Wednesday, July 3, 2013

A Tale of Two Conglomerates: Chapter 3

This is Chapter 3 of a 3 part blog post.  Click here to read part 1 or here to read part 2.

Chapter 3: Metamorphism
When we last left our conglomerate heroes Mount Battie was hanging out around the South Pole, and Mars Hill near the equator.  As you can probably guess they didn't stay put.  Mars Hill and what is called the Laurentian Continent sped north, but Mount Battie and the Avalonian microcontinent sped faster.  The result: a Mack truck v. Geo Metro collision of geologic proportions (in other words, incredibly powerful, and extremely slow).  This last segment of the post is the claims adjuster's report, with metamorphism in the rocks marking the damage.

Metamorphism is a process in which rocks are changed by heat and pressure, and the Midcoast certainly endured both during the collision.  Before the collision, the sandstone that made up both the clasts, and the matrix, would have been indiscernible from sand except for the fact that the sand was cemented together into a rock.  After the collision geologists find a set of minerals called amphibolite.  Unable to form under different conditions, amphibolite tells our adjuster that the collision caused a pressure equivalent of between one and six of those Mack trucks resting on every inch of rock, while the temperature rose to around 1000 degrees fahrenheit.  Under these conditions a literal Geo Metro would be obliterated.  Our figurative car is merely transformed.  The heat and pressure of the continents colliding is enough to recrystallize the sand - converting them from discernible grains into an interlocking mass, which is called quartzite.

This is not to say that the Laurentian truck did not take damage.  Androscoggin County endured an equivalent amount of metamorphism.  But if the Midcoast and Androscoggin County were the respective front bumpers of our Metro and our Mack Truck then Mars Hill is the back end of the truck, experiencing little damage.  The clay particles in the shale clasts, under similar conditions to Mount Battie, would have recrystallized.  This would have made the pebbles look like miniature disco balls, called schist, within their matrix.  Limestone can endure a lot of heat. The limestone matrix may have remained limestone, however, it is also possible that other materials like quartz may have been injected through the rock.  In this scenario quartz, or silica, replace some of the elements in a calcium carbonate limestone creating what is called a calc-silicate rock. Instead, what we are left with looks like what we started with a mixture of limestone mud and pieces of rock all piled together heated scarcely above the temperature necessary to turn the amalgamation into stone.

Bartok, Peter. "Geology of Ireland and the United Kingdom." Ireland and United Kingdom. Tarryton, NY: Marshall Cavendish, 2010. 15-16.

Mottana, Annibale, Rodolfo Crespi, and Giuseppe Liborio. Simon and Schuster's Guide to Rocks and Minerals. Ed. Martin Prinz, George E. Harlow, and Joseph Peters. New York: Simon and Schuster, 1978. Print.

"How Much Does a Mack Truck Weigh?" Web. 03 July 2013.

"Maine Geologic Map Data." Maine Geologic Map Data. 05 Apr. 2013. Web. 03 July 2013.

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