Wednesday, November 26, 2008

The geology of Plymouth Rock

Plymouth Rock, where the Mayflower pilgrims allegedly alighted on the new world, is a large boulder of Dedham granite. Like the pilgrims, this granite has traveled.

It is a glacial erratic, meaning that it was scraped off the ground and picked up by a glacier, and it then dropped into its current position when the glacier melted during the waning years of the last ice age. The Dedham granite from which it is derived is a neoproterozoic intrusive that forms much of the crystalline basement of the Esmond-Dedham terrane.

This terrane forms the bedrock of much of eastern Massachusetts and most of Rhode Island. The 680 Ma granites intrude older clastic sediments, which contain predominantly Grenville-aged zircons. But the basement on which those sediments settled is a mystery.

The Esmond-Dedham does not just have a mysterious bedrock; it has an exotic origin as well. It is an accreted terrane. It is a sliver of the southern supercontinent of gondwana, and only arrived at its present North American position in the last gasp of the Alleghenian orogeny, the last of the three mountain-building events that formed the Appalachian mountains.

So when the pilgrims stepped onto that rock, those transported souls were disembarking onto an erratic dropstone of a microcontinent, which itself had traveled from Africa, back before the Atlantic Ocean even started to form.

Here on planet earth, even the solid ground moves around the globe. Splitting, sliding past, colliding and attaching to each other, these continental fragments travel around the globe at the whims of convection.

In is not in the nature of this planet to be static; we leave that to the dead moons and asteroids of the solar system. But for an evening, tomorrow, we can cease our wanderings, and gather together for company, cheer, chow, and thankfulness. Unlike Plymouth Rock, most of us are not born of lava, witnesses to the birth of mountains and seas, and surfers of continental glaciers. But we can all tell stories, give thanks, and enjoy this great American holiday.

Happy Thanksgiving.


Callan Bentley said...

John McPhee wrote up the geology of Plymouth Rock in an entertaining essay called "Travels of the Rock," which appeared in the compendium Irons in the Fire. I recommend it.

Chris Phoenix said...

Look at the craters, then tell me the moon is static.


C W Magee said...

I should have realized that an eloquent person would have picked up on this.

And Chris, if the moon was dynamic, those craters would have disappeared.

C W Magee said...

McPhee's essay, as a sample chapter for his book:

Kirsten said...

Chris and I went to see Plymouth Rock once. He threw a penny at it and got it to stay on top. Also, it's smaller than we thought it would be. But the things you said about Plymouth Rock are nice too.