River landscape evolution

I’ve developed a little cartoon diagram to show four stages of river landscape evolution. I use this image in Physical Geology when discussing how running water erodes the land. Check it out:

There are two rows, and four columns. The columns are the four stages of river landscape evolution: youth, maturity, old age, and rejuvenation. The rows offer different perspectives on the landscape: the upper row is a map view, and the lower row is a cross-sectional view.

The first two columns are shown here in more detail:

When they are young, rivers ideally start out relatively straight in map view, entrenched in V-shaped valleys. You’ll also find plenty of waterfalls and rapids at this “Youth” stage. As time goes by, the river erodes downward to base level, and loses the gravitational impetus to incise any deeper. The river now begins to meander side to side, and as it does so, enlarges the size of its valley by lateral erosion at cut banks. It is “Mature.” As time goes by, the valley walls get further and further apart. …Then what?

If enough time goes by, the river can enlarge the size of its valley so much that you can’t really tell it’s a valley any more. At this stage, meandering can get pronounced enough to fold back on itself and create oxbow lakes (visible in the map view of the “Old Age” stage). The story could conceivably end here. However, if base level were to drop anew, the river will begin to incise again, producing a valley profile (cross-section) that looks pretty much identical to the “Youth” stage. It has been made young again, or “Rejuvenated.” In map view, however, you can see from the meandering shape of the re-incised valley that the river must once have been at the “Old Age” stage. There are no more oxbow lakes in the “Rejuvenated” stage, as the river’s energy is going into downcutting rather than lateral meandering.

My experience is that this nice neat sequence works as a conceptual model for Physical Geology students. Nature, of course, is more complicated, but this serves me well as a foundational framework. What do you think? Is this scheme appropriate for an introductory audience, or is it too simple?

Graph beauty: T vs. viscosity for lavas

I spent the day lazily reading the igneous petrology chapters of Petrology by Blatt, Tracy, and Owens (third edition, 2006). Last time I read it, I didn’t get all that much from the igneous section, but this time around that’s the thing that motivated me to delve into it again. I don’t remember enough about igneous petrology from my school days, and while I have a little breathing room this summer, it seemed to me that I could bone up on it a bit.

One thing that caught my eye this afternoon was Figure 4-15, on page 78. I have redrawn it for you here:

I love beautiful graphs like this. It compares viscosity (resistance to flow, as measured in pascal-seconds; each pa-s is the same as 10 poise) to temperature (as measured in degrees Celsius). Five different compositions of lava are plotted: komatiite, basalt, andesite, dacite, and rhyolite.

First off, you’re no doubt struck by the inverse relationship between temperature and viscosity. The hotter the lava is, the less viscous it is (more runny; easier to flow).

Second, higher-silica-content lavas (rhyolites, dacites) are much more viscous than lower-silica content lavas (basalts, komatiites). Silica (and, to a lesser extent, alumina) form polymer-like chains. The more silica there is (up to 75 wt% in some rhyolites), the more of these sticky, web-like chains can form. This is why you can see lava dripping off the molten basalt in this video, but the molten granite clings to its source rock. Water actually interrupts the formation of these silica polymers, and thus lowers viscosity when it is present.

I’m also struck, looking at this graph, of the difference in temperatures plotted from left to right. This corresponds with observed lava eruption temperatures of different compositions. Low-silica lavas erupt at high temperatures, as they are chock full of high-crystallization-temperature mineral components. (They wouldn’t erupt at all at lower temperatures, because they would be solid.) High-silica lavas erupt at relatively low temperatures, as the components they contain will crystallize into minerals like quartz at relatively low temperatures.

Ironically, though we might think “high temperatures = more dangerous,” the opposite is true. Low-silica lavas tend to erupt effusively, with relatively little risk for human life. You can outrun a lava flow — with rare exceptions, even low viscosity lavas still only flow a few hundred meters per hour. The lower-temperature lavas, on the other hand, are the ones to worry about — because they’re viscous. This “stickiness” means they tend to clog up magmatic plumbing and allow greater pressures to build up. Couple this idea with the tendency of high-silica lavas to also be rich in dissolved gases, and you get a much more explosive style of eruption.

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Reference

Spera, F.J. (2000) “Physical properties of magma,” in Encyclopedia of Volcanoes, H. Siggurdsson, ed. San Diego CA: Academic Press. {Fig. 4}

What does Callan see here?

Tell me why I took this iPhone picture, and I’ll mail you a GEOLOGY ROCKS bumper sticker! Answer in the comments below…

Do they make a MORB sticker too?

A day in the field

I spent last Thursday on a long field trip in the Valley and Ridge province of northernwestern Virginia. Leading the trip was Dan Doctor of the USGS-Reston. Accompanying Dan was a UVA environmental science student named Nathan. And the NOVA crew rounded it out: professor Ken Rasmussen from the Annandale campus, associate professor Victor Zabielski from the Alexandria campus, and me. We met at the Survey at 9am, and headed west towards Strasburg, site of my Massanutten field trip.

We started off by examining three Ordovician carbonate units (all above the Knox Unconformity) on the I-81 exit ramp at Route 11. This is the same sequence seen at the classic Tumbling Run outcrop: the New Market limestone, the Lincolnshire limestone, and the overlying Edinburg Formation. We looked at fossils, stratigraphy, some minor structures, and some interesting lithified gunk on the inside of some solution cavities (small caves). Dan interpreted it as collapse breccia: lithified sediment from inside the cave. The question was: when did it form? We wrestled with the best way to test its age, and didn’t come to any clear conclusions. I love moments like that one: out in the field, one geologist shows another something that’s caught his or her attention, and the other geologist reacts, and the two toy with the idea, batting it around like a cat with an unknown object. Like the cat, geologists will either then get really excited and attack the new idea, or get bored, shrug, and walk away.

Our next stop was Crystal Caverns, a commercial cave that is in ownership limbo. Our spirited guide Babs said that it was likely the last time she would lead a tour down in the cave. She was busy liquidating the artifacts of the adjacent Stonewall Jackson Museum, which had recently been shut down by its board of directors. The cave is accessed via a small building that has been built over its mouth. It was a cool cave with a significant 3D aspect: we descended in a corkscrew like fashion, then came back up via a different route. Very cool. A shame that it is being closed (at least temporarily) to the public.

We followed the cave with lunch at a local Mexican restaurant, and while we were there, a big thunderstorm rolled through. Victor, Dan, and I played dueling iPhones to get imagery of the weather front and plot out our plan for the rest of the afternoon.

The afternoon was spent visiting outcrops on the west side of the Great Valley, working our way up to Route 50, and then west to Gore, VA. I wasn’t especially fastidious about photographing everything we saw, but here’s a sample of where I opened the camera shutter…

Ooids in the Conococheague Formation:

Same shot, zoomed in to the middle:

Fossil (blastoid? crinoid?) stem, Needmore Formation:

There were some lovely Opuntia cactus blooming among the vetch at this Needmore outcrop:

From there, we checked out the Chaneysville Member of the Mahantango Formation, where we saw some snail fossils…

…and some spiriferid brachiopod fossils:

Our last stop of the day was at the Clearville Member of the Mahantango Formation, which had lots of lovely coral fossils in it:

Dan put together a Google Map of our 17 stops; if you’re interested in checking out some of these places yourself, then this is a great resource.

I’d like to publicly thank Dan for taking a work day to contribute to our understanding. It was a lot of fun!

Uniformitarian

Heat-stressed map of the Chesapeake Bay / Washington, DC region, as seen at Kenilworth Aquatic Gardens. Looks like mudcracks, eh?

Similar stresses; similar strains.

Straight nautiloid fossil

It seems I forgot to show this fossil when I found it in February of last year with my MSSE advisor John Graves. We were out in the Needmore Formation of the Fort Valley then. The Needmore is a formation I visited again yesterday with some colleagues in other outcrops further to the west.

In shade (penny for scale):

In sideways-angled sunlight (thumb for scale):

I’ll debrief yesterday’s field trip when I get some more time… for now, let me just toss this little fellow out there for your enjoyment.

If anyone can I.D. it based on these two images, please leave your assessment in the comments section.

Blogspring

World! …I have an announcement!

Three of my structural geology students from this past semester are now geoblogging… can’t say I had anything to do with that, but there it is.

They are:

Joe Maloney at Fossiliferous Weekly

Aaron Barth at Got The Time

and

“AlanP” at Not Necessarily Geology

Please check them out, and give them positive reinforcement. These are three bright young men with strong geological careers ahead of them.

1453, by Roger Crowley

So,  I think I dropped a hint here that I was planning to travel to Turkey this summer. Lily and I will be there from the end of June until the middle of July. (And I’ll be going back in October for the Tectonic Crossroads conference.) In preparation for a trip like this, I enjoy doing some research and reading some books. There are a lot of books about Turkey, and I’ve got at least two more I want to get through before I go, but I wanted to tell you about one that I just finished.

It’s called 1453, and the author is Roger Crowley. The book is a nonfiction account of the battle for Constantinople in the titular year, just one among many attempts by Muslims to conquer this Christian enclave in their part of the world. Situated on a triangle of land between the Bosporus and the inlet called the “Golden Horn,” Constantinople has fended off many attacks over the years. In a position to control commerce between the Black Sea and the Mediterranean, the city was a fabled jewel: it inspired some men’s covetousness and other men’s wholehearted defense.

Image modified from the NASA image (public domain) here

The geography of Constantinople protected it from assault on two sides due to sea (Sea of Marmara side + Golden Horn side), and the third side (facing west) was protected by several layers of defense: a moat, an outer wall, and a taller inner wall. For a thousand years, they had defended their city from outside aggressors. In the year 1453, the Byzantine empire (a.k.a. the “Greeks”) was defending Constantinople from the imperial advance of the Ottoman Empire (a.k.a. the “Turks”). Constantine XI was leader of the Byzantines; Mehmet II was sultan of the Ottomans. After a two-month siege, the Ottomans triumphed, and the city was sacked; its inhabitants enslaved or slaughtered. The city itself was transformed: The Cathedral of St. Sophia was remade as a Muslim mosque, the Hagia Sophia. The name “Constantinople” was replaced with “Islambol,” which became “Istanbul” with time.

It was a battle between two “great” world religions, with “great” in this case meaning “big.” Both prayed to their respective ideas of God, beseeching the deity that their cause triumph over their enemies’. Every time something positive happened to advance their cause, they sang the praises of their chosen deity. Of course, when things didn’t go their way, they did not infer that their deity had forsaken them (or that no deities were involved), but that they only needed to pray harder.

Powered by their deep religious faiths, they cut one another’s heads off, chopped one another into pieces, impaled one another on pikes. In addition, the raiding Muslims speared babies in Constantinople and raped women. (If it were Christians were to have attacked a Muslim stronghold instead, it seems clear that the “morals” would have led to similar depravity and atrocity in the other direction. That didn’t happen in 1453, since the Muslim army was all men, and the residents of Constantinople were mixed men, women, and children. For example, the Fourth Crusade, a group of warrior Christians, sacked Constantinople in 1204: this Christian attack on other Christians was the only previous occasion when the city had fallen to invaders. )

I came away from this book:

1. …thinking “wow” — all this shit played out more than 650 years ago, and Constantinople was already a thousand years old when that happened! History is deep in some parts of the world. Even “civilized” history…
2. …thinking “wow” — the siege of Constantinople is an epic saga, far more compelling than Troy. The back and forth, the ingenuity, the personalities involved in the 1453 battle: they are epic. This book would make an amazing movie. Read it to find out how Mehmet gets his boats into the Golden Horn, or how Constantine protects the city walls from Mehmet’s cannonfire. How does the Pope react to the Ottoman threat? How to the Genoese merchants on the north shore of the Golden Horn attempt to remain neutral while secretly pulling for Constantine? How close did Mehmet come to giving up?  I won’t reveal this wild stuff here but it’s a roller coaster ride.
3. …thinking that Roger Crowley is an exceptionally talented writer, if he can make me care about these people, this event, lost in the netherworld between 10,000 BCE and the present. This man can write. What else has he written? Bring them to me!
4. …thinking that I am very motivated to visit some of the critical locales mentioned in this book, where the battle for Constantinople took historical turns one way and the other. I’ll be in Istanbul a month from now; Stay tuned.
5. …thinking how astonishing it is that people continue to think that their religion is the right one, in spite of being surrounded by other people thinking the same dang thing about different deities. This tribal thinking (“my people are the chosen ones; your people are infidels”) leads to tremendous suffering and bloodshed: people who don’t think the same things you do are by definition no longer people, and may be treated as non-human. [begin rant] I am reminded of a quote from Steven Weinberg: “With or without religion, good people can behave well and bad people can do evil; but for good people to do evil—that takes religion.” For thousands of years, our species has fought itself with tribal self-righteousness. It’s always “us” and “them,” and religion is the most frequently-adopted tribal cloak. In their own minds, religion absolves its practitioners from their atrocities;  by supplanting reason, it leads to unspeakable acts and horrific history. I am impressed by those religious individuals who think critically about their faith’s offerings, and apply the theological precepts with a modicum of common sense and an independent sense of ethics. But many religious people disappoint me deeply, with a series of actions that wreck the world along with any notion of consistency or moral “high ground.”  After reading about the battle for Constantinople, or experiencing the 9/11 attacks, or following the daily news, I can’t help but think the world would be a better place with a purely natural sense of ethics, and supernatural moral frameworks banished to the dustbin of thought. “Hypothesis not supported.” [end of rant]

If you are at all into history, or at all into Turkey, read 1453. As a rule, I’m not into history, but I am very grateful that fellow Turkey traveler Greg Willis recommended this book to me. In return, I loaned him Orhan Pamuk’s Istanbul: Memories and the City. I look forward to offering my review of that tome in the weeks to come.

Butter Buster animation

A million years ago, I posted about my inaugural attempt to use the Butter Buster to illustrate shear zone deformation to my structural geology students.

Today, using the UnFREEz program to make an animated GIF (Thanks, Lockwood!), I give you the Butter Buster animation: