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Hydrothermal Activity

Introduction

Hydrothermal activity; what is it, and how does it relate to volcanology? Well, to begin, we should dissect the word to find its definition: hydro- water, thermal- heat. Hydrothermal activity simply refers to the movement of heated water, usually beneath the surface of the earth. The most common source for hydrothermal activity is a volcanic magma chamber or pluton, which heats the surrounding water, causing it to migrate. Monitoring this activity may tell us a number of important things, such as the depth/temperature of the magma, size of the chamber, and changes in groundwater behavior arising as a result. This subsurface activity can also create perfect surface conditions for lahars and debris avalanches, making it important to understand these processes to the best of our ability.

Events Beneath the Surface

So the process is fairly simple, right? The magma heats the water and the water moves around. Well, not really…there’s a bit more to it than that.

The magma, deep within its chamber, is heating the surrounding wall rock (contact zones are gradational with crystalline mush layers). The water in the surrounding rock is heated as well, which lowers its density and causes it to flow toward the surface (through pores and cracks). Cooler water, from further away, is drawn into these empty pores and cracks to be heated and moved up as well. These are not singular events, but an ongoing process that results in convection currents near the magma chamber. In reality, you can picture the process as a three-dimensional “tube” that surrounds the chamber, where water is rising up and away from the contact zones; but then, this isn’t exactly right either. In some cases, this may be how things work, but in a good convection current, that heated water will cool and come back down to go through the process again.

The water in a magma-driven hydrothermal system comes from one of two places: the surface or the magma. Surface waters are called meteoric (meaning from the atmosphere, much like meteorology or meteorites). At the surface, these waters are frequently most common, though this is not always the case. Deep below the earth, near the magma chamber however, the waters may be predominantly magmatic. The most common gas in magma is water vapor and it is the most quickly exsolved. The water, as it leaves the magma chamber, may carry out other minerals (but in fairly small quantities as it is leaving as vapor). The heated water, once liquefied, continues to pick up more minerals from the surrounding rock as it rises toward the surface. As it ascends, meteoric water joins the action in large quantities, is heated, and picks up more minerals from the surrounding rocks. Magmatic waters may exsolve carried carbon dioxide, sulfur compounds, hydrochloric acid, hydrofluoric acid, mercury, and arsenic. As the water circulates through the surrounding rock, it may pick up chlorine, fluorine, bromine, boron, sulfate, carbolic acid, and silica. As the waters continue to rise, hydrogen sulfide reacts to form a mixture of sulfuric acid and hydrogen sulfide, which anyone who’s been near a hydrothermal system should be very familiar with. For those of you who have been near geysers, hydrogen sulfide is the source of that horrible rotten eggs smell (you needn’t worry, however; it takes much higher concentrations of hydrogen sulfide than this to be dangerous). When this water reaches the surface, you may have a number of things happening. If you have a shallow active heat source, you may have fumaroles. A fumarole is a vent that expels gas; mostly water, but also all of the other associated gasses. If the heat source is deeper or cooler, the water may be mixed with vapor or simply water, such as you see with geysers and hotsprings. The phrase “simply water” is misleading, however, as this water may be highly acidic and thus hazardous to the surrounding environment.

So is this all that happens? Certainly not. The study of hydrothermal systems can be extremely complicated and our understanding of them is fairly poor. If you’d like to learn more about the subsurface activity of these systems, I would suggest taking advanced courses on the subject or finding more detailed texts such as technical books or scientific reports. For instance, we know that the subsurface shows three-dimensional zoning in the temperature, phase, movement, and composition of water. It is thought that there are frequently sections that are analogous (not literally) to acid pools, contained by cap rocks within the earth. Consider that in steam-dominated areas, the water may be liquid for most of the cycle until it depressurizes at the surface and returns to vapor! This is no simple subject by any means.

At the Surface

So what’s going on at the surface in these hydrothermal systems? Well, that depends a bit, but we’ll work through it. If it’s a vapor-dominated system, you’ll have fumaroles. As mentioned earlier, fumaroles are vents that expel the water vapor and magmatic gasses. Fumaroles frequently occur along cracks that extend a fair distance into the earth. The reason for this is fairly simple, though. In many areas, the gas may be seeping through the soil rather diffusely, but in order for a fumarole to form, you must have a significant amount of gas coming up in order to clear a vent from which it can escape. Thus, a deep crack is perfect for the formation of fumaroles in these areas. Most of the gas and vapor, however, is released diffusely through the soil, which creates another problem. Recall that the water can be extremely acidic (frequently, the hotter it is the more acidic it is) and so it interacts with the soil, altering it to an extremely weak yellow clay. On the sides of a volcano this can be a huge problem and lead to the formation of lahars and landslides, not to mention a tough climb!

At cooler systems, there is more diverse activity. At the hotter end of the scale, you can have the formation of acid crater lakes or acid springs! In some places one can even find acid streams. As the system continues to cool, you have more neutral hot springs, geysers, and mudpots forming. Geysers can be one of the most impressive sights at a hydrothermal field as these produce jets of water and vapor that may rise over one hundred feet into the air. The manner in which geysers erupt is a rather interesting subject as well. (The diagram w/ explanation may help greatly) The eruption exits through a vertical or near-vertical pipe that may have a diameter of inches or feet. The pipe extends a variable distance, usually hundreds of feet, into the earth. At the base is a chamber that acts to store extremely hot water. The pipe intersects the side of the chamber, meaning the ceiling is much higher than the exit point. Boiling water builds in the chamber and begins to pressurize (the drawing will help as I explain). The water continues to pressurize as the vapor cap increases in size, moving downward toward the exit. Finally, the vapor cap reaches the exit and begins to escape. This escape of gas triggers the rapid depressurization of the chamber which, in turn, causes the water to flash boil and literally explode. Thus, the boiling water (both liquid and vapor) rushes toward the surface, creating an impressive display. After the eruption has fairly emptied the chamber, water begins to reenter and the process is repeated. That is how many geysers can be predicted; by knowing the rate at which water reenters the chamber, we can know how long it will take to pressurize and erupt. Changes in the regional hydrology (subsurface water flow) can cause this to change. An earthquake in Yellowstone National Park several years ago caused several geysers to become extinct and several new ones to form. It also changed the timing of many famous geysers, including Old Faithful, which no longer erupts as often (though still consistent).

As the warm hydrothermal waters reach the surface, they rapidly cool and deposit their mineral loads. To non-geologists, this description may be meaningless, but to those of us in the field, such information may be very useful. Sulfur minerals are most common, but minerals that may occur include: alunite, natrolunite, jarosite, gypsum, kaolinite, dickite, diaspore, pyrophyllite, silica residue, cristobalite, hematite, barite, epidote, wairakite, prehnite, travertine, halloysite, pyrite, aragonite, and trona. Kaolin and smectite clays are common, depending on the acidity the fluid reaching the surface. In some areas, ore grade deposits of metal are deposited in mineable quantities. These metals include gold, silver, arsenic, antimony, and thallium (though ore quality deposits of this kind are pretty rare).

Deep Sea Activity

For most of the Earth’s history, we’ve had no idea that anything was going on volcanically at the oceans’ bottom. As the theory of plate tectonics developed through the 1960’s, it became apparent that something was going on down there. The first hydrothermal vents were not discovered, however, until 1977. The systems are identifiable by the presence of black smokers and chimneys that rise high above the sea floor. Black smokers are vents that expel mineral laden fluids up to and occasionally over 400 degrees C, so named for the dark color of fluids expelled. Chimneys are tall vents which have been built through the deposition of minerals from the cooling water escaping from it. The size of a vent depends on the amount of water expelled, the mineral content of the water, and the amount of time the vent has had to grow. When we see pictures of these vents, it is easy to think that the hydrothermal field is roughly limited to this area, but this is far from the case. In fact, a very large area is involved in the hydrothermal system producing these features.

Deep sea hydrothermal systems are composed of 3 basic zones: the recharge zone, reaction zone, and upwelling zone. The recharge zone, the surface/near-surface area in which water enters the ocean floor, can extend for miles. This zone is nearly the sole source of water for the hydrothermal system. At the vents, there is invariably some magmatic water exiting, but it is an extremely small amount in proportion to the sea water traveling through. The reaction zone is a relatively small (compared to other zones) region within the earth, near the magma chamber, in which the water is heated to the point at which it begins to rise toward the surface. This then turns into the upwelling zone, where water is carried upward by its lower density and the push of water below it. The upwelling zone ends at the surface where the water is expelled from vents of various shapes and sizes. Unlike subaerial hydrothermal fields, water cannot rise diffusely through the surface. Moreover, the fluids will not rise in vaporous form as occurs above sea level. Each of these peculiarities has a specific reason, however.

Water does not rise diffusely through the surface; neither does it enter diffusely through the recharge zone. The reason for this is the high density of the basalt under which the system is located. Basalt (with the exception of flow tops) has almost no pore spaces to store water, and those few that exist are very poorly connected. Basalt, however, is extremely brittle meaning that there are numerous cracks through which water can travel very quickly. As the heated water from the reaction zone rises upward, water from elsewhere is drawn into the empty cracks and so water is constantly being drawn in to the cracks of the recharge zone. Thus a cycle is created, fueled purely by the heat of the magma chamber and transported along the fissure system of the basalt. Neither do we find water vapor at deep sea hydrothermal fields, due to the pressure at these depths. Here water remains a liquid at temperatures of up to 400 degrees C, which can be reached at these vents.

As water moves through the rocks, a number of chemical reactions occur. As water first enters the rocks in the recharge zone, reactions occur between the mineral load of the fluid and the host basalt. Initially, oxygen, nitrate, and sulfate are lost, followed closely by the precipitation of magnesium as various minerals. This transformation of magnesium generates an acid, which causes the basalt to release calcium, iron, and other positively charged elements into the fluid. At cooler temperatures, boron, potassium, lithium, rubidium, and cesium are lost to the basalt; as temperatures rise above 50 degrees C, however, the reaction reverses completely. This process continues as the zone transitions to the reaction zone; at 150 degrees C, however, a dramatic barrier is passed. At this temperature, anhydrite (CaSO4), is precipitated from the fluid, affecting chemical reactions dramatically. At 400 degrees C, near the magma chamber, trace elements naturally found in the water (in tiny proportions) become readily picked up from the surrounding rock, increasing in quantity by three to seven orders of magnitude (decimal places). Thus the water becomes extremely rich in rare or unusual metals while hydrogen sulfide is picked up in large quantities, more than making up for the amount of sulfur lost in the recharge zone. The chemistry of the fluid is very site specific, depending on temperature, pressure, ph, and redox potential. To get more specific requires delving into more complicated subjects in mineralogy. As the water rises to the upwelling zone, it begins to cool again, resulting in the precipitation of metal sulfides (such as pyrite). If the solution becomes cool enough, precipitation of these minerals will cause the fluid to become more acidic and eat away further at the surrounding basalt. This fluid eventually reaches the surface, and can be extremely variable between systems. They are usually loaded mostly with chlorine, lithium, sodium, potassium, calcium, quartz, manganese, iron, copper, zinc, lead, hydrogen sulfide, carbon dioxide, methane, and nitrate. If you were to look at every element carried by the hydrothermal fluids, however, it’d look more like a periodic table! At subaqueous hydrothermal fields, large sulfur deposits form which can later be mined, but that is a subject for another time.

Conclusions

Hydrothermal activity is a fairly common occurrence, but it is different at every location at which it occurs. It occurs in a very localized manner around active or dormant volcanoes, and can cause problems with slope stability due to acidic alteration of the soils. Hydrothermal activity is far more abundant in active and dormant calderas such as the extremely popular Yellowstone National Park, though most are not on this scale (in size or degree of activity). Hydrothermal systems are most common where we cannot easily see them, however: at the bottom of the ocean. Spreading centers along the ocean bottom, where continental plates move away from each other, contain abundant hydrothermal features, though further exploration is needed to determine just how many there are and the degree of variability among them.

Hopefully by reading this article, you’ve gained a better understanding of just what’s going on beneath the surface in volcanically active areas. If you are interested in pursuing the subject further, I would suggest taking advanced geology courses on the subject or looking to scientific journals for articles published on the subject. There are several good texts that elaborate much more fully on the subject, though these can be harder to get your hands on. I don’t, however, see the importance of knowing these subjects more completely if you are not a university student of geology or a professional geologist. If you are one of these, you should be able to get your hands on better resources pretty easily. As always, I encourage you to take a look at the other articles in this website; this is just a small slice of what goes on at active volcanoes all over the world.