These daily podcasts build upon previous episodes, so the best way to work through them is by starting with the oldest, January 1. But you don't have to do that.
Most episodes are two to 10 minutes long. It's November, so we're moving through the Cretaceous Period.

Sunday, November 23, 2014

November 23. The Richest Hill on Earth



The subduction that became quite active in the Jurassic and continued into the Cretaceous and eventually created the Sierra Nevada Batholith was probably related to the two styles of mountain building we have talked about recently, the Laramide and Sevier Orogenies. The subduction itself was also complicated.

By Cretaceous time, some subduction was taking place much further east than the magmatic arc where the Sierra Nevada Mountains are today. It’s 750 kilometers, 450 miles, from Sacramento, California, to Boise, Idaho, but that’s about how far you have to go to find the continuation of the batholiths that resulted from Cretaceous subduction. In central Idaho, the granitic igneous rock is called the Idaho Batholith. It’s only about half as long as the Sierra Nevada Batholith – still huge, about 200 miles long, and generally it’s younger – formed about 100 to 54 million years ago, mostly Late Cretaceous, but some of it dates to the time a few million years after the Cretaceous ended.

Geologic map (from USGS, National Atlas) with batholiths emphasized.
Sierra Nevada is mostly Jurassic; Idaho and Boulder Batholiths
are mostly late Cretaceous in age.
Why is the Idaho Batholith so far east? There must have been a sharp break in the shape of western North America, or the stuff colliding, or both, to account for this. Because the Sierra Nevada and Idaho Batholiths are separated in time as well as space, there’s plenty of opportunity for things to change. Bottom line, subduction was taking place further east during Late Cretaceous time than it was during the Jurassic.

Even further east there’s an even smaller batholith in southwestern Montana, the Boulder Batholith. It’s only about 75 miles by 25 miles in size, extending from Helena, Montana to the Highland Mountains south of Butte. Butte, where I live, here in the Boulder Batholith, is today’s topic, because it holds “The Richest Hill On Earth.” That’s a nickname applied to the mineral district at Butte, and it might even be true. In the United States, there is no question – the US Geological Survey has calculated the value of the big mineral districts, and Butte is definitely the most valuable. It’s a little harder to say for sure in the entire world, because we don’t really know the value of mineral production from the Roman Empire, Incas, or whatever. But for one little mineral district, only about 6 square miles in area, it probably is the most valuable on the planet. I admit that I’m a little prejudiced about it, but still, it’s pretty likely.

There are rich metalliferous deposits scattered through the Rockies, the mountains of Mexico, and the Andes, all related to subduction of various oceanic plates beneath the North and South American continents. In terms of US production, Butte ranks #2 in copper, but #1 in produced copper plus reserves, #2 in silver, but #1 in terms of produced silver plus reserves, #6 in Zinc, and among the leading producers of manganese, lead, and molybdenum. It’s also produced large amounts of gold, cadmium, bismuth, and other metals. In terms of weight, Butte has produced about 24 billion pounds of copper, 5 billion pounds of zinc, almost 4 billion pounds of manganese, and almost three-quarters of a billion ounces of silver. That silver production probably places Butte third in the world, after Potosi, Bolivia, and Coeur d’Alene-Kellogg, Idaho.

Why? Why, within the 75 by 25 mile Boulder Batholith, is so much mineral wealth concentrated in a 6-square-mile area? The bottom line is, we don’t know. We know all sorts of things about how the veins formed, the way the mineral deposit is zoned, with more copper toward the center and more silver and zinc around the margin. We can talk about intersecting fractures that helped channel the hot waters carrying the minerals, concentrating them into this one area. But the ultimate question of why is unanswered. One idea suggests that in the subducting oceanic crust, there was a zone that was very rich in copper and the other minerals. It could have developed above a mantle hot spot that conveyed the minerals into the crust. This is happening today along many mid-ocean ridges, where things called black smokers are essentially underwater geysers on the sea floor, erupting superheated water rich in copper, zinc, and more, to deposit it on the sea floor. A long-lived system of black smokers might have put the mineral wealth into the oceanic crust that subducted. When it got hot, waters brought those minerals up into the overlying continental crust, where the rock melted, and when it solidified into granite, the last cracks got filled up with the mineral-rich veins.

Another possibility – and this is what I tell tourists when they visit Butte – is that it was just luck of the draw in the early earth. If the early, semi-solid earth was something like a plum pudding, with the plums representing blobs of minerals concentrated in spots that were distributed with no particular regularity, those blobs might still be hanging around, to some extent. They’ve been heated, partially melted, sliced and diced and faulted, uplifted and eroded, but still might be more or less in their original plums. About one-third of all the mercury known on the planet is in one deposit in Spain. I just don’t know of a reasonable way to concentrate all that mercury in one place – but it might have done so in the early almost molten earth, with mercury, or copper or whatever, coming together in a relatively few plum-like blobs.

In any case, there’s a great mineral wealth here in Butte. You recall that the word ‘batholith’ means ‘deep rock’ because the granitic rock solidified down in the earth – probably several miles down. It’s here at the surface today for the same reason the Sierra Nevada Batholith is at the surface – much later uplift and erosion to expose the once deep-seated granite. The volcanoes that were once above the granite, and much of the granite itself, have eroded away, so that the mineral deposit is now exposed at the surface. A happy circumstance for the prospectors who came here in 1864.

The Boulder Batholith cooled about 78 to 76 million years ago, and the mineral veins formed between then and about 61 million years ago, all in the last part of the Late Cretaceous. Big Butte, the eroded neck of an extinct volcano that gives the city of Butte its name, is from a later episode of igneous activity, about 49 million years ago, in the Cenozoic.

I have a link below to an article I wrote on this topic, called The Nature-Built Landscape: Geological Underpinnings of Butte. It’s a PDF of an article that appeared in the Vernacular Architecture Forum Guidebook.
—Richard I. Gibson

Links:
Geological underpinnings of Butte

Idaho Batholith 

Talk by John Dilles on the Geology of the Butte Mineral District

Saturday, November 22, 2014

November 22. Sevier Orogeny



A few days ago we talked about the Laramide Orogeny, the brittle breaking of the continental crust into huge uplifts along relatively high-angle faults with miles and miles of throw. And I mentioned a different style of mountain building that took place partially at the same time and in some areas, in the same places. That aspect of the deformation is called the Sevier Orogeny – and again that’s not “severe” but “Sevier,” from a place in Utah. Just to keep things confusing.


Sevier vs Laramide (source)

The basic difference between the Sevier part of the activity and the Laramide part is that instead of those big brittle breaks in the crust, in the Sevier we had much thinner slices of rock – mostly the bedded sedimentary rock – being pushed over each other in generally low-angle thrust faults with often only a few thousand feet of displacement, but sometimes more. This is the fairly typical result of collisions.

Think of a short pile of carpets and sheets and bedspreads all nicely on top of each other in horizontal layers. That’s the sedimentary package of rocks in western North America, ranging in age from Precambrian to the early Cretaceous. There’s been some bumping and breaking and so on, but on the whole, those carpets and sheets are still more or less intact and relatively horizontal. Now set a file cabinet on one side of the pile, and start pushing. The fabrics will fold and push up and over each other. In the real world of rocks, they are brittle enough to eventually break and slide over each other, and those breaks are called thrust faults. But the floor, the crystalline granitic rocks underneath the sediments, does not break. Well, it did in the Laramide Orogeny that we talked about the other day. But not in this more straightforward pushing we had during the Sevier Orogeny.

In the real world, as the rocks that are the equivalent of our carpets and sheets piled up on top of each other, two things happened. First, the tops start eroding, with the eroded sediments shedding off to the east of the uplifted, thrusted mountains. And second, the weight of the thrust sheets, plus the sediment, was enough to bow down the crystalline granitic floor. Not really enough to break it, but to make it sag.

We’ve just created a classic foreland basin. The foreland is the territory toward the craton, in this case the North American continent, inboard from the mountains created by the collision off to the west. The deepest part was in the west closest to the collision zone, where the thrust sheets and sediment piles were thickest, then it gets shallower and shallower, really pretty quickly, as you head east onto the strong craton.

We can see the evidence for this in changes in sedimentation from west to east. Here in Montana, where I live, along the Big Hole River, the Kootenai Formation of early Cretaceous age is something like 3,000 feet thick. Just 50 miles to the east, the Kootenai is about 400 feet thick. That change in thickness reflects the bottom falling out – the crust subsiding – in the western part of the foreland basin. To an extent it’s a self-perpetuating event: as more and more sediment and thrust sheets come in, the crust bows down more and more, allowing for more and more sediment to pile into the basin. Eventually, of course, it slows down or stops, once the collision has stopped and the mountains have been eroded to a level where they don’t contribute sediments in huge volume any more.

One important difference between the Laramide style and the Sevier style of mountain building is that the Laramide was pretty much a case of brittle breaking. The crust is thick, relatively uniform, and brittle, so it tends to break rather than bend. The sedimentary pile, on the other hand, was a package of diverse rocks that overall was a bit more plastic, and could fold before it broke. In detail, of course, there are brittle units and more plastic units – sandstones and limestones tend to be stronger and more brittle, and shales tend to be weaker and behave more plastically. Sometimes shales essentially even flow, because the rock is so weak and the particles are both small and mobile.

Part of the Sevier Orogenic Belt in Montana (USGS SIM 2860)
What this resulted in during the Sevier Orogeny was big folds, like the folded carpet pushed by the filing cabinet. They tend to be asymmetrical, reflecting the push direction, from the west, and most of them are above faults. Take your two hands, palm to palm, held horizontally pointing away from yourself. Push the upper hand over the flat of your lower palm, letting the ends of your fingers in the upper hand drag or stick on the lower palm as you push your upper hand away. The curl created in your upper hand is one of these folds, an anticline, and the surface of your lower palm is the fault surface.

Anticlines in general are excellent traps for oil and gas, if the other requirements are present, and in the Sevier Orogeny of western Wyoming everything came together. It’s called the fold and thrust belt there, with long, gently sloping thrust faults underlying big rounded anticlines, many of which hold oil and gas fields.

In other places along the Sevier Orogenic belt, the structure wasn’t enough – something was missing to make a hydrocarbon province, either source rocks, a seal, or something particular in the burial history to mature the organic matter into oil or gas. Consequently, for example, in the Montana Disturbed Belt, part of the Sevier Orogeny, there’s very little oil and gas. But the action did help create some spectacular scenery. The jagged alternating mountain crests of the Bob Marshall Wilderness in many cases reflect alternating thrust sheets. And all of Glacier National Park rides on one of the thrust faults, a special kind called an overthrust because it is very low angle, maybe 5° or less, and the rocks have been pushed over underlying rocks a great distance, tens of miles. The Canadian Rockies also are underpinned by Sevier Orogeny thrust faults, although like Glacier National Park, the sculpted mountains are the more direct result of glaciation in the last 2 million years or so.

I’ve said that there was some overlap in both time and space between the Sevier and Laramide styles of mountain building, but it’s fair to generalize some and say that the Sevier was mostly somewhat earlier than the Laramide, and generally took place to the west of the places deformed by the Laramide Orogeny. This makes sense if we buy the idea that an initial subducting oceanic slab, off to the west in what is now California and Idaho, began somewhat conventionally, with a magmatic arc and thrusted pieces pushing off the collision belt, followed by that change in the angle of the descending, subducting oceanic crust so that the forces were transmitted further east where they created the brittle breaks of the Laramide.

In places where the two types of deformation overlap, it can be challenging to unravel the sequence. Was a strong crystalline basement cored block already uplifted when the weaker, thinner thrust faults impinged on it? If so, the Laramide block might serve as a buttress, with the Sevier faulting swinging around it rather than through it. But you can get essentially the same expression by having the Sevier faults present first, and the massive Laramide uplift coming second, folding the earlier faulted rocks and faults around it. In some places, I really think it is correct to think that the two different styles of deformation were actually happening at the same time. 

East of the main action of the Sevier Orogeny, where the crust was not depressed so far as it was in the west, it was pushed down low enough to help create the Cretaceous Interior Seaway. This was a long-lived shallow ocean that extended from Arctic Canada to Mexico, and it covered much of what is now the eastern Rocky Mountain Region as well as much of the Great Plains, from the Dakotas through Kansas and Texas. It connected with the Gulf Coast where Cretaceous rocks were being laid down to ultimately become today’s Gulf Coastal Plain in Texas and Louisiana. The presence of the seaway was the result of the crustal depression caused by the Sevier Orogeny, but it was enhanced by the ongoing worldwide transgression or rising sea level, throughout the Cretaceous.

That’s enough for today. Tomorrow, we’ll talk a bit about the igneous rocks associated with all this action in western North America.
—Richard I. Gibson

Cross-section from Geologic Map of the Canyon Ferry Dam 30´× 60´ Quadrangle, West-Central Montana, By Mitchell W. Reynolds and Theodore R. Brandt, 2005: USGS Scientific Investigations Map 2860.

Seaway map from USGS (public domain)

Green River Basin cross section by Richard Gibson (source)

Friday, November 21, 2014

November 21. Cretaceous magnetic quiet period




One of the findings that helped convince geologists on the American side of the Atlantic  that continental drift was a reality was the discovery of sea-floor spreading – the idea that new oceanic crust is generated at the mid-ocean ridges, and that new crust pushes older crust out of the way, away from the mid-ocean ridge, thereby making the ocean basins wider and wider at about the rate your fingernails grow.

The discovery of sea-floor spreading came through geophysics – specifically, measurements of the earth’s magnetic field as it is preserved in formerly molten rock. When molten rock, magma, solidifies, tiny particles of magnetic minerals, mostly the mineral magnetite, an iron oxide, become frozen in place with the orientation of the magnetic field that’s present at the time the rock solidified.


That’s all well and good, but so what? It’s useful because the earth’s magnetic field has changed over time, and is changing right now. The positions of the north and south magnetic poles change, the strength of the magnetic field changes, and the whole system even reverses its polarity, so that the north magnetic pole becomes the south magnetic pole and vice versa.

There’s a lot of research and a lot of debate about how a magnetic field reversal happens. Increasing evidence shows that it may take place over a period of a few thousand or even a few hundred years – instantaneous, geologically. It must have something to do with flow in the molten outer core of the earth, where the magnetic field is generated by electrical currents in the liquid rock there. For now, let’s just recognize that these reversals have indeed happened in the geologic past. We can use measurements of the rocks with opposite polarity to figure out a lot of geological things.

Reversal pattern at mid-ocean ridge
As new oceanic crust is generated at a mid-ocean ridge by upwelling magma, each new intrusion splits the previous rock into two, one slice on each side of the rift. The two slices are pushed aside by the new intrusion. Do this hundreds of times, and on both sides of a mid-ocean ridge you have pairs of almost identical stripes of rock representing the continual intrusion of new rock at the ridge axis.

As the different intrusive magmas solidify, they record the magnetic field in place at that time. Because the field reverses, we end up with alternating high and low magnetic values, reflecting the alternating polarity of the field resulting from reversals. In practice, this gives a uniform striping to the map of the magnetic field along mid-ocean ridges – symmetrical, alternating, long linear magnetic highs and lows. We can figure out things like the direction of spreading, its speed, and more. An incredibly useful tool for understanding plate tectonics.

Reversal chart
There have been at least 180 magnetic field reversals in the past 80 or so million years, with a seemingly random periodicity. The length of time that the magnetic field remains stable in either north or south polarity is also pretty variable, ranging from a million years or more to a few hundred years. The last major magnetic field reversal happened about 780,000 years ago, although there was a short-lived event about 41,000 years ago as well.

I mentioned back in the Jurassic that almost all the present-day oceanic crust is of Jurassic age or younger, because most of the older oceanic crust has been subducted, recycled and melted down inside the earth. So we can’t use this tool for times before the Jurassic, at least not using oceanic crust, but we can look at the magnetic field frozen into other magnetite-bearing igneous rocks that are of all ages.

I’ve brought this topic up now, during the Cretaceous, because the Cretaceous was a time when the earth’s magnetic field did not reverse as it has so often at other times. For 38 million years, 121 to 83 million years ago, the earth’s magnetic field didn’t flip, a time called the Cretaceous Normal Superchron, normal because it was the same as the field orientation of the present day, and ‘chron’ refers to a period of one polarity or the other. A superchron is a long period with one polarity. We know Cretaceous superchron exists because we can measure the magnetic field in oceanic crust of Cretaceous age, and for that time interval, there are no alternating highs and lows that would reflect the reversal of polarity. Why was the earth’s magnetic field quiet for 38 million years, when it usually flips on average about every half-million years or so?

Since we don’t really understand how or why the field flips at all, we also don’t really understand why it would stay so stable for so long. Modeling of the dynamo, the earth’s electro-magnetic generator in the fluid outer core, gives some suggestions – maybe there is some kind of trigger mechanism to start a reversal – which begs the question, why were there no triggers during the Cretaceous superchron. Bottom line, we don’t know why there was a long period of no magnetic field reversals during the Cretaceous.

There’s lots of research going on into the earth’s magnetic field, including the phenomenon of reversals. We may be entering one now, as the overall field has been decreasing – a characteristic of a reversal – and the position of the north magnetic pole as it moves around in Arctic Canada has increased its annual rate of migration. But we really don’t know if those are real precursors, or evidence for the start of a magnetic reversal. In any case, there is no correlation between reversals and extinction events, though it has to be admitted that no reversal has happened since humans have had our wonderful electronic world. It’s kind of hard to imagine there would be no impact other than a compass needle pointing the opposite direction. If mankind can survive its other threats, perhaps someone will see what those impacts may be in the next few hundred or few thousand years.
—Richard I. Gibson

Why do periods of stable magnetic field exist?

Reversal chart from Wikipedia (public domain) 

Sea floor spreading diagram from USGS (public domain)