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.
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.