Episode 385 The Magnetic Field Anomaly

Most of my career was in analyzing features of the earth’s gravity and magnetic fields, to infer geologic structures for oil exploration. But that doesn’t mean I really understand the whole earth’s fields – and for some aspects of it, neither do folks much more knowledgeable than I am.

You’ve probably seen the images that show the earth’s magnetic field as like that of a dipole magnet, with north and south poles that don’t coincide exactly with the poles of rotation. That’s fine as a starting point, but in detail, we find that the earth’s field is not smooth and uniform, but it has bumps and changes over time and in space. Today I want to talk about some of the anomalies in magnetic field intensity.

At the earth’s surface, the magnetic field varies from about 23,000 to 65,000 nanoteslas, with a tesla being the standard unit of magnetic field strength. It’s not surprising that a body as complex and heterogeneous as the earth would have variations in its physical properties, but a range of 40,000 out of a maximum of 65,000 might seem to be a wide range.

The highest highs are over Siberia, northern Canada, and the ocean between Antarctica and Australia, while the one big low is over central South America and the South Atlantic Ocean. That South Atlantic Anomaly has gotten some serious study lately.

The weakness of the magnetic field at the South Atlantic Anomaly is enough that increases in radiation – which the magnetic field protects us from – can affect satellites like the Hubble telescope, and the International Space Station has extra shielding just because of the South Atlantic Anomaly. Even at ground level, communications can be disrupted during solar storms.

Jay Shah, a student at Imperial College London, studied rocks on the volcanic island Tristan da Cunha, right in the middle of the anomaly, and found that the magnetic field there has probably been weaker than elsewhere on earth for at least 46,000 to as much as 90 thousand years ago, indicating that the South Atlantic Anomaly is probably a fairly persistent feature of the magnetic field.

One speculation about the nature of the South Atlantic Anomaly had been that it somehow was an expression of an impending reversal of the magnetic field. We know that these inversions happen, and have happened dozens of times in earth’s geologic past, but we know very little about the actual mechanism of a reversal. The finding that the South Atlantic Anomaly is fairly old doesn’t say it’s not related to a reversal, but it maybe reduces the chances. The evidence suggests that reversals probably happen over a fairly short time span, a few thousand years or even fewer, and probably not a time as long as 50,000 years or more.

The earth’s magnetic field is probably generated by electrical currents mostly in the liquid outer core. You can imagine that a fluid, even one as dense and hot and deep as the outer core, would have variations in flow and geometry that would be reflected in the magnetic field generated, and this is almost certainly the case. Models suggest that the South Atlantic Anomaly might be related to some kind of disturbance or variation at the boundary between the outer core and the base of the mantle, but that position is about 2900 kilometers – 1800 miles – beneath the surface. It’s studied mostly by looking at variations in seismic waves, although information about earth’s gravity and magnetic fields also comes from specialized satellites.

This is a field of study that’s very much in flux, with new ideas and models coming out yearly. Stay tuned.

—Richard I. Gibson

 Links:

Interview with Jay Shah

The South Atlantic Anomaly
A really good look at the nature and implications of reversals

Episode 384 Kyanite

Today’s topic is three minerals with the same chemical formula: Kyanite, Andalusite, and Sillimanite.

 

How can three things with the exact same chemical formula, Al₂SiO₅, be different minerals? Many of you probably recall that besides a distinct chemical composition, a mineral has a definite crystalline structure. And these three minerals each have completely different crystallography.

The basic reason for the different crystal structure is that the chemicals aluminum and silicon, arrange themselves differently depending on conditions of pressure and temperature. Kyanite forms at relatively low temperatures over a wide range of pressures while sillimanite crystallizes at relatively high temperatures, generally above 700º C over a similar range of pressures to kyanite. Andalusite develops in a more limited temperature-pressure field, call it medium temperatures but always relatively low pressures.

All that variety happens under metamorphic conditions, when rocks are undergoing lots of changes such as those that happen when continents collide, or when subduction scrunches some parts of the crust against others. So that means these minerals are usually found in metamorphic rocks, and in fact they are called index minerals for the particular conditions that they represent.

Kyanite is probably the most familiar of the three. It’s often a beautiful blue color, making long, lath-like crystals, so it’s popular with collectors. Kyanite also has a nearly unique, and diagnostic property. Whereas most minerals have a particular hardness, kyanite has two. On the Mohs hardness scale, kyanite is 5 in the direction along the length of the crystals, but 7 across them. Together with the color and crystal habit, this makes kyanite pretty easy to identify.

Andalusite and sillimanite are less common. But andalusite also makes interesting crystals, especially when carbon gets included in the growing crystals. That can produce a distinctive elongate four-armed cross, a variety called chiastolite that is sometimes polished to make jewelry. Sillimanite certainly can also make nice crystals, but I guess I’ve led a sheltered life, or maybe I just haven’t mapped enough metamorphic rocks. I’ve never seen a large sillimanite crystal in the wild, just fibrous, wispy, almost feathery coatings in metamorphic rocks like schist and gneiss.

So these things are cool collectible minerals and they help geologists figure out the pressures and temperatures that formed rocks, helping unravel the geologic history of the places where they are found. But they also have economic value.

Kyanite and andalusite especially are mined to make mullite, another aluminum silicate that’s pretty rare in the natural world but pretty common as a synthetic material made from kyanite. Toilet bowls, which you might call porcelain, are more or less mullite. Most of it is made from a clay mineral, but kyanite can be added to improve its toughness and stability. And small amounts of kyanite go to making abrasives in things like automobile brake shoes. But by far the greatest use of kyanite and andalusite is in making mullite for refractories – ceramics that retain their strength and remain chemically inert at very high temperatures. Furnaces, kilns, and crucibles in the iron and steel industries are often constructed with mullite bricks, and steel making consumes something like 70% of all the aluminum silicates produced worldwide.

The United States is the world leader in producing kyanite. It’s mined at four places in Alabama and Georgia, where the metamorphic rocks of the Appalachian Mountains contain abundant reserves. US mine production of kyanite, at about 100,000 metric tons a year, is more than we need, so we export about a third of what we produce – one of only a handful of mineral commodities that the US is self-sufficient in. The total value is around $30 million a year. South Africa produces more andalusite than the US produces kyanite, so it’s the world leader in producing this stuff, and India and Peru are the only other significant commercial producers of aluminosilicates in the world.

Price and production of kyanite is sensitive to the world economy because of variations in the steel industry, but for the past few years the price of kyanite in the US has been fairly steady at around $300 per ton. Kyanite mines in the US employ about 150 workers, and mullite plants account for about 240 more.

Kyanite’s name is from the Greek word kyanos, meaning blue. Think “cyan.” Andalusite was originally described from specimens thought to be from Andalusia, in Spain, but actually from a nearby province. But the name stuck. Benjamin Silliman, a geologist at Yale and founder of the American Journal of Science, gives his name to Sillimanite.

—Richard I. Gibson

Kyanite - USGS mineral commodities
Gigapan image of kyanite

Episode 383 Himalayas, Catskills, and more

For today’s episode of the podcast I’m introducing you to Dr. Petr Yakovlev, a friend and geologist here in Butte at the Montana Bureau of Mines and Geology. Petr will be doing occasional guest episodes to give you all a break from my voice, as well as information about some of the diverse things he's worked on.

Petr Yakovlev with a Cenozoic conglomerate near Cardwell, Montana.
Photo by Dick Gibson

Petr got his undergrad geologic education at Boston College and his PhD at the University of Michigan.

In this episode Petr and I talk about his work in Tibet, which has implications for the fundamental nature of the India-Eurasia collision; another structural geology project he worked on in the Catskills of New York; and the projects he’s working on here in Montana. And he gives us some teasers about the kinds of topics he plans to cover for History of the Earth.

Running time 13 minutes.

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