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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Episode 382 The Riversleigh Lagerstätte

Today’s Episode takes us to Australia, the home of the Riversleigh Lagerstätte. Lagerstätten, you may recall, are fossil assemblages that typically have extraordinary diversity as well as extraordinary preservation. The Riversleigh Lagerstätte fits that definition well, containing fossils of 15- to 25-million-year-old mammals, including an extinct giant platypus, as well as birds, reptiles, frogs, and lungfish. But the story begins about 500 million years ago, not 25 million years ago.

In Early to Middle Cambrian time, more than half a billion years ago, the supercontinent of Gondwana was pretty much assembled, with South America, Africa, India, Antarctica, and Australia comprising a wide, long continent that stretched from the South Pole to north of the Equator. The prong of the continent including Australia was the part that was in the tropics, and much of what is now northern Australia was covered by a warm, shallow sea.

In what is now northwestern Queensland, where the Riversleigh Lagerstätte is found, the shallow sea transgressed and regressed – meaning it came and went – and among other things, sediments included grainy shallow-water limestones. The resulting Thorntonia Limestone contains fossils that I’m sure are interesting and informative, including trilobites, brachiopods, and stromatolites, pretty typical of Cambrian rocks. But that’s not the lagerstätte.

Nimbadon, a koala-like tree-dweller. From Wikipedia.

Fast forward about 475 million years, to the Oligocene epoch of the Paleogene Period of the Cenozoic Era. The rocks of northern Queensland were dissolving in a warm, wet, tropical environment, and the limestones of the Thorntonia formation were developing karst topography – caves and surficial pools rich in dissolved calcium carbonate. Starting about 25 million years ago, near the end of the Oligocene, animals began to die in those pools and become trapped in the caves, where ongoing calcium carbonate deposition preserved them remarkably well. So even though the origin of the calcium carbonate was a really ancient rock, the Cambrian Thorntonia formation, it was the much more recent dissolution of those rocks that provided the material that preserved the fossils at Riversleigh. Ages of the fossils extend from late Oligocene time into Miocene, around 15 million years ago, but there are younger fossils as well since the caves are still there and deposition is continuing.  

The Riversleigh site is not just one location, but many, spread out over a hundred-square-kilometer area that was named a World Heritage Site in 1994.

Riversleigh contains the richest assemblage of bat fossils in the world – at least 35 different species. Besides bats, mammal fossils include extinct koalas, marsupial lions, wombats, herbivores the size of sheep, at least 14 species of opossum, and 15 different kangaroo species. While many fossils are remarkably well preserved, some, such as Yalkaparidon, are only known from a few teeth and scattered bones, making their relationships to other families uncertain. In fact, some are grouped colloquially as Thingodonta, meaning “toothed thing”. Some researchers have suggested Yalkaparidon was a mammalian woodpecker.

Bird fossils at Riversleigh range from extinct flightless rails to storks and lyrebirds. Reptiles are represented by tree-dwelling crocodiles, horned turtles, and dragon lizards, which were probably related to iguanas. There are even snakes and frogs and two species of lungfish, all of which are really rare in the fossil record, especially as long ago as the Riversleigh assemblage, 15 to 25 million years.

Some lagerstätten record essentially an instant in geologic time, or a relatively short period. An example of such an assemblage is the fish of the Green River Formation in Wyoming, many of which may have been killed in a single event – a heavy fall of ash into the lake where they lived, from an erupting volcano. So Riversleigh, in addition to its diversity and excellent preservation is also special because it spans such a long period of time – more than 10 million years – offering a remarkable insight into the evolution of the unique fauna of Australia.

—Richard I. Gibson

Links:

Cambrian fossils in the Thorntonia

A good overview

Lists of species

Episode 381 Zealandia

Today for episode 381, we’re going to Zealandia. No, it’s not a quirky TV show modeled after Portlandia. It’s the 7^th largest continent on earth.

We’re not talking continents in the geographic sense, really large land masses like Africa and South America, but we are talking about continental material in the geologic sense, even though most of it is submerged beneath the ocean.

Public Domain via Wikipedia

Zealandia is centered on New Zealand, and in some ways, it’s been known for decades, ever since good charts of subsurface bathymetry were created. Those data show a complicated mix of rises and troughs on the sea floor around New Zealand, including the Lord Howe Rise in the Tasman Sea between New Zealand and Australia, the Campbell Plateau east of New Zealand, and other highs and lows.

Although the ocean floor around the globe is diverse, with seamounts and trenches, fault zones and piles of volcanic rocks here and there, on the whole oceanic crust is deep and uniform, or at least varying broadly and predictably. These relatively small-scale high plateaus on the ocean floor around New Zealand are unusual.

Let’s define a continent. Geographically, it usually means a large land mass. “Large” is subjective, but Australia, at about 9 million square kilometers, is considered to be the smallest continent. And the count depends on whether you separate Europe from Asia, or just say Eurasia as one, and other conventions. Many would say that a degree of isolation and separation also defines a continent, so Eurasia should be one, together with Africa, North America, South America, Antarctica, and Australia, for a total of six. Forget all that. We’re looking at places made of continental crust, the generally lighter, silica-rich material in contrast to oceanic crust, denser and more iron-rich.

By that geologic definition, all the conventional continents really are mostly continental crust too. All well and good. There are numerous smaller, separated areas of continental crust that are usually called microcontinents. Madagascar is probably the best example of a microcontinent, but there are lots of them, and you can get into plenty of arguments about whether a piece like, say, Greenland, is or is not fully a part of the North American continent.

So, let’s go to Zealandia. In 2017, Nick Mortimer and colleagues, writing in GSA Today, pretty much laid any argument to rest. There’s abundant evidence to say that there is a large, diverse terrain centered on New Zealand but 94% submerged, that is mostly continental crust. It adds up to about 5 million square kilometers, comparable in size to Arabia and India, sub-continents that are connected in one way or another with full-fledged continents. Zealandia is close to, but clearly separated from Australia, and Mortimer and others make a convincing case that it should be considered the seventh continent, six times larger than Madagascar, the biggest microcontinent.

So if continental rocks are lighter, which accounts for them being for the most part above sea level, why is most of Zealandia under water? The crust there is thinner, so it doesn’t rise as high as that on most continents, and it has been stretched and broken, partly rifted apart during a long complex history.

Zealandia was probably originally part of the supercontinent of Gondwana, attached to eastern Australia and West Antarctica, when those two continents were fully attached to each other about 105 million years ago, in Early Cretaceous time. That margin of Gondwana, thousands of kilometers long, was a subduction zone somewhat like today’s Andes, with an old Pacific oceanic plate diving beneath the Gondwana continental crust. There would have been a volcanic mountain range there, recorded in Zealandia as a surviving string of granitic rocks, batholiths similar to those that developed in western North and South America during Mesozoic and Cenozoic time and continuing to this day.

Gondwana began to be rifted apart by about 85 million years ago, Late Cretaceous time. It appears that one of the first parts to go from this part of the supercontinent was what is now Zealandia, a long narrow ribbon along the coast of the continent. Australia and Antarctica probably began to separate at about the same time, but that was initially a rather slow break-up that wasn’t complete for close to fifty million years.

The idea of a long narrow ribbon of continental crust rifting off a continent has plenty of precedents. During the Ordovician, the Avalonian ribbon rifted away from what is now northwest Africa and ultimately collided with North America, where it forms parts of Newfoundland, Nova Scotia, and New England today. More recently, a string of continental fragments rifted off what is now eastern Arabia and Africa, to collide with Eurasia to form the cores of Turkey, Iran and Afghanistan today. It’s also possible that one or more “ribbon” continents broke off what is now northern India, to begin to amalgamate with Eurasia in today’s Himalayas, even before India itself collided.

The rifting that split Zealandia off Gondwana is recorded in sedimentary rocks. The initial rifting may have begun the crustal thinning and breaking of Zealandia that we see today, but some of that may have developed later. Because Zealandia was (and is) in a zone of complex interactions among small and large continental and oceanic plates, it’s not really surprising that it would have undergone quite a bit of tectonic and crustal modification. Today, the main part of Zealandia that’s above sea level, New Zealand, contains the Alpine Fault, a huge strike-slip fault similar to the San Andreas. The Alpine Fault is ripping Zealandia apart, but even that helps us recognize Zealandia as a small continent. Mortimer and his colleagues identified rock types in the sea floor on both sides of the Alpine Fault – rock types that can be correlated with each other, and that reflect the predicted offsets you’d expect from the Alpine Fault.

I think Mortimer and his colleagues have made the case quite convincingly that we should indeed think of Zealandia as a continent, geologically speaking, even though most of it is under water. 

Mortimer et al. paper 

Book, Zealandia:Our Continent Revealed, published in 2014. 

—Richard I. Gibson