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History of the Earth Podcast by Richard I. Gibson

History of the Earth Podcast

by Richard I. Gibson

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366 snapshots of earth history in the form of a perpetual calendar, with daily episodes for 2014 and weekly thereafter. Find all the posts at http://historyoftheearthcalendar.blogspot.com.


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Episode 375: Garnets

rigibson@earthlink.net (Richard I. Gibson)Author: Richard I. Gibson
Fri, Jan 08, 2016







Garnet is the birthstone for January, but there really isn’t a single mineral named ‘garnet.’ It’s a group of at least 15 minerals of differing chemistry, but only six are common. They are three aluminosilicates combined with iron, magnesium, or manganese, and three calcium silicates in combination with either iron, aluminum, or chromium. The definition of a mineral is something with a specific chemistry and a definite crystalline structure.

All six garnet minerals have essentially identical crystal structure. They are in the isometric or cubic crystal system, meaning their three crystal axes are perpendicular and the same length – that’s what isometric means, same length. But garnets seldom form cubes. The molecules are of sizes that result in many-sided forms, including dodecahedrons, with 12 faces, and trapezohedrons with 24 faces, and more complicated shapes, all of which tend to make garnets almost spherical. Often, they look like little soccer balls in the rock.

The word garnet is from an early English word that meant dark red, as many garnets are, but it may ultimately be derived from the words pomum granatum, Latin for pomegranate – because pomegranates have bright red seed covers similar in appearance to common garnets.

And they’re pretty. Red, purple, yellow, brown, orange, bright green for uvarovite, the chromium garnet – and they’re hard, usually around 7 or higher on the Mohs hardness scale. This makes them great for two things: gemstones and abrasives, and they’ve been used for both since the Bronze age.

By far the greatest use of garnets, by volume, is as abrasives. The US uses about 190,000 tons of garnets every year, with about 17% of that mined in New York’s Adirondack Mountains and in Idaho. The rest, 83% in 2014, was imported mostly from Australia and India and a bit from China. India produces about half the world’s industrial garnet, with China second with a third and Australia 15%. The US is a distant fourth with only 2% of world production.

You benefit from garnets every day, even if not directly in the rough surface of an emery board or sandpaper. Waterjet cutting is used to cut and shape metals from steel to aluminum as well as plastics and glass. Besides their use as abrasives, about a fifth of garnet consumption in the United States goes to filtration in water wells.

The value of industrial garnet in the US is about $9 million. Garnets are used for gemstones, but because they are so common, their value is nothing like that of diamonds, sapphires, rubies, and others, most of which are also harder than garnet and much rarer. But garnets can still make beautiful jewelry.

Do I have any garnets in my collection? Yup. Hundreds of thousands of them, but most of them are microscopic, in a rock with a blue mineral called glaucophane from the California Coast Ranges. The biggest one I have is maybe an inch or so across, from Ontario, but at Gore Mountain, New York, there are garnets as much as two feet across in what is possibly the largest garnet deposit in the world. They are in metamorphic rocks related to a continental collision about a billion years ago, called the Grenville Orogeny.

So happy birthday to everyone with a January birthday. Have a garnetiferous day!

—Richard I. Gibson



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The Cenozoic Episodes

rigibson@earthlink.net (Richard I. Gibson)Author: Richard I. Gibson
Sat, Nov 28, 2015




Running time 3 hours

We are up to the Cenozoic Era in the monthly episodes. This one combines the 31 episodes from December 2014, covering the Cenozoic, into one episode, and it completes the packaging of each month of episodes from the original series.

As usual for this monthly compilation, I’ve left the references to specific dates in the podcast so that you can, if you want, go to the specific blog post that has links and illustrations for that episode. They are all indexed on the right-hand side of the blog.

Thanks for your interest in this project. If you have questions or comments, please let me know, either here on the blog – there’s a page for Questions– or contact me by email at rigibson at earthlink.net. I’ll try to respond. You can of course also leave a review on iTunes. I really do appreciate your feedback.

For 2016, I’m going to TRY to have more frequent new programs than I did in 2015. Some upcoming topics, many suggested by you listeners, include fluorescence in minerals, questions about the oxygen crisis back in the Precambrian, the Tepuis in South America, and more. Thank you!

—Richard I. Gibson


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The Cretaceous Episodes

rigibson@earthlink.net (Richard I. Gibson)Author: Richard I. Gibson
Tue, Nov 24, 2015




Running time 2 hours 30 minutes

We are up to the Cretaceous Period of the Mesozoic Era in the monthly episodes. This one combines the 30 episodes from November 2014, covering the Cretaceous, into one episode.

As usual for this monthly compilation, I’ve left the references to specific dates in the podcast so that you can, if you want, go to the specific blog post that has links and illustrations for that episode. They are all indexed on the right-hand side of the blog.

If you have questions or comments, please let me know, either here on the blog – there’s a page for Questions – or contact me by email at rigibson@earthlink.net. I’ll try to respond. You can of course also leave a review on iTunes. I really do appreciate your feedback.

—Richard I. Gibson


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Episode 374. Twins

rigibson@earthlink.net (Richard I. Gibson)Author: Richard I. Gibson
Tue, Nov 24, 2015







This week, our topic is twins. Twinned crystals.

When two, or sometimes more, crystals of the same mineral share volumes or surfaces within the crystal lattice, they grow together in interesting ways. One common kind of twin is essentially a reflection across a plane, a plane that is shared by both crystals. They can end up growing perpendicular to each other, or at a specific angle in what are called contact twins. One common type of contact twin is in the common mineral quartz, silicon dioxide, when two crystals grow at almost right angles to each other, sharing a common plane. This kind of twin is called a Japan Law twin because they are found frequently in quartz crystals from Japan, although they were first described from localities in the French Alps.

Sometimes multiple crystals grow together. Aragonite, calcium carbonate, crystallizes in the orthorhombic crystal system, which means it has three unequal but perpendicular crystal axes that form the basis of the molecular crystal lattice. Think of a shoe box, with three different edge lengths all at right angles to each other, that’s an orthorhombic crystal lattice. But in aragonite, sometimes particular planes within the crystals serve as common surfaces for crystallographically distinct crystals, and they grow together to form near-perfect hexagons. This can be confusing when you’re trying to identify a mineral, like aragonite, which you know is not hexagonal but there you have these nice hexagons. They’re multiple twins, actually composed of three separate crystals of aragonite grown together in a pseudohexagonal form.

The other primary type of twin is called a penetration twin, where two distinct crystals share not just a plane, but an entire volume within the combined twin crystal. They end up looking like the two crystals penetrate each other. Probably the most famous example of a penetration twin is the mineral staurolite, an iron-aluminum silicate. It’s also orthorhombic and usually forms little box-like crystals, typically like the shoe box but with one dimension often a lot thinner than the thinnest dimension of a shoe box. If two crystals share the center of the box, you end up with twins that look for all the world as if one crystal of staurolite has penetrated through the other. You can get two different angles in this sharing that make forms that look like crosses. The two crystals can cross at a 60-degree angle or at 90 degrees, making what are called cruciform – cross-shaped – twins. Such twins in staurolite are relatively common.

Fluorite, calcium fluoride, is another mineral that often forms beautiful penetration twins. Fluorite is isometric, meaning it has three perpendicular but equal crystal axes. In terms of fluorite crystals, this means that instead of that shoe-box shape, you often get perfect cubes where all the edges have the same length. If two cubes grow together and share a volume of molecules, you get twins that can be pretty cool.

There are quite a few different twin laws, and they really are “laws” to the extent that these kinds of intergrowth can only follow specific patterns, all controlled by the geometry of the crystal and the size and arrangement of the molecules in it.

Thanks for your interest!
—Richard I. Gibson 


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The Jurassic Episodes

rigibson@earthlink.net (Richard I. Gibson)Author: Richard I. Gibson
Mon, Sep 28, 2015







Running time 2 hours 5 minutes

We are up to the Jurassic Period of the Mesozoic Era in the monthly episodes. This one combines the 31 episodes from October 2014, covering the Jurassic, into one episode.

As usual, I’ve left the references to specific dates in the podcast so that you can, if you want, go to the specific blog post that has links and illustrations for that episode. They are all indexed on the right-hand side of the blog.

If you have questions or comments, please let me know, either here on the blog – there’s a page for Questions– or contact me by email at rigibson at earthlink.net. I’ll try to respond. You can of course also leave a review on iTunes. I really do appreciate your feedback.

—Richard I. Gibson



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The Triassic Episodes

rigibson@earthlink.net (Richard I. Gibson)Author: Richard I. Gibson
Sat, Aug 22, 2015




Running time 1 hour 45 minutes

Shonisaurus from the Triassic of Nevada. Maximum length, 49 feet. Drawing by Nobu Tamura http://spinops.blogspot.com used under Creative Commons license.
We are up to the Triassic Period of the Mesozoic Era in the monthly episodes. This one combines the 30 episodes from September 2014, covering the Triassic, into one episode.

As usual, I’ve left the references to specific dates in the podcast so that you can, if you want, go to the specific blog post that has links and illustrations for that episode. They are all indexed on the right-hand side of the blog.

If you have questions or comments, please let me know, either here on the blog – there’s a page for Questions– or contact me by email at rigibson at earthlink.net. I’ll try to respond. You can of course also leave a review on iTunes. I really do appreciate your feedback.

—Richard I. Gibson


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Episode 373. A walk to Branham Lakes

rigibson@earthlink.net (Richard I. Gibson)Author: Richard I. Gibson
Sat, Aug 15, 2015




Upper Branham Lake
Today’s episode will be a little different from what you are used to. I’m going to try to give some of the story of the Precambrian here in southwestern Montana, but I’ll do it in the context of a little hike I did yesterday to the Branham Lakes in the Tobacco Root Mountains. So there will be some of the usual narration, but also some snippets that I recorded while I was on the walk, which are not included in the script below. You can expect some huffing and puffing. See also this blog post by Pat Munday.

probably hypersthene (Mg Fe silicate)
When I was learning the geology of this region back in 1969, the Precambrian rocks of the Tobacco Root Mountains were considered to be Archean, older than 2.5 billion years. They were (and are) the northwestern-most corner of the Wyoming Craton, one of the ancient, fundamental building blocks of North America that we talked about last year. And the Wyoming Craton is definitely Archean in age. At least most of it is.

More recent analyses of age dates in southwestern Montana gave rise to another interpretation, by Tekla Harms and her colleagues a few years ago, that the zone through the Tobacco Roots, Highland Mountains south of Butte, and Ruby Range east of Dillon, Montana, represents the old margin of the craton, where a pile of sedimentary rocks formed – possibly during Archean time, but if it was then, it wasn’t long before the 2.5-billion-year cutoff date for the Archean. The sediments might have been early Proterozoic, called Paleoproterozoic. In any case, Harms and colleagues interpret age dates in some of these rocks at about 1.75 to 1.9 billion years to represent the collision between the northwestern corner of the Wyoming Province and another terrane, now mostly in the subsurface of central Montana. There isn’t much doubt that such a collision happened, but there remain questions as to whether the Precambrian metamorphic rocks of southwestern Montana were already there, Archean, or if they were sedimentary rocks that got caught up in that collision and metamorphosed a few hundred million years after they were deposited.

Geologic Map of part of the Tobacco Root Mountains. Reds and oranges are igneous rocks of the Tobacco Root Batholith, about 75 million years old. Grays are Precambrian rocks, about 1700 to 2500 million years old. Both maps from Vuke et al., 2014, Geologic Map of the Bozeman quad, Montana Bureau of Mines and Geology Open-file map 648. Black box in lower left corner is enlarged below. 
Oranges (Khto) are Tobacco Root Batholith, grays are Precambrian. X=Paleoproterozoic, about 1.7 to 1.9 billion years old; A = Archean, over 2.5 billion. XA means we aren't really sure. qfg = quartzofeldspathic gneiss, ah = amphibolite and hornblende gneiss. Xsp = Spuhler Peak formation. Branham lakes are blue. 
There isn’t much doubt that the metamorphic rocks there were originally mostly sedimentary rocks, sandstones, shales, siltstones, maybe even a few limestones, and that they were intruded by some igneous rocks like basalt, all before they were metamorphosed. We can infer what these protoliths, the original rocks, were, from the chemistry and mineralogy of the rocks today. So it’s a question that doesn’t matter too much, although it has big implications for the detailed story of this part of the world – when were sediments laid down, when were they metamorphosed. That in turn has implications for the structural and tectonic history, and understanding THAT helps us explore for mineral resources and understand things like earthquake fault distributions.

I’m not going to solve the question by walking up to the Branham Lakes. This beautiful location is about 9 miles or so up Mill Creek, east from Sheridan, Montana.

Most of the major valleys on the flanks of the mountains of southwest Montana held glaciers during the most recent glacial period that ended about 12,000 years ago or so.


Kyanite, Aluminum Silicate
Sediments like silts and muds usually contain plenty of silica, fine-grained quartz, but often they have fragments of feldspars or the clays that weather from feldspar, and those minerals contain a lot of aluminum. Under metamorphic conditions, high temperatures and pressures, the aluminum and other chemicals reorganize into minerals that are stable at those temperatures and pressures. There are three minerals, kyanite, andalusite, and sillimanite, which are chemically identical aluminum silicates, Al2SiO5, but which have different crystallography that reflects the details of the pressure-temperature regime in which the aluminum and silica were mobilized. Kyanite is stable at relatively low temperatures, 200 to 700°C, and low to high pressures; Andalusite forms at low pressures and medium temperatures, and Sillimanite forms at high temperatures across a range of pressures. The boundaries between these phases are well known so we can use their occurrence to infer the temperatures and pressures that the rocks reached during metamorphism.

Tobacco Root Batholith granite with dark xenolith of older rock
The Archean and early Proterozoic metamorphic events, about 2.5 billion to maybe 1.7 billion years ago, were ancient when the next potential metamorphic event took place, about 76 million years ago.

In the next clip, I made a mistake – I say epidote when I meant to say enstatite. They both start with an E, that’s my excuse! Enstatite is magnesium silicate, and hypersthene, also mentioned in the next clip, is enstatite with iron in it. Both are the kinds of minerals you can get by metamorphosing rocks that have a lot of iron and magnesium, probably NOT simple sediments like shale.

The road to the Branham Lakes, about 9 miles from Sheridan, Montana, is pretty good, and you could probably make it almost all the way in a 2-wheel-drive vehicle if you have decent clearance. I chose to leave my Prius about 2? miles from the lakes just to be safe, as there are a few dicey stretches, and because it was such a fine day I really preferred to walk. If you go, it would be an unusual year that you’d find the road and lakes snow-free before late June at the earliest, but the setting is spectacular in July and August. I have a few photos from my walk on the blog, history of the earth calendar dot blogspot dot com.

I hope you have enjoyed this little ramble from the Precambrian to the Cretaceous to the glacial period of the Pleistocene. Thanks for listening!

Lower Branham Lake

—Richard I. Gibson

More photos on Facebook


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Episode 372 Satellite-derived gravity

rigibson@earthlink.net (Richard I. Gibson)Author: Richard I. Gibson
Wed, Jul 29, 2015





Welcome to the History of the Earth, which has now evolved into a general podcast covering all things geological. I’m your host, geologist Dick Gibson.

Today I’m going to talk a bit about one of my specialties, interpretation of gravity data. Specifically, gravity data derived from a satellite. Measurements of the earth’s gravity field are essentially measurements of the attraction of the earth on a spring – the more the spring extends, the stronger the pull of gravity, and the stronger pull of gravity occurs where denser materials are present beneath that spring. We can actually measure those attractions with such precision that we can identify areas where there are varying distributions of rocks of different density – or more correctly, we can identify locations of density contrast, where there is a change from one density to another. A classic example is a salt dome. Salt, the mineral halite, has a density of around 2.15 grams per cubic centimeter, while common rocks like shale and sandstone have densities of anywhere from 2.4 to 2.6 grams per cubic centimeter, within an even larger range. So when a low-density, buoyant salt dome rises up through shales and sandstones, it creates a pretty significant density contrast, and a salt dome often produces a strikingly intense, circular gravity low, representing the low-density salt versus the surrounding denser rocks.

Satellite gravity map of western India, from Technical University of Delft.
The gravity low discussed in the podcast is circled.
Since the 1920s we’ve had gravity meters that can measure the earth’s gravity field, and maps of the distribution of gravity data have guided oil exploration as well as our understanding of regional geology and tectonics ever since. Most of those gravity data were acquired by people driving or hiking across country, sitting a gravity meter down, and making a measurement. Time intensive and expensive. Eventually we developed technologies to allow the gravity field to be measured from a moving aircraft or from a moving boat – such measurements are lower quality, but they’re a lot cheaper.

In the middle 1990s incredibly accurate radar altimeters were developed and deployed on satellites. A radar altimeter is basically a range-finder, an extremely accurate tool for measuring distance. The radar signal goes out and bounces back, and the time it takes for the trip is proportional to the distance the radar beam traveled. So you can visualize a sensitive radar altimeter on a satellite as something that can give incredibly accurate measurements of the height of the land – topography. The satellite-borne radar altimeters had centimeter-scale accuracy.

But it can do more. Over the oceans, the radar altimeter measures the distance from the satellite to the surface of the ocean. That’s cool, but so what? Ocean surfaces are really very irregular, with waves, currents, and so on to make any measurements at the level of centimeters irrelevant, right? Right. But if you make the measurements dozens, hundreds, thousands of times, you effectively average out things like waves and currents. You get an average measurement of the height of the surface of the ocean. OK, really it’s the distance from the satellite – whose elevation is precisely known – to the ocean surface, but it’s OK to think of that as the sea’s height.

Again, so what? Well, the average height of the sea surface on a perfectly uniform sphere, the earth, would be a uniform surface, and it actually has a name, the geoid. But the earth is anything but uniform. And in fact we can use the satellite radar altimeter measurements to make maps that are essentially representations of the attraction of gravity – and the accuracy is high enough that we can actually see geological features.

Imagine the sharp slope on the sub-sea edge of a continent – the position where the water gets abruptly deeper. This happens around all the continents. The dramatic contrast in density between water and any rock, any rock at all, it huge, from 1.0 to more than 2 grams per cubic centimeter, so the under-water topography, called bathymetry, is by far the strongest component of the gravity maps that are made from satellite radar measurements. But where the water bottom is relatively flat, the variations in the radar measurements, which translate to gravity values, really do represent geology.

Yes, these data are lower quality than the gravity data that come from gravity meters sitting out there on the land surface. But they are essentially free – all this comes from data acquired from satellites paid for by tax money, so they are in the public domain. And they are often much better data sets than anything that might have been acquired from a ship or aircraft, just because there are not many such data sets.

These satellite-derived gravity maps are really useful for strategic planning for oil companies. Most of the consulting work I did from 1997 to 2002 was interpretation of such data. I did projects that covered all the coastal areas of Africa and India, the east coast of South America, the Maritime Provinces of Canada, the Gulf of Mexico, the South China Sea, and all the waters offshore Indonesia. The information is pretty amazing, if you can figure out how to read the data, and that was my job.

Some of it is pretty unsurprising. For example, off the coast of Bangladesh, in the Bay of Bengal arm of the Indian Ocean, there’s a really wide, flat continental shelf. The Ganges River has carved a submarine channel across that submarine shelf, and no surprise – the channel, which in terms of density is a narrow canyon of water surrounded by rock and sediment that are much denser, shows up in the satellite gravity map as a long, sinuous gravity low. Offshore southwest Africa, there are features in the gravity map that represent huge igneous intrusives that might be wonderful places to explore for minerals, if they were not 150 kilometers offshore and under 200 meters of water. But figuring out where they are located can guide our understanding of the structural geology and tectonics that may help with exploration onshore.

On the west coast of India there is a nearly circular gravity low. You should think of that as low-density rock, but compared to what? It turns out it is really a block of granitic crust, rifted away from the Indian Subcontinent, but it shows up as a low because even at a density of about 2.7 grams per cc, it’s really low compared to the oceanic crust, basalt at maybe 3.3 grams per cc, that surrounds it. This block is actually a high-standing bit of continent that tried to rift away from India – but never really managed to separate. And guess what – today, the largest oil field in India, the Bombay Field, sits right on top of that gravity low. So knowing that, we can look for similar, perhaps less obvious places where there might be additional oil or gas fields. That was the nature of the work I did in the late 1990s for oil companies interested in evaluating the strategic potential of offshore India. It wasn’t really a question of “where do we drill,” – it was more a question of “should we be interested in this region or not.”

Because the satellite data are essentially free, and a geological interpreter like me is relatively cheap in oil company terms, there was a lot of analysis of these data sets back about 15 years ago. The data are still useful and oil companies routinely use them as they plan their exploration programs.

Thanks for listening. I appreciate your interest and support.

—Richard I. Gibson


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The Permian Episodes

rigibson@earthlink.net (Richard I. Gibson)Author: Richard I. Gibson
Fri, Jul 24, 2015


Running time 2 hours




We are up to the Permian Period of the Paleozoic Era in the monthly episodes. This one combines the 31 episodes from August 2014, covering the Permian, into one two-hour episode.

As usual, I’ve left the references to specific dates in the podcast so that you can, if you want, go to the specific blog post that has links and illustrations for that episode. They are all indexed on the right-hand side of the blog.

Thanks for your interest in this project. As the year continues, I really do hope to have occasional new episodes in addition to these monthly assemblies. I know I haven’t accomplished that as much as I wanted to – sorry, but I plead a complicated year.

If you have questions or comments, please let me know, either here on the blog – there’s a page for Questions– or contact me by email at rigibson at earthlink.net. I’ll try to respond. You can of course also leave a review on iTunes. I really do appreciate your feedback.

—Richard I. Gibson


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The Pennsylvanian Episodes

rigibson@earthlink.net (Richard I. Gibson)Author: Richard I. Gibson
Sun, Jul 12, 2015


Running time 1 hour 45 minutes



This episode is the seventh monthly package that combines the daily episodes for July 2014, covering the Pennsylvanian Period of the Paleozoic Era. As with the Mississippian, I apologize for not getting this assemblage done on time – my summer has been pretty complicated.

As usual, I’ve left the references to specific dates in the podcast so that you can, if you want, go to the specific blog post that has links and illustrations for that episode. They are all indexed on the right-hand side of the blog.

As the year continues, there will be occasional new episodes in addition to these monthly assemblies.

If you have questions or comments, please let me know, either here on the blog – there’s a page for Questions– or contact me by email at rigibson at earthlink.net. I’ll try to respond. You can of course also leave a review on iTunes. I really do appreciate your feedback.
—Richard I. Gibson


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The Mississippian Episodes

rigibson@earthlink.net (Richard I. Gibson)Author: Richard I. Gibson
Tue, Jul 07, 2015







Running time 1 hour 30 minutes

This episode is the sixth monthly package that combines the daily episodes for June 2014, covering the Mississippian Period of the Paleozoic Era. I apologize for not getting this assemblage done on time – my summer has been pretty complicated.

As usual, I’ve left the references to specific dates in the podcast so that you can, if you want, go to the specific blog post that has links and illustrations for that episode. They are all indexed on the right-hand side of the blog.

Thanks for your interest in this project. As the year continues, there will be occasional new episodes in addition to these monthly assemblies.

If you have questions or comments, please let me know, either here on the blog – there’s a page for Questions– or contact me by email at rigibson at earthlink.net. I’ll try to respond. You can of course also leave a review on iTunes. I really do appreciate your feedback.

—Richard I. Gibson



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Episode 371: Listener Questions

rigibson@earthlink.net (Richard I. Gibson)Author: Richard I. Gibson
Fri, May 08, 2015




Today for Episode number 371 I’m going to try to address some of the questions I've received on the blog or through the podcast. A fair number of these were addressed in comments here on the blog, and I usually tried to answer them here, but listeners may not have seen or been aware of them.

Thanks for listening!
—Richard I. Gibson


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The Devonian Episodes

rigibson@earthlink.net (Richard I. Gibson)Author: Richard I. Gibson
Thu, Apr 30, 2015




Running time 2 hours 20 minutes

This episode is the fifth monthly package that combines the daily episodes for May 2014, covering the Devonian Period of the Paleozoic Era.

As usual, I’ve left the references to specific dates in the podcast so that you can, if you want, go to the specific blog post that has links and illustrations for that episode. They are all indexed on the right-hand side of the blog.

If you have questions or comments, please let me know, either here on the blog or contact me by email at rigibson at earthlink.net. I’ll try to respond. You can of course also leave a review on iTunes. I really do appreciate your feedback.

—Richard I. Gibson


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The Silurian Episodes

rigibson@earthlink.net (Richard I. Gibson)Author: Richard I. Gibson
Wed, Apr 01, 2015






Running time 2 hours

This episode is the fourth monthly package in the History of the Earth Calendar. It combines the daily episodes for April 2014, covering the Silurian Period of the Paleozoic Era.

As usual, I've left the references to specific dates in the podcast so that you can, if you want, go to the specific blog post that has links and illustrations for that episode. They are all indexed on the right-hand side of the blog.

As the year continues, there will be occasional new episodes in addition to these monthly assemblies.

If you have questions or comments, please let me know, either here on the blog – see the tab page for Questions, above – or contact me by email at rigibson at earthlink.net. I’ll try to respond. You can of course also leave a review on iTunes. I really do appreciate your feedback. Thanks for your interest and support.
—Richard I. Gibson



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Episode 370: Pseudomorphs

rigibson@earthlink.net (Richard I. Gibson)Author: Richard I. Gibson
Mon, Mar 23, 2015




Malachite pseudomorph after cuprite showing octagon and dodecahedron faces.
From Chessy, France. Specimen is just over 1 cm in maximum dimension.
Photo by Richard I. Gibson.
Today for Mineral Monday my special guest is Kyle Eastman, mineral collector and geologist, here to discuss with me some special minerals called pseudomorphs. These are “false form” minerals that have the shape of one mineral but are actually something else because of replacement, substitution, encrustation, or some other process.

We discuss some classic pseudos from Chessy, France and Corocoro, Bolivia, as well as one of Kyle’s favorite skarns from Arizona.

Running time, 19 minutes.
—Richard I. Gibson



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The Mineral Episodes

rigibson@earthlink.net (Richard I. Gibson)Author: Richard I. Gibson
Thu, Mar 12, 2015





This episode is a package containing all the previous episodes of the podcast related to minerals and mineral deposits. There are 16 of them, and the running time is about an hour and thirty minutes. Thanks very much for your interest.

—Richard I. Gibson


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The Ordovician Episodes

rigibson@earthlink.net (Richard I. Gibson)Author: Richard I. Gibson
Wed, Feb 25, 2015






Running time 2 hours 15 minutes

This episode is the third monthly package of the History of the Earth calendar. It combines the daily episodes for March 2014, covering the Ordovician Period of the Paleozoic Era.

You’ll find that I’ve left the references to specific dates in the podcast so that you can, if you want, go to the specific blog post that has links and illustrations for that episode. They are all indexed on the right-hand side of the blog.

As the year continues, there will be occasional new episodes in addition to these monthly assemblies.
If you have questions or comments, please let me know, either here on the blog – there’s a page for asking questions tabbed above – or contact me by email at rigibson at earthlink.net. I’ll try to respond. You can of course also leave a review on iTunes. I really do appreciate your feedback.


—Richard I. Gibson


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Episode 369: Gallium

rigibson@earthlink.net (Richard I. Gibson)Author: Richard I. Gibson
Sat, Feb 14, 2015




On my What Things Are Made of blog, by far the most popular post is one on the element gallium. I don’t know why this is so, unless there are a lot of middle schools assigning homework on gallium. So I thought I’d update that post here for the podcast.

Gallium is an element isolated in 1875 by French chemist Paul Lecoq de Boisbaudran, who named it for his native France or Gaul, Gallia in Latin. It proved the predictive validity of Mendeleyev’s then new Periodic Table of the Elements, in which Mendeleyev had predicted the element in 1870. Gallium is extremely rare in terms of gallium minerals. There’s one, gallite, a copper gallium sulfide, but most gallium occurs as traces in other minerals, and most of it is recovered during processing of aluminum and zinc ores, bauxite and sphalerite, where it can occur at up to 50 parts per million. Not much, but enough to be economically recoverable.

So what? Well, virtually every American uses gallium virtually every day. Not much, but it’s critical in things like semi-conductors and integrated circuits in computers and televisions, cell phones, LEDs in street lights, solar panels, and more. About three-quarters of the gallium used in the United States goes to integrated circuits in the forms of gallium arsenide and gallium nitride. Most of the rest is in devices such as lasers, LEDs, telecommunications, and solar cells. Gallium has also been used as an additive to ski wax, where it helps reduce friction on the surface.

The United States hasn’t produced gallium since 1987, so it’s entirely dependent on imports. As with many mineral products, China is the world leader in gallium production with about 80% of the total, and they’ve really ramped up their production in recent years, leading to a decline in the price of gallium from almost $700 per kilogram in 2011 to $240 per kilogram in September 2014. Even though supply increased by 26% in 2014 over 2013, so did demand, especially as Asia increases its electronics usage. An increasing use for gallium is in thin-film solar cells, made with copper-indium-gallium diselenide, an alloy with photoelectric properties.

About a third of U.S. imports come from metal refineries in Germany, with another quarter from the U.K. and the same amount from China. Ukraine provides about 6% of U.S. gallium. Total U.S. consumption is about 40,000 kilograms a year, or about 88,000 pounds. Not much compared to, say, almost a million tons of zinc the U.S. consumes annually – but as I suggested earlier, gallium is pretty critical to the modern American lifestyle.

—Richard I. Gibson

Resource: USGS Mineral Commodities Summary – Gallium


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The Cambrian Episodes

rigibson@earthlink.net (Richard I. Gibson)Author: Richard I. Gibson
Sun, Feb 01, 2015






Running time 2 hours 10 minutes

For 2014, the podcast consisted of daily productions averaging about 5 minutes each. The individual episodes for each month are now being assembled into packages for each month, representing various spans of time in the history of the earth. 

This episode is the second package, which combines the daily episodes for February, covering the Cambrian Period of the Paleozoic Era. This includes all the individual daily episodes from February 2014.

There is variable audio quality in some of the individual recordings, which are conversations between me and other geologists – the nerds in a bar episodes. And I’m leaving the references to specific dates in the podcast so that you can, if you want, go to the specific blog post that has links and illustrations for that episode. They are all indexed on the right-hand side of the blog.


As the year continues, there will be new episodes, maybe every week or two, in addition to these monthly assemblies.

If you have questions or comments, please let me know, either here on the blog on the ask a question page, or contact me by email at rigibson at earthlink.net. I’ll try to respond.

—Richard I. Gibson


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The Oily Episodes from 2014

rigibson@earthlink.net (Richard I. Gibson)Author: Richard I. Gibson
Thu, Jan 29, 2015




My friend Larry Smith, a geology professor at Montana Tech here in Butte, Montana, suggested that I assemble the podcasts from 2014 into thematic packages as well as the month by month packages, which is ongoing. I thought that was a good enough idea to buy Larry a beer, and here’s the first of these packages.

This group contains all the 2014 episodes tagged with oil or oil shale keywords. There are 15 of them, including two that are mostly about oil shale deposits. Running time is about an hour and twenty minutes.

Thanks very much for your interest.
—Richard I. Gibson


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Episode 368: Alaska, 1898

rigibson@earthlink.net (Richard I. Gibson)Author: Richard I. Gibson
Tue, Jan 27, 2015




Today’s episode is from my book, What Things Are Made Of and the chapter that includes gold.

THE LAST GREAT GOLD RUSH began in August 1896, when prospector George Washington Carmack and his two Indian companions, Skookum Jim and Tagish Charlie, found gold in the Klondike River basin in Canada’s Yukon Territory. Two years later, thousands of the gold seekers who had climbed the perilous slopes of Chilkoot Pass were gone, either back to the states or on to other diggings in Alaska. But that summer of 1898 saw a rush of another sort: the rush to understand Alaska’s resources.

The 19-year-old United States Geological Survey dispatched four parties that spring, and the War Department sent two more teams accompanied by geologists. Among other things, they were to “observe and note all occurrences of valuable minerals, giving special attention to the presence or absence of gold, whether in placers or veins.” These early scientific expeditions guided later exploitation of Alaska’s mineral wealth, and established the careers of several USGS geologists.

Spurr's 1898 geologic map of southwestern Alaska

Josiah Spurr was just 28 years old when he led a reconnaissance in southwestern Alaska, but he knew what he was looking for: his previous geological experience in the Yukon gold fields prepared him for any exploration dealing with gold. Spurr’s 1425-mile Alaskan journey with a handful of other scientists in three lightweight cedar canoes resulted in new geologic and topographic maps covering a vast territory. Spurr’s report reflects the dangers of exploration as the 19th Century came to a close. The team arrived at Tyonek on Cook Inlet, on April 26, 1898, but could not head upriver until the ice began to break up on May 4. After reaching the Susitna River mouth, their intended portal to the interior, the weather forced a delay until May 20 when the river became sufficiently ice-free for them to travel. Even then, after they “had gone several miles, we were surprised by a solid wall of ice bearing swiftly down upon us, and we had only time to throw our load upon the banks and drag the boats out of the water before the ice jam swept past, piling over upon the banks in places and grinding off trees.” Spurr’s narrative reads more like an adventure story than a scientific document.

The Spurr Expedition coined the term alaskite, a word for a particular light-colored granitic rock. The scientists observed considerable mineralization associated with southwestern Alaska’s granites, though gold occurred only sparingly. Nonetheless, in 2007 Alaska was the second-leading gold producing state in the U.S., with more than 700,000 ounces, mostly from mines near Fairbanks and Juneau. Alaska’s production is a distant second to Nevada, the heavyweight in the U.S. gold-mining industry with around 5,000,000 to 7,000,000 ounces per year. The United States exports gold, in a virtual three-way tie with Australia and South Africa for second, third and fourth place in the world after China.

Josiah Spurr’s work on western U.S. mineral deposits gained him considerable fame, and he wrote a book on economic geology. In the 1940s his work focused on the moon – earning him a crater named Spurr to go with Spurr Volcano in Alaska and the mineral spurrite, a complex calcium silicate. Another 1898 Alaska explorer, Walter Mendenhall, became the fifth Director of the U.S. Geological Survey in 1930 and gave his name to a glacier near Juneau. Alfred Hulse Brooks, chief Alaska geologist for the USGS from 1903 until he died in 1924, explored Alaska’s interior Tanana River valley in 1898 when he was just 27 years old, and was honored by the naming of the Brooks Range in 1925. The rare mineral hulsite, an iron-magnesium-tin borate, discovered at Brooks Mountain on the Seward Peninsula, also bears his name. George Eldridge and Robert Muldrow led an 1898 expedition that accurately pegged the height of Mt. McKinley, or Denali, at 20,464 feet – remarkably close to today’s value, 20,306 feet. Glaciers descending from Denali’s flanks recall their names. In this way, gold set the stage for Alaskan geological investigations that continue into the 21st Century, and pointed ultimately to the United States’ largest oil field, Prudhoe Bay, and the world’s largest known zinc deposit, at Red Dog, in the western Brooks Range of northern Alaska.
—Richard I. Gibson

Spurr's complete report: Spurr, J.E., 1900, A reconnaissance in southwestern Alaska, 1898, in Walcott, C.D., Twentieth annual report of the United States Geological Survey, 1898-1899: Part VII - explorations in Alaska in 1898: U.S. Geological Survey Annual Report 20-VII, p. 31-264.


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Episode 367 – Kidney stones

rigibson@earthlink.net (Richard I. Gibson)Author: Richard I. Gibson
Tue, Jan 20, 2015




Today I want to talk about some of the youngest minerals on earth – kidney stones. Some might argue whether these should be considered minerals at all, since part of the definition of a mineral is other than man-made, but that’s usually phrased as “naturally occurring” and these mineral deposits really are natural, even if they are not welcome.

My first professional job was analyzing kidney stones. My mineralogy professor, Carl Beck at Indiana University, had died, and his wife asked me, his only grad student, if I would continue the analytical business he had been operating. I said yes, setting out on a four-year, 20,000-kidney-stone start to my geological career.

First, please accept this disclaimer. I am not a medical doctor. I’m not even a geological doctor. So don’t take anything I say as medical advice.

Apatite is a common mineral in nature. Chemically it is a complex calcium phosphate with attached molecules of hydroxyl (OH), fluorine (F), and sometimes other elements. Apatite is the fundamental mineral component in bones and teeth, and when apatite has fluorine in its crystal structure, it is stronger. This is why fluorine is added to water and toothpaste. In kidney stones, carbonate (CO3) substitutes for some of the phosphate, making a mineral that is relatively poorly crystallized. Its formula in kidney stones is usually given as Ca5(PO4,CO3)3(F, OH, Cl). Well-crystallized or not, apatite often forms the nucleus upon which other urinary minerals are deposited. It usually occurs as a white powdery mineral deposit, one of the most common components of kidney stones.

Two minerals that are really common in human urinary stones but that are exceedingly rare in nature are whewellite and weddellite, calcium oxalates. Oxalate is C2O4, not too different chemically from carbonate, CO3, the common constituent of limestone.

Whewellite (CaC2O4.H2O) is known to occur in septarian nodules from marine shale near Havre, Montana, with golden calcite at Custer, South Dakota, and as a fault filling with celestite near Moab, Utah. It is found in hydrothermal veins with calcite and silver in Europe, and it often occurs in association with carbonaceous materials like coal, particularly in Saxony, the former Czechoslovakia, and Alsace. Whewellite was named for William Whewell, an English poet, mathematician, and naturalist who is credited with the first use of the word ‘scientist,’ in 1833.

It is one of the most common kidney stone minerals, where it typically occurs as small, smooth, botryoidal – which means like a mass of grapes –  to globular yellow-green to brown, radially fibrous crystals. Whewellite stones larger than ? inch across are quite unusual. Often whewellite is deposited upon a tiny nucleus of apatite, which may form as build-ups on the tips of tiny papillae in the kidney. Those papillae are little points where ducts convey the urine produced by the kidney into the open part of the kidney.

Weddellite, CaC2O4.2H2O, was named for occurrences of millimeter-sized crystals found in bottom sediments of the Weddell Sea, off Antarctica. Unfortunately the sharp yellow crystals that urinary weddellite forms are often much larger than that, and they are frequently the cause of the pain experienced in passing a kidney stone. Rarely, weddellite crystals may occur that are nearly a half inch on an edge, but most are somewhat smaller. The yellow crystals are commonly deposited upon the outer surface of a smooth whewellite stone. Like whewellite, weddellite is a calcium oxalate. They differ in the amount of water that is included in their crystal structures, and this gives them very different crystal habits. Occasionally, weddellite partially dehydrates to whewellite, forming excellent pseudomorphs of grainy whewellite after weddellite's short tetragonal dipyramids. Together, apatite, whewellite, and weddellite are probably the most common urinary stones.

Struvite is a hydrous magnesium ammonium phosphate, Mg(NH4)(PO4).6H2O, that forms distinctive coffin-shaped crystals. Often masses of tiny crystals grow together with powdery apatite to form huge branching stones called "staghorns," which may be several inches long. They may even fill up the entire open area of a kidney. Struvite stones are sometimes associated with bacterial infections of the urinary system. They also require non-acid systems to form, as indicated by the presence of ammonium (a basic, non-acidic compound) in the crystalline structure. The only common occurrence of struvite outside the urinary system is in bat guano. Certain dogs (especially Dalmatians) can produce remarkable large, smooth, milky-white tetrahedrons of well-crystallized struvite.

Brushite is a calcium phosphate compound, CaHPO4.2H2O that is very similar to the common mineral gypsum (calcium sulfate, CaSO4.2H2O). Gypsum finds its greatest use in sheetrock and other wallboards used in home construction. Brushite is a rare mineral outside the urinary tract, and even there it probably occurs in fewer than 10% of all stones. It is a soft, silky mineral, usually honey-brown and showing a fine radial fibrous structure. It can only crystallize in a limited range of pH (acidity), so treatment may include changing the acid-base balance of people who make brushite kidney stones.

Whitlockite is very rarely found in the urinary system, but it is the most common mineral found in prostate stones. It is a calcium phosphate with small amounts of magnesium, Ca9(Mg,Fe)H(PO4)7, or Ca9(Mg Fe)(PO4)6(PO3)OH and its occurrence may be stabilized by trace amounts of zinc. Prostate fluid has a very high zinc content. The mineral is a resinous, brown, hackly-fracturing material, and it commonly forms multiple small stones in the prostate. It also sometimes precipitates as deposits on teeth in cases of periodontal disease. In nature, it’s pretty rare – it wasn’t formally described until 1941. It occurs in pegmatites, igneous bodies that often contain complex and unusual minerals, and in lunar samples and meteorites, where it is known as merrillite. Whitlockite also forms in bat guano.

The other moderately common crystalline compound in kidney stones is cystine, the amino acid. It forms maybe 1% of the total spectrum, and crystallizes as beautiful, soft, honey-colored hexagonal masses. It is the least soluble of all the amino acids which is probably why it can form kidney stones. So far as I know it never occurs outside of biological systems.

There are a handful of really uncommon kidney-stone minerals, including newberyite and hannayite, both magnesium phosphates, monetite, a calcium phosphate, and calcite and aragonite – calcium carbonate, really common in nature but really rare in kidney stones. Chemical analysis sometimes reports calcium carbonate in kidney stones, but that’s probably wrong. The analysis picks up the CO3, carbonate, that is incorporated in the apatite crystal structure. The chemicals are there, but not the minerals. And that can make a big difference in treatment.

If you have kidney stones, you have my sympathy! Drink lots of water.

My kidney stone web page (source of most of the text here, and more photos)
—Richard I. Gibson


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The Precambrian - the daily episodes from 2014 combined

rigibson@earthlink.net (Richard I. Gibson)Author: Richard I. Gibson
Fri, Jan 16, 2015






For 2014, the podcast consisted of daily productions averaging about 5 minutes each. The individual episodes for each month are now being assembled into packages for each month, representing various spans of time in the history of the earth.

This episode is the first package, which combines the 31 daily episodes for January, covering the Precambrian – the time from the origin of planet earth up to about 543 million years ago. This episode is an hour and 15 minutes long and includes all the individual daily episodes from January 2014.

If you listened to last year’s daily episodes, you’ll find that I’ve re-recorded some of them. Especially in early January, when I was just getting started, some of the episodes were technically lacking. I hope I have improved on that for this assembly of 31 episodes covering the Precambrian. Thanks very much for your interest in this project. As the year continues, there will be new episodes, maybe every week or two, in addition to these monthly assemblies.

If you have questions or comments, please let me know, either here on the blog – there’s a page for Question of the Week – or contact me by email at rigibson at earthlink.net. I’ll try to respond.
—Richard I. Gibson


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December 31. The 6th Extinction

rigibson@earthlink.net (Richard I. Gibson)Author: Richard I. Gibson
Wed, Dec 31, 2014




So we’ve made it to the end of our geological year, covering 4.6 billion years of geologic time. 366 episodes, close to 180,000 words and a total of about 25 or 30 hours of programs. I hope you’ve enjoyed it! The podcast will not end, but the structure will change as we go into next year. It won’t be daily any more, sorry to say – there were times when it was really touch and go in terms of me getting the episodes out, but I’m happy to say I managed. Thanks for your interest – that was my main motivation once things got going. 

I’m not certain exactly what sorts of topics next year will bring. I’m not going to try to cover “current events” in any particular way because there are many good blogs and podcasts that do that. I expect I may do a few more that use my own work as a basis, and some posts will probably be based on topics in my other book, What Things Are Made Of. I want to try to do a few interviews with geoscientists working on interesting topics, and that may give it a Montana-centric flavor, but we’ll try to make things pertinent to a wide audience. Since I only covered 366 topics – and they were selected largely based on my own prejudices – there’s certainly plenty more that we can look at. Feel free to submit questions or suggestions, either through the blog or email me at rigibson at earthlink.net. You’ll get a spamblocker message, but I’ll find your email. I’m not going to make any promises, but I am going to shoot for at least one episode every week or ten days next year. 

I also will be assembling the existing podcasts into single recordings. I’m not sure how it will work in terms of file sizes, but I’m hoping that I can make each month of the previous series into one or two packages – without the repetition of the intro music and exit tagline. I’ll try to edit the episodes into those assemblies early in the coming year and make them available in the usual way, through the blog. 

To close the year, I thought I might address what has been called the Sixth Extinction. Of all the mass extinctions in earth history, only five have been really, really devastating. Those are the ones at the end of the Ordovician, in the Late Devonian, the biggest of all at the end of the Permian, the Triassic-Jurassic extinction, and the one that ended the Cretaceous Period and the Mesozoic Era. The case has been made that we are presently in the midst of another mass extinction, the Sixth great one.

There is an entire book, titled The Sixth Extinction: An Unnatural History, by Elizabeth Kolbert. It just came out in 2014, and I recommend it highly. The New York Times Book Review listed it as one of the ten best books of 2014. The book makes the case for a present-day massive die-off of organisms, ranging from bugs and bats to corals and rhinos. We’ve seen in the podcasts in this series that things like climate change certainly affect extinction rates, and there is no question that earth’s climate is changing at high rates right now. There’s also no question that human activities are affecting many of the things that contribute to climate change. It’s happening.

Kolbert integrates our knowledge of past extinctions, such as the disappearance of the ammonites, which you have heard about in this podcast series, as lessons for the present.

We don’t really know how many species of plants and animals exist on earth today, although we have good ball-park estimates. New species, even new large animals, are being found all the time. So it’s hard to say with certainty what kind of extinction rate is in progress, but some estimates say that as far as we can tell, extinction rates today are as much as 1,000 times those that typified most of earth’s history. We might argue about whether what’s happening now is on a scale comparable to the Big Five Extinctions, but at some scale, an extinction event is assuredly in progress.

The big, charismatic animals, like rhinos, elephants, tigers, and whales, that need extensive spaces for their lifestyles, are probably most threatened by human pollution and invasion of habitat, but who knows what’s going on in the insect world or the frog world? Those who study frogs are concerned. Is it possible to help all, or even most, species survive in a human-dominated world? I certainly don’t know. Should we even try? From both the altruistic and selfish points of view, it would probably be advantageous to try. Even human-centric people can’t deny the continual discoveries of beneficial products that come from obscure plants and animals.

For me as a geologist, I sometimes take the long view – the earth does not care. If humans kill off lots of things, including perhaps themselves, earth, and life, will continue. It always has, and until the sun burns out or some incredible catastrophe happens, it always will.

Thanks for joining me on this journey. If you’ve learned a tenth of what I have learned in putting these talks together, you’ve learned a lot! I hope it was fun!

—Richard I. Gibson

LINKS:
Extinction rates

Animals that went extinct in 2014


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December 30. Modern Plate Tectonics

rigibson@earthlink.net (Richard I. Gibson)Author: Richard I. Gibson
Tue, Dec 30, 2014




Through the year, we’ve talked about events that broke apart and combined the various tectonic plates on the earth, but today, as we’ve almost reached the present, I wanted to just summarize the way things are today. 



First, I know I talked repeatedly about oceanic crust and continental crust. They are quite different from each other, in density, thickness, and mechanical behavior, and those differences drive subduction and plate tectonics. But the two types of crust also move together, a lot. The North American Plate includes all of the North American continent – except the bit of California west of the San Andreas Fault – but it also includes the oceanic crust beneath the North Atlantic Ocean, all the way out to the Mid-Atlantic Ridge. Iceland straddles the mid-ocean ridge, a pile of volcanic material erupted because of a hotspot at depth, but the west half of Iceland is part of the North American Plate, and the east half is on the Eurasian Plate.  

Depending on exactly how you want to define “major,” there are 9 to 16 major plates. Africa has two sub-plates – Arabia, which is tectonically separated from Africa, but only by the width of the young Red Sea, and the Somalia Plate, breaking away along the East African Rift. The Somalia Plate is certainly separating from Africa, but in many ways and in many places, they’re still attached to each other too.

Then there’s the North American and South American Plates, both of which include the continent and the western half of the Atlantic Ocean. There is only a vague boundary between the North and South American Plates, because they are to a large extent moving together at a similar speed and direction. There is an extensional rift between North America and Greenland – but it failed, and Greenland is now completely attached to North America, and is moving with it.

Australia and India have their own plates and include more oceanic crust than continental, but like the Americas, they are almost locked together now that India has collided with Eurasia and slowed down. Antarctica also has its own plate, and except in some small sections, Antarctica is entirely surrounded by oceanic rifts. Everything else is pulling away. How can that be possible? You can’t pull away everywhere. Well, you can, for a while – the excess is taken up by collisions elsewhere on the globe, but eventually, even if the active rift between West and East Antarctica opens up, part of the Antarctica Plate will start colliding somewhere.

The Pacific Plate is entirely oceanic – no continental material except the bit of California west of the San Andreas Fault, which is traveling with the Pacific Plate. And the Pacific Ocean is really underlain by two large plates – the larger Pacific, and east of the East Pacific Rise spreading center, the Nazca Plate which is subducting beneath South America to lift up the Andes. The eastern half of the oceanic plate in the North Pacific was called the Farallon Plate, but it has been almost entirely subducted beneath North America. That subduction made for the various mountain-building events that created the Rockies, the Sierra Nevada, and the Coast Ranges.

The other big plate is Eurasia – Europe, and Asia except for Arabia and India and the far eastern tip of Siberia, plus the east half of the North Atlantic Ocean.

The smaller plates make for some interesting geography and tectonic activity. Arabia, breaking away from Africa, makes the Red Sea, and colliding with Asia makes the mountains of Turkey, Iraq, and Iran, and the Caucasus. The Philippine Plate is a small oceanic plate in the western Pacific Ocean. The volcanoes of the Philippines, Taiwan, and southern Japan are the result of the Philippine Plate subducting beneath the complex eastern margin of Asia, and the Mariana Trench – the deepest point on the ocean floor – is on the opposite side of the Philippine Plate, where the Pacific Plate is subducting beneath it. When two oceanic plates collide as they are doing here, all bets are off. One may subduct beneath the other, or the other way around, and the directions of subduction can even flip.

Between North America and South America, the Caribbean Plate is mostly oceanic, but there are some continental blocks on it too, in Nicaragua. This small plate is overriding the oceanic part of the Americas Plate, resulting in the volcanoes of the West Indies. There’s a similar narrow plate between the southern part the South Atlantic Ocean and Antarctica, called the Scotia Plate.

There are a bunch of little plates along the west coast of North America that are essentially the remnants of the old Farallon Plate – the remnants that haven’t yet been subducted. There’s the Cocos Plate, off southern Mexico, the Rivera Plate a bit further north, and the Gorda and Juan de Fuca Plates offshore from Oregon, Washington, and British Columbia. Where these plates in the northwest continue their ongoing subduction beneath North America, the subduction is producing a volcanic chain – the Cascade Mountains.

Beyond that, it starts to become a question of semantics – what’s a plate? In a way, every single zone bounded by active faults is an active plate – the fault separates two regions that are moving in different ways. But plates are really much grander objects, and they are separated from each other by really major breaks – not just a fault, but a change in the way the rocks behave and move. Even the small plates I described are considered to be plates because they are pretty clearly the left-over pieces of a once much larger plate.

Today, every possible kind of interaction between plates is ongoing simultaneously. The Pacific Plate slides past North America on the San Andreas Fault, but the Pacific Plate also is subducting beneath North America in Alaska and Mexico. North America and Eurasia are pulling apart along the Mid-Atlantic Ridge and along the Nansen Ridge in the Arctic Ocean, but the two plates are locked together in far eastern Siberia.

There are failed rifts all over the place, some of which were never much more than sags in the crust, such as the oil-rich Sirte Basin in Libya, and some of which became true oceanic spreading centers only to stop fairly quickly. That happened in what is now the South China Sea. Some subduction zones continue for tens of millions of years, and some abort after just a few million. The earth is an incredibly dynamic system – and what happens in one part of the globe will be accommodated, one way or another, even if the result is thousands of miles away. The dynamic earth isn’t just a recycling system for rocks, but it generates things that humans rely on daily, from oil to copper to salt. Plate tectonics is the basic underlying engine that drives the diversification of life, as well as its extinction.

—Richard I. Gibson

Maps from USGS or NASA, public domain


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