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.
Source:
Leaverite News Articles by William S. Cordua UWRF – Mellen “Black
Granite” 1995