Sunday, March 17, 2013

The History of Maine: Part 3 The Acadian Orogeny

Note: This program first aired on March 9, 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 430 million years ago. A microcontinent had just crashed into the proto North American plate, and became welded to the coast. This was the Taconic orogeny, and it gave us the Green Mountains of Vermont, as well as some of the land mass of New Hampshire and western Maine.

If you look at any bedrock geology map of the state of Maine, you can see the general pattern of land forms these and subsequent tectonic collisions left on the state. Most of our major geologic forms run from south west to north east, in a diagonal across the state. This is the aspect of the coastline from the original proto North American continent. As additional crust was either welded on to or subducted underneath North America, this diagonal was mostly maintained. Thus the volcanic plutons that resulted from the subduction associated with the Taconic orogeny run from northern New Hampshire (and are in part, named for Hurricane Mountain on the Maine/New Hampshire border) up through northern Maine.

After the afore mentioned collision of the Taconic orogeny, there was a period that was mostly marked by erosion (again!). The Taconic orogeny resulted in very large mountains, which can be thought of simply fodder for the forces of erosion. The bigger the mountain, the more material there is to erode. For our purposes here in tracing the history of Maine, the erosion we are interested in was coming from land and flowing what would now be east to southeast. Terrestrial sediments accumulated of the new shoreline of the proto North American continent. There was a bit of rifting that started to occur to our north and west, so erosion filled that basin as well, which shows up now in far northern Maine. All the while the Iapetus Ocean was still slowly closing, and the continent that would become Europe was still on its collision course with North America.

As the Iapetus closed, the crust that made up the bottom of this ocean was subducted at trenches in the middle of the shrinking ocean, as well as potentially at the edge of either or both of the continents involved. Volcanoes always accompany subduction zones; two bands of volcanoes formed during this period that relate to Maine. One was on the edge of Europe—land that would eventually impact North America, the other was on the edge of what would become central Maine, as it was the leading edge of the North American continent at that time. From about 430 to 400 million years ago, this is what was going on. Erosion was adding to the sediment load in the ocean off the coast of North America, and Europe was drawing ever closer to North America. And I know the anticipation is killing you, but, it takes a while for a collision like this to happen, because the plates only move on the order of 10’s of centimeters a year.

Around 400 million years ago, the collision finally happened. Europe impacted what would become New England. The worst of it was in southern Maine, we can tell because the rocks down there are the most heavily metamorphosed, telling us that they took the hardest hit. Rocks in northern Maine show the least metamorphism, indicating that the force of the impact was minimal there. The impact transformed the mud and silt stones that were forming off the coast before the collision into slate and other metamorphic sedimentary rocks we see in central Maine today.

This impact finished the accumulation of land that would become Maine, and not just due to the folding and morphing of those sedimentary rocks off the coast. It turns out that Europe had its own little love child. Much like the micro continent that had previously impacted North America, Europe had a similar skeleton in its closet. The microcontinent that was welded on to the leading edge of Europe even had a cool name: Avalon* . It was Avalon that slow motion smashed into the coast of Maine, metamorphosing the sediments that were trapped in between the impacting continents. As the last of the Iapetus Ocean disappeared in this collision, the last of the Iapetus Ocean crust was suducted beneath both North America and Avalon, and the melting of that rock gave us the plutons that have become some of our most beloved mountains, the mountians of Baxter State Park  to the north and those of Acadia National Park on the coast. The remnants of the Avalon plate itself make up much of the downeast coast and interior.

This impact and the associated mountain building that accompanied it is called the Acadian Orogeny, and it’s importance in creating the land forms we enjoy in Maine can’t be understated. 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.

*Actually if you look in the literature, you will see that this microcontinent is referred to by many cool names (don’t you love it when scientists fight?). To me “Avalon” has the best backstory, and is thus the one I prefer.


Yep, these references really hit all the key points:  Refer to the list for “The History of Maine: Part 1”.

Maine’s own Geological Survey has a wealth of resources available online:
This is a link to a pdf of a simplified bedrock geology map of the state of Maine, clearly showing the southwest/northeast trend of bedrock:

 Our friends at the US Geological Survey have some nice material on plate tectonic basics, including information about just how they know how fast the plates move--
 (This is exactly the kind of initiative I want my tax dollars spent on! Thanks USGS!)