This is an Earthcache – as such, there is no physical geocache to find here. Instead, after examining the Duluth Gabbro at the posted coordinates, you will answer 3 questions, and then message me the answers to claim a find. The National Museum of the Marine Corps is open from 9:00 AM (0900) to 5:00 PM (1700) every day except Thanksgiving Day and Christmas Day.
Welcome to the Timothy T. Day Overlook. Built in 2013, it offers an impressive, elevated view of the Semper Fidelis Memorial Park. Mesabi Black Granite was used for the sign, steps, and circle. Though it is sold as a "granite" commercially, technically this stone is a Anorthositic Gabbro from Babbitt, Minnesota. As you enter take a look at the polished gabbro signs on each side of the entrance. Polished stone bring out the colors in stone more, and as you approach the steps near the front of the Overlook you will be able to compare between the polished and unpolished stones.
Gabbro is an intrusive igneous rock, meaning it formed from the slow cooling magma into a holocrystalline mass deep beneath the Earth's surface.
The large mineral crystals in gabbro are evidence that these rocks cooled slowly from molten rock. That slow cooling occurred beneath the earth's surface and required a long period of time. Igneous Rocks that form closer to the earth's surface (where it is exposed to air or water) cools down more rapidly, which results in the stone having smaller crystals.
This rock is part of the Duluth Gabbro Complex, a body of igneous rocks that curves 150 miles northeast from Duluth. This geological complex of intrusive rocks formed about 1.1 billion years ago during the Midcontinent Rift, a geological time where continents collided, fused together, and split apart along rifts.
To be classified as anorthosite gabbro, the gabbroic rock must consist of less than 10% mafic mineral content. Plagioclase feldspar is usually a white to gray color, so having a gabbro rock dominated by a large percentage of plagioclase feldspar makes it more similar to anorthosite.
Igneous Rocks come in a wide variety of colors--greens, whites, browns, yellows, blues, blacks, reds, grays, and more. The relative proportion of the different colored minerals in a stone is largely due to the original source of molten rock that cooled to form that rock.
So, if the molten rock was abundant in potassium feldspar, the igneous rock is more likely to take on a pink color, and if the molten rock was abundant in quartz and minerals that make up amphibole you will likely get a black and white speckled rock. Duluth gabbro is dark gray rock with calcic feldspars, black hornblende, and green olivine crystals.
LOGGING REQUIREMENTS:
To log this Earthcache: Read the geology lesson above. Answer all three questions posted below. Answers can be sent via e-mail or messenger contacts on my Geocaching profile within a reasonable time. Group answers are fine, but do not post the answers to the questions in your logs.
QUESTION 1. To be classified as anorthosite gabbro, the gabbroic rock must consist of less than what percent mafic mineral content?
QUESTION 2. Look closely at the gabbro, you will see splotches of colors. What other minerals can you identify that make up this stone?
QUESTION 3. Now that you are an "expert" on this stone, stop by the bronze German Shepherd statue standing watch. Is "Caesar" sitting on a slab of Duluth Gabbro?
OPTIONAL PHOTO: Posting a photo that readily indicates that you (and anyone else logging the find) are at the location.
Awesnap has earned GSA's highest level:
REFERENCES:
1. The Duluth Complex in the Gabbro Lake Quadrangle, Minnesota W.C.Phinney, Report of Investigations 9, Minnesota Geological Survey, University of Minnesota, 1969, PDF, conservancy.umn.edu
2. Minnesota Geological Survey , College of Science and Engineering, University of Minnesota, website, cse.umn.edu
3. Site Visit to Mesabi Black Quarry, Lake County, MN, Nov., 2003, website, johnroloff.com
4. National Marine Museum, Case Studies, Coldspring USA, website, (Link no longer valid) coldspringusa.com