Facelift

I've tried freshening up the blog by replacing the stock photo with a picture of some 3.3 billion year old zircons from my PhD research. The holes are from laser ICP analyses. The zircons are from a sandstone unit in the Jacobina group, which is a paleoproterozoic cover sequence on the Sao Francisco craton, in Brazil. What I can't figure out is what color to use for the font, in order for it to be readable without being ugly.

Mantle melting

Much of the island of Tasmania is covered in basalt, which sometimes shows columnar jointing

There are a few processes in geology that are so fundamental and thoroughly taught that we forget that they are counterintuitive to normal people. One of these is mantle melting. The Earth consists of compositional layers of increasing density from surface to center. Near the surface is the crust, which is about 5-70 km thick. The crust has a complex composition I don’t want to get into now, but it is mostly silicates- complex metal oxides that include at least some silicon. Below that is the mantle. The mantle is mostly magnesium silicates, with some iron substituting for magnesium, and a few less common calcium and aluminum bearing minerals. The core is metallic iron, and is mostly molten. This, in itself, is fairly intuitive.

When the mantle partially melts, the resulting magma is less dense, and rises either to the base of the crust, or through the crust to the surface. For reasons I’ll skip for now, the composition of this partial melt is different to the unmelted material left behind. This mantle-derived magma* is known as basalt, and a lot of the Earth’s crust is made of this material. You can see basalt above, in the post before my last one, in volcanoes like Hawaii. Mot of the ocean floor is covered in basalt. So, when the mantle partially melts, we get a molten rock called basalt. And that is not particularly unintuitive either.

The tricky bit is this: Unlike most of the melting we see in our daily lives, the melting of the Earth’s mantle is never caused by heating it up. Most of the mantle is solid rock, and it melts fairly regularly, but most of the processes that melt it actually cool it, instead of melting it.

We geologists are so used to this that we don’t bat an eye, but for normal people, melting without heating seems a tad unusual. But then, the mantle is quite a different place than the kitchen counter.

The mantle is very hot, fairly dry, and under extremely high pressure. This pressure ranges from a few thousand times atmospheric pressure at the top of the mantle to about 1.3 million times atmospheric at the bottom. And one of the effects of increased pressure is that it also increases the melting point.

In fact, most of the solid mantle is so hot that, if you suddenly released the pressure around it, it would spontaneously melt. It is only the pressure that keeps it solid. The mantle can also flow- at high temperatures and pressures, solids become slightly ductile, and over long periods of time, the mantle can ooze around at speed of a few centimeters each year.

When a very deep, very hot part of the mantle rises close to the surface, if it rises faster than it can cool, it will generally start to melt once the pressure drops to around 15-25 thousand atmospheres. This is called decompression melting, and is the main cause of basalt magmatism in mid-ocean ridges and hotspots.

In some cases, mantle rises so slowly that it cools faster than it rises. When this happens under a spreading center, then you get ocean floor made of mantle, not basalt, because no melt was produced. This is rare, but there are known occurrences, mostly at very slow spreading ridges, such as the Arctic Ocean or the ridge between Africa and Antarctica. An expedition to a newly discovered crustless region was recently summarized by the rockbandit here.

The second main cause of melting is caused by water. When ocean floor sinks down into the mantle, it can carry water with it, which will eventually escape into surrounding mantle. Wet mantle has a lower melting temperature than dry mantle, so the introduction of water into warm dry mantle triggers melting. This is what produces arc volcanism above subduction zones in places like Japan or Chile, although these wet magmas sometimes interact with the overlying crust in ways that changes their composition so that they are no longer basalt.

They key point is that unlike the melting butter or ice at home, melting in the mantle is not caused by the addition of heat. It is caused by lowering the melting temperature of material that is already hot.

*By far the most common one. There are some rare, volumetrically insignificant mantle melts that are not basalts, but we can ignore them for now.

Sexual Sin and Christians (repost)

I originally wrote this for a usenet group as an undergrad back in 1993. This is the edited, cleaned up version. The original can probably be found floating around cyberspace somewhere, but it has more typos, so I prefer this version:

It is a well known phenomenon that Christians, particularly dogmatic, fundamentalist Christians, have a disturbing tendency towards homophobia. There have been many suggestions as to why this is, including direct quotes from scripture, statements by powerful theological figures, and other such religious ideas, but in order for a non-Christian to really understand the nature of this deep-seated prejudice, a non-religious model must be constructed to show exactly why it is that traditional Christian beliefs often seem to conflict with homosexual activity. The following model, which is principally based on traditional Newtonian physics, should do much to explain this phenomenon. Unfortunately, while it may bring understanding to the agnostic physicist or computer scientist, its physical nature will probably enlighten the godless historian or other liberal artist no more than the traditional explanations. However, since such folk are usually excellent at coming up with dubious explanations of variable credibility (abbreviated by biologists as B.S.), we are confident that they can create their own creative explanations, and thus have no use for this one.

We will consider God is a point mass, centered at the origin of our xyz space. Christ, we will assume, is at the right hand of God, or about 100 centimeters away. His mass is probably around 75 kilograms. Since God has a very large mass (a bit less than infinity), Christ, who we will assume is in a circular orbit around God, has a very large momentum, and hence has a very small wavelength. This means that Christ's uncertainty is quite small, so we can therefore conclude that He is fairly certain in all that He does. Now let us consider a sinner. We shall place him at a large distance from God, say one inch and 45 million light-years. He, also being in a circular orbit, will be traveling significantly slower than Christ, and will therefore be more uncertain about it. One should also consider, however, that since Christ's orbit could fit in a kiddie pool, while the sinner's would encompass not only our galaxy, but a few of the nearby ones as well, that the sinner gets around more, sees more, and is generally a more knowledgeable guy than the Savior. This fits in with traditional wisdom. From this situation we can draw a few conclusions. The first is that Mary, the mother of Christ, being a fairly pure person is close to God. This means that she must be a fast woman. The second conclusion that can be drawn is that sinners have a lot more potential than saints, since less of their energy is stored as kinetic energy. Further insights can be gained when we look at the situation of the heathen.

A heathen is someone who is not affected by God. This means that they are at least a infinite distance from Him. Now, assuming that one of these folk starts to travel towards God, he will convert his potential energy to kinetic energy during the approach, or descent. Since he started out an infinite distance away, but with some kinetic energy of his own, he will approach God on a hyperbolic trajectory and then disappear into space again, never to be seen again. If his approach is such that it brings him inside the orbit of the Son of God, then right after his closest approach, the sinner's velocity will be greater than Jesus', which means that he will be more sure of himself in his escape than Christ is in orbit. This is an interesting notion, but some of the side ramifications are even more intriguing.

Without any orbiters, therefore, God would not be able to attract anyone - all approaching bodies would have either parabolic or hyperbolic trajectories. However, once God has an orbiter, the two of them could collaborate to capture other bodies. This means that heathens that get too close to believers in their approaches might get trapped, and by the same token, believers who are buzzed by heathens could be ejected. And what, the reader asks at this point, does any of this have to do with sex? It is after all, that, and not Newtonian physics that gets Christians so agitated. Well, the answer is this: Sex, as we all know, is the union of two or more people. This, in our analogy, would be represented as a collision. Now, in Christianity, almost all of the holy figures are male. For God, a collision between any of these close-in folk would be disastrous, because, even if we assume they are indestructible, such a high energy collision would

eject one of the men in it,
cause one of them to fall into God, or
give them highly irregular elliptical orbits.
All of these would be bad for God, because in the first two He would lose orbiters, making His chance at capturing new ones less, and in the third case He would have a much greater chance of more collisions, as the elliptical orbiters would cross many of the unaffected circular orbits. Therefore, God probably disapproves of these collisions.

Unfortunately, this theory is far from robust. It does not, for example, contain a method for experimentation whereby one can determine its validity. It also assumes that religious figures are sufficiently slow that they do not obtain relativistic speeds. Considering the large mass of God, this seems improbable. In fact, if God is as large as we suggest, the orbit of Christ would probably lie inside of His Schwartzchild radius. This would make figuring out what those two are doing very difficult, since none of the rest of us in the outside universe would be able to see beyond that limit, but because the bond between Them would be incredibly powerful, the evidence all points towards something that the Bible is not in favor of. On the other hand, it is the opinion of this author that whatever one does inside of one's personal black hole is one's own business, and therefore, I shall turn my attention to other matters.

Scooping the thrust sheet

A while back, Highly Allochthonous promised us some juicy field photos of South Africa. Since he has yet to produce the goods, I thought I'd whet y'all's appetite with this picture of the Drakensberg. The Drakensberg mountains are the 2 km wall formed by the eastern edge of the Karoo flood basalts. That's them to the left, and some of the underlying sandstone outcrops in the valleys below.

OHS and pregnancy

Sciencewoman recently blogged about reasons women leave science, and one of her commenters brought up the issue of pregnancy and laboratory safety requirements. The way I see it, there are three basic approaches to this issue. I will lay them out as dispassionately and factually as scientifically possible.

1. The Victorian chauvinist approach. This approach assumes that womenfolk are vital to the health of the country as bearers of young men who we desperately need to send into the trenches against the Germans. As such, it is vital to protect the childbearing resource at all costs, and any pesky activity like earning a living could endanger the ability of society to breed a new generation of pig-headed, antediluvian assholes.

2. The lawyerphillic approach. This approach assumes that any risk, however minute, must be avoided in order to protect the university gold. Instead of resources to protect, pregnant women are liabilities to minimize. Other than that distinction, this approach is identical to the Victorian chauvinist approach.

3. The sensible scientific approach. Under this system, pregnant research staff are informed of potential risks, they have those risks compared to more familiar, out-of-lab dangers in order to make them comprehensible. The lab then makes arrangements so that if the pregnant researcher chooses not to take a risk, it does not impact on her work. She then makes an informed choice about how to proceed.

Here’s an example of how the third way works, taken from a long time ago in a university far far away…

Dr. XX, a pregnant post-doc, wanted to know if exposure to her ²³³U spike would constitute a radiological hazard risk to her unborn child. The lab supervisor, Dr. XY, showed Dr. XX the math to determine what the decay rate was. He then pulled out a Geiger counter, put it next to her spike (tick tick tick….) to demonstrate. In order to compare this with the radiological risk from real-life items, he them put the counter next to a cement wall (tictictictictic…) Dr. XY then reminded Dr. XX what the biological effects of ionizing radiation were, and told her that if she chose not to spike her own samples, he or someone else would be happy to do it for her. Dr. XX then made her informed decision.

Note to Dr. XY wannabes. If you are looking for a slightly radioactive everyday item to compare a low level radiohazard to, DO NOT USE the woman’s bump! While it may seem perfectly logical to point out, “Look, your baby’s already way more radioactive than your sample,” in practice this approach is asking for trouble. So unless you want your Geiger counter forcibly inserted into your low photon environment, find something else.

Isotope dilution

Sciencewoman has an interesting article enumerating the barriers to people in general, and women in particular, who wish to be professional researchers. There has been some interesting discussion on this issue. I would like to contribute. Unfortunately, before I can do so, I need to explain isotope dilution.

Isotope dilution is a way of getting very accurate concentration numbers from mass spectrometers. One of the problems with mass spectrometry is that you generally don’t know what proportion of the total sample you introduce actually gets to the detector. This depends on a number of different things, including the ionization efficiency and transmission efficiency of the instrument. In general, the relative efficiencies of different elements are not equal and not constant. As a result, tricks must be employed to account for this.

Isotope dilution is one of these tricks. In isotope dilution, you add a known amount of a single isotope of the element you wish to measure to the sample. The isotope you add is called the spike. The action of adding it is called spiking. Since you know how much spike you added (this is measured very carefully), the ratio of spike sampled to spike detected gives the detection efficiency of the instrument, and the ratio of spike to natural isotope gives the concentration of that isotope.

Spikes are often, but no always, short-lived radioactive isotopes not found in nature. For example, ²³³U is commonly used to spike U solutions for measuring U concentrations. Because separating and/or creating spikes via nucleosynthesis is difficult, they are often expensive, and the radioactive ones can be a potential radiation hazard.

Obviously there are all sorts of further improvements on the process- double spiking to determine mass bias, changing dilutions to determine detector linearity, etc. But the basic idea is the same. You add a known amount of something unique to account for the screwy processes that occur inside the machine.

A dark day for science blogging

"I felt a great disturbance in the Force, as if millions of voices suddenly cried out in terror and were suddenly silenced."

Frink tank has gone dark, no doubt frozen in carbonite by the evil minions of taste and respectability.

The blogosphere has lost its most sophisticated, precise voice.

I will try to lower the level of discourse here at the lounge, as a sign of respect. It is the least I can do.

Gender representation of my blogroll

My blogroll got mangled when I upgraded to the new format, so I’ve been putting off fixing it. One method of procrastination was to break down the links that haven’t disappeared, in order to look for inadvertent bias. It appears that I have 11 men, 10 women, and one robot linked. Interestingly, the two “et al.” blogs on there are written by either all women (inkycircus) or all men (realclimate). I have no idea what the gender breakdown of the blogosphere is, or whether it is desirable or even sensible to have a target. I thought I might just float this little observation out there without any means to evaluate it.