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?" Ask.com. Web. 03 July 2013.

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

A Tale of Two Conglomerates: Chapter 2

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

Chapter 2: The Matrix
Conglomerates share there stories in several ways.  The pebbles that fall together to make the rock tell a story of what came before.  The stuff that holds the rock together - the matrix - speaks about what was happening when the rock came together.

Four hundred and fifty million years ago, Mars Hill would have been pretty close to the Equator.  This time period also happened to be when coral were distributed widely around the world.  These facts were unknown to me at the time I first visited Mars Hill.  What I did know, however, was that Mars Hill was not far from a town called Limestone.  I looked at those clasts, and I looked at the stuff that held them together (called the matrix in the geology world), and I wondered.  I have since broken my piece of conglomerate and, logically, dropped it in vinegar.  The neat thing about limestone is that you don't have to wonder for long.  Dropping limestone in vinegar causes a reaction between the acetic acid of vinegar and the limestone base, causing carbon dioxide to fizz off.  Soon after the rock hit the bottom of the mug, bubbles started rising to the surface of the vinegar.  Four hundred and fifty million years ago, about the same time that ocean bottom rock was being torn apart, a coral reef, not far from Mars Hill was breaking down as well.  While the majority of this calcium carbonate piled up on flat ocean bottom, creating the substrate for Aroostook County's potatoes, some followed the flow of water over some sort of cliff into some kind of deep water canyon allowing shale pebbles and limestone mud to mix together.

The pebbles that make up the Mount Battie conglomerate are held together by something else.  500 million years ago, Mount Battie was in a part of the world not very likely to host coral.  The Avalonian microcontinent, perhaps similar in form to today's Japan, was on the bottom part of the world, below the 60th parallel.  The matrix here, insoluble in acid, is quartzite, indicating a sandy environment.  Layering at the site indicates the sand gathered in an ocean basin, where wave action swept away most of the smaller sediments, leaving behind sandstone pebbles in a matrix of sand.  This shore line would have been a bit cold for developing coral reefs, excluding the development of limestone in the area.  This gravelly beach the stage for our future coastal mountain.      

Modern World Coral Reef Locations - Credit: NASA

Berry, Henry N., IV. "The Bedrock Geology of Mount Battie, Camden." Maine Geological Survey: , Maine. Maine Geological Survey, 19 Apr. 2012. Web. 01 July 2013. <http://www.maine.gov/doc/nrimc/mgs/explore/bedrock/sites/jul01.htm>.

Scotese, C. R. "Earth History." Plate Tectonic Maps and Continental Drift Animations. PALEOMAP Project. Web. 01 July 2013. <http://www.scotese.com/earth.htm>.

Wang, Chunzeng, Gary Boone, and Bill Forbes. "Geology of Mars Hill Mountain and Vicinity." Http://goaroostookoutdoors.com/. Web. 1 July 2013. <http://goaroostookoutdoors.com/sites/default/files/trails/maps/mars_hill_geology2.pdf>.

A Tale of Two Conglomerates: Chapter 1

A name should tell you something.  When it comes to rocks this is generally the case.  In sedimentary rocks, the name tends to give away the energy of the sedimentary environment.  Sandstone forms wherever sand might gather - like a beach with strong waves or a fast moving stream.  Shale evolves where clay might gather, say the bottom of the ocean.  Conglomerate, along similar lines, is birthed where gravel collects - perhaps a raging river or a bar in a particularly active part of the ocean.  The name conglomerate tells you that there was enough energy to carry rocks there, but the rock tells you a lot more.

Last summer I visited two conglomerate outcrops; one was Mount Battie in Camden, the other Mars Hill Mountain in Mars Hill.  These two rocks couldn't be any different while sharing the same name.  Each rock tells this history of its own place in three distinct chapters.

Chapter 1: Clasts

Mount Battie Conglomerate - Round Quartzite Clasts Circled

One neat thing about conglomerates is there are two sets of rocks with stories to tell: the conglomerate itself, and the rocks parts that are cemented together within it.  The rock parts (or in geology speak, clasts) at the two sites are surely at odds.  The Mount Battie conglomerate is made up of rounded, light colored clasts about the size and appearance of a pale gray gumdrop.  The gumdrops are quartzite.  If we could turn back the clock we would see that these chunks (like all quartzite) were once sandstone.  Imagine a wave rocked ocean off the coast of some forgotten continent.  Sand piles up (pure sand; we know this because the quartzite is so light in color) and eventually, chemical intrusion turns it from packed sand to sandstone.  Once loose sand, now solid rock, as they say, even this shall pass.  Perhaps the land gets raised and rivers cut through.  The slab of rock, once a beach, is broken by time and water into smaller pieces.  As the water courses the rocks get rounder and rounder.

Mars Hill Conglomerate - Flat Black Clasts Squared, Jagged
Edge Visible on Top Piece

Mars Hill has a different story.  The rocks that comprise this conglomerate are flat and black.  The flatness (and perhaps the darkness) suggests that this rock may be a shale formed in a  low energy environment.  This rock probably sat at the bottom of some ocean.  Layer upon layer of fine-grained sediment coasted to the seafloor at a ridiculously slow pace, with added phytoplankton and low oxygen levels coloring the sediment black.  Here too the land becomes elevated.  Streams shatter the shale and push it downstream.  The current softens the edges a bit before the slivers of rock reach an endpoint.

The endpoint, however is just an endpoint, not the end.  The beach, the ocean bottom, they are just the first step in the stories told by these rocks.  Check back soon for chapter 2 and 3.

Dietrich, Richard Vincent, and Brian J. Skinner. Rocks and Rock Minerals. New York: Wiley, 1979. Print.

Way, Bryan. "Black Shales." 1 Dec. 2006. Web. 28 June 2013. <http://faculty.umf.maine.edu/eastler/public.www/Black%20Shales.pdf>.