The 366 daily episodes in 2014 were chronological snapshots of earth history, beginning with the Precambrian in January and on to the Cenozoic in December. You can find them all in the index in the right sidebar. Now, 2015, the blog/podcast is on a few per month schedule with diverse topics.

Saturday, August 22, 2015

The Triassic Episodes



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

Monday, August 10, 2015

Episode 373. A walk to Branham Lakes



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 2½ 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

Wednesday, July 29, 2015

Episode 372 Satellite-derived gravity




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