Monday, March 19, 2012

Minnesota Geology Monday - Anorthosite

Anorthosite is an intrusive igneous rock made up of more than 90% of the mineral plagioclase.  The minerals with Anorthosite crystallize at depth within Earth's crust.  Because Anorthosite contains such a small percentage of minerals that tend to oxidize, this rock is extremely resistant to forms of chemical weather.

The Anorthosite has been brought to the surface in several locations along the North Shore of Lake Superior by associated magmas and lavas of the Mid-Continent Rift 1,100 years ago as large blocks that formed at great depths in the crust.  As the magmas of the rift rose, they carried the blocks of Anorthosite upwards.  The contact of the Anorthosite and mafic lavas can be very distinct.

Near Carlton Peak, within the Temperance River State Park ( is an inactive Anorthosite quarry.  Rock quarried here were used for breakwaters at locations near the quarry.  The quarry has several distinct levels that were active in the past.

From the quarry or nearby Carlton Peak, views out over Lake Superior are fantastic on clear days.  Carlton Peak has an elevation of 1526 feet above sea level, this is 924 feet above the surface of Lake Superior.  To reach the quarry or parking area to Carlton Peak, you must drive a winding 1.5 mile un-improved gravel road inland from MN highway 61.

At all levels of the inactive quarry are large waste rock piles and the impressive vertical faces where rock material was removed.

Another location with a large block of Anorthosite is at Split Rock Lighthouse.  The lighthouse sits on one block of Anorthosite during the rifting event, more will come later on the geology of the rift and the Split Rock Lighthouse area.