It's rather odd that, in volcanic eruptions, most people tend to forget about the ash. They remember the lava and the pyroclastic flows. They may remember domes and dikes and columns of debris above the crater, but the ash and what happens to it just seems to slip our minds. Perhaps it's not as exciting as a lahar, or as interesting as the gasses that are released. Maybe we see it travel up into the atmosphere and simply forget that it has to come back down. Whatever the reason, ash is one of the most important aspects of volcanology. It's effects are not blaringly obvious right off of the bat, but it is crucial to understand the behavior of ash in relation with humans and the environment. Here we are going to discuss the characteristics of ash fall deposits, how they are formed, and what health risks, if any, they pose to local people and agriculture.
First of all, we should start with their formation. Ash is actually very tiny (typically ~5-10 microns, though sometimes meters) particles of rock. The ash can either be composed of the original magma or the volcano's wall rocks that got blasted out with it; an eruption column usually contains both. To truly understand the fragmentation process of the ash, you have to go a bit into the internal processes of a volcano. While these are discussed in detail elsewhere, we will lightly cover them here so that we can explore how the ash forms in the first place. As the magma rises, gasses trapped inside begin to expand and coalesce to form bubbles. The bubbles, being less dense than the surrounding magma, rise to the top and so form a frothy head on the rising magma column. Now, when the magma is very fluid, you can get hawaiian style eruptions with lava fountains as the gasses explode outward. Because the magma is so fluid, however, you get almost no ash associated with this. As the silica content of the magma rises, though, so does the viscosity. In intermediate to silicic magmas (andesite-dacite) the bubble walls are very rigid and do not simply pop upon reaching the surface, but rather shatter and explode. As the bubble walls shatter, the pieces are carried upward by the force of the exploding gas. The bubble shards are tiny and are carried easily upward. The actual explosion of the bubble is important as well, however, as the shockwave blasts out material in its path that may have already been part of the volcano. The material is disintegrated and carried up with the eruption column.
Well, that's the end of the story, right? Well, not exactly; we've only covered the first 10 seconds or so of a story that can last months, years, and even millennia. Next we'll talk about transport.
There are three to four stages of an ash particle's travels. The first stage is the trip up. We know that the ash is carried upward by the force of the exploding bubbles, but ash columns are tens of kilometers high... no amount of naturally occurring bubbles could send an ash cloud that high; there has to be another mechanism. What do you know, there is, and it's called thermal atmospheric convection. As the column rises, it entrains air from around it and incorporates it with the volcanic ash and gasses making it more dilute. In addition, the column rapidly heats the surrounding air and as we should all know, warm air is less dense than cold and so rises. As the heated air rises, it leaves a very brief and weak vacuum that draws in air from further away. As the heated air rises, it begins to cool and sink back down, but it drops further away from column and is later sucked back in toward the rising air again to make another circulation. This altered USGS photo may help you to visualize the process. Hence you have a thermally induced convecting current of atmospheric air and if you put these all around the column you'll notice an effect: all the air that the column touches is rising. The rising air pushes the column up even higher until it reaches a maximum height that may be kilometers above the volcano. The combination of explosive force, heated dilute/low density material, and convection cells come together to send plumes of ash high into the atmosphere and complete the first phase of our ash particles' transport.
The second stage is time spent traveling through the atmosphere. The third stage is falling, and the two are inexorably intertwined and so they'll be discussed here together. The key here is that the moment the particles stop rising, they begin to fall. Gravity doesn't take breaks and so it immediately begins pulling down the tiny particles, but they don't all fall the same way. The particles all have important differences in density, surface area, mass, and volume, which affect the way they behave in the atmosphere. Right away after an eruption, larger material drops out and lands on the sides of the volcano. As you move outward from the base of the volcano, you'll find that the ash is getting progressively finer and is taking longer to reach the ground. That is because smaller particles drop out of the atmosphere slower than heavy ones because the relative effect of air-resistance on them is greater (the smaller stuff gets carried further by the wind). This gradation outward from the volcano continues until there is so little ash falling that you can not even measure it anymore; in fact, in large eruptions ash has been known to circumnavigate the globe several times before finally dropping out or simply disappearing in our atmosphere. There is more than one factor in how the ash drops out, however (come on, nothing in science is really that simple!). We still have particle surface area, electro-static/van der waals forces, and water cohesion to briefly discuss.
There are a number of formulas available for calculating resistance of a media on a falling object and so if you know the density of the medium (air) and the size and density of the falling object, you can get a rate at which the object will drop out. It's fairly easy to do, but when you apply it to ash you get numbers that are extremely wrong; they are all too low. The problem lies with assumptions in the calculations; it is assumed that the falling object is a sphere. Our ash, however, is predominantly composed of bubble fragments which are irregularly shaped and far from spherical. A sphere has the minimum amount of surface area for any shape, and the more surface area you have- the more friction you've got between the object and the air. I hope you see what I'm getting at here. The bubble shards are roughly curvi-planar; that is, they are mainly flat, but not completely so. Take two pieces of scrap paper and crumple one of them up into a ball. Stand up and drop the two pieces of paper to the floor. The crumpled up ball hits far earlier than the flat paper, which floats back-and-forth as it descends toward the ground. Now you understand the problem- those pieces of paper both had the same weight and density, but the amount of surface area created a massive effect on it's settling speed. As of this time, there are no perfect formulas for calculating settling speed, although models for it do exist.
The other two factors are a little less complex, but still just as important. Talking about the electro-static forces seems awfully technical, but it's really not as bad as it sounds. When you have the ash cloud traveling through the atmosphere, particles rub against each other and build up an electric charge (this is much like what happens in a thunderstorm). As the charges build up, you have your opposites attracting. The oppositely charged particles come together and literally stick to each other. Now, for all practical purposes, you have a single particle that is about twice as heavy and that has about the same amount of surface area as each one of the individual particles. This, of course, means it is going to drop out sooner. The water factor comes into play when you have a great deal of moisture in the atmosphere (which is common). The water causes the particles to stick to each other, similar to the electro-static effect. Thus you have ash falling out earlier in this manner. In some cases there can be storms in the area which actually entrain the ash into the water droplets and you literally have mud falling from the sky (accretionary lapilli).
All of these factors mentioned above are known to influence atmospheric transport of ash. As soon as the column is erupted, it begins to be pushed by the wind and the ash deposits reflect this by being much thicker and extensive in down-wind regions. We are still far from fully understanding how everything works, however. In deposits, locally heavy variations in the amount of deposited ash exist that are yet to be fully explained by science. The study of ash clouds remains a very integral part of the field of volcanology.
Remember way back toward the beginning of the article where I mentioned there were four stages during the transport of ash? Well, the fourth is it's reworking after it reaches the earth. Erosion, both via wind and water, causes the ash to be moved into different places after the initial deposition. This can cause major problems with local communities and also cause problems in estimating the size of past eruptions. On a field excursion with Dr. Bill Rose to ash-fall beds of Nebraska, it was discovered that the ash at Ash Fall Fossil Beds was not deposited directly from the atmosphere, as was previously thought, but actually partially deposited by local rivers! This is vastly important in understanding how the corresponding extinction took place (you may be able to find more in depth info on the ash-induced extinction elsewhere).
Next we should talk about how the deposits look once they have been set down semi-permanently. I say semi-permanently because ash deposits tend to move around a lot. When the ash settles out from the atmosphere, it lands with the particles' long axis pointing in all sorts of directions meaning that there is a great deal of empty space between the pieces. The ash will compact as more falls, but the deposits maintain a very low density and so can be easily eroded by passing water or wind. This makes good undisturbed ash deposits a bit of a rarity which is a pain because they are a good way of understanding how violent or long a past eruption was if there was no-one there to see it. The ash beds have a number of telltale characteristics, however. First of all, and the most characteristic, is the fact that they mantle the topography. This is very important to remember; if the bed does not perfectly mantle the area's topography, it is not a direct ash-fall deposit. What do I mean by that? Well, if you have a river depositing sediments, you find more material left in depressions than you do in high places. If you have a pyroclastic surge or blast, they follow the valleys and deposit more material at the base than at the top. In ash-fall deposits, however, you simply have small particles falling directly out of the air onto whatever surface is beneath them. That means that the thickness of the deposit will be exactly the same at the top of the hill as it is at the bottom of the hill. Now, the deposit will thin out as you get further away from the volcano, but this is generally very gradual and not significant in large deposits studied in small regions. The really interesting thing about the deposits is that you can watch how the eruption changed as you move up through it. You start at the bottom of the deposit and you may have some lithic fragments that were blasted off the volcano when the eruption began. As you move up, you will find variations in the grain size of the ash. Larger grain sizes mean that the eruption has become more violent and is blasting out larger pieces even farther than before, while finer material indicates that the eruption is quieting down.
The last aspect of ash this article intends to deal with is the effect of ash on human health. You may have seen pictures of people around erupting volcanoes wearing those white surgical masks or bandanas over their mouths; there is good reason for this: ash can be extremely dangerous to inhale. In fact, ash can be deadly in a large number of ways. One of the more common is roof collapse; when ash is deposited on a community, it lands on roofs as much as streets and is generally very light. If your house has a flat roof, however, that ash can build up; and if it is raining, the ash becomes water-logged and extremely heavy. Roof collapse is a larger problem in under-developed or rural communities where structures may not be very stable to begin with. The weight of the ash becomes too much for the roof to support and so crashes through to the interior of the house. People inside may be crushed to death by the debris or buried alive by the ash (which, if wet, may have the consistency of wet cement). Another, more direct threat, is remobilization of the ash; that is, erosion by water into mudflows called lahars (which are dealt with in a later section). Another problem that most people don't really think about, though volcanologists do, is air traffic. When aircraft pass through an ash cloud, the tiny particles can be sucked into the jet engine. Inside the engine, the temperature is hot enough to remelt the ash which then sticks to the turbines and causes the engine to break down. This can be deadly dangerous, though fortunately no aircraft have actually crashed due to this (there have been several emergency landings). In addition to this, the ash may "sand blast" the windows, blurring the view or even making them impossible to see out of (which is dangerous for the pilots). It may erode at the front edges of the wings and block the engine cooling vents while shutting down nearby airports that can't clear the runways fast enough. To deal with this problem, many volcanologists work at or with VAACs: Volcanic Ash Advisory Centers. There are nine VAACs, located in: Tokyo, Japan; Darwin, Australia; Wellington, New Zealand; Anchorage, Alaska; Washington D.C.; Buenos Aires, Argentina; Montreal, Canada; London, England; Toulousse, France. These are responsible for monitoring eruptions and ash clouds over their respective regions, which may span continents. Thus far they have been extremely successful.
The last manner in which people's lives are affected by ash is inhalation. When people inhale the fine ash particles, the tiny glass fragments tear at the lungs, which may cause scarring. This scarring is known as silicosis and can be a deadly condition. The scar tissue does not readily expand with the rest of the lung, limiting the person's ability to breathe. This can be a major problem for livestock and grazing animals which keep their heads low to the ground. The loose ash particles coating their food are easily inhaled through the nostrils and can lead to the deaths of entire herds and devastate local farmers. For humans, small ash falls can aggravate asthma, but more serious conditions such as silicosis require long term or repeated exposure to large quantities of ambient ash with a high cristoballite content. Studies are in progress to determine just how much a health risk inhalation of ash truly is.
So let's recap what we've covered here. We know how ash forms and how it travels, how it is deposited and what those deposits look like, and we know the effect it has on people living within the affected area. Despite the length of this article, it was by no means an intensive look at the characteristics of ash; rather, it was more a short overview or summary. For more in depth information, there are resources you can find in libraries or in journals. This may be of particular interest to those going into volcanology or living near active volcanoes. It may also be relevant to those living on volcanic ash beds, which may not be near an active volcano at all. Now, at least, you know the basics of how ash behaves.