By: magic-gigapans
Image: Robin Rohrback, Mid-Atlantic Geo-Image Collection
License: Creative Commons Non Commercial ⧉

Uploaded: 13 May 2020
Last Updated: 15 May 2020

777 megapixels
26,440 x 29,392 pixels
88.1 in X 98.0 in at 300dpi
284 pixels per inch

As North America collided with the Chopawamsic Terrane during the Taconian orogeny, temperatures within the crust started to rise. As things got warmer, partial melting occured. Minerals present in the graywacke that had low melting temperatures, like quartz, potassium feldspar, and muscovite mica (felsic minerals) all melted, and trickled out of the area. Minerals with higher melting temperatures, like plagioclase feldspar, augite, hornblende, and olivine, were left behind as solids. These minerals (the mafic ones) didn't melt. Mobilized granitic magma rises until it reaches neutral buoyancy, or something stops it. At whatever level in the crust it stops, it cools and solidifies into solid granite. To sum up, granite is generated through partial melting at depth in the crust, and then rises to cool and recrystallize at a more shallow depth in the crust. Felsic instrusive rocks are one of the signature characteristics of mountain belts. Rocks along the Billy Goat Trail show this process of partial melting caught in the act. These rocks are called migmatite, rock that has partially melted and then recooled. As such, migmatite straddles the boundary between metamorphic and igneous rocks; it's partially metamorphic and partially igneous. The granite derived from partial melting then mobilizes as big blobs of low-density liquid. Like the blobs in a lava lamp, the granite magma rises through denser rock, cutting across it and working its way upwards. The dark-colored rock in this image is metagraywacke. The light-colored rock is granite that has been "sweated out" of the metagraywacke.