the Titanic is gone. Continents are indestructable, unable to subduct or sink back into the mantle. Not for them the failure of King Canute to stop the tides. They are the winners in the struggle against gravity.

The air we breathe

But there is another winner, one we rarely recognise. It is in the very air we breathe. The volatiles that come out of the magma can reach the air long before the magma does. They can go on their own journey. The CO₂ that comes out of the liquid rock joins the atmosphere. 40 per cent of the CO₂ in the atmosphere comes from us, but the remainder comes from volcanoes. Water has a harder time traveling separately and comes up more with the eruption itself, but if you wondered why after 4 billion years of letting wet rock subduct we still have an ocean left, this the answer: it is brought back to earth by volcanoes. All of our atmosphere, with the exception of O₂ comes from degassing. Degassing of rock requires heat, and this is provided by the rising magma. Your air has changed over time. The Earth had a very different atmosphere shortly after its formation, which we lost and which was replaced by something closer to what we have now. Only argon, which strangely accounts for 1 per cent of our air, survives from those hadean early days.

This is the journey of magma. It is a slow dance, a sarabande with gravity, driven by pressure and fighting inertia, ending by returning to the Earth. But it shapes our world, creates our air and builds our continents. Physics it may be, but the journey is a human one. Verne would be pleased.

Albert, Good Friday, April 2025

“Wherever he saw a hole he always wanted
to know the depth of it. To him this was important.”
― Jules Verne, Journey to the Center of the Earth

(Verne quotes from www.goodread.com)

Euer Grab und Leichenstein soll dem ängstlichen Gewissen ein bequemes Ruhekissen und der Seelen Ruhstatt sein.Höchst vergnügt, schlummern da die Augen ein. Wir setzen uns mit Tränen nieder und rufen dir im Grabe zu: Ruhe sanfte, sanfte ruh, ruhe sanfte, sanfte ruh!