Kyrgyzstan – Part I: What/Why/Where?

I’ve started to develop a backlog of things to write about, and seemingly less and less time to actually write. Most significantly, probably, I need to write something (anything!) about my trip to Kyrgyzstan. I’ll start with some overview stuff here, and talk about specifics in later posts.

Q: Whaaat? You went where? Why? How? Where the heck is Kyrgyzstan, anyway?
A: Our department has a fund for sending a bunch of students to study geology in a remote place every couple of years. It’s always a place where one of our faculty has an active research project, and it switches each time. The last trip was just before I got to UO, my adviser led a trip to Spain and Morocco. This year the destination was Kyrgyzstan. Kyrgyzstan is a small central Asian country sandwiched between Kazakhstan to the North, China to the East, and Tajikistan and Uzbekistan to the South and West respectively. More intuitively, it’s here:

Kyrgyzstan Map

Q: What active research are UO workers doing in Kyrgyzstan?
A: Ray Weldon, one of the UO geology faculty members, has been working for over 20 years on a continental dynamics project with Dr. Kanat Abdrakhmatov, the director of the Institute of Seismology at the Kyrgyz National Academy of Sciences. They’re trying to understand the mechanisms of deformation that have uplifted the Tien Shan mountain range, and by proxy understand seismic hazards associated with the active tectonic deformation. There have also been countless grad students over those 20 years who have done their dissertation work on that project. Most of which has been geologic mapping of some of the major fault zones. Most recently, Ray has been trying to interest the paleontology group in helping to pin down absolute ages on some of the important rock layers by identifying fossils in them. This year my friend, Win, got a grant through the Fulbright Program to work towards that goal by looking for fossils in the Tien Shan mountains for 10 months next year.

Q: Why research Kyrgyzstan? What’s significant about those mountains?
A: The Tien Shan mountains are the northernmost extent of the Tethyan Orogeny, a result of the continental collision between India and Eurasia which also created the Himalaya. Both mountain ranges are the result of horizontal compression of the Asian continent: as it gets squished horizontally, there’s nowhere to go but up (and down, but these structural geologists don’t usually care about that). The Himalaya have been around for a long time now, and the Tien Shan are relatively young. The Himalaya are now increasingly inactive, and most of the modern deformation is taking place farther North.

Currently, around 50% of the total shortening of the Eurasian continent, is taking place in the Tien Shan [1]. That shortening has to be accommodated somewhere. Deep in the crust, the rock is warm enough, and under high enough pressure, that it can deform slowly over time, but near the surface that stress is accommodated through sudden ruptures, ie. earthquakes.

Just as an example of the active seismicity, according to the USGS earthquake catalog, http://earthquake.usgs.gov/, there have been 21 earthquakes greater than magnitude 4.0 in the Tien Shan in the past 30 days.
Tien Shan Mountains

Kyrgyzstan might be the most seismically active region in Asia, but the record of historic earthquakes there is surprisingly sparse. As the population has grown, and urbanization increased since the beginning of what everyone there just calls “soviet times”, building and infrastructure regulation needs to start taking in to account the possibility of very large earthquakes, but nobody knows how often such an earthquake might happen, or how big it could be.

During our trip we learned a bit of everything about Kyrgyz geology, but for most of the trip we focused on understanding the history of uplift and deformation, to better constrain the net rate of movement on the major faults, and ultimately determine which ones could present significant seismic hazard.

Of course we also got to meet lots of very cool people, and see all kinds of beautiful places; and meet some shady people, and see some very ugly places, but more on all that in later posts!

[1] Abdrakhmatov, K. Ye, et al. “Relatively recent construction of the Tien Shan inferred from GPS measurements of present-day crustal deformation rates.” (1996): 450-453.

Trip Report: North Sister (May 31, 2014)

This is a few months old now, but I think it’s still worth posting about.

In an effort to train for our upcoming trip to the Tien Shan mountains in Kyrgyzstan, my roommate, Dylan, and I have been planning out climbs on some of the more technically challenging Cascade volcanoes. Of course none of them can prepare us for the altitude, but that wouldn’t last anyway. Mainly we want to get some experience with technical climbing in the mountains, as most of our climbing experience is bouldering, sport, and small trad routes. Between us we have effectively zero practical experience with technical alpine climbing, but that needs to change soon, and fast.

We thought North Sister, out near Bend, might be a good place to start because its standard route is apparently considered the most challenging of the standard routes in the Oregon Cascades. Also, it doesn’t involve any mandatory glacier travel.

We watched the weather during the week leading up to the trip, and it wasn’t looking promising. The NOAA forecast predicted a chance of thunderstorms, and the overnight low temperatures were barely going to get down to freezing. We were ready to call off the plan until the day of, when we made the last minute call to at least drive out and see for ourselves what the conditions were like.

We stopped by Backcountry Gear on the way out of town to grab some alpine gear. We picked up two small snow pickets, one big snow picket, a snow fluke, and an ice screw. Unfortunately we only had one 60m non-dry rope, which we knew would be less than ideal, but neither of us was ready to drop a few hundred bucks on a set of dry treated half ropes.

We left Eugene mid afternoon and got to the trail head around 6pm, less than ideal considering how far in we were hoping to make it to set up camp. The trail was really easy to follow. It goes more or less straight most of the way. It cuts across the width of the Pole Creek Fire burn. The beginning of the hike is really surreal because of it. There is literally nothing left alive in most of that area. The soil has been burned to dust and there is not a sign of any regrowth in the middle of it. Towards the edge of the burn area there is regrowth working its way in, but you can really see the effects of decades of fire suppression in that area; most of it is just completely dead.
DSCN1729

Once we made it past the burn a ways we started hitting patches of snow. It wasn’t long before we completely lost the trail and started route finding based on what short glimpses of North sister we could get. The hike from there to camp did have a few spectacular views of the sisters, and of the North side of Broken Top. We were determined to make it past tree line before camping, which turns out to be a pretty big hike when you get a 6pm start, but we finally made it to the South-East flank of the mountain just past treeline just before dusk.

The next morning we got up around 3:30am and were hiking by 4. We started out the day with a classic Dylan moment. He was trying to figure out a way to carry the big snow picket that would be stable and not awkward, by wedging it sideways between his pack and his back. At the top of the first hill we hiked up, the picket slipped out and skated on the snow all the way back to the bottom. He ran all the way down after it but wasn’t able to find it. We had left the ice screw at camp so we went the rest of the way with two small snow pickets, and a fluke.
DSCN1734

The hike felt long, mostly because of the endless scree you find on all of the Cascade volcanoes. North sister is made of particularly chossy rock, which makes the situation extra bad. In some ways it’s surprising that the mountain is standing at all. The whole mountain is basically just a pile of gravel at the angle of repose. It seems to be precariously held in place by a series of slightly more competent dikes[1]. Hiking gets easier once you’re high enough to travel on only snow, which was relatively high on the mountain by late May.

We hiked up the Southeast ridge to the summit pinnacle. Route finding was was easy through this section, except one point where we missed an obvious path over a gendarme and traversed some steep snow below it instead.

The summit pinnacle is the only technically challenging part of the climb. It starts with a section called “The Terrible Traverse” which didn’t quite live up to its name, but is certainly more difficult than anything lower on the mountain. The snow was getting soft by the time we got to it and I almost called off the whole thing at that point except that there were still a ton of people heading out on the traverse which gave me hope that we weren’t too ridiculously late. We roped up and pitched out the terrible traverse (with some simul climbing) on our now-soaking-wet 60m sport climbing rope. That was a hassle, and we moved slowly, but I think we were pretty efficient in general. The traverse its self is just a steep snow slope. Crampons and two ice tools were good to have, but there wasn’t anything technical, and no solid ice to speak of. The only thing that made the terrible traverse nerve racking was the constant rock fall from the summit pinnacle. The mountain is actively trying to self destruct.
P1040578

The last pitch of the climb is, aptly, named “The Bowling Alley” because of the constant rock and ice cascading down from the true summit. Fortunately, the climbing route is actually pretty much free of rock fall, the real show is going on about fifteen feet to your left. The climb is a short pitch of 70 degree alpine ice through a chute on the Northwest side of the summit pinnacle. This is the section where I really wished we had brought the ice screw. Dylan belayed me on an anchor made of one small picket and both of his ice tools. I placed the other picket low on the pitch, before I hit the solid ice. Then I effectively soloed the rest of the pitch, with the added risk of dragging Dylan down the mountain with me in the case of a fall. Fortunately the climbing isn’t technically very difficult, but the exposure freaked me out a bit. I mostly just tried to not think about it.

Somebody graciously left a fixed webbing sling at the top of the pitch which I clipped in to as soon as I was close enough to reach it and belayed Dylan from there. There’s about 25 feet to the end of the real climbing, which we belayed as another very short pitch. We could have easily linked the two pitches with the 60m rope but I knew I wouldn’t be able to build an anchor that solid at the top with just the snow fluke so I chose to belay Dylan through the difficult stuff from the sling.
DSCN1744

We were on the summit at about 12-12:30. Good views, nice weather.
DSCN1751
P1040573

The descent was actually harder than the ascent in many ways. We were a little burned out, and without double ropes, we couldn’t rappel through the whole bowling alley. We basically down-lead climbed to the anchor, rappelled the main ice section, and down-lead climbed the lower snow part. Then we pitched out the terrible traverse, the same way we came across it in the morning. It took a long time, and by the time we were done the snow conditions had gotten pretty bad. The rock fall had increased as it had warmed up, and the sun was almost on the traverse so the snow was getting mushy. I didn’t check the time, but I’d say we were back to the beginning of the terrible traverse by about 1:30 or so. It’s not much distance, but we were moving pretty slow.

Hiking out was a bit of a chore, too. Dylan was really crashing because he decided to eat all his food before we even got to the terrible traverse. We made it back down to camp by about 4 and packed up all of our stuff.
P1040581

The rest of the hike out was uneventful, but felt long. We got back to the car after dark, around 27 hours after we started, with about 3.5 hours of sleep under our belts and a lot of walking, but it was good to be back and I was actually feeling very awake. I bought a ton of candy bars and chips at the gas station in Sisters and drove back to Eugene. By the time we got home we were both pretty burned out, but in a good way.

Our roommate, Ben, didn’t believe that we had taken over 24 hours to climb just one mountain and was convinced that we had sneakily linked up Middle Sister too. Everything has to be really fast with him. He was pretty bitter that he hadn’t gotten to join. I don’t think he would have enjoyed it.

[1] Schmidt, Mariek E., and Anita L. Grunder. “The evolution of North Sister: A volcano shaped by extension and ice in the central Oregon Cascade Arc.” Geological Society of America Bulletin 121.5-6 (2009): 643-662.

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Oregon Dunes

This gallery contains 9 photos.

All of my roommates are gone right now for various reasons, so with nothing better to do last weekend I drove out to the coast to camp by myself for a night. I stayed at Jesse Honeyman state park, just … Continue reading

Synchronizing Documents Across Computers

This post is a bit different than what I’ve been posting lately, but I’ve spent a lot of time thinking about this problem recently and I wanted to post what I’ve found for the sake of documenting something that I think is useful.

As anyone with more than one computer knows, keeping documents synchronized between computers is a problem which, considering how common it is, is surprisingly poorly addressed by existing solutions. Most common problems like this have dozens of implemented solutions and agreed upon best practices but everyone seems to have different ideas about this particular one, and none of them seem very well implemented.

Of course I’m aware that services like DropBox and Google Docs do exactly what I’m about to describe, but I’m skeptical of any service that describes its self using the word “cloud.”

If “cloud computing” has a meaning, it is not a way of doing computing, but rather a way of thinking about computing: a devil-may-care approach which says, “Don’t ask questions. Don’t worry about who controls your computing or who holds your data. Don’t check for a hook hidden inside our service before you swallow it. Trust companies without hesitation.” In other words, “Be a sucker.” A cloud in the mind is an obstacle to clear thinking. For the sake of clear thinking about computing, let’s avoid the term “cloud.”

-Richard Stallman, Who does that server really serve?

I’d prefer to keep my data safely on my own computers, and not give it away to be mined for advertising keywords. The solution I’ve come up with is just a combination of commonly available tools to build ones own dropbox-like service.

First, a little on some alternatives I’ve looked at. Of course there are DropBox and Google Docs and the like, but those come with privacy concerns which I’d rather avoid.

Then there are services like SpiderOak, which is closer to what I want. I used their service for a few months back in 2011 and the client eventually randomly corrupted all of my documents, which were between backups so I lost a lot of important stuff. Maybe it’s gotten better since then, but that left a bad taste in my mouth. Also, their client is mostly proprietary software. They claim to be willing to open it up, but they have yet to do so.

Then there are the half dozen packages intended to run on ones own server which all claim to do exactly what I’m looking for. Unfortunately, none of them worked out for me:

  • NFS/sshfs. One option would obviously be to keep all of my files on a network mounted filesystem. That way, the exact same files are accessible from anywhere. This solution works fine, but the files load slowly if you’re not physically close to the server and they won’t be accessible when away from internet access.
  • SparkleShare is a great tool, and I like that it’s just a thin interface to a standard git repository. The downside is that the git tree grows out of control if you have files that change very often. I wanted to be able to synchronize my Thunderbird profile, and within 24 hours my 5GB profile had grown to 35GB.
  • OwnCloud. Also a cool idea, but it has lots of features I don’t need and the synchronization protocol (csync) is too slow.
  • Bittorrent Sync is proprietary software so it has the same problems as dropbox, etc.

What I need is something that transparently fits in a small script; based on simple, well-tested utilities; is easy to maintain across heterogeneous systems; and is easy to back up in case something goes wrong.

What I found is a utility called Unison that uses the rsync protocol for efficient transfers, and never leaves the system in an unusable state. If there are conflicts it simply does nothing to avoid making simplistic assumptions. Fortunately I rarely create conflicting files between devices because I don’t often use two computers at once. I typically do things like change files at home, and like to have those same files automatically updated on my work computer by the time I bike in.

I set up my network in a “star” topology where there is one server that holds the master copy, and all the others synchronize with it. The server keeps hourly backups of the home directory and has duplicated hard drives in case one breaks. I generally don’t like centralized systems like this, but every client has its own full copy of every file so there is plenty of redundancy.

I could spend a while explaining my whole file server setup, but the basics are that I have a server called “Andromeda”, for which all of my other computers have passwordless SSH keys. To keep them synchronized, each computer runs this small script through cron every ten minutes to sync up with Andromeda.

#!/bin/bash
 
args="-batch"
 
if [ $# -gt 0 ]; then
    case $1 in
        "check")
            # Just print out the status of the last unison run
            grep "^Synchronization" ~/.unison/unison.log | \
                tail -n 1 | awk '{print($1,$2,$3,$4)}'
            exit
            ;;
        "manual")
            # Do not use Unison's -batch option,
            # ask for user confirmations
            args=""
            ;;
        *)
            echo "Unknown option: $1"
            exit
            ;;
    esac
fi
 
args="$args -sshargs -C"
 
# Use different ssh profile if I'm on home LAN
ap=`/sbin/iwconfig 2>/dev/null | \
    grep "Access Point" | \
    awk '{print($6)}'`
if [ $ap == "1C:7E:E5:3B:BD:68" ]; then
    echo "At home, using LAN IP address."
    ip="Andromeda"
else
    echo "Away from home, using DNS IP."
    ip="Andromeda-remote"
fi
 
## Wait until server is done syncing with other clients
echo -n "Checking whether unison is already running on server..."
isrunning=`ssh $ip "ps aux" | grep -c "unison"`
niter=6
while [ $isrunning -gt 0 ]; do
    echo ""; echo -n "Waiting 10 seconds for server to free up..."
    sleep 10
    isrunning=`ssh $ip "ps aux" | grep -c "unison"`
    if [ $niter -le 0 ]; then echo " Failed"; exit; fi
    niter=$(( $niter - 1 ))
done
echo " OK"
 
## Finally, sync with server
unison $args /home/jmp ssh://$ip//home/jmp \
    -path Documents \
    -path .thunderbird \
    -path .gnupg

To take this even further, I wanted to make sure that if I updated this script, it would run the same way on all of my computers. In other words, I wanted to synchronize the synchronization process. To do that I planned on uploading the above script to a server, then making a smaller script which wouldn’t ever need to be changed, that would download and execute the actual commands.

I realize that there are security concerns associated with automatically executing commands downloaded from the internet. For instance, if someone were to intercept the script on its way to me, they could change the commands and have my computer automatically execute arbitrary code. To avoid that hypothetical scenario I could have just put the actual script on my server, and access it by scp. Instead I chose a slightly more involved method.

I first sign the script with my PGP key, then upload it to the public internet:

gpg --sign sink.sh
scp sink.sh.gpg jonper24@invertedearth.net:invertedearth.net/content/
rm sink.sh.gpg

and when my main script accesses it, it verifies the signature before executing the content:

#!/bin/bash
 
## Check internet connection
echo -n "Checking network connection..."
ping -c 1 invertedearth.net >/dev/null 2>/dev/null
if [ ! $? -eq 0 ]; then echo " Failed"; exit; fi
echo " OK"
 
# Download and verify authenticity of script
script=`mktemp`
siginfo=`mktemp`
 
echo -n "Downloading script..."
curl http://www.invertedearth.net/content/sink.sh.gpg 2>/dev/null | \
    gpg --decrypt 2>$siginfo > $script
goodsig=$?
echo " OK"
 
echo -n "Verifying signature..."
key=`awk '/Primary key fingerprint/{$1=$2=$3="";sub(/^[ \t]+/, ""); \
  print $0}' $siginfo`
if [ $goodsig -eq 0 ] & \
   [ "$key" == "4F7D 3BC7 C595 DE5D 7A13 1EBC 67BA C49D 4561 1A51" ]
then
    echo " OK"
    chmod +x $script
    $script $@
else
    echo " Bad signature!!!"
fi
 
rm $script $siginfo

One could argue that this is unnecessarily nerdy, which is true. There are simpler ways to do this, but I like that this solution could also be used for sharing more public scripts online, etc. Also, it’s a nice use of GnuPG, which I always like.

Anyway, whenever I think about problems like this that should have been solved once and for all a long time ago, I’m reminded of this particular XKCD comic:

File Transfer

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Charlotte’s Visit: Broken Top + McKenzie River Trail

This gallery contains 85 photos.

My sister, Charlotte, visited this past weekend with her girlfriend, Lizzie. We had a big weekend hiking up Broken Top mountain near the Three Sisters and Bend, and also mountain biking the McKenzie River Trail. Broken Top was really nice. … Continue reading

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Bishop, CA Spring Break 2014

This gallery contains 15 photos.

During spring break I headed down to Bishop to boulder for a week with a couple of friends and climb Mt. Tom if weather permitted. We were lucky enough to get a nice day on Mt. Tom and plenty of … Continue reading

Lithospheric Delamination in the Western U.S. – Part IV: Delam Styles / Type Localities

In my last post I tried to explain the fundamentals of delamination. I used a floating wooden board being weighed down by metal as an analogy for the state of the Earth’s lithosphere. In the Earth, the crust is buoyantly floating on top of the mantle and is being weighed down by cold mantle material stuck to its bottom. If the two materials were to separate, the cold mantle material would sink off in to the warmer mantle below and the buoyant crust would rebound, rapidly building mountain ranges. So, in the most general sense, delamination refers to the process of heavy material peeling of the lithosphere and sinking down in to the mantle. This can be manifested in a few different forms.

The first form I’ll mention is the one which most resembles the analogy I gave before. In this case the cold mantle lithosphere peels back from the crust and sinks away in to the mantle (Figure 1). Hotter mantle material from deeper in the Earth replaces the cold lithosphere as it recedes.

Peel-back delamination

Figure 1. Mantle lithosphere can peel off from the base of the crust, forcing hotter mantle to flow in to its place.

This sort of thing is most likely happening beneath the edges of the Colorado Plateau, contributing the dish-like shape to its edges [1]. Whether or not this is actually the case is a contentious question but the evidence is convincing enough to mention it here.

Secondly, it is possible for the mantle lithosphere to drip off, with similar shape to the form which cold wax at the top of a lava lamp takes as it convects back toward the bottom. This is known as a “Rayleigh-Taylor Instability.”

Delam-Drip

Figure 2. Mantle lithosphere can drip off of the crust as a Rayleigh-Taylor instability.

This appears to have happened as a late stage in the formation of the Wallowa Mountains in northeastern Oregon [2].

Lastly, lithosphere can be forced to depth by horizontal tectonic forces from the plates. In this case, the lithosphere isn’t really delaminating because of its own inherent density instability, but because it is being pushed to depth by the material above it.

Delam-compression

Figure 3 Horizontal tectonic forces can push lithosphere deep in to the mantle.

This is apparently the case for lithosphere beneath the transverse ranges of southern California, around the Los Angeles basin for instance [3].

My research is focused on understanding the mechanisms that enable each of these processes to happen. Assuming the mantle lithosphere is always buoyantly unstable, can it actually ever stably stick around? Is delamination the exception or the rule? And what controls whether the lithosphere will drip off in small Rayleigh-Taylor stabilities or in massive intact slab roll-backs.

All of these questions have impacts on, and feedback from other, associated tectonic and magmatic processes. I’m interested in untangling the causes and effects of each of these mechanisms and placing them in real-world context using the western US as a natural laboratory. All of my experiments have already been run for me, I just need to sort out what happened.

References

[1] Levander, A., et al. “Continuing Colorado plateau uplift by delamination-style convective lithospheric downwelling.” Nature 472.7344 (2011): 461-465.

[2] Darold, Amberlee, and Eugene Humphreys. “Upper mantle seismic structure beneath the Pacific Northwest: A plume-triggered delamination origin for the Columbia River flood basalt eruptions.” Earth and Planetary Science Letters 365 (2013): 232-242.

[3] Houseman, Gregory A., Emily A. Neil, and Monica D. Kohler. “Lithospheric instability beneath the Transverse Ranges of California.” Journal of Geophysical Research: Solid Earth (1978–2012) 105.B7 (2000): 16237-16250.

Lithospheric Delamination in the Western U.S. – Part III: Delamination

In my last post I covered the basics of mantle dynamics and now I should be just about ready to dive in to the actual content of this series of posts: lithospheric delamination. I’ll start by just explaining the concept. In my next post in this series I’ll talk about type-localities in the western U.S. and some basics on the methods I use to study the process. I’ll finish the series by explaining specifics of my research and where I hope to go with it.

In geodynamics, delamination refers to a process in which something that was formerly attached to the lithosphere becomes unstable and falls off, sinking deep into the mantle. This can happen if the material is substantially more dense than the material below it. We call this a buoyant instability.

A nice way to visualize what’s happening, and an analogy that I’ll return to in a moment, is to imagine a piece of wood floating in water with a heavy chunk of metal glued to its bottom. The wood in this case represents the buoyant crust and the metal represents the cold upper mantle that’s stuck to its bottom. The water it’s floating on represents the rest of the mantle. Delamination would be the process of the glue that holds the two together coming undone and allowing the metal to sink away.

So, why would we care? This is a process that happens tens or hundreds of kilometers below the surface of the Earth so how does it affect anything else on Earth?

First of all, in Earth sciences even the processes which seem most esoteric and intangible usually have massive consequences to other processes and systems which directly affect life on Earth. Secondly, it’s just interesting, okay? It shouldn’t need justification to be interesting.

In this case however there is a direct, tangible consequence that’s fairly interesting in its own right. Let’s go back to the floating board being weighed down by the chunk of metal analogy for a moment. Basic physics tells us that the buoyant force pushing the board up equals the weight of the displaced water and that the system will find equilibrium when the weight of the floating stuff equals this buoyant force. Since the metal is dense, when the two are attached the whole thing will float relatively low (assuming it’s not so heavy that it just sinks all together). When the metal comes loose and sinks away, the board is much less dense and will float higher in the water. It’s similar to a boat weighed down with cargo floating super low until the cargo is unloaded.

So, what does this look like in geophysical terms? If there’s heavy material attached to the bottom of the crust, it’s being weighed down and will float relatively low, but if the heavy material falls off the crust will float up higher. This is usually expressed on the surface as (relatively) rapid uplift. In other words, it can build mountains.

We suspect that this has happened in the southern Sierra Nevada (adding to an already impressive mountain range), the Wallowa Mountains in eastern Oregon (building a mountain range entirely from scratch), and around the edges of the Colorado Plateau (responsible for its dish-like topographic expression).

I think I’ll cut this post short here and talk about the different forms of delamination in my next post. As illustrations, I’ll give specifics on the three type-localities I just mentioned: the southern Sierra Nevada, Wallowas and Colorado Plateau.

Lithospheric Delamination in the Western U.S. – Part II: Mantle Dynamics

In my last post I tried to do a quick overview of seismic tomography. The takeaway message being that it is possible, through seismology, to “see” structures inside the Earth. It’s how we know that the Earth has layers, for example.

Other significant structures that we observe with seismic imaging are seismically fast (cold) regions beneath subduction zones where we can see tectonic plates being subducted deep into the mantle, and seismically slow (hot) regions beneath mid-ocean ridges. All of these examples are to be expected from the basics of plate tectonic theory and the fact that we see them seismically nicely supports both plate tectonic theory and seismic imaging theory.

More interestingly, in some areas we see hot and cold blobs in places that aren’t obviously associated with plate tectonics. Some examples from the western U.S. are circled in the seismic image from the last post.

This would be a nice point to lead me in to my topic, lithospheric delamination, but I feel like I should first take a tangent to explain something that I think is non-obvious about the way the Earth operates, and come back to my main point in my next post.

The Earth’s mantle makes up a huge percentage of the mass of the Earth. It’s a chemically distinct layer between the crust and the core, encompassing almost 85% of Earth’s total volume. It’s very hot but is also under enormous amounts of pressure, the deeper you go, the more pressure the mantle is under. This has weird mineralogical and rheological consequences. The mantle can be divided in to several distinct layers in which pressure and temperature trade off differently. From the top down:

At the top of the mantle there is a cold region which is, for the most part, frozen on to the bottom of the crust. The crust floats at the surface because it is chemically buoyant (made of much lighter materials) and the top of the mantle hangs on to the bottom of it. If it weren’t being held up by the buoyant crust it would prefer to sink deep in to the mantle because it is cold and dense. This frozen mantle along with the crust is collectively called the lithosphere. The part of the lithosphere which is made of mantle rock and not crustal rock is referred to (not very imaginatively) as the mantle lithosphere.

Earth Layers

Relative geometries of the compositional layers of the Earth, drawn to scale.

Below the lithosphere, the mantle is hotter but it isn’t under too much pressure so it’s able to flow relatively easily. It’s not melted (at least not entirely, this is an area of some debate) because the pressure forces it to be a solid. It is hot enough and under sufficiently low pressure that it can “flow” by allowing crystal grains to slide past each other and deform. This layer is called the asthenosphere and it allows plate motion by decoupling the lithosphere from the rest of the mantle..

Below the asthenosphere, the mantle is very hot but is also under much more pressure. It can still flow but significantly more viscously. Flow in this part of the mantle is dominated by whole-mantle scale convection. Much like the way water flows around in a pot to equalize temperature, the mantle is heated from below and cooled from above so there is a constant flux of material rising and falling throughout.

Mantle convection

The mantle is able to convect similarly to a pot of water being heated from below and cooled from above. [Figure generated with ASPECT mantle convection code]

The convecting interior of the mantle is all made of the same elements but is actually divided in to layers of its own because higher pressure at depth can force the minerals to reorganize their structure (think graphite -> diamond style transition). There are mineralogical transitions at 410km and 660km depth, as well as a possible transition right near the core-mantle-boundary at around 3000km. For the purpose of my story though, we’ll assume that the mantle is homogeneous below the asthenosphere.

Now that I’ve given a quick overview of the way the mantle acts and moves, I think this is a reasonable place to stop for now. Next time I’ll get to the actual content of my research and talk about the ways in which the cold mantle lithosphere can come detached from the crust to sink back down in to the mantle.

Lithospheric Delamination in the Western U.S. – Part I: Seismic Imaging

I recently put together a poster about lithospheric delamination in the western U.S. for the CIDER workshop that’s coming up in Berkeley and I thought I’d share the content here too.

I’d like to start with the basics to tell the whole story but it’s a long one so I’m gonna split this in to several installments. This first one is just going to fill in some background on methods we use in Geophysics, starting with teleseismic imaging of the Western U.S.

Seismic Imaging: What Is It And Why Do We Care?

As a geodynamicist I’m interested in deep-Earth processes. Most of my work is focused on understanding what’s inside the Earth, where it came from and where it’s going. More specifically, I’m interested in modeling deep-Earth processes, i.e. using analogy of a simplified system to explain the behavior of a complex one like the Earth’s mantle. Of course, just making up analogies between simple systems and complex ones isn’t really science. To relate our simplified system to the real world we need an idea of what the real system looks like and how it behaves.

Geophysicists have relatively few tools at their disposal for seeing the structure of the Earth at depth: geochemists can tell us about the origin of rocks once they emerge at the surface, and geomorphologists can tell us about the evolution of landscapes which can both give hints at the processes happening at depth (or at least processes that have happened at depth in the past) but neither gets at the current state of the Earth below a few kilometers.

That’s where geophysical imaging comes in.  I’ll be specifically talking about teleseismic imaging. The concept is relatively simple: seismic waves from earthquakes all over the planet propagate through the Earth’s interior and interact with the different structures they encounter along the way. Namely, they travel faster through some materials than through others. When the waves arrive somewhere far away we can measure the relative delays among arrival times at different seismometers and use that information to reconstruct an image of the structures through which they passed.

Seismic wave paths

As seismic waves propagate through the earth, they pass through lots of different structures and interact with the Earth’s interior in many different ways.

Techniques for seismic imaging have been meticulously developed over the past century or more, but in the past decade advancements in, and access to, computational hardware as well as amazing data from the EarthScope project have enabled unprecedented progress in imaging structures beneath the U.S.

Seismic image at 195km depth below the western U.S.

Horizontal slice of seismic wave velocity through the upper mantle in the western U.S. (Schmandt, 2010)

Things that affect the wave speed include chemical properties, pressure, and directional strain but the one we’re most interested in studying is usually the temperature. The other effects on seismic velocity are either easy enough to correct for (like pressure) or small enough effects over large distances that we usually assume all seismic velocity perturbations are caused by temperature perturbations. Typically, a rock at low temperature conducts seismic waves more rapidly than the same rock at higher temperature. Temperature is an interesting property because it can tell a lot about where an object came from and where it’s headed.

Since most materials, including rock, are less dense when they’re hot, temperature is typically a good indicator of relative buoyancy. For example, when we see a cold object we can guess it’s heavy and therefore actively sinking deeper in to the Earth, or it at least wants to. There are other methods of using seismic waves to measure different properties about the Earth but temperature and buoyancy of objects in the mantle are the most interesting to me.

In my next installment I’ll talk about the significance of some of the objects we see in the most recent seismic images, such as the figure above.

References

Schmandt, Brandon, and Eugene Humphreys. “Complex subduction and small-scale convection revealed by body-wave tomography of the western United States upper mantle.” Earth and Planetary Science Letters 297.3 (2010): 435-445.