Monday, April 21, 2014

Besting Goliath: The Formation of the Pawtuckaway Ring Dike

The Massabesic Gneiss looks like ice cream
swirled with fudge because of heat and
pressure from 500 million years on the surface
It's hard to dodge the mythological when you look at maps of Pawtuckaway State Park.  Some refer to it as the Dragon's Eye, and for good reason; the park features concentric rings of mountains, identifiable as the iris and cornea.  To see it might even conjure up thoughts of a volcanic Mordor, and as it turns out, 150 million years ago, you wouldn't have been all wrong.  Perhaps the best metaphor for the origin of this strange structure is not literary, but biblical.

The Massabesic Gneiss was Goliath.  Forged in the tectonic rift that cleaved coastal New England from its former mooring point in an ancient Africa, it survived a trek across a long vanished ocean and 500 million years of survival at the surface of the Earth.  Its bold endeavors are catalogued in folds left by heat and pressure along the way.
The ice on the trail is a good metaphor for the park.  As the pool
of magma melted the land below, and added weight above.the
land gave way, allowing magma to rise up through the crack.
Massabesic's David, was not a sling, but a hotspot.  A sedentary warm point deep below Earth's surface regularly created blobs of magma that rose upward.  As the continent shifted with the movement of tectonic plates, these blobs left a string of volcanic mountains including Mount Royal in Montreal and Mount Washington farther north in New Hampshire.  Piercing the Massabesic Gneiss would be different.  Per usual the blob rose toward the surface.  Instead of erupting, the magma sat near the surface, weakening the gneiss's structure from below.  A small eruption may have penetrated the enduring rock.  The added weight, like our misplaced feet, was enough to plunge the Massabesic gneiss into the magma below.  David had bested Goliath.

The gabbro that makes up Meloon Hill flowed up through the
cracks left when the roof of the magma chamber collapsed
Over ten million years, this crack in the armor became a passageway for eruptions of magma that had bided their time.  Magma would have oozed up through the cracks that separated the sunken gneiss from that which remained.  An arc of dark colored, large grained rock, called gabbro, confines the southwest part of the Dragon's Eye.  A disk of salt and peppered rock, called diorite, underlies the lowlands.  These darker rocks may have recollapsed and remelted.  This newer, purer magma would have seeped through cracks to form the whitest, hardest rocks in the park: monzonite. 

Millions of years of erosion laid waste to the softer diorite, and shaved quite a bit off of the gabbro.  The monzonite, more resilient than the rest, remained.  Its stark cliffs are now a monument to the epic battle between a seasoned champion and a literal  That is, until a new champion arises.

The view from one monzonite ridge to another.
In the middle are the diorite lowlands. 
Dorais, M. J.. "The Massabesic Gneiss Complex, New Hampshire: a study of a portion of the Avalon Terrane." American Journal of Science 301.7 (2001): 657-682. Print. 

Eby, G.N.. "Mount Pawtuckaway Ring Dike Complex." Geology of the coastal lowlands, Boston to Kennebunk, Maine. S.l.: New England Intercollegiate Geologic Conference, 1984. 240-248. Print. 

McGarry, MaryAnn. "Volcanoes in New Hampshire ." Plymouth Portfolio. Plymouth State University, 17 Nov. 2012. Web. 19 Apr. 2014. <>. 

Reidy, Daniel E.. "Jurassic Period." New Hampshire Geology Home Page. N.p., n.d. Web. 19 Apr. 2014. <>. 

1 comment:

  1. This is great! Thank you so much! Had no idea about the hot spot.