When visiting volcanoes, the first thing to strike you is the desolation. The ground is black without any sign of green, a lifeless wasteland. In Hawaii, walking through the rain forest, the sickness in the trees is the first thing that becomes evident. A little later, the forest opens up and disappears as the path reaches the lava flows. The desolation is not just from the burning by the lava flows. The sickness in the trees comes from the sulphates in the air. Death comes both from the ground and the air. Life will not return until the volcano goes dormant – preferably extinct.
But this is not true everywhere. The oceanic rifts underneath the oceans are more active than any land volcano. But rather than extinguishing life, these volcanoes attract them. The hydrothermal vents are surrounded by fields of fecundity. To live here means to live fast, in a precarious balance. Just a meter too close, and you burn. Too far, and you run out of energy. But like goldilocks, pick the right location and there is a good, if temporary, living to be made.
The first evidence for volcanically-powered life was found in 1977, in an area close to the Galapagos islands. Crucial to the discovery was a submersible called Alvin. Submersibles are expensive tools which are used over many decades. Alvin was first used in 1964 and I believe is still going. The name is in honour of one of the engineers, Allyn Vine. It is an impressive vehicle. Alvin is 7 meter long, 2.5 meter wide, and 3.5 meter high; even with a titanium hull it still weighs 16 tons. It can carry three people (including the pilot), has life support for 3 days (although a dive would not normally exceed 10 hours) and reach a depth of 4.5 kilometer. It can hover, or it can move around at a speed of 2-3 km/hr propelled by its five hydraulic thrusters. There are three thick portholes, designed to cope with the extreme pressure. A variety of powerful external viewing lights provide the illumination, essential at a depth where no sunlight will ever reach.
Alvin has numerous claims to fame. It was one of three submersibles that in 1974 obtained the data that confirmed the theory of ocean floor spreading. More prosaically, in 1966 it recovered the hydrogen bomb lost during an accident in the Mediterranean. And best of all, during a 1977 dive near the Galapagos Islands, it came across the phenomenon of the hydrothermal vents. Two years later, it discovered the black smokers.
The first sign had come from an autonomous sled which was exploring the sea floor and had recorded a sudden temperature spike. It was dismissed as an instrument error, until the scientists noticed that the still photo taken at the same time showed a completely unexpected landscape. Two days later, 17 February 1977, Alvin was send down. It found an oasis in what had been believed to be a desert, 2.5 kilometer deep. The presence of hydrothermal vents associated with the spreading ridges had been predicted, if not actually seen until that time, but their effect on the environment came as a complete surprise. The deep sea would never be the same.
The presence of hydrothermal vents had been predicted in the 1960’s. After all, spreading ridges require volcanic activity. On land, approaching magma gives you hot pools, fumaroles and geysers – all the result of underground heat without requiring the magma to actually reach the surface. The heat is carried up by water circulating underground. You would expect the same mechanism to happen under water. Indeed, the hydrothermal vents are the oceanic equivalent of geysers and fumaroles, and they share the same cause.
In the central valleys of a mid-ocean spreading ridge, the magma may reach as close as 1 kilometer to the ocean surface. Elsewhere it may be 5-10 kilometer. Water enters the ocean floors through cracks and begins to circulate below ground. Some reaches near the magma and becomes heated. Hot water rises, and it pushes its way back to the surface. The vents are where it comes out, either as a gentle flow of warm water, or as a super-hot jet spurting out of a small hole at high pressure.
The temperature of the vent depends mainly on how deep the magma is. Where the magma is 10 kilometer below or more, the vent may only be 40-60 C. Where it reaches within 1-2 kilometers, it can be much hotter, in places over 400 C. And remember, the surrounding ocean has a temperature of only 2-3 C. The hot water rises vertically through the surrounding ocean.
The first hydrothermal vent found was a mild one, operating at around room temperature. Two years later, a far more extreme example was found, a vent ejecting black water at hundreds of degrees C. The pilots of Alvin put a temperature probe into the vertical jet. It melted. The pilots realized that the perspex cover of the probe was made of the same material as their viewing ports. They beat a hasty retreat. One paper describes how a 2-cm thick electrical cable accidentally became draped across such a jet. It melted and split within 1 second! The heat capacity of water is much higher than that of air: the heating is therefore much faster than it would be even inside a flame. Calling the black smokers dangerous is a severe understatement.
Luckily, the location of the hottest vents tended to be quite obvious, at least after their discovery. First, their water is pitch black; second, it comes out from chimneys which can be meters – sometimes tens of meters – tall.
The black jets were quickly called ‘black smokers’, a name which now shows its age. Mike Ross famously told us that volcanoes don’t smoke – true for terrestrial volcanoes, but these hydrothermal vents most definitely do! The chimneys which they build up, when taken to the surface, even feel like cigarette ash. Later, it was found that not all submarine smokers are the same. Some produce white smoke rather than black.
The chimneys come in fields, just like fumaroles tend to occur in groups. The fields have evocative names: one is called the Lost City. The chimneys can grow as tall as 40 meters. But once they get too large, they break under their own weight. The vents are also temperamental – they stop, and may restart from another hole nearby, just like the geysers of Yellowstone can change behaviour with every earthquake. A field of vents is a very dynamic environment and exploring them is a lot like walking in the crater of an active volcano. The ground underneath your feet may suddenly open up.
This constantly changing activity gives a series of chimneys of varying height and activity, surrounded by the debris of the glorious fallen – in some cases toppled deliberately by the scientists trying to measure the water as close as possible to the source. Beyond the chimneys the ground is covered in a dark, ash-like substance. It is an image of unreal beauty: the fractal-like chimneys, surrounded by an utter, dark desolation. It is in fact remarkably like the landscape around terrestrial volcanoes.
So what on earth are the vents smoking? The answer comes from the rocks below. Rocks do not dissolve easily. But the hotter the water, the easier the minerals dissolve. Especially iron sulphides, copper and zinc react with the water and dissolve into it. What comes out of the vent is, in some ways, like a liquid mine.
As soon as the water shoots out, it meets the near-freezing water around and begins to cool. As the temperature drops, within seconds the dissolved minerals come out of solution and the water becomes suddenly full of microscopic solid particles – it is in fact exactly like smoke particles. The particles in black smokers are mainly a mix of sulfides (e.g. pyrrhotite FeS, sphalerite ZnS, chalcopyrite CuFeS2, etc.) and sulphates (anhydrite CaSO4, barite BaSO4). The solid particles, especially iron and iron sulphides, absorb the light in a way that the water didn’t – and the water turns black. Wherever a surface is available, the particles stick to it. This is what builds up the chimneys: the water jets build up their own cocoons. And the chimneys can grow fast, at meters per year. The jets also carry the particles further away, and some condense only when the temperature has dropped closer to ambient. These particles rain out further away and give the ashy cover of iron sulphide and other particles around the chimney fields.
The process has been demonstrated in the laboratory by Laura Barge. She injected alkaline solutions into iron-rich acidic solutions. It made mini chimneys of iron hydroxide and iron sulphide.
Temperature has a strong bearing on what minerals can dissolve. If the water is not hot enough, the iron sulphites do not dissolve. Instead, the water becomes laden with calcium and barium. Once they reach the surface, they can’t produce the black particles. Instead, the particles that precipitate out are white in colour. These are the white smokers.
If the magma is much further from the vent, the water can cool already underground while traveling towards the exit. The cooling can happen by conduction, losing heat to the surrounding rock, or by mixing with cold sea water circulating through the ocean crust. As a result, the metal and sulphide particles fall out below ground, and never reaches the surface. The water chemistry is very different in black and white smokers. Black smokers are highly acidic, worse than battery acid. White smokers are highly alkaline. The combination of dissolved elements, the acidic nature and the turbulent mixing with the alkaline sea water makes the black smokers sites of extreme chemistry.
Most vents are close to the ridge axis, within the valleys on top of the ridge valleys. Not all do. Some may be as far as 10 kilometer from the ridge, well away on the slope where the surface is a million years old or older. The same, of course, is seen on land. Neither Bardarbunga nor Grimsvotn is on the spreading rift: they are on either side. Once perhaps they were near the axis, but as the new crust spread it took the volcanic conduits with it. They will now slowly drift away from the spreading centre, until they conduit becomes blocked, the volcano dies and a new one forms closer to the axis. In the sea, the vents located on the ridges are much hotter than those far away. Black smokers are found at the centre. White smokers dominate farther away.
Water can exist in three main phases: solid, liquid, and gas. (Actually, the solid phase is not a single one but there are many different solids which can be formed from frozen water, each a different phase but few can exist on Earth.) In a geyser, the super-heated water boils just below ground, the vapour massively expands in volume and up shoots the jet. As it cools, a bit of the water re-condenses above ground, giving the mist that typifies a geyser and that gives the audience forgetting about wind direction a drenching.
The phase diagram of water shows the well-known fact that ice can only exist at sub-zero C. (Interesting, that means that the only place in the entire planet where water can be solid is on the surface!). Similarly, water as gas (or steam) only exists if the pressure is less than 218 atmospheres. That pressure is found at a depth of just over 2 kilometers (the pressure under water goes up by 1 atmosphere for every ten meters). Hydrothermal vents that are deeper can not form gas, neither directly nor as bubbles. Neither can you get a phreatic explosion at these depths: it is against the laws of physics. The deepest vents, in the Caribbean, are some 5 kilometers below sea level. A these depths, the vents can only ever vent liquid water.
But there is more. Pure water can only be a liquid below a temperature of 374 C. Salt water can remain a liquid to slightly higher temperatures, just above 400 C. But above that temperature, the water has to boil and turn into steam. But what if the super-hot vent is more than 2 kilometers deep? It is a catch-22: both phases are against the law.
Physics has a solution. The water becomes a supercritical fluid, neither a gas nor a liquid but somehow a bit of both. It behaves like a liquid, but unlike a liquid it is compressible. Water does not compress. Try standing on it: it won’t give in. A supercritical fluid can happily expand when heated – a liquid does so only very reluctantly. In practice, this means that when the superheated water spurting out of the deep vent is a supercritical liquid, its density is much less than that of normal water. And so it rises.
Normally, the jets from hydrothermal vents rise some 200-400 meters before coming to an end. But supercritical vents can rise much further, because their water has much lower density than the surrounding sea: the highest value measured is 1100 meter. The rising stops when the temperature of the jet has dropped below 400 C, where the water comes out of its peculiar state and behaves like the ambient sea water.
The rising allows the mineral-laden water to spread out far and wide. Much of the chemical wealth of sea water, from gold to uranium, comes from this. It was brought to the bottom by hydrothermal vents, and as long as it remained soluble, polluted to the entire water column, top to bottom. And life loves these minerals. Especially the iron is pure fertilizer.
The discovery of 1977 wasn’t so much the vents themselves. In fact, the black smokers weren’t seen until 1979. It was the life around the vents that drew the world’s attention. The deep sea is in many ways a desert. There is very little life, and what there is congregates around wind falls, such as the occasional whale carcass but more generally, the slow rain of nutrients descending from the top layer of the ocean. But the vent near the Galapagos showed a veritable rain forest. It was a luxurious garden, albeit of creatures very different from the norm. Some things were familiar: the octopi (they get everywhere), and the crabs. But the forests of tube worms and fields of Yeti crabs were unworldly.
Both the density and diversity of the vent life is staggering. It really feels like a rain forest of the deep. But what makes this happen? Nutrients are of course a necessity. But they are not sufficient. Besides the raw material, life also needs energy. Life builds complexity. On land, the energy ultimately comes from sunlight. Plants have learned to build up calorific molecules with photosynthesis. The basic formula is
CO2 + 6H2O -> C6H12O6 + 6O2.
The crucial aspect of this equation is that it takes energy to run, and for this energy plants use sunlight. There are costs: the process is not particularly efficient, and plants needs a lot of surface area, and they do not generate enough energy to do energetic things like move around. Animals do, but they obtain it by eating a lot of plants. There is far more biomass in plants than in animals: it cannot be otherwise, for reasons of energy.
The deep sea vents lie out of reach of sunlight, so where do their rain forests get their energy from? It turns out that this is a complex question. There are two aspects to it. The food chain is build on sulphide, through the basic equation
CO2 + 4H2S + O2 -> CH20 + 4S + 3H2O.
The vent bacteria oxidize hydrogen sulphide, add carbon dioxide and oxygen, and produce sugar, sulphur, and water. Other bacteria make organic matter by reducing sulphide or oxidizing methane. All other life here is based on this food supplied by the bacteria. But you need a lot of bacteria to provide the animals with enough energy. So the animals take the same approach as trees, by providing as much surface area as possible. This explains the tube worms: the tubes provide the surface area, and gills make sure that water continuously flows over the inner surface. The bacteria love it. Molluscs have similar techniques, although mussels also are filter feeders, which makes them less dependent on the bacteria. Yeti crabs grow bacteria on their own legs. Other crabs feed mainly as predators or scavengers. It is a complex chemo-ecology.
The varying temperature causes tremendous problems. The tube worms need to stay close enough to the vent to get the exhaust, but far enough not to burn. If the vent moves, the worm dies. The crab can move around and is a bit more robust. But even the crab will not survive if the entire vent field goes extinct. But the community very quickly re-establishes itself when another vent field appears – clearly the animals, in some form, move around the ocean quite easily. Not unlimited though: the hydrothermal vents in the Arctic ocean have quite different communities. The Arctic ocean is isolated from the deep conveyor currents of the world ocean, and it shows this in the vents.
The hydrothermal life is often used as an example of how life on other planets or even on the earliest Earth may develop. But that doesn’t fully work. Yes, the chemo-synthesis does not require sunlight and provides an alternative to photosynthesis. But the reactions do need oxygen. The water that comes out of the vent is anoxic. The oxygen is mixed in by turbulence, coming from the oxygen-rich cold waters of the ocean. The scavenging crabs, also, can cope for some time with anoxic conditions as they move into the vented water. But they do need to go out at times to breath oxygen again. The tube worms can’t move well and need to grow on the interface where the vent and ocean waters mix. The early Earth had no oxygen: that did not come until photosynthesis had developed. The communities around hydrothermal vents could not have existed in their current form without oxygen being produced at the surface. There are organisms that use anoxic reactions for their metabolism, the archea, but their role in the hydrothermal vent communities is not well known.
Volcanoes are anathema to life. In the long term, life benefits from the mineral riches they bring, but in the short term they are deadly. But life in the deep oceans has a much closer relation to volcanic activity. They have turned super-heated smoke enriched in toxic metals into rich oases. It is a different world, and one about which we know rather little. There are many discoveries to be made, new vents, new species, new life. The deep ocean is a final frontier which calls out for explorers to boldly go where only tube worms have gone before. It is volcanics with a difference.
Albert, Jan 2018