The next time you glance up at the Moon (if more than just a narrow crescent is visible), take an extended look at what you see. Even with the unaided eye, or better still, with a pair of binoculars, you’ll note that there are two different types of landforms visible on its surface.
One of these is the lighter-colored, more rugged lunar highlands, while the other consists of the darker-colored, relatively smooth lava plains or lunar maria (plural of mare – the Latin word for “sea”). The older (and heavily cratered) highlands resulted from the cooling of a “magma ocean” that once blanketed the Moon’s surface. By contrast, the younger and roughly circular maria are remnants of former giant impact basins that afterward became filled with flood basalts from the Moon’s interior.
Six pairs of Apollo astronauts visited the Moon’s surface from 1969 to 1972 and returned with more than eight hundred pounds of lunar rocks (see Steve Voynick’s article, “Minerals of the Moon,” January 2020 Rock & Gem). Scientific analysis of these samples (which remains an ongoing endeavor) has told us a great deal about the makeup of our nearest neighbor in space. In turn, radioactive dating of these rocks has enabled us to reconstruct an accurate chronology of the Moon’s leading geological events –– a history stretching back roughly 4.5 billion years before the present. At the same time, these lunar rocks provided evidence for a dramatic new theory of the Moon’s origin (see ahead), one that likewise explains the cause of its former magma ocean.
The lunar highlands are composed chiefly of aluminum- and calcium-rich silicate rocks of a type known on Earth as anorthosite –– a coarse-grained, intrusive igneous rock “consisting mostly of calcic plagioclase (> 90%, usually labradorite) with < 10% mafics” (i.e., darker, ferro-magnesian minerals such as pyroxenes). Another name for “calcic plagioclase” is the mineral, anorthite (CaAl2 Si2 O8 ), from which the rock’s name itself is derived. Anorthosites differ from most other igneous rocks (e.g., granites) because of their almost complete absence of quartz. On Earth, anorthosites are relatively rare and have been broadly classified in the same group as the gabbros (low-silica, mafic intrusive rocks) and diorites (those of intermediate composition between mafic and lighter-colored felsic rocks) (Mindat.org).
Terrestrial anorthosites are usually found among the oldest parts of the continental crust, although why this should be so isn’t clear. According to one earlier theory (perhaps no longer supported), anorthosites might have composed the first solid crust on Earth (Peter Francis, Volcanoes (1976), p. 335). In North America, anorthosites are most readily found within the Precambrian Grenville Province of southeastern Canada and Labrador, where they have ages of around 1.6 billion years. But they are much more widespread than that. Significant occurrences are also found in New York State’s Adirondack Mountains and Montana’s Stillwater Complex, along with northeastern Minnesota and east-central Wisconsin (part of the Superior Province). It is these latter occurrences to which we will devote most of our attention in the remainder of this article.
SPLIT ROCK LIGHTHOUSE STATE PARK
One of the largest and most accessible exposures of anorthosite in the world is the headland on which Split Rock Lighthouse in Minnesota was constructed. The lighthouse is located on State Highway 61, about 20 miles northeast of Two Harbors and roughly 50 miles northeast of Duluth. These magnificent, one-hundred-feet high cliffs form a small portion of the extensive rock sequences comprising the Beaver Bay intrusive complex that borders the northern shore of Lake Superior from Split Rock Point to Little Marais.
The need to build a lighthouse to warn mariners about this headland arose from a disastrous storm that sank or damaged 29 ships on western Lake Superior in 1905. Victims of that November gale included a commercial ship, the Edenborn, plus a barge, the Madiera, that was being towed. With support from the federal government, this navigational aid was built and became operational on July 31, 1910. The Split Rock Light Station (as it was originally called) primarily served freighters that hauled iron ores that had been mined in northern Minnesota and shipped from Duluth and Superior, Wisconsin. After decades of service, the lighthouse was decommissioned in 1969 and ownership transferred it to the State of Minnesota for preservation.
It was made the centerpiece of Split Rock Lighthouse State Park in the 1970s and has been managed by the Minnesota Historical Society. It is listed on the registry of National Historic Landmarks.
GETTING TO SPLIT ROCK LIGHTHOUSE
From Duluth, take Minnesota Highway 61 northeast along the Lake Superior shoreline and proceed another 20 miles past the town of Two Harbors. The entrance to Split Rock Lighthouse State Park is on the right (follow the signs). Free parking is provided. Tickets for admission may be purchased at the lighthouse. If you simply wish to tour the lighthouse and park grounds for the day, then no reservations are needed. But overnight camping at the park does require advanced reservations, which can quickly fill up for the season. Call in advance to see if campsites are available: (218) 226-6372. Photography is permitted; special events are hosted on occasion. Hours: May 15 – late October: DailY, 10–6; late October – May 14: Thurs. – Mon., 11–4.
ADDRESS: 3713 Split Rock Lighthouse Road, Two Harbors, MN 55616. WEBSITE: http://www.splitrocklighthouse.org E-MAIL: firstname.lastname@example.org GPS COORDINATES: 47.2010, –91.3671
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