Episode 386 Dynamic Topography

What is dynamic topography? Well, it depends on who you ask. Dynamic topography is similar to other terms, like uplift, that have been used in so many different ways that you really have to look at the document you’re reading to understand what the author is talking about. This term has been applied to places around the world, like the Colorado Plateau in the United States, South Africa, the Aegean, and East Asia, which makes it even more complicated to tease out its meaning.

Most broadly, dynamic topography refers to a change in the elevation of the surface of the earth in response to something going on in the mantle. This “something” can include both the flow of the mantle, as well as differences in mantle temperature or density. For the purposes of this podcast, I will use a more strict definition: Dynamic topography is the change in the elevation of the surface of the earth in response to the upward or downward flow of the mantle.

How much higher or lower can dynamic topography make the earth’s surface? Well, that’s a matter of debate. Earlier studies have suggested that several kilometers, or over 6000 feet of modern elevations can be explained by things going on in the mantle. More recent work instead suggests that dynamic topography creates changes of at most a three hundred meters, or a thousand feet.

A good example of a place where this process is thought to be active is Yellowstone. As Dick Gibson discussed in the December 19th, 2014 episode, Yellowstone is thought to be a hot spot. That is, an area of the earth where hot material moves from deep within the mantle to the base of the crust, causing significant volcanism at the surface of the earth. Other well-known hot spots are located in Hawaii, and Iceland.

So how can a hot spot like Yellowstone cause dynamic topography? Well, you’ve probably seen a similar process at play the last time you played in a pool or a lake. Think of the surface of the pool like the surface of the earth. If you start moving your hands up and down under water, the surface of the pool starts to move up and down. If you ever tried to shoot a water gun upwards underwater when you were a kid, you probably remember it pushing up the surface of the water, and being disappointed that it didn’t shoot out at your friend or sibling. As an adult, you could try holding a hose upwards in a pool. Again, it probably won’t shoot out, but will gently push upwards on the surface of the pool.

Dynamic topography concept. © Commonwealth of Australia (Geoscience Australia)
 2017, used under Creative Commons Attribution 4.0 International Licence 

The principle for a hot spot creating dynamic topography is the same. The flow of the mantle pushes upwards, warping the crust and increasing the elevation of the earth’s surface above the hot spot. Near Yellowstone, this results in an area of high elevation which lies next to the Snake River plain.

But Dynamic Topography doesn’t just cause increases in elevation, it can also pull the earth’s surface downward. In North America, dynamic topography is thought to have been in part responsible for the creation of the Cretaceous interior seaway. 

As a reminder, the Cretaceous interior seaway was a shallow sea that covered parts of western North America, in middle to late Cretaceous time, about 100 to 79 million years ago. Its size varied, but at its greatest extent the seaway stretched through Texas and Wyoming in the US, and Alberta and to the Northwest Territories in Canada. It was widest near the US-Canadian border, where it stretched from Montana to western Minnesota.

Low elevations in western North America that allowed the ocean to flood in and form this shallow sea may have been caused by downwards flow in the mantle. This downwards flow was likely caused by oceanic crust that was subducted at the western margin of North America. That is, oceanic crust that went underneath the North American plate and into the mantle. Because this crust was part of the Farallon oceanic plate, it is often referred to as the Farallon slab.

As oceanic crust associated with the Farallon plate continued to sink into the mantle, it continued to cause changes in the elevation of North America. This drop in elevation likely decreased in size as the Farallon slab moved towards the eastern edge of North America, and deeper into the mantle.

Since Eocene time, or about 55 million years ago, dynamic topography associated with the Farallon slab is thought to have been in part responsible for lower elevations in the eastern United States. A wave cut escarpment called the Orangeburg Scarp is now located 50 to 100 miles inland from the coasts of Virginia, Georgia and the Carolinas. It formed at sea level and now lies up to 50 meters, or about 165 feet above the modern coast line. In fact, a good part of the southeastern US to the east of this escarpment contains marine sediments, and smooth topography as a record of its time underwater.

Differences in the elevation of the Orangeburg Scarp along its length suggest that rather than just going up and down, the Atlantic coast experienced a broad warping caused by mantle flow. The most recent phase of warping brought this area to modern elevations, as warm material moved into the upper mantle beneath the Atlantic coast. This warm material helped push the crust up to higher elevations, creating the southeastern US as we see it today.

This example also highlights an important part of dynamic topography: If you are already at really high or really low elevations, you might not notice it much. If you are near the coast, it can have a big impact as the sea starts to flood in and out due to changes in the mantle. Provided of course, you’re there for the millions to tens of millions of years it takes for the mantle to flow this way and that. That’s why geologists typically rely on the rock record to provide evidence for processes like dynamic topography.

—Petr Yakovlev

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This episode was recorded at the studios of KBMF-LP 102.5 in beautiful Butte, Montana. KBMF is a local low-power community radio station with twin missions of social justice and education. Listen live at butteamericaradio.org.

Links:

What is Dynamic Topography?

Surface expressions of mantle dynamics

Dynamic topography, eastern US

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