Tuesday, March 5, 2013

The History of Maine: Part 2 The Taconic Orogeny

Note: This program first aired on March 2, 2013.

We’re spending a few weeks here on the world around us, tracing the deep history of Maine, from its geological genesis to the current day. We left off last week about 500 million years ago; an ocean called the Iapetus was open in essentially the same spot relative to the continental landmasses as today’s Atlantic ocean, and during that time terrestrial sediments were slowly building up on the continental margins of what would become North America.

At this time the edge of the continental mass that will become North America was located much further inland than it is today, around the St. Lawrence seaway. Some of the rocks that would become greater New England were formed as sedimentary rock being deposited off the coast, the result of millions of years of erosion during the relatively quiet time of the Iapetus Ocean. All good things must come to an end however, and about 500 million years ago, the Iapetus Ocean reversed course and began to close. An ocean doesn’t just close on its own accord, it closes because the continents on either side of it start moving towards each other, propelled by what is not exactly worked out. Heat is constantly moving in the deeper layers of the Earth, what you learned as the mantle and the core back in grade school. It is thought that that heat melts the material of the mantle and core, and that molten rock moves in ultra slow convection currents deep inside the earth. When that moving material gets close enough to the crust it can actually drag the crust along. That is the idea anyway. What the closing of the Iapetus actually looked like is the subject of much speculation, as it is likely that the spreading center at the mid ocean ridge in the middle of the Iapetus continued to function in some fashion, so for the ocean basin to actually close, oceanic crust had to get destroyed at a high rate, which happened at the subduction zones that formed, unusually, out at sea.

A subduction zone is a place where one piece of crust is forced underneath another one. The one that goes underneath, the subducted one, travels down until it meets the very hot material from inner Earth, where it generally melts. We up here on the surface call melted rock lava, and yes, volcanoes form as a result of subduction zones. Think of the Cascade Mountains, or the volcanic islands of the Aleutian Island chain, or the Andes. These volcanic mountains are all formed from subduction zones, where oceanic crust is being forced down into the mantle. *

So back during the closing of the Iapetus, subduction zones were forming along with their accompanying volcanoes, and the crust that made up the bottom of the Iapetus Ocean was disappearing (I like to think of it as being recycled) as the area of the ocean decreased. The Iapetus Ocean had one other significant feature that plays into this story as well. It wasn’t an uncluttered ocean like today’s Atlantic, the Iapetus featured at least one, if not more micro continents, also called terranes, as well as all the volcanic islands that were forming from the subduction of the ocean crust. I imagine that a modern day equivalent would be the sea around Indonesia and the island of New Guinea. These are small chunks of continental crust, not attached to a larger continental land mass (though they likely were, and were simply torn asunder during one of the supercontinent break up cycles).

So now we have all the pieces for what happened next, sedimentary rock and oceanic crust off the continental margin of proto North America, and volcanic islands and random continental terranes in the closing Iapetus. Approximately 480 to 430 million years ago, as the Iapetus basin continued to close, and the land masses of proto Europe and proto North America moved towards each other, one of these smaller terranes crashed into proto North America (it is a slow motion crash, much like the slow motion crashes going on in south east Asia today). It was a messy crash and as the terrane was being sutured or welded on to the North American plate, lots of that sedimentary rock that formed off the continental margin, as well as big chunks of oceanic crust were thrust up and pinched in between the two colliding plates. You can see evidence of this suture in central Vermont and southern Quebec and much the Boundary Mountains province of western Maine is made of that small terrane that collided with proto North America. Its important to remember that at this point, the proto European continent hasn’t arrived to the tectonic party yet, the only collision that had occurred at this point was between proto North America, and the small terrane from somewhere in the middle of the ocean. This event is known as the Taconic Orogeny, orogeny meaning mountain building event, and as far a Maine is concerned, it gave us a good amount of our current land mass.

We’ll leave it off there for today, but join us in the coming weeks as we continue piecing together the story of the long and fascinating history of the land that we Mainers call home.

* Subduction zones are also home to the deepest parts of the ocean. As the two plates meet and one gets sucked down under the other, that downward movement forms a trench, and these trenches get quite deep. The deepest spot of the ocean is in the Marianas Trench, where the Pacific plate is being subducted under the Philippine Plate, and is nearly 37,000 ft. deep. We send a “manned” submersible down there to explore in 1960, and then it was not visited by people again until 2012, when movie director and apparent aquanaut James Cameron financed his own expedition and made history by being the third human to get to the bottom of the deepest part of the ocean.


Same as for “The History of Maine: Part 1”.