August 26, 2004
The metamorphic rocks exposed in this outcrop near Michigamme, Michigan are over 3 billion years old, making them some of the oldest in North America. Most of the outcrop consists of a migmatite gneiss. Migmatite looks like a marble cake. It is a mixed rock in which at least one component is igneous, sometimes injected through fractures in the host rock. Migmatites also occur when rocks are partially melted during metamorphism, and the molten mineral assemblage resolidifies as igneous rock. In this outcrop, the migmatite gneiss is cut by 2.7 billion year old granite.