December 7, 1941. A date that will live in infamy. The City of San Diego erected the Pear Harbor Memorial in 1984, on the 40th anniversary of the attack. The memorial consist of two large boulders, both are metemorphic rocks (more specifically called mica schist) which contain bronzes plaques on both sides. That said, the geology of this memorial makes it an interesting place for an earthcache.
Mica schist is a crystalline metamorphic rock, meaning it contains crystals, formed deep within the earth. Its protolith rock (meaning the rock before it was metamorphosed into schist is shale, a common sedimentary rock found off shore in costal and lacustrine (lake) environments. Shale can contain fossils, but through the transformation process of creating metamorphic rock all remnants and structures of the fossils are lost.
Schists typically are named first by their most dominant mineral. For example, mica is a common silica mineral which has perfect basal cleavage. Cleavage is the tendency of a mineral structure to break along predictable planes and angles. Mica, in this case white muscovite, tends to split into sheets of paper, giving the rock an overall glossy and sparkly appearance if viewed from the right angle with the sun's reflection. Go one and give it a try. If you can see the sun's reflection, you are looking at a cleavage plane.
Below is a chart of common minerals
Below is a chart of common metamorphic minerals
Schists are classified by 50% of its minerals being elongated (foliated) due to the environment (pressure and depth) the rock was metamorphosed in. In addition, mineral grains must be visible to the naked eye. Other common minerals in schists include hornblende (brownish black), talc (white), chlorite (dark green), graphite (grey), garnet (rich orange-purple), and glaucophane (light blue). As this is a metamorphic rock, the protolith rock did not melt or liquify. Instead, the heat and pressure allowed the atoms of the protolith minerals to restructure and reorganize, changing the mineral and chemical makeup of the rock.
A common way shale can metamorphosize into slate and marble occurs at a convergent plate boundary where two tectonic plates come together. Typically, this is where an oceanic plate subducts under the continental plate. As the cold oceanic plate dives under, it heats up which changes the mineralogy of the rock eventually. Depending on the specific heat, pressure, and protoliths a wide range of metamorphic rocks and minerals can form. Slate is considered to be low grade because it has smaller grains. This means it has had less time to form larger, more distinctive crystals which would otherwise be classified as high grade. Slate forms in low pressure and low temperature conditions (low p/t). Marble can have a variable grade, depending on the time it underwent metamorphism. The longer a rock is subjected to high temperature and high pressure, the more linear the minerals become. This is because the rocks are large amounts of pressure (kms of rock) that squeeze newly formed crystals into lines. The first image below shows varying types of grades that show mineral sizes (and linearities) under a petrographic microscope. The second image shows hand samples. Note the linearity and size of grains as the grade gets higher.
TO LOG A FIND ON THIS CACHE YOU MUST ANSWER ALL THE QUESTIONS BELOW. YOU CAN CONTACT ME THROUGH MY EMAIL OR THE GEOCACHING MESSAGE CENTER TO SEND YOUR ANSWERS. ANY INCORRECT ANSWERS MAY RESULT IN A CLARIFICATION RESPONSE FROM ME.
1. "Pearl Harbor Memorial" on the first line of your email AND list all geocaching names of your party so I can match your answers to them. If you all want to learn something, I would prefer each cacher send me individual emails in the spirt of earthcaching.
2. Take a close up photo of the mineral grains of the rock using some sort of scale. Geologists often use coins, GPS', pens, shoes, notebooks, ect. You might want to be creative in what you use for your scale, however, the scale that you choose is entirely up to you. Make sure your photo is close enough to capture foliations that can be clearly seen. This photo MUST be attached to your "found it" log. Please do not send any photos through the message center as Groundspeak compresses images. Each log MUST have a unique and different photo. No two logs may contain the same photo.
3. Describe the physical characteristics of the mica schist boulders. Specifically, tell me their (a) colors, (b) textures, and (c) average grain sizes.
4. Based on your answers for 3, do you think these two boulders were originally the same rock, or did they form and originate in different parts of the Earth? Explain.
5. Using the mineral identification chart in this cache page, name two minerals that are prevalent (excluding mica).
6. What grade would you classify these mica schists? What makes you think so? Explain.
Works Cited
http://chc.sbg.ac.at/sri/thesaurus/node.php?id=37
https://miningeology.blogspot.com/2015/05/metamorphic-ore-deposits.html
https://usenaturalstone.org/schist-happens-get-know-brilliant-stone/
http://geologycafe.com/gems/chapter9.html