Hunting for microscopic fossils at the dinner table. Inside the circle is 100x magnification; outside the circle the magnification is 1.
My students will tell you that I’m never happier than when I have my cup of tea. On the night after our visit to Coon Creek, I put a tiny sample, about the size of a matchstick’s head, of sediment matrix on a microscope slide, and added a drop of water to disperse the grains. Then I sat there, while the chaos of dinner-making swirled around me, and searched for tiny, microscopic fossils of creatures that died long ago. With my cup of hot tea beside me, it was like sitting in the eye of a storm, flaming hamburgers be damned, a modicum of sanity in the asylum.
Quartz grains from Coon Creek Formation sediment seen under the microscope at 100x magnification. Quartz is easy to identify because of the way it breaks with curved fractures.
The first thing I noticed though were the quartz grains. They’re very small, silt-sized, but are the largest grains in the sediment. They’re pretty easy to identify because they break like glass, with curved, conchoidal fractures. They’re also pretty little things under the microscope; little, sharp-peaked, transparent mountains.
Other minerals are visible in the sediments. Though they're relatively large they're still dwarfed by the quartz (100x magnification).
Other minerals are visible in the slides, but they’re dwarfed in size and quantity by the quartz. Yet there is enough of the dark green, glauconite clay to bind the quartz grains together and protect the shells embedded in the sediment from dissolution by the universal solvent, water.
It’s interesting to observe these other minerals, because they take the more classic crystalline shapes and forms. The sharp edges are parallel to one another because of the alignment of the atoms in the mineral crystal.
Snail like shape of what's probably a planktonic (lives in the water) foram. (100x magnification).
Finding the micro-fossils took a little patience. The entire slide had only four obvious specimens. Since they’re so small that meant a lot of going back and forth under the small field of view of the 100 magnification objective lens. They look like foraminfera to me, but it’s been a while since I’ve encountered them. Foraminfera, or forams for short, are tiny organisms that secrete beautiful calcium carbonate shells. They can be found in, or in the sediments beneath, most of the world’s oceans, particularly in the warmer areas.
Finding forams in the Coon Creek Matrix is a nice little exercise. One of my students, seeing what I was doing, wanted to try it too, so she made her own slide and searched until she found her own specimen. It was somewhat inspiring, so I’ve put together a more detailed post about finding microfossils.
We also found a neat little shell that looks like the overlapping scales on a pine cone. We were disconnected from the internet, so I was only able to look it up when I got back to school.
What looks like a type of bolivina foraminfera. It's benthic, which means that it lives in the sediments not in the water. (100x magnification).
Dr. J Bret Bennington at Hofstra has posted a nice PowerPoint of his introduction to marine microfossils lecture. As a basic introduction, it’s quite comprehensible to middle-school students, or people like myself who did not pay as much attention as they should have during that part of Paleontology. Anyway, based on these notes, the pine-cone-shaped thing is probably a variety of bolivina, a benthic foraminfera. The Foraminifera.eu-Project, is a wonderful, volunteer-produced resource for pictures and identifying forams.
Bolivina are benthic, which means they spend most, if not all of their time in the mud. Planktonic micro-organisms, on the other hand, spend their lives floating around in the water.
Foraminfera have calcium carbonate shells, as do clams and oysters. In the shallow oceans there is a slow rain of them that cover the sea-bed over the millenia. You can end up with thick layers. In fact, the white cliffs of Dover are white because of all the microscopic calcium carbonate shells. In the deeper reaches of the oceans there are much fewer of these shells because they dissolve under high pressure. As a result, down there you tend to find microfossils of diatoms and radiolarians, things with silica shells. Silica is that same material from which glass is made, and is the same material in quartz.
Finding microfossils has actually been quite important for understanding the history of the Earth’s oceans and climate. But that’s another story.
70 million year old shell and its imprint in a clay matrix, collected at the Coon Creek Science Center.
Collecting the amazingly well-preserved Cretaceous molluscs and arthropods at the Coon Creek Science Center was an excellent way to learn about fossils and the geology of the Mississippi Embayment.
Consider: the actual shell of an actual organism that actually lived 70 million years ago; not the form of the shell, petrified in silica; not the silent imprint of ridges and grooves in the mud of some bivalve’s test, long dissolved by the silent flux of millenia of groundwater flow, although you can find those, too; but to stand in the daylight, on the gravel bar of a creek, and hold the actual shell of an actual marine organism that lived here when it was six meters under water.
When we got to Coon Creek, Pat Broadbent did her typical, excellent presentation, starting with the very basics question of, “What are fossils?” Apart from the aforementioned actual preserved shells, you can also find trace fossils, like, for example, where the imprints of the an organism is left in the mud while the shell itself has long dissolved away. They can be imprints, or molds of the shells. One of my students found the mold of a crab’s claw along the creek bed; the mud filling in the claw had solidified into rock but you could clearly see where the pincer once articulated.
Pat also talked about the Mississippi Embayment, which is the long, broad valley through which the Mississippi River flows.
The breakup of the supercontinent, Pangea. Notice how the North Atlantic Ocean is opening as North America pulls away from Europe and Africa. You can also see the flooded Mississippi Embayment. (Image from Scotese, C.R., 2002, http://www.scotese.com, (PALEOMAP website)).
When the supercontinent Pangea started to break up, North America pulled away from Europe and Africa. This created a rift that eventually became the North Atlantic Ocean. At about the same time, North America tried to split into two as a second rift was created, right where the Mississippi Embayment is today.
How the coastline of North America, has changed over the last 100 million years. The sediments at Coon Creek were deposited in the Cretaceous (black line). The current coastline is shown in blue. (Image from Wikipedia).
But the rift failed (Cox and Van Arsdale, 2007). It did, however, stretch and thin the continental crust enough to create a large inland sea running up the middle of North America. Over the 100 million years since, the rift formed, the Mississippi Embayment has filled in, first with oceanic sediment, but then with terrestrial sands and silts as the mountains to the east and west were eroded away and washed into the inland sea.
The layer of silt and glauconite clay that encases the fossils at Coon Creek is called the Coon Creek Formation. Pat was very clear that we should refer to this material surrounding the fossils as “matrix”. The “d” word was prohibited. These sediments were deposited while the sea still flooded the embayment. They formed a sand bank, several kilometers offshore.
I vaguely remember doing some research on glauconite a long time ago. Glauconite pellets are found in shallow marine waters, usually far enough away from the coastline so that sediment is deposited slowly, and it’s the finer materials, such as silts and clays, that are deposited. The water also needs to be deep enough to protect the fine sediment from the force of the waves. These are ideal conditions for clams, mussels, conchs, and their Cretaceous relatives.
A simple smear of the sediment across a microscope slide is enough to show that the matrix is has a lot of quartz. You need a microscope because the mineral grains are tiny, silt sized or smaller.
But the best part of looking at the slides is finding the microscopic fossils. They’re not as ubiquitous as you might think, but they’re there if you look. I found a couple of forams, a snail-like one and another that looks like a bolivina species.
What looks like a type of bolivina foraminfera. It's benthic, which means that it lives in the sediments not in the water. It is surrounded by silt-sized grains of quartz.
However, the smear slides came later. After Pat’s talk, she took us out to a small mound of matrix that had been excavated for sampling. Everyone grabbed chunks of matrix and pared away at them until they found something promising. These promising samples were wrapped in aluminum foil so we could clean them up under more controlled conditions.
Cleaning samples.
Cleaning takes time and patience, so Pat showed us how to do it, and each student worked on a single sample. The main idea is to create a display of the fossil using the matrix as a base. The general procedure is to:
Use a small pick, paintbrush and spray-bottle of water, to wash and wipe away the matrix from the fossil.
Let it dry out well, which usually takes about five days.
Paint the entire thing with a 50-50 mix of acrylic floor wax and water. Pat recommends Future Floor Wax, but that seems to have been rebranded out of existance.
Repeat that last step three times (let it dry for about 15 minutes inbetween) to get a well preserved, robust sample.
After the instructions on cleaning, we broke for lunch. For most of us lunch could not have come early enough, not because we were particularly hungry, but because it was quite cold outside. Just the week before the temperature had been above 20 °C, t-shirt weather. Now students were clustering around a couple space heaters trying to ward off frostbite (or at least that’s what they claimed). I did offer that they could stay inside after lunch while the rest of the class walked along the creek, but no-one took me up on it. I don’t know if it’s specific to this group or just to adolescents in general, but if there a chance to walk through water, and get dirty and wet, they’ll take it no matter what the consequence.
Students looking for fossils in gravel bar.
Walking the creek, pulling shells and molds out of the gravel bars, was the best part of the visit.
Students standing in the creek, testing their rubber boots.
The water was shallow, not getting up above the shins, despite the rain showers of the preceding days. A few students borrowed rubber boots, which half of them proceeded to fill with water.
There were quite a lot of fossils. Some of the bivalves have really thick strong shells that not only survived the 70 million years since the Cretaceous, but being washed out of the matrix and tumbled down a stream bed with all sorts of sand and gravel. Some of the casts, like the aforementioned arthropod claw, are also pretty robust.
Snail shell that's been in the ground for millions of years and then got washed out into a gravel bar.
A couple of the more interesting finds are the rather elongate tube like structures that are believed to be either fossilized burrows, or fish feces (coporolite). The material in the coporolite has been replaced by minerals, which is why it survived, but it still retains a little of the ick factor.
There’s an awful lot to learn at Coon Creek. I did not even mention the mesosaur skeletons that have been found there, but there is a nice IMAX movie, Sea Monsters, that’s a nice complement to the field trip because it’s set at the same time, and in the same marine environment as the Coon Creek Formation.
Between 6 and 25 km thick, the Earth’s crust is an excruciatingly thin skin on a 6400 km globe. Yet even drilling to the bottom of the crust would require a remarkable feat of engineering. Some geologists want to try.
NPR’s Science Friday interviews Damon Teagle, one of the architects of the project. They want to drill in the ocean because oceanic crust is thinner than continental crust (on the other hand, it’s denser too, which is why it subducts).
Using different types of chocolate covered candy, they also have this wonderful video of the basalts, sheeted dykes and gabbros that make up the crust.
Our trip was not without difficulties, however. The group learned a bit more about self-regulation, governance and the balance of powers, as a consequence of “The Great Brownie Incident,” and the, “P.E. Fiasco.”
We were also fairly well cut off from the “cloud”: no internet, and you could only get cell reception if you were standing in the middle of the road in just the right spot in front of Cabin #3.
But more on these later. I have some sleep to catch up on.
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.
With a little help to get started, the water erodes a channel, transporting sediment to the ocean.
For what it’s worth (and it seems a reasonable explanation to me):
The beach sits at the base of a valley which has a small stream running through it. Due to wave action, sand gets pushed up into a large hill in front of the stream each winter. This creates a natural dam that the stream water collects behind for months which is about 20 feet above the level of the ocean on the other side of the sand berm. Every year some one digs a trench through the sand releasing millions of gallons of fresh water into the ocean.
– YouTube User:Hackfleischhasser comments on the video Waimea River
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.
