History of the Earth

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

Upper Branham LakeToday’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 SilicateSediments like silts and muds usually contain plent

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

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

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

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

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

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

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

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

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

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

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

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

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