There are a number of quarries near Mellen that produce a stone locally called "black granite". The rock is more correctly identified as anorthosite and can also be seen in many roadcut exposures in the area, such as this one.
Anorthosite is a coarse-grained igneous rock like granite, but contains mostly plagioclase feldspar, usually with a bit of pyroxene, olivine or magnetite. Anorthosite makes up about 60% of Earth's crust. It was found in all the rocks returned from the Moon, including the oldest (dating back 4.4 to 4.5 billion years), and is believed to make up a significant fraction of the lunar crust. The anorthosite near Mellen is part of a larger body of igneous intrusions formed by the same magma that popped through to form the lava flows around Lake Superior. Instead of erupting, the Mellen magma cooled slowly below the ground, forming coarser crystals. The plagioclase near Mellen is dark gray to nearly black and crystals can reach several inches in length. This forms a dark but sparkly rock similar in appearance to larvikite.
One quarry is easily visible from the road leading from Mellen to Copper Falls State Park (see WP). The anorthosite in the quarry is very dark, and is cut by dikes of a white rock of more granitic composition. It’s also been said that Mellen black granite was used on Kennedy’s eternal flame gravesite
TO LOG THIS CACHE:
1) Identify a piece of “black granite” and upon closer examination estimate the percentage of white/grey/black crystals in the specimen and their average crystal size.
E-mail me the answer HERE.
Leaverite News Articles by William S. Cordua UWRF – Mellen “Black Granite” 1995