Students take a break for journaling during our canoe trip on the Current River.
It was not all dark and stormy on our Outdoor Education canoe trip. The first afternoon was warm and bright; the first splashes of fall color spicing up the deep, textured greens of the lush, natural vegetation. It was so nice that, in the middle of the afternoon, we took a short break, just shy of half an hour, to reflect and journal.
A time an a place for reflection.
Our guides chose to park our boats at a beautiful bend in the river. Most of my students chose to sit in the canoes or on the sandy point-bar on the inside of the meander, but a few to be ferried across the river to a limestone cliff on the cut-bank of the curve. An enormous, flat-topped boulder had fallen into the water to make a wonderfully picturesque site for quiet reflection for two students. A third student chose to sit in a round alcove sculpted by the solution weathering of the carbonate rock itself.
A shady place to stop and think.
The cut-bank of a river’s meander tends to be deeper than the inside of the curve, because the water is forced to flow faster on the outside of the bend where it has more distance to travel. This proved to be quite convenient for my students, because it meant that the stream-bed around their boulder was deep enough that they could jump into the water after the hot work of writing while sitting in the sun. And they did.
Rain falls.
Some runs off,
Some seeps into the ground.
Water drips from the tips of limestone straws on the roof of Twins Cave.
It trickles through soil.
Leaching acids, organic,
Out of the leaf litter,
But even without these,
It’s already, every so slightly, corrosive, From just the carbon dioxide in the air.
Gravity driven,
The seeping water seeks the bedrock,
Where it might find,
In the Ozark Mountains,
Limestone.
Planktonic shell (from Coon Creek which is 30 million years old, compared to the limestone rocks in the Ozarks which are 300 million years old.)
Limestone:
Microscopic shells, of plankton,
Raining down, over millenia,
Compacting into rocks,
In a closing ocean,
As North America and Africa collide,
From the Devonian to the Carboniferous.
Orogenic uplift,
Ocean-floor rocks,
Become mountains,
Appalachians, Ouachitas,
The Ozark Plateau.
The collision of North America and Africa uplifted the limestone rocks from the closing ocean (the Rheic Ocean) to create the Ouachita Mountains and Ozark Plateau. (Figure adapted from iimage by Dr. Ron Blakey - http://jan.ucc.nau.edu/~rcb7).
Limestone dissolves,
In acid water.
Shaping holes; caves in bedrock,
Where we go,
Exploring.
Crude oil is extracted from layers of sand that can be deep beneath the land surface, but it was not created there. Oil comes from organic material, dead plants and animals, that sink to the bottom of the ocean or large lakes. Since organic material is not very dense, it only reaches the bottom of ocean in calm places where there are not a lot of currents or waves that can mix it back into the water. In these calm places, other very small particles like clay can also settle down.
Figure 1. Formation of sandstone (reservoir) and shale (source bed).
Over time, millions of years, this material gets buried beneath other sediments, compressing it and heating it up. Together the organic material and the clay form a type of sedimentary rock called shale. As the shale gets buried deeper and deeper and it gets hotter and hotter, and the organic matter gets cooked which causes it to release the chemical we know as natural gas (methane) and the mixture of organic chemicals we call crude oil (see the formation of oil and natural gas).
Figure 2. The trapping of oil and natural gas by a fault.
Shale beds tend to be pretty tightly packed, and they slowly release the oil and natural gas into the layers of sediment around them. If these layers are made of sandstone, where there is much more space for fluids to move between the grains of sand, the hydrocarbons will flow along the beds until they are trapped (Figure 2).
In this exercise, we will use the wave tank to simulate the formation of the geologic layers that produce oil.
Materials
Wave tank
Play sand (10x 20kg bags)
Colored sand (2 bags)
Observations
For your observations, you will sketch what happens to the delta in the tank every time something significant changes.
Procedure
Fill the upper half of the tank with sand leaving the lower half empty.
Fill the empty part with water until it starts to overflow at the lower outlet.
Move the hose to the higher end so that it creates a stream and washes sand down to the bottom end — observe the formation of the delta.
Observe how the delta builds out (progrades) into the water.
After about 10 minutes dump the colored sand into the stream and let it be transported onto the delta.
After most of the colored sand has been transported, raise the outlet so that the water level in the tank rises to the higher level. — Note how the delta forms at a new place.
After about 10 more minutes dump another set of colored sand and allow it to be deposited on the delta.
Now lower the outlet to the original, low level and observe what happens.
After about 10 minutes, turn off the hose and drain all of the water from the tank.
When the tank is dry, use the shovel to excavate a trench down the middle of the sand tank to expose the cross-section of the delta.
Analysis
1. How did changing the water level affect the formation of the delta.
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2. When you excavated the trench, did you observe the layers of different colored sand in the delta? Draw a diagram showing what you observed. Describe what you observed here.
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3. Was this a realistic simulation of the way oil reservoirs are formed. Please describe all of the things you think are accurate, and all of the things you think are not realistic?
Note to self: The Tavern Rock Cave, where Meriwether Lewis almost fell to his death, is 45 minutes from the St. Albans Lake (walking at a fair pace mind you), not “just 20”, no matter what the students claim.
It’s a little tricky to get to, and we couldn’t spend more than a few minutes there because of the longer than expected walk, but it’s a nice place to visit because of the history and beautiful geology (jointed limestone and dolomite outcrops; scree slopes at the angle of repose; Ordovician fossil imprints).
Rapelling down an overgrown scree slope.
P.S. The National Park Service has an excellent site on the Lewis and Clark Expedition that includes maps, their itinerary, and a long list of sites they visited.
Composition of the atmosphere from the formation of the Earth. Image ᔥJoel Cayford ↬Ethan Siegal
Joel Cayford has posted a nice image showing the composition of the atmosphere over time — since the formation of the Earth.
Note that, although the Earth is 4.5 billion years old, and life has been around for over 4 billion years, there has only been oxygen in the atmosphere for about 2 billion years.
Oxygen is an extremely reactive gas, which is why we use it when we “burn” carbohydrates for energy. But it also means that any free oxygen added to the atmosphere would easily react with rocks, water, and other gasses in the atmosphere, so would not be available in the quantities needed for air breathing organisms until it slowly accumulated.
Also, you need a lot of oxygen in the atmosphere to produce enough ozone to form the ozone layer that protects life at the surface from high-energy, cancer-inducing, ultra-violet radiation from the Sun.
Clays and Silts (mud) deposited when the river floods.
Bedrock. Mostly limestone:
Can be found outcropping on the hills.
Mississippian Limestones (USGS ref.) (330-360 million years old.): found on some of the hilltops.
Ordovician Dolomites and Limestones (USGS ref.) (435-500 million years old)
The geology of St. Albans consists of young floodplain sediments (yellow), and old limestone bedrock (blue). Image adapted from the USGS.
Geologic History
The continents form
To reconstruct the geologic history, we can start a bit deeper, with the fact that we’re sitting in the middle of a continent, which means that if you drill deep enough you’ll get to some of the original, granitic rocks that formed just after the crust of the Earth cooled — about four and a half billion years ago.
The froth that floats on top of the boiling jam is a bit like the continental crust.
The continental crust is a bit like the froth that forms on moving water (or the top of boiling jam), and just like froth it tends not to want to sink. So there’s some pretty old continental crust beneath the continents.
However, also just like froth on water, the continental crust is pushed around on the surface of the Earth. This is called continental drift (which is part of the theory of plate tectonics). Sometimes, the continental crust can split apart, making space for seas and oceans between the drifting continents, and causing parts of the continent to subside beneath the oceans.
At other times, such as when two continents collide, they can push each other up to mountains out of areas that were once seas.
And that’s how we ended up with limestone rocks in the middle of Missouri.
Forming Limestone Rocks (Ordovician)
Five hundred million years ago (500,000,000 years ago) the continents were in different places, and Missouri was under a shallow part of the Iapetus Ocean.
The location of Missouri 458 million years ago. Image from: "Plate tectonic maps and Continental drift animations by C. R. Scotese, PALEOMAP Project (www.scotese.com)"
Many of the micro-organisms that lived in that ocean made shells out of calcium carbonate.
100 million year old, calcium carbonate shell (from Coon Creek).
When you accumulate billions of these shells over the course of millions of years, and then bury them, compress them, and even heat them up a bit, you’ll end up with a rock made of calcium carbonate. We call that type of rock: limestone.
Limestone outcrop on St. Albans Road (Ordovician).
Emerging from the Oceans: The Formation of Pangea.
The Mississippian limestone rocks formed in the same way, but about 360 million years ago. Why is there a gap between the Ordovician rocks (450 million years ago) and the Mississippian ones? Good question. You should look it up (I haven’t). There may have been rocks formed between the two times but they may have been eroded away.
I can make a good guess as to why there are no limestone rocks younger than about 300 million years old, however. At that time the continents, which had been slowly sidling toward each other, finally collided to form a super-continent called Pangea.
What would become North America (called Laurentia), ran into the combined South America/Africa continent (called Gondwana) pushing up the region, and creating the Ozarks and Appalachian Mountains.
Laurentia collides with Gondwana. Image from "Plate tectonic maps and Continental drift animations by C. R. Scotese, PALEOMAP Project (www.scotese.com)"
And that’s the story the geology can tell.
References
The USGS has good, detailed, interactive maps of the geology of the states in the US.
A nice geologic map of St. Louis County can be found here.
A geologic time scale from the USGS. The geologic time scale. From the USGS via Wikipedia.
The Earth’s magnetic field results from the movement of molten metal in the Earth’s core. The outer core actually. It’s mostly molten iron, which conducts electricity, and as it convects up and down, like boiling water in a pot, the moving electrical charges create the Earth’s magnetic field. Its a bit like a dynamo.
The internal structure of the Earth. Movement in the liquid metal outer core (green arrows) generates the earth's magnetic field.
What drives the convection of the outer core? The heat released from the freezing of the liquid metal to the solid inner core. The inner core is ever expanding, and the outer core is getting smaller and smaller. Ultimately, when the entire outer core freezes the Earth’s magnetic field should disappear. But we’ve got some hundreds of millions of years left so we don’t have to worry quite yet.
The Other Planets
The question came up: Do Mars and the other planets have magnetic fields?
Astronomynotes has compiled a table of Planet Atmospheres and Magnetic Fields that shows that of the inner planets — Mercury, Venus, Earth and Mars — only the Earth has a significant magnetic field; Mercury does have its own field but it has less than 1% the strength of the Earth’s.
At present, Mars does not have a magnetic field, but it does have remnant magnetism imprinted on its rocks, indicating that it used to have one in the past. It’s internal dynamo died away long ago. Interestingly, the pattern of Mars’ remnant magnetism indicates that it’s interior was once molten enough that the surface had tectonic plates just like the Earth.
Comparison of Earth's healthy magnetic field and the local, remnant magnetism of Mars. Image via NASA.
On the other hand, the outer planets have much larger magnetic fields; Jupiter’s is almost 20 times larger than Earth’s. The gas giants’ magnetic fields are also generated by fluid motion in their interiors (Stevenson, 1983 (pdf)). It’s likely, however, that in some of these bigger planets, at least, the electrically conductive fluid is not liquid metal like in the Earth’s core, but either liquid hydrogen, or a water solution with dissolved electrolytes.
Jupiter's strong magnetic field interacts dramatically with its moons, inducing magnetic fields in some. Image via NASA.
