Gneiss Smile

August 01, 2018

Gneiss smiling

Photographer: Rob Sheridan 
Summary Author: Rob Sheridan 

Looking a bit like a smiling frog, this weathered rock is a fine example of granite gneiss (pronounced "nice"), formed when granite undergoes metamorphic change deep below the surface. Granite usually forms when a deep intrusion of magma cools slowly, allowing individual crystals of feldspar, quartz and mica to grow large enough to see. When later subjected to the intense pressure, temperature, and particularly shearing forces associated with major tectonic events (such as orogenies) granite will metamorphose to become gneiss.

Eastern Massachusetts is a complex assembly of volcanic subduction zone islands and seabeds compressed against the central continental craton during closure of the Iapetus Ocean (proto-Atlantic) and formation of Pangea between about 500 and 250 million years ago. Deep underground granite intrusions were metamorphosed during this time. The opening of today’s Atlantic Ocean and erosion have exposed some of this rock. Granite gneiss is characterized by small elliptical feldspar inclusions, sparkling small crystals of dark biotite mica, compositional banding, and a foliated (layered) structure, all of which are seen in this specimen, that was subsequently subjected to smoothing by wind and water. Photo taken on July 25, 2017.

Photo Details: Camera: Apple iPhone 5s; Exposure Time: 0.033s (1/30); Aperture: ƒ/2.2; ISO equivalent: 125; Focal Length (35mm): 29