Tipping the Clown: Changing Density in the Deer Isle Granite

When I was young I had an inflatable clown with weights on the bottom, so you could administer whatever childhood battering you cared to, and the clown would bob back upright.  I recently read about a feature of the Deer Isle Granite that got me thinking about that clown.

Deer Isle Granite: Naskeag Point

The granite that underlies Deer Isle is long.  It extends from Flye Point on the Blue Hill Peninsula to the southern tip of Stonington in the south.  While the rock is all clearly Deer Isle Granite, it is not homogenous.  Going to Naskeag Point on the mainland presents a deep pink, while a visit to Stonington displays a much wanner stone.  The middle ground of Oak point shows something in between.  The source of the redness may lie in oxidized (rusty) iron that replaces aluminum ions typically present in a mineral called feldspar.

Deer Isle Granite: Oak Point

Liquid rock under the surface cools to form solid granite.  As a result of 4.6 billion years of sorting by density, most granite bodies tends to have fairly uniform consistency.  Deer Isle Granite is different.  For some reason, during its formation, two types of magma were mixed together.  Imagine a nice Italian dressing, shaken before being added to salad. The vigorous mixing swirled everything together, but before it could harden there was time to settle.  Less dense materials, high in silicon content drifted to the top, while the more dense, high aluminum content stuff sank to the bottom.  The aluminum portion took on its iron and its rusty hue.

Deer Isle Granite: Stonington

Under normal conditions the weighted bottom of the clown would remain pointed downward.  The Acadian mountain building event was not normal conditions.  A small continent, and the tectonic plate it rode upon, glided across the fluid mantle toward the prehistoric Maine coast and rammed the landmass.  The collision was not a child's smack, but a match full of heavyweight boxer's jabs.  This impact was enough to permanently tip the clown on its side, revealing the changing color.

Dietrich, Richard Vincent, and Brian J. Skinner. Rocks and Rock Minerals. New York: Wiley, 1979. Print.

Hooke, Roger Leb.. "A Geologic History of Deer Isle, Maine." College of the Atlantic, Serpentine Ecology Conference. July 2007. Web. 14 Oct. 2013. <www.coacommunity.net/downloads/serpentine08

A Piece of Deer Isle: Rapakivi Fingerprints

When I first started learning about rocks I remember being impressed that each rock formation was unique.  The reddish color of a brownstone cobble in Finlayson, Minnesota informed me that the rock had made the 50 mile trip from the iron rich shores of Lake Superior.  I've started to look for fingerprints in rocks, and there is none more common in Maine than the rapakivi crystals of Deer Isle granite.

Deer Isle Stonework in Falmouth, Maine

While the formation has its home on its namesake island downeast, Deer Isle granite is everywhere.  You can hardly take a step in the L.L. Bean flagship store without resting your sole on a slab. I've noted its presence in kitchen counters, cutting boards, and outdoor stonework.  It even secreted itself into the foundation of Yankee Stadium, a long trip for a stone from Red Sox territory.  In any of these locations the rock would be instantly recognizable by its round pink crystals wrapped in a ring of white.

Close up of Deer Isle granite.  Notice the rounded pink crystal
in the center, and the white rim surrounding it.

The ring not only gives up the source location, it tells a story.  The pink mineral, microcline feldspar, is not normally round. When it developed deep under the surface it would have taken the form of a prismatic rod with regular angles. But this was under pressure.  Where the microcline first solidified may have been 10 miles under the surface. That means 10 miles of rock weight are pressing down on our magma stream like the grasp of Superman.  Just like the hero's clutch could turn coal into diamond, the added pressure that comes with this weight turned liquid rock into solid.  But the crystal had formed before its time.

As this slurry of magma and loose crystals rose up toward the surface, the weight pressing on it subsided.  Without the added pressure, the geometric crystals began to melt, leaving only their rounded centers behind. As the ooze ascended, crystal formation conditions changed.  The microcline, more stable in the deeper conditions, would now be replaced by a different mineral, white plagioclase feldspar.  Because the two minerals are very similar, the plagioclase quickly continues the pattern disrupted by the pressure drop.

Conditions must be just so to create this pattern. A cooler magma channel, the white mineral never forms.  The pressure drops too quickly and the pink feldspar melts altogether. Deer Isle granite's unmistakeable fingerprint is a result of its unique story of formation, dissolution, and restoration. This distinctness may be one trait that makes the stone desirable, but it is certainly one that makes it recognizable.

Eklund, O., and A.D. Shebanov. "The origin of rapakivi texture by sub-isothermal decompression." Precambrian Research 95.1-2 (1999): 129-146. Print.

Hooke, Roger Leb.. "A Geologic History of Deer Isle, Maine." College of the Atlantic, Serpentine Ecology Conference. July 2007. Web. 14 Oct. 2013. <www.coacommunity.net/downloads/serpentine08

Nekvasil, Hanna. "Ascent of Felsic Minerals and Formation of Rapakivi." American Mineralogist 76 (1991): 1279-1290. Print.

Prinz, Martin. Rocks and Minerals Simon & Schuster's Guide to Rocks and Minerals.. New York, NY: Simon and Schuster, 1978. Print.

A Trip to Baxter Four Hundred Million Years Ago

To visit Baxter State Park during Devonian time would have been quite an experience.  Devonian time extends from 419 million years ago, when fish were just starting to widen their grip as rulers of the ocean, to 359 million years ago, when amphibians were testing their new toes on continental soil.  It is during this period of prehistory when almost the entirety of Baxter's bedrock was lain down.

419 million years ago, to travel the path that one takes to the north entrance of the park from Patten would require a boat.  Paddling north on the route that 159 takes, you'd hit land not far south of Shin Pond and a long portage would take you over the island arc and continue you on your way.  The island extends into the realm of the park only in so much as the rains tearing apart the island, at a snails pace, were delivering the islands sedimentary fragments into the surrounding ocean.  The heavier sand dropped first in a wide delta, while the smaller silt and clay drifted farther into the ocean, only to be dropped when the stream's energy had been almost fully spent.  The sandstone that was once the delta can be found along the eastern edge of the park, while the old ocean bottom wraps the northern and western sides.

The trip up Katahdin would have, in fact, been a descent.  While the portage island was being torn apart, southern Maine was plunging beneath northern. As the ocean bottom sank, it melted.  As it melted it rose, creating an upside down tear drop of magma not far from the surface, but still a ways down from Baxter Peak's current stature.  As the magma cooled, minerals formed creating the small, but visible crystals of the Katahdin granite.  The magmatic elements paired off, leaving behind the ingredients of water vapor.  The bubbling gas rose to the top of the magma chamber.  As the magma hardened around the bubbles it left cavities in which different minerals could form.  The change from the liquid magma chamber which formed the base of Katahdin to the frothy top, which formed the peaks is visible today as the white, even grained granite evolves into reddish, multi-textured granite.

While the Travelers are smaller in stature now, they literally rose out of Katahdin during the Devonian.  The Traveler Rhyolite was the volcano to Katahdin's magma chamber.  A trip there means braving molten lava, but also burning ash.  A hellish expedition to be sure, but at least there wouldn't be any black flies.  The drifting ash interbedded with the lava and then flowed down slope.  In modern times the flow is visible because the gray ash is flattened amidst the white rhyolite.

Later in the Devonian, a trip down the South Branch Pond Brook, geologically, wouldn't have been too much different than it is now.  The towering volcanoes, like the mountains that now stand, would have provided a prime environment for raging rivers powerful enough to break apart and then round the edges of chunks of rhyolite.  Smaller particles would be taken farther off to sea.   This order is preserved in the sequence of rocks below the falls - rhyolite, conglomerate, finer-grained sedimentary rock.

With current technology as a limit, an adventure in Devonian Baxter State Park is of course an impossibility.  The current landscape becomes our only time machine through which to view this exciting period in Maine's history.

Osberg, Philip H., Hussey, Arthur M., II, and Boone, Gary M. (editors), 1985, Bedrock geologic map of Maine; Maine Geological Survey (Department of Conservation), scale 1:500,000

Rankin, Douglas W., and Dabney W. Caldwell. A Guide to the Geology of Baxter State Park and Katahdin. Augusta, Me.: Maine Geological Survey, Dept. of Conservation, 2010. Print.

"Maine Geologic Map Data." Maine Geologic Map Data. 05 Apr. 2013. Web. 03 July 2013.