Tuesday, February 22, 2011

Delaware Bay - Part II

About 280-230 million years ago (Late Paleozoic Era until the late Triassic) the continent we now know as North America was continuous with Africa, South America, and Europe.  They were all part of an enormous supercontinent known as Pangaea.  Then, as part of an enormous crustal dance, this supercontinent began to break up sometime in the early Mesozoic Era (251.0 to 65.5 million years ago).  It was like a colossal cosmic fork cutting into a slice of our cherry pie – separating this wedge of geological dessert into two parts.  As one section slowly separates from the other, cherry filling oozes and seeps from the crack – a temporary “valley” has formed between the two segments.
In the evolving earth, a three-pronged fissure deliberately grew between Africa, South America, and North America – gradually ripping Pangaea apart.  Cleaving began as magma (cherry filling) welled up through a weakness in the crust, creating a volcanic rift zone.  Powerful eruptions spewed enormous masses of ash and volcanic debris across the landscape as the severed continent-sized fragments of Pangaea slowly, ever so slowly, diverged.
Then, sometime during the Jurassic Period (199.6 to 145.5 million years ago) the Atlantic Ocean began to form between North America and Europe (To carry our geological metaphor one more step let’s imagine some vanilla ice cream melting into the rift zone between our two portions of cherry pie.).  The Eurasian Plate and the North American Plate continued to separate.  Between them was a plate boundary (now known as the mid-Atlantic ridge)« which provided the raw volcanic materials for the constantly expanding ocean basin.  Somewhat later, South America and Africa also began to drift apart (look at a map of the world and you will see how they might fit together, even today, much like two gargantuan jigsaw puzzle pieces; or, if you will, two oversized portions of slightly deformed cherry pie). 
Meanwhile, what is now North America was pulled westward - away from the rift zone.  The thick continental crust that made up the new east coast eventually collapsed into a series of down-dropped fault blocks that roughly parallel today's coastline.  At first, the hot, faulted edge of the continent was high and buoyant relative to the new ocean basin.  But then, as the edge of North America moved away from the hot rift zone, it began to cool and subside beneath the new Atlantic Ocean.  This once-active plate boundary now became the passive, trailing edge of westward moving North America.

[Part III of our story will continue in the next posting.]

« This ridge now extends from a point northeast of Greenland to the Bouvet Triple Junction in the South Atlantic – a distance of approximately 6,200 miles.  It is currently the longest mountain range in the world. 

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