Plate Tectonics from the Middle of the Plate

Plate Tectonics:
In the past generation, geologists have discovered that the earth's crust does not remain stationary.  Instead, it breaks into large plates which float on the flexible aesthenosphere and move relative to one another.  Follow this link for a review of how plate tectonics works.  Follow this link to see a history of the movement of earth's continents since the Cambrian Period.

Most of the obvious evidence of plate tectonics, volcanoes and earthquakes, are found on the edges of the earth's tectonic plates. Active volcanoes and fault lines mark the edges of current plates.

Nashville is nowhere near the edge of a plate, and never has been. However, even here, ash from long extinct volcanoes can provice evidence of the movement of continents.

Evidence from Volcanic Ash

At several sites in Nashville, you can see layers of a clay called bentonite.  Bentonite is aged volcanic ash, and in Nashville, geologists find ash from five separate volcanoes. (I have found bentonite from two of them).  To see the ash, go on the virtual field trip, two earthquakes and two volcanoes.

By analyzing the bentonite from these volcanoes, we can discover a great deal about the world at the time the volcanoes erupted.
 
 

Necessary Background

To be able to understand our volcanoes, you may need a quick review on some basic geology.

The ash found in Nashville has been dated at about 454 million years old. Geologists traditionally divide this time into eras, periods and smaller units.

What era and period does Nashville's volcanic ash come from. Follow this link for a review.

Kinds of Volcanoes:

Classifying volcanoes can be confusing.  They can be classified by

• Where on tectonic plates they are located
• where plates converge
• where plates diverge (rift valleys)
• in the middle of plates (hot spots)
• What kind of rock comes from the volcano
• basaltic rock - liquid - low silica
• granitic rock - viscous - high silica
• What form of rock comes from the volcano
• Hawaiian - lava flows of liquid basaltic lava (very low silica content) - creates shield volcanoes
• Strombolian - explosive eruptions of high silica lava - forms lava flows and cinder cones.
• Vulcanian - violent eruptions, often after long dormancy, emits mainly ash.
• Pelean - give off high silica lava and gas flows (nuee ardente), form strato volcanoes
• Plinian - most powerful of all, erupt after long dormancy, occur when viscous high silica magma combines with gas. shoot huge ash clouds high in the atmosphere.
• submarine - occur at sea floor rifts.  low silica lava forms flows.  high pressure prevents explosions.

hydrovulcanian -  emits ash and steam as well as lava bombs.
• What the composition of the cone
• cinder cone - generally small, common volcanoes, cause primarily by strombolian volcanoes
• composite cone (also called strato volcanoes) - composed of alternating layers of lava and rock fragments
• shield volcanoes - formed by many layers basaltic lava.  They form at hot spots and on diverging plates
• scatter cone - occur when hot erupting lava contains just enough explosive gas to prevent the formation of lava flow.
• complex cone - this is what we call volcanoes which have more than one characteristic.

How Volcanic Ash Moves:
To learn about this, I want to go to the EarthComm site, and go through an exercise on volcanic ash.

Compare the movement of ash fall patterns of Mount St. Helens with those of Mount Pinatubo, Philippines.  How do you explain the difference?

This is a map of the ash fall from the eruption of Mt. St. Helens (1981).  What is the latitude and longitude of the volcano? What direction is the ash blowing?

map is from:  Sarna-Wojcicki, et al, 1981, Areal distribution, thickness, mass, volume, and grain size of air fall ash from the six major eruptions of 1980: p. 583, in The 1980 eruptions of Mount St. Helens:  Lipman, Peter W., and Donal R. Mullineaux (eds), United States Geological Survey Professional Paper 1250.
 
 
 
 
 
 
  
 

This is a map of the ash fall from the eruption of Mt. Pinatubo (1991) What is the latitude and longitude of the volcano?  What direction is the ash blowing?

This map is from:  Wiesner, Martin G., Y. Wang, and l Zheng, 1995, Fallout of volcanic ash to the deep South China Sea induced by the 1991 eruption of Mount Pinatubo (Philippines):  Geolgy, v 23; no. 10; p. 885-888
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 
 
 
 
 
 

What does a volcanic ash fall look like?
In 1981, the eruption of Mt St. Helens was well documented.  Follow the link for picture before and after the eruption.
 
 
 

Now let's look at our volcanic ash?

How big were the eruptions?
The bentonite beds that are found in Nashville are part of a huge collection of bentonite beds found throughout the eastern part of the United States.  In recent years, geologists have developed techniques for identifying ash from a specific volcano.  Studies of these bentonite beds have identified sixty separate volcanic eruptions.  Bentonite from the two largest eruptions, the Millbrig eruption and the Deicke, is exposed in Nashville.  If you look at the picture at the top of the page,  the thin layer of clay marking the change in the rock (massively bedded Lower Carters below, thinner bedded Upper Carters above) is the Diecke Bentonite.  A second bentonite layer, from the smaller Ooltewah eruption is visible at the bottom of the exposure.

These eruptions were immense.  Their ash is found over half of the United States and parts of northern Europe.  The Millbrig eruptions threw the equivalent of 1140km^3 into the air over an area of several million square kilometers.  This is one of the largest ash producing eruption in the past 590 million years, the same order of magnitude as the eruption at Yellowstone 630,000 years ago.
 

Where were the volcanoes?
Deicke and Millbrig bentonite have been found throughout the eastern US.  Using the data reported in Kolata, Huff & Bergstrom's book, we can map the location and thickness of the bentonite beds.

I have used the following research in the section above. Huff, W.D., Bergstrom, S.M., and Kolata, D.R. 1992 Gigantic Ordovician volcanic ash fall in North America and Europe:  Biological, tectonomagmatic, and event-stratigraphy significance, Geology 20:875-878 Kolata, Huff & Bergstrom, Ordovician K-bentonites of Eastern North America, Geological Society of America, Special Paper 313, 1996

What conclusions can you draw from studying our volcanic ash?

1.  What kind of eruptions were they?

2.  Judging from what you learned in the Earthcomm exercise and the location and depth of the bentonite, where were the Deicke and Millbrig volcanoes?

3.  If a volcano, located in the same place, erupted today, how much ash would it deposit on middle Tennessee?

4.  What was the latitude of North America when these volcanoes erupted?
 

Try to answer these questions yourself, before looking at the answers.
 

For more information on Tennessee's volcanic history visit this USGS site.
 
 
Evidence from Fossil Distribution

When Alfred Wegener first presented his theory of continental drift, one of his lines of evidence. Wegener pointed out that the same fossils were found in Africa and South America, and that this was only likely, if the two continents had been connected at the time the animals and plants were alive. Look here to see the fossil evidence Wegener used to support his theory.

There are many fossils found in Nashville. Can we learn something about the location of tectonic plates in the Ordovician Period by seeing where else they were found.

I have collected 35 common Nashville fossils, to try it.

First visit the Paleobiology Database. This amazing site compiles evidence from paleontologists around the world about fossil species. It includes classification, descriptions, bibiliographies, data on when the animal lived and a map showing where its fossils have been found. Enter the genus name of the animal to find all of the information about it.

Below, I have adapted the maps from the database for ease of comparison. I did this for two reasons. First, the program which draws the maps chooses the largest scale possible. Thus, a fossil which is found only in North America is presented on a map of North America. This makes comparison difficult. Second, many of these animals survived beyond the end of the Ordovician Period, and I wanted to limit the data to sites where the animal lived during the Ordovician.