Dude, what happened to Cenozoic stratigraphy when I wasn't looking?

There's all this discussion going on about the existence (or not) of the Quaternary Period, and whether we ought to declare a new epoch called the Anthropocene. Now, it's pretty scary, realizing that humans have had geologic-scale effects, especially on climate. But I've heard talk about the Anthropocene already. (Elizabeth Kolbert discusses it in her excellent book Field Notes From a Catastrophe.)

But I was shocked by another, much more mundane comment:

In recent decades, ICS--with the consent of the International Union of Geological Sciences (IUGS), the world's ruling body on such matters--dropped the Tertiary as well. (From the Science article about the Quaternary.)

Now, I've never worked on Cenozoic rocks. (Mesozoic, Paleozoic, and Precambrian, yeah, but never young stuff.) But my house is sitting on rocks that are mapped, even in very recent maps, as Tertiary. I live in a state where "Tertiary volcanics" is as common a phrase as "Quaternary alluvium." And, probably most embarrassingly, I've made students memorize a geologic time scale that included the Tertiary. (Not recently; my current institution has an excellent Historical Geology class, and I let the sed/strat/paleo guy deal with the time scale.)

I understand that stratigraphers have good reasons for wanting names that make sense, and for wanting worldwide correlations to be possible. But I like my Tertiary volcanics. And the K-T boundary.

Of course, I still haven't gotten over the change from "sphene" to "titanite"...

Pie and misconceptions: accretionary wedge #5 is posted

Accretionary Wedge #5 is up at Green Gabbro.

The big question is: if we were talking about pie, why did so many people write about the mantle instead of the crust?

Rock of the (not quite mid) week: disharmonic folds

Folds are complicated structures. These are in marble just below the Snake Range Decollement. The quartzites below them are mylonitic, stretched to flat slabs. But the marbles have folds like these:



"Disharmonic" folds have different wavelengths, different hinge locations, and so forth in different layers. The marble looks for all the world like it's a pot of paint that has been stirred. And yet it hasn't been a liquid. It was calcium carbonate - easy to dissolve in water, and easier to deform by intracrystalline deformation than many other minerals, but still solid.

Why does it look like this? Heck if I know. But it looks cool.

Folds and crustal thickening

The intro textbook that I use implies that folded rocks usually mean the crust has been thickened.

But I ask you: has this crust been thickened? Hmmmmm?

(Apologies for not writing a real post this time...)

Ice crystals on the snow

It was cold last week. Colder than I remember since moving to Colorado. Below zero (F) cold. Kinda wimpy for Vermont or Minnesota, but I'm here on a south-facing slope, with 300 sunny days a year, and it doesn't have to get to 20 below for me to feel it.



The cold, clear weather came on top of a couple feet of snow. And that has meant surface hoar.



I'm not a snow scientist. I'm not a backcountry skier, either, for that matter. My students could tell you much more about surface hoar and what it means for avalanche danger next time it snows.

Me, I just know that the surface of the snow sparkles, and that there are these tiny little tree-like crystals sticking up everywhere, and they are beautiful.



(The surface hoar photos are mine; the moon photo was taken by my husband.)

On a related note, I caught the beginning of this story on NPR about snowflakes. (I wanted to listen to the entire thing, but I have a four-year-old, and he wanted to discuss urinating polar bears. Some people hear stories about polar bears and think "oh, no, climate change!" My four-year-old thinks of the time his grandparents saw polar bears peeing at the zoo.) There are some beautiful photos linked from the story.

Visualizing the birth of the San Andreas

Tanya Atwater (who figured out the San Andreas Fault back in the day) was just here, speaking to our local geological society and talking to classes. She's been working a lot on making animations to help people understand geology, and I wanted to share a couple that she showed to my structure class.

These are Quicktime movies that can be either viewed on Tanya's site or downloaded.

Here's the Pacific Ocean from 80 million years ago to present, and here is western North America from 38 million years ago until now. The first movie shows three big plates in the Pacific Ocean: the long-lost Kula plate, the Farallon plate (which is mostly gone, too, except for that little piece now known at the Juan de Fuca plate), and the Pacific plate. You can see the Kula and Farallon plates get smaller and smaller, until the Pacific plate touches North America... and somehow the plate motions were almost right for a strike-slip plate boundary. The second one is a close-up of North America, and you can watch the San Andreas form and grow through time. (You can also see some of the other big changes in western North America, especially the stretching of Nevada to double its original width.)

If you've been told about the story of the San Andreas Fault, but weren't able to picture it, check it out. My students loved these (and the professionals at the meeting did, too).

Rock of the (mid) week: Wasatch Fault rock

When I first started planning work for Advanced Structural Geology, I considered focusing the entire course on faults and shear zones. In part, I was trying to make up for things that I worried were deficiencies of my own - I still remember staring blankly at my field camp TA as he sat on the Darby Thrust, asking leading questions until I finally realized that the stratigraphic section was not in the correct order.

Now, I've got some spectacular exposures of minor faults in my backyard. But they aren't quite the same thing. So I decided I wanted to take students to see some of the rocks found in major fault zones.

My brittle example was the Wasatch Fault, the big normal fault east of Salt Lake City. It's active, and there are good geomorphic signs that it's capable of earthquakes, including a nicely offset glacial moraine at the mouth of Little Cottonwood Canyon.

This outcrop, of an actual slip surface, is on the edge of Provo. The rock is limestone, but it's been broken into tiny pieces, and then cemented back together by more calcite. The striations on the surface may be slickenlines, but they may also be scratches from the equipment that stripped this surface bare (in an aborted plan to build a ski lift).

Is this rock the result of earthquakes on the fault? In the past, the only dead-certain evidence for earthquakes on exhumed fault rock was pseudotachylite, rock that melted and then froze during an earthquake. But at last fall's GSA meeting, during a session on Co-Seismic Fault Zone Structures, a number of groups suggested that maybe cataclasites form during earthquakes, as well.

Threshold concepts in geology - the poll

I'm curious how many people agree with some of the threshold concepts in geology suggested by commenters (and by members of the UK group). So I figured out how to add a poll to Blogger. Please vote - I'm curious how many people have similar experiences.

(Edit: I've borrowed the idea of "threshold concepts" from a Journal of Geoscience Ed article, and it seems as though the idea has been floating around other science education communities for some time. Perhaps such things don't exist, and there are simply things that some students find difficult. But it they do exist, they might help me choose which things to spend more time on.)

What geological concepts are "thresholds"?

While cleaning out my office for the beginning of the semester, I ran across an article in the November 2007 Journal of Geoscience Education on "threshold concepts," and I've been wanting to talk about it for a couple weeks. The article itself isn't going to be available online until next year, but there are lots and lots of related articles here.

According to the authors, a "threshold concept" is something that is difficult to grasp, but which transforms one's understanding once understood. It makes other things make sense, but only if the student really gets it. If the student doesn't get it - well, maybe he/she will get by with parroting answers, but it will be difficult to use the concepts.

A meeting in the UK came up with a list of some possible threshold concepts in the earth sciences (including geography). It's a really long list, and I'm not sure I buy all (or even most) of it.

But I'm curious what you think. Professors, post-docs, grad students, undergrads, interested non-geologists: what concepts have made parts of geology suddenly make sense to you? And do you think that there are any universally confusing concepts, or would the list depend on where a person grew up, and what a person learned as a child, and what kinds of cultural stories the person believes?

For instance, I always tell geology majors that there are two classes that, in my opinion, really transform their understanding of geology. Mineralogy is the class in which rocks are transformed from dull grey things into storytellers. Structural geology is the class in which geology acquires a third dimension (or at least, a third dimension underground). But I may be biased - after all, I do structural geology on metamorphic minerals, and clearly I'm the one interested in the coolest and most fundamental stuff in the discipline!

And my big ah-ha moment in my introductory class had nothing to do with minerals or structures. It had to do with correlation of flat-lying sedimentary rocks. I simply couldn't believe that it was possible to know the relationship between rocks under two different hilltops - at least, until I went on a field trip and made a map and saw it for myself. But then I grew up in Maine, on top of glacial till and multiply deformed metamorphic rocks, and flat-lying stratigraphy was not part of my experience. Somehow I expect that my students from the Colorado Plateau don't have that same problem.

So I'm curious: what were your big ah-ha moments? What concepts made it all suddenly make sense?

Because if we could really figure out what the fundamentally important stuff is, I could spend the time on it, and leave out things that fall into place more easily.

Reference: Stokes, A., King, H., and Libarkin, J.C., 2007, Research in science education: threshold concepts: Journal of Geoscience Education, v. 55, n. 5, p. 434-438.

Does subduction quit every now and then?

Callan Bentley at NOVA Geoblog has a good discussion of an article published in Science by Paul Silver and Mark Behn about the possibility that plate tectonics has been intermittent, rather than a continuous feature of Earth’s geology. I was intrigued by the press releases, but the paper itself seems to have problems, and I’m curious whether I’m simply being dense, or whether these problems really exist.

The basic premise of the Silver and Behn paper is this: when continents collide, a subduction zone goes out of existence. Cartoons of pre-Mesozoic* collisions (such as the Appalachian collisions that Callan shows on his blog) usually show subduction initiating someplace in a nearby ocean, to balance sea-floor spreading somewhere else on the planet. But what if that doesn’t happen? And what if there aren’t major subduction zones on the opposite sides of the colliding continents to take up the motion? Is it possible that world-wide plate motions could simply stop, until enough heat built up in the mantle to force continents to rift (through initiation of mantle plumes, if they really exist)?

It’s an intriguing idea. It’s true that subduction zones don’t seem to regularly start in places where oceanic crust is particularly old, and that collisions don’t always result in the formation of new subduction zones. There are only two dinky little subduction zones in the Atlantic (the Caribbean and Scotia arcs), and there is no new subduction zone south of India. On the other hand, there is a new subduction zone forming on the north side of Papua New Guinea, after the northern continental shelf of Australia clogged the subduction zone. And the idea that supercontinents could insulate the mantle beneath them, leading to increased heat flow, could explain one of the mysterious features of the Precambrian basement in my neck of the woods: the ~1.4 billion-year-old “anorogenic” granites, and the pervasive high temperature/low pressure metamorphism that can be found even far from the contact of any exposed granite.

But the proposal implies a particular sequence of events... and the data used to support the model suggests that things actually happened in a different order.

The order ought to be this:

1) A Pacific-type ocean basin (in which the ocean basin existed before breakup of the previous supercontinent) closes, creating a supercontinent.

2) The majority of worldwide subduction zones are caught in the middle of these collisions, and disappear.

3) The lack of subduction causes mid-ocean ridges to slow, as well, and plate motion decreases (or, possibly, stops).

4) Heat builds up in the mantle until plate tectonics resumes (presumably driven by the rising of hot material and some kind of rifting; subduction would have to begin at the same time to keep the Earth the same size).

Silver and Behn argue that Pangea shouldn’t fit this model, because when the Caledonian/Appalachian/Variscan/Ural collisions occurred to form Pangea, the oceans that closed were young. (And there were many active subduction zones around the edges of Pangea, off the west coasts of present-day North and South America, among other places.) Instead, they focus on two older supercontinents: Rodinia, which formed as a result of the Grenville orogeny around a billion years ago, and Pannotia/Gondwana, which formed slightly later.

Their primary way of testing the model is to look at the geochemical evolution of the mantle as a way of measuring the amount of subduction that occurs. They use two different measures of mantle chemistry: the Nb/Th ratio (which should increase in mantle-derived rocks through time, because Th is preferentially incorporated into subduction zone melts), and the ratio between two isotopes of He.**

The test is summarized in their figure 1, which I can’t show because it’s behind a paywall. So I’ll draw some sketches to show why I’m confused. The approximate shape of their estimated subduction flux looks like this:


There are two peaks, one in the late Archean (around 2.5 billion years ago) and one in the Phanerozoic. And there’s one minimum, around 1 billion years ago.

If you add the times during which Rodinia and Pannotia/Gondwana formed by closure of Pacific-type basins, the diagram looks approximately like this:


Do you see why I’m confused? Yes, there appears that global subduction decreased (if the proxies are correct) between 2.5 and 1.0 billion years ago. But the minimum subduction rate occurred just before Rodinia formed – exactly the opposite order than one should expect if the formation of Rodinia caused subduction zones to shut down. In fact, the formation of Rodinia occurred just before the worldwide subduction rate began to increase, as if the formation of supercontinents actually caused new subduction zones to form in other parts of the world.

There is another, even older supercontinent (Nuna) proposed, but it isn’t clear exactly what kind of ocean basin closed to form it. And it formed during the middle of the decline in subduction rates.


In fact, the only supercontinent whose formation correlates to a decrease in the proxies for subduction rate is Pangea – and we know that subduction occurred before, during, and after Pangea existed. Pangea was the one supercontinent they excluded from consideration.

So... interesting idea. But their measures of subduction flux don’t seem to support it.

I have no trouble with the idea that plate tectonic rates may have varied through time (though I would love to see, say, a compilation of paleomag data as an independent test of the geochemical proxies). But their explanation – that the formation of supercontinents causes subduction to halt – doesn’t fit the data.

If this study is correct, it would have interesting implications for things like the driving mechanism of plate tectonics. But the data doesn’t seem to fit the conclusions. Am I missing something critical about their reasoning – some reason why the decline in subduction should precede the collisions?

I’m not teaching plate tectonics again until 2010, so there’s time for comments, replies, and discussions before I would discuss this in class.

Reference: Silver, Paul G., and Behn, Mark D., 2008, Intermittent plate tectonics?: Science, v. 319, p. 85-88, doi: 10.1126/science.1148397.

*Plate movements from the Jurassic to the present are much better constrained than pre-Mesozoic motions, because magnetic lineations in the ocean floor provide a quite detailed record. So the changes in plate motions after the Himalayan collision can be examined worldwide.

**I don’t know how quickly mantle chemistry should respond to the amount of subduction – that gets into questions about how quickly the mantle mixes, and I can’t even say how readily the mantle mixes vertically, let alone how easy it is for an aesthenospheric wedge above a long-lived subduction zone to mix with the wider mantle. But we’re talking about lots of time in the Precambrian, so maybe that isn’t really an issue.

Teaching (some) controversies

My undergrad plate tectonics professor gave us a great exercise, one that really made an impression on me. He had us choose one article or book from a list of articles skeptical about plate tectonics, and write a short paper responding to one of the arguments. I think that was the first assignment I had had in which I evaluated the argument of some other scientist. (I had had plenty of assignments in which I was supposed to figure something out for myself, but I hadn’t really questioned the published literature.)

I stole that exercise the first time I taught tectonics. But it has lost its usefulness with time. The scientists who didn’t buy plate tectonics have retired, and many have passed away, and they haven’t been replaced by younger skeptics. Plate tectonics is old, and settled, and the stuff of Magic Schoolbus books now. Bringing up the old skeptics would be... well, it would be like attacking evolution. Which is another reason why I don’t use the exercise.

But I still want students to encounter disagreements between scientists. For one thing, I figure that if they know how scientists behave when they genuinely disagree, they’ll be able to recognize manufactured controversy. (See: "intelligent design". Or arguments that human production of carbon dioxide does not play a major role in climate.) But that isn't the only reason - genuine scientific arguments are just plain interesting. Science is fun because we don't know everything.

But the assignments can be hard to structure. When the science is at its most interesting, when there’s something unexplained that lots of people are trying to make sense of, the literature... well, it’s chaotic. Every paper has a different explanation, and in many cases, the papers even set up the background in different ways, trying to build the case that their model is best. And it takes a lot of time, and a lot of background, to sort out what data support which model, and to decide which (if any) models are best.

So I tend to choose topics in which groups of scientists have taken sides. Maybe there’s a comment and reply published. Maybe there’s one paper that’s clearly a response to an earlier paper. Maybe there are a number of papers that come down clearly on one side or the other of an issue.

I’ve got a number of different ways to set up discussions. If the papers are fairly short and accessible (like a couple GSA Today articles about whether continental crust behaves more like a jelly sandwich or creme brulee), I have all the students read both papers, and then split the class into two groups to summarize the arguments on either side. If the papers are long, I might spend one day on one paper, and another day on the other. (I did this in Advanced Structure last semester. It worked well in a seminar-style class, but I’m not sure I would do it with anything but a group of very motivated, ready-for-grad-school students.) If there are a lot of papers (such as the papers from the mid-to-late 90’s about the cause of the Laramide Orogeny), I divide the class in half beforehand, and tell each group which side they’re going to be in charge of investigating and presenting.

Sometimes these work really well. The first time I did the Laramide debate with a class, the arguments were pretty new, and I set the class loose in the library, using the Science Citation Index to try to find new articles on one side or the other. I ran into the two teams talking trash to each other in the Science Library – not ideal professional behavior, but enthusiastic, at least.

But sometimes I wonder: am I giving them a misleading impression about the nature of disagreements? When an argument develops two clear sides, it seems like it stagnates. (The Laramide debate has become less interesting, not because the problem is solved, but because it doesn’t seem to be going anywhere new.)

And sometimes the students come away from the discussions with impressions that surprise me. They often believe the most recently published paper – they seem to think that the newest idea must be the most correct one. (But sometimes new ideas get proposed, and published, and talked about... and then rejected. Not big things like gravity or evolution or plate tectonics, but littler things, like the importance of escape tectonics versus orogenic collapse versus lithospheric de-blobbing in the development of a given mountain belt.) Or, worse, the students view the arguments as personal, rather than scientific. Yes, sometimes the arguments do become personal... but that’s not the point. The point should be for the students to evaluate the arguments and figure out which ones are strongest.

Because someday they will be faced with new ideas, and they will need to decide for themselves whether they agree or disagree.

Rock of the (mid) week: mylonitic quartzite

I'm going to jump on the rock-pictures bandwagon, and try to post a rock photo (or outcrop, or thin section - rocks are interesting at all scales) once a week. I was almost ready to post these photos Saturday night, but then my power went out.

I've got two photos this week, of the same rock - one from the side, and one looking down from the top. (I just told my Structural Geology class how important it is to look at an outcrop from all sides. I figure I can try to model that behavior in public.)





These are the rocks that I blogged about in my first post here (which is going to be included in The Open Lab 2007). At first (and probably second and third) glance it looks like a typical quartzite - quartz, and maybe some thin layers of hematite or magnetite or some other heavy mineral, and not much else. And, well, that's what it is... except that it's been thinned to somewhere in the neighborhood of 1/10 its original thickness. Those black lines were probably originally cross-bedding, but I certainly can't tell it from the outcrop. (The evidence for the stretching comes from a comparison with undeformed rocks from the same unit, the Prospect Mountain Quartzite, in nearby ranges, and from the shapes of deformed pebbles in a conglomerate below this unit.)

The top surface shows the black streaks that define the lineation in the rock - the evidence of the direction in which the rock was stretched. It's subtle in this photo, and is often a really subtle feature. (Thermochronic's garnet-sillimanite gneiss also shows a fabulous stretching lineation, maybe more obvious than this one.)

The third piece, which I guess I'm just going to have to supply after I cut a thin section of the rock, would be to show the microscopic texture.

If you want to visit this location, follow the directions from either of these field guides:

Gans, Phillip B and Miller, Elizabeth L, 1983, Field trip 6; Style of mid-Tertiary extension in east-central Nevada, in Gurgel, Klaus D., ed., Geologic excursions in the Overthrust Belt and metamorphic core complexes of the Intermountain region; Guidebook, Part I: Special Studies - Utah Geological and Mineral Survey, vol.59, pp.107-160.

Miller, Elizabeth L, Gans, Phillip B, and Lee, Jeffrey, 1987, The Snake Range decollement, eastern Nevada, in the GSA Cordilleran Section's DNAG field trip guide.

In both guides, it's the Hendry's Creek stop. Out in the middle of nowhere, but still a great field trip stop.

Another fun game: what on Google Earth

Chris Rowan has a fun game up: what on Google Earth. He's posted an image from Google Earth, and is asking commenters to speculate about the nature of the feature.

I've told my structural geology class to look at it, so I'm not going to say what I think it is. Though I really, really want to.

*sits on hands*

*squirms*

Power-less

URGENT - WINTER WEATHER MESSAGE
NATIONAL WEATHER SERVICE GRAND JUNCTION CO
1025 AM MST MON JAN 7 2008

...ONE MORE ROUND OF HEAVY SNOW FOR SOUTHWEST COLORADO AND HEAVY
SNOW EXPECTED FOR EXTREME SOUTHEAST COLORADO...

.HEAVY SNOW WILL INCREASE FROM EXTREME SOUTHEAST UTAH INTO
SOUTHWEST COLORADO. SNOW WILL BE HEAVY AT TIMES THIS AFTERNOON
BEFORE TAPERING OFF IN THE EVENING. VALLEY AREAS OF EXTREME
SOUTHEAST UTAH AND SOUTHWEST COLORADO WILL RECEIVE ANOTHER 4 TO 8
INCHES TODAY AND THIS EVENING. NEW SNOW IN THE SAN JUAN FOOTHILLS
COULD PICK UP ANOTHER 8 TO 12 INCHES.

We've already had a foot and a half of snow at my house (less than half the accumulation at Purgatory, in case you're interested in skiing when the passes open). It's wet and heavy, and the beetle-damaged trees aren't taking it very well. I haven't had electricity since Saturday evening (which means, most importantly, no water - though it also means no internet). I would like to blog more about it, but it's the first day of classes, and I need to teach a topo maps lab (to the students who made it to town), and then go home and hibernate some more.

I hope Ron is doing all right with his southwestern photography expedition. If you read this, Ron: this is not a good time to take photos of rocks around Durango. :D

Mysteries of technology

Our campus has entered the Age of Course-Management Software. (That would be what comes after the Age of Mammals, I'm beginning to suspect.) We're using Moodle, which is an open-source tool. That would be cool, if I programmed php. Which I don't. But I've never used WebCT or Blackboard, so I don't have anything to compare it to.

I've been messing around with it over break, and after backing up and restoring my work a number of times, I've got one question:

Why, exactly, is it possible to assign a due date of 1970?