CONNECTICUT TECTONIC HISTORY
NAME:_______________________DATE:___________PER:____________
Also put your name on your Bedrock Geology Map.
Purpose --- Part 1: Before the Crunch --- Part 2: Ocean Subduction --- Part 3: Continental Collision --- Part 4: Rifting --- References
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Purpose:
To best manage the natural resources of CT, we need to understand how those resources formed. The bedrock geological map of Connecticut describes not only the kinds of rock beneath our feet, it gives us clues to how this area formed.
As you read through this story, compare the descriptions and diagrams with your text (Standard classes p442), your notes and handouts from the videos.
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PART 1.
Two continents approached each other, propelled by the forces of plate tectonics that we studied earlier. Geologists named the original, central part of our continent Proto-North America. Geologists have named the other continent Avalonia, after the mysterious, legendary land where King Arthur ruled Camelot. But the Camelot stories were set in a time when there were people. Avalon approached and collided with Proto-North America before there were even dinosaurs.
On the CT Bedrock Geology map, the rocks of Avalon are shown in the southeast part of the state. The rocks of the northwest corner of CT are from Proto-North America.
The Proto-North American rocks include some that formed along on a continental shelf from the sediments washed off the land and from a coral reef along the edge of the ocean. Similar rocks are now forming along the coast in areas such as Florida and the Bahamas.
1.
Locate the rocks from Proto-North America on your map by finding their colors in the key. LABEL your map with "PNA" in the upper left and draw arrows to the three kinds of rock from Proto-North America.
2.
Locate the rocks from Avalonia on your map by finding their colors in the key. LABEL your map with "AV" in the lower right and draw arrows to the two kinds of rock from Avalonia.
2.
Label Proto-North America and Avalonia on the diagram above with the names of the rocks from the map.
3.
Marble forms when limestone rock from a coral reef is heated and squeezed. Use your text to find out where reefs form (standard classes p266). What does the presence of marble indicate about the climate before the rocks were caught between continents?
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PART 2
Between Avalon and Proto-North America was an ocean named Iapetos by geologists (in Greek mythology, Iapetus was a Titan who was the father of Atlas and an ancestor of the human race). The ocean crust of heavy basalt was subducted into the mantle where it melted and the magma found its way back to the surface to form volcanic islands just like Japan and the Philippines are forming now.
4.
What kind of plate boundary is this?
5.
Where in your text is this kind of boundary explained (page and paragraph number)?
6. When basalt is heated and squeezed in a continental collision, it forms the metamorphic rocks schist, phyllite and gniess. Locate the colors of the Iapetos rocks in the key, and number them 1-5. Draw arrows from the five kinds of rocks listed in the key to their locations on the map.
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PART 3
Now we move forward in time to the collision of P-North America with Avalonia. The oceanic, basaltic crust of the Iapetos ocean was subducted under the continents. The smaller continents of that time all crunched together to form the super-continent Pangea. Islands and ocean sediments became trapped in between the continents to make sandwiches of continental and oceanic rocks. The same thing occurred when the Indian sub-continent rammed into Asia, uplifting the Himalayan Mountains in the process.
This collision of land masses raised a huge range of mountains, the ancient Appalachians. During the collision, the rocks were baked and squeezed into the metamorphic rocks we now find in CT.
8.
What kind of plate boundary is this? Reference to your text.
9.
Label the above diagram of Pangea with the names and numbers of the rocks from P-N America, Avalonia and Iapetos.
10.
The heat and pressure that baked the rocks into the metamorphic rocks we now have also hardened them. The rocks we now have exposed the surface were deep under the mountains, even below sea level. Why are they now where they are at the surface?
11.
The fault between the ancient Iapetos and P-N America rocks is called Cameron's Line, after a geologist who traced it along the eastern seaboard. It was located in nw CT when Route 8 was blasted through the rock in Torrington just south of the intersection with Rt 4 at the nw corner of Harwinton. Speculate what you would see where Cameron's line is exposed there.
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PART 4
Soon after Pangea formed, it began to break up again. The rifting and splitting of the continent began in several places before shifting to the final split closer to where we now have the edge of the continental shelf. One place the rifting began was right through the Iapetos rocks in central Connecticut.
A deep rift zone opened through the mountains, similar to what is now happening in eastern Africa. Sediment washing down from the mountains filled the bottom of the rift, then was covered by lava flows.
12.
On the drawing above, label the rocks you labeled on the other drawings.
13.
What kind of a plate boundary is this? Reference to your text.
14.
On the geologic map, the central rift valley is called a Newark terrane because a similar rift zone was first identified in New Jersey. Label the rift zone on your map.
15.
What kinds of rock are found in the rift valley? Label them on the diagram above.
When you travel down Rt 4 from the center of Burlington down the steep slope to the bottom of the Farmington River valley, you are riding down the western edge of the rift valley. If the plates had continued to diverge there, that would now be the edge of the continental slope, under water.
16.
The rock of the rift zone eroded more than the rock of the uplands, forming the central valley of CT. Why?
17.
Label the traprock ridges on the geologic map.
The red soil of our central valley comes from the red sedimentary rock known as brownstone, which formed in a more tropical climate. Where in CT would you expect to find the best soil for farming? Why?
16.
Draw your own diagram showing the formation of the features of the central valley.
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REFERENCES
(available in class):
Bell, Michael The Face of Connecticut, State Geological and Natural History Survey of Connecticut Bulletin 110. 1985
Generalized Bedrock Geologic Map of Connecticut, at http:// www.wesleyan.edu/ ctgeology/, by the
Connecticut Geological and Natural History Survey, Department of Environmental Protection, 1990.
Rodgers, John. Bedrock Geological Map of Connecticut, Connecticut Geological and Natural History Survey, Department of Environmental Protection in cooperation with US Geological Survey, Department of the Interior, 1985.
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