A NOTE TO THE CACHER:
As you discover more about this iconic stone, please follow all posted signs, stay on the marked footpaths and do NOT attempt to touch the stone. Practice good geocaching princilples and ensure the longevity of this historic monument.
GEOLOGIC HISTORY:
Plymouth rock is a prime example of a geologic substrate that covers a majority of Eastern Massachusets: Dedham Granite. Named after the nearby town where it was first classified, Dedham Granite is characterized by its greenish grey coloring and fine crystaline structure. Radioactive dating of this similar examples of this granite show it cooled from a molten state about 620 million years ago. Chemical studies show this granite hardened fairly close to the Earth's surface, since it is mixed with surface volcanic lava of similar composition. Stare at Plymouth Rock. Not only are you staring at an American symbol, you're staring at a 620 million year-old chunk of Senegal, Liberia, or Sierra Leone from a time when no life on land existed on Earth and nothing bigger than your hand lived in the world's oceans.
All of eastern Massachusetts, from Route 495 to the Atlantic, is a chunk of West Africa which began to split from that continent around 600 million years ago and started moving towards ancestral North America, which at the time had a coastline alongside the Adirondacks in the north and followed the western side of the Appalachians to the south, neither of which existed then. This early, smaller continent we now call North America is called "Laurentia." At this time, Africa and much of Europe and Asia were part of one giant continent called "Gondwana."
Somewhere around 650-600 million years ago the westernmost end of Gondwana did an Ishmael, took a boat ride and began to split from Africa. As this splitting occurred, magma from the Earth's mantle surged up through the cracks, creating volcano chains and lava flows. Some of the magma never surfaced but formed huge pools and ponds underground where it slowly cooled into solid, fine-grained rock. This is the rock, the Dedham granodiorite, which comprises what we now call Plymouth Rock.
Somewhere between 375 and 325 million years ago, Dedham finally made landfall and glued itself to North America. This event is called the Acadian Orogeny. The Gondwana-orginating Dedham Granodiorite met with the much courser grained granites and bedrocks of the existing North American continent. The meeting of the drifting continent and the fledgling North American continent resulted in the formation of the current geological substrate of modern day New England. Millions of years later, during the last ice age, Plymouth Rock was deposited on its current beach by a retreating glacier.
Today the Atlantic Ocean is getting wider by a couple thumbnails (10-15 millimeters) each year. If you dial that rate back, you find that around 290 million years ago, Africa was just a hop-step off Plymouth Rock. These observations, compounded with observations of fossil records and metalic alignment due to polar shifts, help support the theory of Techtonic Drift, first proposed under a different name, by Alfred Wegner in 1912.
Dedham Granite, has been used for hundreds of years in New England, it adorns the walls of St. Mary's Church and St. Paul's Church, the Boston and Providence Rail Stations, and the Trinity Church in Copley Square.
Millions of years of travel, have brought this iconic symbol of the New World to its final resting place. How ironic that the boulder upon which the Pilgrims landed in 1620 as they sought to escape the "old world" actually originated from the place they left.
TO CLAIM THIS EARTHCACHE ANSWER THE FOLLOWING QUESTIONS:
- Is the crystalline structure of the rock fine or course? Does this match with typical formations of rock farther west? In your own words how can the grain size be used to help support the idea of plate tectonics?
- How many years ago did this chunk of Dedham Granite split from Africa?
- In your own words explain Plate Tectonics.
- (Optional) Was the rock bigger or smaller than you expected?!
- (Optional) Take a photo of yourself at GZ (please do not include photos of the rock in your picture)
Send the answers to me via the geocaching messaging system. You do not have to wait for me to respond to log your smiley. If I have issues with your answers I will contact and discuss them with you. If I do not recieve your answers within a week of logging, I reserve the right to delete your log. Please send the answers! Thats how I know you actually visited the site!
Sources:
- USGS Mineral Resources Spatial Data
- The American Journal of Science - Volume 188
- Contributions to the Stratigraphy of New England
Special thanks to blogger Douglas Watts who inspired this Earthcache with his blog pointing out the irony of the Old World origins of Plymouth Rock.