Volcanos and convergent margins go together. Typically, the plate being subducted melts as it is pushed deeper into the Earth and temperatures rise. It also helps that the water in the crust and sediment of the subducting plate makes it easier to melt, and makes the resulting magma much more volatile and explosive.
The subducting plate melts producing volatile magma.
But although Shinmoedake is in Japan, it is not on the same tectonic boundary as the earthquake. The northern parts of Japan are where the Pacific Plate is being subducted beneath the Okhotsk Plate. This volcano is connected to the subduction of the Philippine Plate to the south.
The large earthquake's epicenter and the Shinmoedake volcano are on different plate margins. Image adapted from Wikimedia Commonsuser Sting.
This does not necessarily mean that the two occurrences are totally unrelated. Seismic waves from the big earthquake could have been enough to incite magma chambers that were just about ready to blow anyway.
The map below is centered on the series of craters in the region of the erupting volcano.
The magnitude 8.9 earthquake that devastated coastal areas in Japan shows up very clearly on the United States Geologic Survey’s recent earthquake page.
The big red square marks an aftershock of the magnitude 8.9 earthquake off Japan. (Image via USGS). Note that most of the earthquakes occur around the edge of the Pacific Ocean (and the Pacific Plate).
Based on our studies of plate tectonics, we can see why Japan is so prone to earthquakes, and we can also see why the earthquake occurred exactly where it did.
The obvious trench to the east and the mountains and volcanoes of the Japanese islands indicate that this is a convergent margin. The Pacific plate is moving westward and being subducted beneath the northern part of Japan, which is on the Okhotsk Plate.
The tectonic plates and their boundaries surrounding Japan. The epicenter of the earthquake is along the convergent margin where the Pacific Plate is being subducted beneath the Okhotsk Plate. Image adapted from Wikimedia Commonsuser Sting.
The epicenter of the earthquake is on the offshore shelf, and not in the trench. Earthquakes are caused by breaking and movement of rocks along the faultline where the two plates collide.
In cross-section the convergent margin would look something like this:
Diagram showing the tectonic plate movement beneath Japan. Note the location of the earthquake is beneath the offshore shelf and not in the trench.
The shaking of the sea-floor from the earthquake creates the tsunamis.
So where are there similar tectonic environments (convergent margins)? You can use the Google Map above to identify trenches and mountain ranges around the world that indicate converging plates, or Wikimedia Commonsuser Sting’s very detailed map, which I’ve taken the liberty of highlighting the convergent margins (the blue lines with teeth are standard geologists’ markings for faults and, in this case, show the direction of subduction):
Convergent plate boundaries (highlighted blue lines) shown on a world map of tectonic boundaries. The blue lines with teeth are standard geologic symbols for faults, with the teeth showing the direction of the fault underground. Image adapted from Wikimedia Commonsuser Sting.
The Daily Dish has a good collection of media relating to the effects of the quake, including footage of the tsunami inundating coastal areas.
Cars being washed away along city streets:
Our thoughts remain with the people of Japan.
UPDATES:
1. Alan Taylor has collected some poignant pictures of the flooding and fires caused by the tsunami and earthquake. TotallyCoolPix has two pages dedicated to the tsunami so far (here and here).
2. Emily Rauhala summarizes Japan’s history of preparing for this type of disaster. They’ve done a lot.
3. Mar 12, 2011. 2:10 GMT: I’ve updated the post to add the map of the tectonic plates surrounding Japan.
4. A CNN interview that includes video of the explosion at the Fukushima nuclear power plant (my full post here).
5. NOAA has an amazing image showing the tsunami wave heights.
Tsunami wave heights modeled by NOAA. Note the colors only go up to 2 meters. The maximum wave heights (shown in black in this image), near the earthquake epicenter, were over 6 meters.
They also have an excellent animation showing the tsunami moving across the Pacific Ocean. (My post with more details here).
6. The United States Geological Survey (USGS) put out a podcast on the day of the earthquake that has interviews with two specialists knowledgeable about the earthquake and the subsequent tsunami, respectively. Over 250 kilometers of coastline moved in the earthquake which is why the tsunami was so big. They also have a shakemap, that shows the area affected by the earthquake.
USGS ShakeMap for the earthquake. Image via the USGS.
9. A detailed article on earthquake warning systems, among which, “Japan’s system is among the most advanced”, was recently posted in Scientific American.
10. Mar 15, 2011. 9:15 GMT: I’ve added a map of tectonic boundaries highlighting convergent margins.
Shinmoedake Volcano.
11. The Shinmoedake Volcano erupted two days after the earthquake, but they may be unrelated.
Fukushima reactor status as of March 16th, 5:00 pm GMT from the Guardian live blog.
12. The Guardian’s live blog has good, up-to-date information on the status of the nuclear reactors at Fukushima.
This short hike that follows a limestone bedded creek, will likely take a while because there’s quite a bit of geology to see.
The start of the hike is on the eastern side of the bridge between the villas and the hotel. Head north (left in the image) toward the lake.
This year, it was on a chilly, rainy morning in February, that we started on our hike. We took a left off the concrete stairway onto the trail that runs parallel to the river flowing in the ravine just below our cabins.
We’d stayed at the villas at Montgomery Bell State Park, which is about an hour east of Nashville. The villas are quite nice. Built into the side of the valley, sitting just across a small river from the park’s hotel/conference center, and designed to be energy efficient, they’re quite comfortable with their geothermal heating and vaulted ceilings.
They’re so nice that some wanted to stay in the warm. Others, however, were eager to get outside, despite, or perhaps even because of, the rain. I gave them the choice, but everyone came.
With the rain, we soon ran into trouble. Runoff from the road and building uphill converted part of the trail into a small stream. The first few brave souls committed to wet feet, and waded through.
The dam and lake at Montgomery Bell.
But the stream along the trail did not last long. Pretty soon we left it behind, and coming out of the valley the lake and dam opened up to the right and left. Though it had been raining for much of the previous night, the lake was still very low after the dry autumn and winter. The line of grass that marks its usual shoreline was over a meter above the level of the water.
Short concrete wall that acts as the outlet level for the dam.
So we crept along the southern edge of the dam to follow the path of the overflow channel. It was quite interesting to see the sediment and debris that choked the reservoir side of the concrete wall that regulates the level of the lake. The other side of the wall, where the water must accelerate as it overtops the barrier, was clean, bare and smooth, looking a lot like concrete until you get close enough to see that it’s hard, dark, limestone bedrock.
Drill-hole with radial shatter pattern.
But not hard enough. Small, round holes pockmark the rock. Clearly artificial, with radial cracks diverging from the center, they remind me of Sarajevo roses.
They’re probably contemporaneous with the building of the dam. In order to have their outflow channel, the dam builders needed to blast away some of the rock, so they drilled holes and filled them with explosives. The blasts crushed the upper layers of rock, but the bedding plane, upon which we are walking, dissipated the force and remained, mostly, intact.
Following the reservoir outflow channel.
The bedding plane is a bit slippery with the rain and light coating of moss, so we take a bit more care with our footing. The sides of the outflow channel are steep, with nice exposures of horizontal layers of limestone rocks.
Though I don’t go into it in detail, the different layers, with their different colors, hardness, and fossils, show the changing environment in which the sediments that created these rocks were deposited. The more friable, tan-colored layers were likely formed at a time when sea-level was lower, when this area was closer to the coastline so more sand and clays could settle out of the muddy waters emerging from fecund deltas. On the other hand, the dark, dense, grey limestone rocks are much more typical of deeper seas, offshore environments.
Tree roots prying apart the bedrock: biological weathering.
I did take the time to elaborate on the topic of weathering when my students pointed out the tree growing on the side of the cliff, with its roots entwining and pulling apart the limestone rock. It’s a part of the rock cycle that we had not spent a whole lot of time talking about in the classroom so I was glad for the opportunity.
Joints in limestone. Notice how the layers on either side of the joint line up.
Weathering also plays a part in the widening of joints, and the joints we saw were obvious and important in shaping the course of the channel. Joints are simply breaks in the rocks. When this region was uplifted, the rocks were squeezed and fractured by tectonic forces. There was not enough tectonism to seriously deform the region, the rocks are after all still close to horizontal, but they did break, creating joints that cut right through the bed of our channel and straight through the wall.
You’ll notice that the layers on either side of the joint line up, so this is just a fracture in the rock. Often, the rock will break and one side will be pushed up relative to the other; that would be considered a fault.
Runnoff from the rain, flowing along and widening joint in limestone.
One of the nice things about being out in the rain, was that you could see the water in action. Gliding along the joints, picking up and eroding small pieces of debris, while slowly, imperceptibly, dissolving away the rock and enlarging the joints. It’s the same process that created the caves we saw last year at Merimec; the reprecipitation of dissolved calcium carbonate from the limestone rocks is what creates the stalactites, stalagmites and other cave formations.
Looking up the channel at exposed bedding planes and joints.
It took a bit of care to follow the channel down. It also took teamwork. We’ve been practicing all year and it’s under these conditions that all the teambuilding, from the challenge course onward, really pays off.
Committing to wet feet.
At the bottom of the bedrock traverse was a big puddle. The water from the regular outflow of the dam creates pushes up sediment that blocks the free flow of the runoff from the current rainfall. Undoubtedly, this gets washed away when the reservoir overflows through the outlet channel, but today there was just a big puddle.
Here we faced a choice. We could have taken a hard right and walked back up to the dam along the edge of the small cliff that overlooked the outlet channel we’d just come down. It’s a nice walk, through last year’s leaf litter, and the overhang is just high enough to provide a small taste of vertigo. But the students wanted to push on, past the confluence, and follow the stream downhill. A second set of students had made the full commitment to wet feet, and any initial reluctance to be outside on a rainy day had disappeared. We followed the stream.
Convergence of the overflow channel and the drainage stream for the reservoir.
Just a few meters downstream from our decision puddle, we ran into the confluence of the regular outflow from the dam and the ephemeral, rainfall driven stream we’d been following. It’s a good place to talk about tributaries, deltas, and sediment transport, deposition and erosion, because the channel deepens into a little pool with lots of small scale features.
Following the stream.
Past the confluence the stream straightens out. It’s remarkably straight. If it weren’t for the fact that we’re in limestone rocks, it would be easy to assume, given the dam and all, that the lack of sinuosity is artificial. But it seemed like the stream was flowing parallel to the joints we’d seen earlier, so it’s not unlikely that the water is following a fracture in the rock. When convergent, tectonic forces fracture rocks, the rocks tend to break at an angle to the direction of the forces (somewhere around 60 degrees to the direction of the forces, if I remember correctly).
Climbing up to the trail that follows the ridge.
Following the stream brought us close to the picnic shelter near the entrance to the park. Just across the water is a pathway up the rocks on the side of the valley that takes you up to the trail that follows the ridge that parallels the valley.
Looking down at the stream and its floodplain from the top of the ridge.
It’s quite peaceful, standing on the ridge while water droplets drip through the sparse winter canopy, with last fall’s leaf litter beneath your feet.
Looking back down into the valley you could see (and talk about) the stream and its flat flood plain. It’s a chance to anthropomorphize. The stream “wants” to meander. It has to be constrained to one side for a reason.
Crossing the dam on the way back to the cabins (upper left).
The ridge trail takes us back to the reservoir and dam, which are quite noticeable if you’re paying attention. We traipsed down the hill and walked a narrow path between the tall, reddish-tan grass that tops the dam, and the bouldery rip-rap that protects the earthen structure from the force of the waves.
Boiling jam will often create a froth that floats on top, much like the granitic continental crust floats on the Earth’s mantle. Also like the boiling jam, the mantle convects (even though the mantle is not liquid).
Convection in jam.
The darker red areas in the image are where the convection cells in the boiling jam reach the surface and push the froth away. It’s a bit like the bulge in the Earth’s crust that occurs beneath hot-spots and the mid-ocean ridges.
Model of convection in the Earth's mantle (image from Wikipedia)
There’s a nice article in the New York Times on the fact that oil, petroleum, did not come form dead dinosaurs, but rather from the microscopic plankton that died and fell to the ocean floor.
The idea that oil came from the terrible lizards that children love to learn about endured for many decades. The Sinclair Oil Company featured a dinosaur in its logo and in its advertisements, and outfitted its gas stations with giant replicas that bore long necks and tails. The publicity gave the term “fossil fuels” new resonance. – Broad, 2010
It’s easy to forget how pervasive is the idea that oil comes from dinosaurs. Broad’s article is a nice reminder that:
Today, a principal tenet of geology is that a vast majority of the world’s oil arose not from lumbering beasts on land but tiny organisms at sea. It holds that blizzards of microscopic life fell into the sunless depths over the ages, producing thick sediments that the planet’s inner heat eventually cooked into oil. It is estimated that 95 percent or more of global oil traces its genesis to the sea. – Broad, 2010
How do we know?
[I]n the 1930s. Alfred E. Treibs, a German chemist, discovered that oil harbored the fossil remains of chlorophyll, the compound in plants that helps convert sunlight into chemical energy. The source appeared to be the tiny plants of ancient seas. – Broad, 2010
Phytoplankton bloom off the Carolina coast. (Image from NASA).
We tend to find a lot of oil in the deltas of the great rivers because the rivers provide nutrients for the microorganisms to survive and layers of sand and clay sediments that trap the oil and natural when they’re produced.
The article also ties the location of oil production to the geography of plate tectonics.
[W]hen Africa and South America slowly pulled apart in the Cretaceous period, forming the narrow beginnings of the South Atlantic. Big rivers poured in nutrients. A biological frenzy on the western shores of the narrow ocean ended up forming the vast oil fields now being discovered and developed off Brazil in deep water. – Broad, 2010
Major tectonic plates (from USGS).Seafloor topography around the Hawaiian Hotspot (from NOAA)
The question came up about where are good places for geothermal energy, and the answer, of course, was to introduce plate tectonics. It was a quick introduction, and a refresher for the 8th graders, but the interest was there and it seemed impactful.
It also provided a link to talk about the Icelandic volcano that’s been disrupting air traffic in Europe. NASA has an amazing picture of the eruption on its Picture of the Day for April 19th.
Google Maps is a great tool for showing features like the mid-ocean ridges (use the satellite view), zooming in and out of the mountain ranges, tracing the Hawaii hotspot and watching East Africa split apart.
On the last morning of our immersion trip we had a choice between going to Hot Springs, Arkansas, with its geothermal springs and another museum, or hiking at Lake Catherine where we had spent the night. Since we’d been to two museums on the previous day, we pretty unanimously chose the hike. And it was great.
Conchoidal fracture in quartz (image by Eurico Zimbres FGEL/UERJ via Wikimedia commons)
We took the Falls Branch Trail, which follows a couple of young, boulder-choked creeks that have are carving steep-sided valleys through nice clean limestone bedrock. The students were constantly bringing me rocks to identify, and they were almost invariable limestone, with a few pieces of quartz thrown in. The limestone was so clean that it was near translucent (fairly close to marble) and the cobbles in the stream bed were easy to mistake for smoky quartz, particularly if they were rounded enough that you could not look for quartz’s characteristic, curved, conchoidal fracture. Quartz also tends to be a lot harder than limestone, but the ultimate test, which the students really wanted to see, is to put acid on the rock. Limestone fizzes. I did not have any limestone on this trip (note to self: get some HCl for next time), but this little experiment is a nice follow up for our discussions of ionic bonding in chemistry. We have some limestone samples back at school so I plan on doing this as a follow-up.
Quartz vein (white) cutting through a limestone boulder.
Because Lake Catherine is very close to Hot Springs, it would not be surprising to find some quartz. The hot water that’s coming out of the springs flows up through cracks (faults) that extend deep beneath the surface. The deeper basement rocks are silicates, the granitic rocks that make up the continental crust, so the hot water dissolves some of that silicate material, and when the water cools down, ever so slightly, as it approaches the surface, some of those silicates will precipitate out to coat the walls of the faults with quartz. Sometimes they even fill up the faults entirely, leaving quartz veins.
The stream bed, being of young geological age, was a series of small waterfalls culminating in the five meter high drop that gives the trail its name. The water was clear and cold but with that beautiful aquamarine tint of dissolved limestone. There’s a whole lot more I could say about plunge pools and migrating nickpoints, but I’d probably go on too long. Besides we did not take the time to talk about those since there was so much else to see.
Interference pattern in ripples.
When we reached estuary of Falls Creek and Lake Catherine, the lake’s water was so calm that the kids started trying to skip stones. This of course provided a beautiful opportunity to look at water waves and interference patterns. As the ripples from each skip of the stone expanded, they melded. The constructive interference was easier to see in the field because it made for bigger ripples. But the photos show the destructive interference very nicely.
This was an excellent hike. We were a little pressed for time since we needed to get back to school before the end of the school day, but next time, I think, I’ll have us pack our food in and have lunch on the trail overlooking the waterfall and the lake. The ability to use these types of outdoor experience to integrate the academic work is one of the main reasons I enjoy the Montessori approach to middle school. All through the trip back though I kept thinking about how I could organize things so that we would never need to see the inside of the classroom again.
There are a lot of Earth Science applications that deal with waves. Seismic waves from earthquakes are a major one that is particularly pertinent after the recent Haitian earthquake. There are quite a number of lesson plans dealing with seismic waves at Larry Braile’s website. Most of the lessons are as practical demonstrations pdf’s and some use downloadable software (Windows only unfortunately), but there are some online applications as well.
In terms of online resources, the IRIS network, produces nice maps of recent earthquake locations. It also has a good page with “Teachable Moments” regarding recent earthquakes. These include the above video of why the Haitian earthquake did not produce a tsunami.
Although it’s not directly related to waves, I particularly like the thermal convection experiment on Braile’s website. It provides, with a baking dish, a sterno can, some water and some thyme, a great example of the convection in the Earth’s mantle that drive plate tectonics.
