Where does gold come from?

Quartz with an exceptionally rich deposit of macroscopic
gold grains
This image from Nevada Outback Gems web page at
http://nevada-outback-gems.com/prospect/gold_specimen/Gold_ores.htm

In general, gold is sparsely distributed in the earths crust, having a typical concentration of about 2 parts per billion (ppb). How does it become concentrated to form deposits in which it is more than a thousand times as abundant, the deposits that we mine? In a new paper* in Nature Geoscience, Dion Weatherley and Richard Henley address this question.

A typical fault jog cavity, now filled with quartz
From the referenced article.

     The observation that underlies the theory is that there is a typical geometry of quartz veins found in fault zones formed during times of mountain building ranging as far back as 3 billion years. Faults are not simple planar surfaces. Rather, they jig and jag forming irregular and complex surfaces. When a fault ruptures, causing an earthquake, some parts of the fault slide past each other smoothly; other parts bump into each other; and, importantly for the gold-story, some parts separate to form open cavities. At the instant of their formation, these cavities have extremely low pressure, causing any fluids in the vicinity to vaporize (so-called "flash vaporization"). When the fluid vaporizes, silica and its dissolved trace elements, including gold, are rapidly deposited.  Nearby fluids flow toward the cavity, and vaporization continues, until the pressure is restored to ambient (presumably lithostatic) conditions. For  case study, the authors took ambient conditions before a fault rupture appropriate to a depth of 11 km: P=290 MPa, T=390 C.  They used a simple piston model for the formation of the cavity during strike-slip motion of a vertical fault, and obtained the change of volume of the cavity in the jog by looking at the seismic moment. They assumed that the cavity initially contained water, and had a width of 100 microns. Using the moment magnitude of an earthquake to calculate a volume change for the cavity, the authors could then, using the perfect gas law linear relationship between volume and pressure change, calculate the pressure drop. They concluded that during a magnitude 4 displacement with a slip of 0.13 m, the pressure would drop from 290 MPa to only 0.2 MPa.
     Quartz solubility is strongly dependent on the density and pressure of the water in which it is dissolved. Such large pressure drops cause extreme supersaturation of SiO2--up to factors of 100,0000--which the authors propose nucleates by forming grains of less than 100 nanometers in diameter.  Silica deposition occurs through a succession of metastable polymorphs including hydrated silica. These phases evolve during the pressure recovery around the initial cavity. The authors propose, but do not develop quantitatively, that these large drops in fluid pressure cause a complicated "cascade of coupled, nonlinear chemical responses" that result in the complex assemblages that are found in veins. Precious metal solubilities depend strongly on pressure.
     Isolated slips do not deposit huge amounts of gold, but rather result in the deposition of only a thin coating of silica-gold vein material. The mass of this material is increased during the recovery stage.  The authors suggest that it takes tens of thousands of years, but less than 100,000 years to form a high-grade deposit.

*Weatherley, D.K. and Henley, R.W., "Flash vaporization during earthquakes evidenced by gold deposits," Nature Geoscience, online March 17, 2013, DOI: 10.1038/NGE01759.

Stealth disasters: Definition and a CALL FOR INTERNATIONAL INPUT ON TERMINOLOGY

(I apologize if followers have gotten multiple alerts about this post--the tables below have caused a real headache, and Blogger is just not letting me correct things.)

I have a project (and a paper to give in two weeks as well as to prepare for publication) on stealth disasters. I'm going to define "stealth disasters" here, and ask for readers to send me their best translation in other languages. Here are some excerpts from my talk and paper:

Natural processes unleash energy in various ways and at differing rates. Sometimes this results in situations that are harmful or, at best, inconvenient, for humans, and we have historically called such events “natural disasters.” If you look up the term “natural disaster” in Wiki (2013) you get the following examples:

avalanches, earthquakes, volcanic eruptions, floods, limnic eruptions, tsunamis, blizzards, cyclonic storms, droughts, hailstorms, droughts, hailstorms, heat waves, tornadoes, wildfires, epidemics, meteorite impacts, solar flares, and gamma-ray bursts. We have historically called such events “natural disasters,” and the insurance industry traditionally has called these “Acts of God.”

Disasters such as these are typically characterized by a relatively sudden onset.  Sometimes there are precursors: seismicity may increase before an earthquake, or the awakening of a volcano may be signaled by small eruptions, often of freshly heated groundwater as hot magma works its way up through the volcano. Landslides may have episodes of creep or small failures prior to a large event.  Typically the precursors of a natural disaster are small compared to the disaster-producing event itself and thus, even though there are precursors, and that is why we can say that this type of disaster has a “sudden onset.” Once the big event occurs, the consequences to humans typically occur within seconds, minutes, hours, or a few days.

     But there are other processes that produce hazards, generally not thought of as “disasters.” These processes involve the natural systems that support us but, rather than being driven primarily by natural non-biological processes, these are driven by human behavior. Examples are climate change, desertification, acidification of the oceans, compaction and erosion of fertile soils, death of coral reefs, and collapse of ocean fisheries and other ecosystems.  These disasters typically have more gradual onsets than natural disasters and, because of this, I refer to these as “stealth disasters.”  

Stealth disasters develop over longer periods of time than natural disasters, often over decades or centuries. Although they do occur at local scales, cumulatively they are at least regional, and often global, in scale simply because our human presence and our human impact on the geosphere is now dense and global. For example, ponds and rivers may be locally polluted, but these ponds and rivers drain into networks that are also polluted and these networks eventually deliver pollution at a regional and global scare. Another way to say this is to say that systems are interconnected on scales ranging from local to regional to even global. Compared to natural disasters, the onset of stealth disasters is gradual and they are for this reason alone, not given prominent attention in the media. 

Unlike natural disasters, the effects of stealth disasters are often not subject to remediation on timescales relevant to us personally or even to our civilizations, that is, their effects are irreversible on any timescale relevant to human survival.  Although we destroy the geosystems that support us at human rates, their recovery will occur at geological rates.

In contrast to natural disasters, press coverage about stealth disasters tends to be sporadic and fails to elicit the attention, sympathy, political will, or economic help that are required to reverse the processes or solve the problems. Although it is possible that by some behavioral changes, some stealth disasters may be minimized or avoided (the ozone hole is the most prominent example of success in this regard), it is now generally acknowledged that, at least for those stealth disasters related to climate change and the current and anticipated human population density, preparation for and behavior during stealth disasters need to be addressed

     In English*, “stealth,” as an adjective, describes an action that is “intended not to attract attention.” Some synonyms listed for “stealth” are “behind-the-scenes,” clandestine, covert, furtive, hush-hush, sneaking, secret, or surreptitious.

            In other languages, Table 1 summarizes the adjective “stealth” or “stealthy”, and the phrase “stealth disaster.” Note: I have this in Hindi, but see note below so do not worry about Hindi. Arabic follows this table.

Readers help in expanding these tables (and in suggesting other examples of stealth disasters) will be much appreciated! Please do not send me samples just taken from Google translator, but only examples for which you personally know. I have found Google Translator can give very misleading results.

Language “Stealth” or “stealthy” (adj.) “Stealth disaster” (noun) German verstohlen verstohlen Katastrophe Spanish furtivo desastre furtivo Hindi  छुपा  or छल चुपके आपदा or गुढ़ आपदा Italian furitivo disastro furitivo French furtif catastrophe furtive Romanian ascuns ascuns dezastru Chinese Russian невидимый скрытая катастрофа Hebrew חשאי אסון חשאי‬ Polish ukradkowy ?? potajemny katastrof ?? Ukranian Прихований ? прихований  лиха    _________________________________________
Arabic “Stealth disaster(s)” (noun)  “disguised disasters” ‫الكوارث المتخفية‬ “invisible disasters” الكوارث اللامرئية “stealth disasters” ‫الكوارث المتسللة‬ “ghostly disasters” ‫الكوارث الشبحية‬ “hidden disasters” ‫الكوارث الخفية‬

The Physics of Making "Smoothies"

Frame from a video on the referenced article. Note the
cloudy region in the bottom of the liquid near the blades.
This is caused by minute bubbles created by cavitation at
the tips of the blades. The brown layer on top of the water
is oil.

Thanks to a colleague for pointing out an interesting article on shock waves in food blenders! Mathematician-turned-chef Chris Young and colleagues have collaborated in a series of articles to explain the science behind various cooking techniques. In this article, Young explains how the blades in the blender cut food up into small pieces, but then that the real work is done by small bubbles created by the blades through a process known as "cavitation."

Cavitation occurs when the pressure in a liquid drops below the equilibrium vapor pressure. It is a common occurrence in industrial settings, such as around the tips of rotating blades. Pressure drops occur in a variety of settings ranging from the flow of rivers around and over objects to flow in nozzles, to the wiggling of the tails and fins of swimming animals, and cavitation is a possibility in any of these settings. A bubble formed by cavitation is unstable. When such bubbles collapse, often asymmetrically, a tiny jet is formed. When these jets impinge on either the opposite sides of the bubble walls or on an adjacent surface, very high pressures, thousands of times atmospheric pressure, can occur. These jets erode the adjacent surfaces, causing structural damage.  Cavitation around the tips of dolphin or tuna tails may limit the speed with which these animals can swim! Cavitation is also a major problem at spillways from dams, and played a role in the near-failure of the bypass tubes during a major flood crisis at Glen Canyon Dam in 1983.

Rocks Flying out of Volcanoes at Breakneck Speed...or do they?

A meter sized bubble bursting
above the vent of Stromboli (top)
and a jet (well-collimated) from a
typical explosion of such a bubble
Figure 1 in the Taddeucci et al paper

I recently came across a year-old Science News article called "Volcanic Rush," by Alexandra Witze. This article summarized the results of two interesting papers (referenced below) on high-speed volcanic ejecta documented in the field, and simulated in the lab.
      The Taddeucci et al. paper reports on direct measurements of pyroclasts ejected from Stromboli at velocities up to 405 m/s, during eruptions from the south-west vent area in 2009. The velocities were obtained from high-speed videos in which centimeter-sized fragments were tracked for 5-10 frames. The velocities cited are believed to be conservative for several reasons.
      This velocity would correspond to a Mach number of 1.2 if the sound speed of air is taken as 330 m/s, and the authors point out that these velocities are "supersonic in ambient air."  It isn't clear whether or not these pyroclasts were actually traveling through ambient air where measured because shock waves which would compress and heat the atmosphere were often observed, and it may also be possible that the blocks were in a steam cloud, in which case their Mach numbers might have been subsonic because the sound speed is higher in steam (e.g., 850 m/s at 1000 C). The authors do not discuss these possibilities.
      The general picture that emerges for the eruption process is of the explosion of a large steam bubble that produces initially the high-velocity burst of small pyroclasts (cm -sized). Within tenths of a second, the velocities decrease and it is only later in the event that larger blocks and bombs are ejected. Only with the use of the high-speed cameras has there been resolution to detect the high-velocity phase of the eruption because lower-speed cameras only capture data later in the event when drag has already acted to slow down the small pyroclasts. The new measured values are a factor of four higher than previously measured. The authors were able to work out from the initial ejection pulses that the distance from the base of the pressurized gas pocket to the camera viewpoint at one of the vents (SW1) was 3-6 m, which corresponds to the size of the bursting bubbles driving this phase of the eruption. In contrast, application of the same analysis to the dominant pulses at the second vent, SW2, gave estimates of bursting length (similar to shock tube dimensions) of 100 m.
      In many older models of reservoirs and the pressures that drive eruptions, including work of this blogger, reservoir pressures are taken as ambient (lithostatic or hydrostatic) pressures, and the pressure difference between the reservoir and atmosphere is assumed to drive the eruption. Alatorre-Ibarguengoitia et al. (2011 and 2010 below) have developed a conceptual model that says that the fragmentation of the magma itself may consume considerable energy, and that the kinetic energy available for driving pyroclasts is significantly less than from these older gas models. In their 2011 paper they put natural volcanic samples of differing porosities into a 1-D shock tube and measure both the fragmentation speed inside the shock tube and ejection velocities. They relate the results using 1-D conservation laws.
     The lab experiments are instructive. In these experiments, a volcanic pumice is capped by a small volume of gas which, in turn, underlies a diaphragm. The duration of the shock-tube experiment is about 300-400 ms after a diaphragm separating the reservoir from the atmosphere is burst. The fragmentation threshold pressures were significant: about 4 Mpa (40 bars) for pumice; 8 Mpa (80 bars) for a denser sample. Below these pressures, the sample either does not fragment, or only partially fragments. From the experiments, the authors developed an empirical relation between the reservoir pressure and the fragmentation velocity (U is a logarithmic function of the ratio of the reservoir pressure to the fragmentation threshold pressure). Using this empirical relation in conjunction with the conservation laws for a pseudogas, they derive the theoretical velocity of eruption of the gas-particle mixture, and found it to be in excellent agreement with the lab measurements. This provides a way, then, to use measured or inferred field velocities to calculate reservoir pressures.
    They then outline some conditions under which the experiments can be applied to volcanic eruptions, one of which is that it is restricted to magmatic eruptions, and produce the following graphs of ejection velocity versus initial reservoir pressure:

Ejection velocity as a function of initial reservoir pressure for two different porosities, and four different values of f,
the fraction of particles in thermal equilbrium with the gas phase. f=100% implies a fine-grained ash eruption; f=0%
would apply to an eruption of scoria or bombs.

There are some interesting implications given the observations of  Taddeucci summarized above (these are my thoughts, not the authors, so they shouldn't be blamed if I'm off track). Ejection velocities on the order of 400 m/s can only be obtained from porous magma initially at high pressure having high heat transfer--this is consistent with the high-velocity ejecta observed being small (1-cm pyroclasts, or less?).  Denser magma can generate only significantly less velocity even with efficient heat transfer, and only low velocities if there are big particles (scoria, bombs) with little possibility of heat transfer.

M.A. Alatorre-Ibargüengoitia, B. Scheu and D.B. Dingwell. Influence of the fragmentation process on the dynamics of Vulcanian eruptions: an experimental approach. Earth and Planetary Science Letters. Vol. 302, February 1, 2011, p. 51. 

J. Taddeucci et al. High-speed imaging of Strombolian explosions: The ejection velocity of pyroclasts. Geophysical Research Letters. Vol. 39, published January 18, 2012, L02301. doi:10.1029/2011GL050404. 

Alatorre-Ibargüengoitia, M.A., Scheu, B., Dingwell, D.B., Delgado-Granados, H., Taddeucci, J., 2010. Energy consumption by magmatic fragmentation and pyroclast ejection during Vulcanian eruptions. Earth Planet. Sci. Lett. 291, 60–69. doi:10.1016/j.epsl.2009.12.051.

Meteor blast over Russia

From the electronic version of the New York Times,
February 15, 2013.

Reports from Russia today tell of hundreds of people hurt during the passage of a meteor. By coincidence or not, this has happened on the very day that a known asteroid, 2012 DA14, is to pass within 17,000 miles of the earth, a widely-publicized happening for which the public has been reassured that there is no danger of the asteroid actually hitting the earth! It will be interesting to follow the speculations: Was this meteor an undetected fragment of the bigger one (it is about 45 meters in diameter)? How big was it? The Russians have given a preliminary estimate of 20 tons. People weren’t hurt by the meteor itself, but by a “blast” from its passage.

What was the “blast?” When an object such as your car, or a bullet, or a meteor, moves through air, a signal also moves through the air “alerting” the air around the object that something is moving. The effect is analogous to ripples spreading away from a pebble dropped in a pond, but this signal is an acoustic wave, just like the waves that carry the sound of your voice across a room. At low velocities, the effect is barely noticeable and the air adjusts to the incoming object by moving away. However, when the object is moving “fast,” the air near the object can’t “get out of the way.” The compression waves emanating from the object pile up through nonlinear effects and form a shock wave.

Pressure, temperature and density all rise across a shock.  When the overpressure is only 1.5-2 pounds per square inch (psi), the effect is annoying; when it is 2-10 psi (about one-tenth of normal atmospheric pressure at sea level), minor structural damage can occur. Sonic booms generate pressures on the order of 20-150 psi (up to about 10 times normal atmospheric pressure) and humans have survived these without injury. Ear damage occurs when the overpressures are over 700 psi (about 50 times atmospheric pressure), and lung damage occurs at about 2000 psi (about 140 times atmospheric pressure). Presumably the pressures from the shock of this meteor were on the order of a few hundred psi when it the ground and broke the windows. It was glass from the broken windows that caused most of the injuries.

How fast is “fast”? This is determined by the speed with which sound propagates through air, about 330 meters per second, about 740 miles per hour. Typical impact velocities for asteroids are 17 km/second and for comets, 51 km/second, much higher than the speed of sound in air. This is the reason that all meteors, if they are big enough, generate sonic booms.

According to the New York Times, the Russians deployed seven airplanes to search for meteorite fragments. More than 20,000 people were sent to search the area on foot for fragments.  An impact crater has been reported on the outskirts of a city “50 miles west of Chelyabinsk,” an area of many military industrial complexes. Since the Cold War days, it has been a concern that an event like this could be mistaken by nervous governments for a bomb attack. If this had happened a few decades ago, the Russian response might have been different, and if it happened over North Korea or Iran in the middle of the night when the contrail in the sky couldn’t be seen, one has to wonder what might have happened. Scary thought…