Tuesday, April 2, 2013

The History of Maine Part 5: The Atlantic Opens

Note: This program first aired on March 30, 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 with the mass extinction event that marked the end of the Permian period, an era of Earth history that came to a dramatic closure about 250 million years ago. All of us animals here on Earth today are some how related to the hearty 10% or so of species that survived this cataclysmic event (plants seemed to have weathered the end Permian much better, mostly surviving into the Mesozoic).

A bit after the end of the Permian age, and possibly related to it, the crust that made up the super continent of Pangea started to weaken. Tectonic forces from the Earth’s mantle and core were awaking, or at least changing direction, and the continents that had been crushed together started to pull apart again. This tension started gradually, about 225 million years ago; by 200 million years ago the continents were separating in earnest. The rifting we are most interested is that which returned the land that would become Maine to the coast. Remember, during the duration of Pangea, Maine was most decidedly inland.

The continents that would eventually become known as North America and Europe had previously drifted towards each other as the Iapetus Ocean closed. Now, they pulled apart from each other, on essentially the same line as the Iapetus Ocean. As the continents pulled apart, a rift formed between them. At first it was likely just an inland valley, much like the East African Rift Valley today, spotted with long narrow lakes and volcanoes. As it grew wider, one end or the other of it eventually contacted the global ocean, and was inundated with sea water. Voila! An ocean was born. To see this in action today, you need look no farther than the Red Sea—an ocean flooded extension of the rift valley from East Africa.

The ocean that formed of course, is our own dear Atlantic Ocean, named for Atlas, the titan of Greek mythology, who was the son of Iapetus. So clever. And I’ve said that it formed in nearly the same location as the ancestral Iapetus. Nearly, but not exactly, and here is why. Some of the small bits of land, the micro continents, terranes and crushed coastal sediments, that were accreted on to the coast lines of North American and Europe, they stayed accreted, or stuck to the larger continental land masses, instead of getting dragged back out to sea. These bits of land added more “continent” to the continents, and gave the land masses new coastlines as the sea filled in the growing gap between North America and Europe. Of particular relevance for Maine, the microcontinent called Avalon stayed stuck to our coast line, and today makes up much of Hancock and Washington Counties, and a thin strip of coast on the west side of Penobscot Bay.

The opening of the Atlantic Ocean was the last major tectonic event to affect the land we now call Maine. The youngest rocks in the state date to this time period. When the continents were under tension and were pulling apart, many cracks formed in the crust. As these cracks formed, they were quickly filled in by hot fluid magma from deep within the crust, forming intrusions called dikes. This magma cooled into basalt, a rock type that makes up most of the crust of the bottom of the ocean. It is typified by being dark in color and fine grained. In many places along the coast, you can recognize these basalt dikes by their dramatic dark color, in an otherwise light granitic rock. After these dikes formed, there was no more igneous activity, no more volcanoes, no more rifting. At this point in history, Maine settled in for about 200 million years of erosion, which removed much of the material that was covering the mountains and hills we see around us today. These rocks started many miles below the surface, and are only on the surface today due to the constant pressure of the forces of erosion.

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. Next week we take a big leap in time, and change the scale of our gaze as well, as we begin to look at the ice age as the next “big thing” in the history of Maine.


Yep, these references really hit all the key points:  same as for “The History of Maine: Part 1”. http://theworldaroundusradio.blogspot.com/2013/02/the-history-of-maine-part-1.html

 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--http://pubs.usgs.gov/gip/dynamic/understanding.html
 (This is exactly the kind of initiative I want my tax dollars spent on! Thanks USGS!)