This earthcache is located in the heart of downtown San Diego, so parking may be difficult especially during certain times of the year. At the above coordinates, you will see a milestone for San Diego, mile zero in fact. Note, as of June 24, 2020 the milestone has been removed. The following is the original cache description, however, the questions are now based on the new location. The Horton Plaza Fountain is made of concrete, granite, and marble per the Smithsonian art inventory. Since the original pacific milestone was made of marble, this earthcache will focus on the marble aspect of the fountain.
Mile zero marked the western terminus of the Old Spanish Trail Auto Route. It's official name is the Pacific Milestone. There is a sister stone on the east coast in St. Augustine, Florida which marks the eastern terminus. Today, this stone marks the official center of San Diego. All roadsigns you see that designate a mileage from San Diego originate here.
This cache is more about the geology of this milestone than the history. See the works cited links at the bottom of this earthcache to learn more about the history and how President Coolidge dedicated this stone in 1923. The stone, made of a solid piece of San Diego County marble, was given to the city by Veruga Marble company which operated its quarry northwest of Ranchita. This stone was mined there and brought here for dedication. Let's explain what exactly is marble, and how it forms in the first place.
Marble is one of the most beautiful naturally occurring rocks. It's why it's in our homes, floors, and art sculptures. The first thing you need to make marble is dead organic matter and a lots of it. This comes from dead sea life that over millions and millions of years have sunk to the bottom of the ocean. Overtime, these layers of dead organic matter hardened due to the weight of more dead material being laid against it. This newly formed rock is called limestone.
Overtime, the ocean moves and the sea floor gets deeper at a subduction zone, also called a convergent plate boundary. Here, the rock subducts under the continental crust because it is much more dense. Eventually, the rock reaches the upper mantel and begins to melt. This in turn makes the less dense liquid magma to create a volcanic arc along the coast, similar to the Cascade Ranges in the north west. Remnants of these volcanoes are still visible in San Diego County. Probably the best example is in Oceanside at this earthcache: GC51CJ5. Factors including length of time, depth, and the type of rock can determine the outcome of the rock produced. Limestone a sedimentary rock, when melted naturally forms marble, thus making it a metamorphic rock.
So how did marble end up in San Diego County? Well, about 40 million years ago the infamous San Andreas Fault wasn't even created yet. It was during this time that much of California was part of the ocean and the entire west side of the continent had a subduction zone. A tectonic plate called the Farallon was actively dipping under the North American Plate. With it, carried limestone that formed in the ocean. Only recently, within the last 25 millions years, was the geologic activity able to bring this marble close enough to the surface. The marble that you see here was trapped underground for millions of years until it was quarried in the 1920s.
Marble contains certain minerals, depending on the conditions in which it formed. Because marble forms directly from existing limestone deposits, it contains mostly calcium carbonate (CaCO3). From there, the other minerals in the rock depends on a few factors. Temperature, depth, pressure, length of exposure, and other existing rock outcrops all play a role into determining the mineral content of marble. Some marble rocks contain pyrite (fools gold), quartz, iron, and graphite. If marble is white or close to it, it has little impurities. This means when the limestone outcrop was very thick or covered a large area. If the rock is slightly off color, then it means the rock was likely at the edge of an outcrop where it was in contact with other rocks.
If the rock is slightly pink, red, or orange, then the marble contains Iron. If the rock sparkles in the light, then it contains quartz minerals. If it contains graphite, the rock is grey or dark in color. If the rock contains pyrite, then it likely contains larger crystals or even gemstones.
Metamorphic Minerals
Calcium Carbonate is a weak base, meaning it will slightly react with acids through neutralization where one of the bi-products is water. Because this stone is located in the 6th largest city on the county, it is subjected to the acid rain. When fossil fuels are burned, it creates SO2 or Sulfur Dioxide. Coal creates a lot of this, but cars do to. SO2 is a gas so it is carried by the wind to the upper atmosphere. Here, it can react in clouds to form H2SO4 or sulfuric acid. When the clouds get heavy enough, it can rain acid which dissolves calcium carbonate from a solid to a liquid in aqueous form. This can be seen on the stone that has been here for many decades, and has seen its fair share of man-made erosion.
Works Cited
https://geology.com/rocks/marble.shtml
https://infograph.venngage.com/p/210465/acid-rain
http://geologycafe.com/gems/chapter9.html
TO LOG A FIND ON THIS CACHE YOU MUST GIVE ME THE CORECT ANSWERS. YOU CAN CONTACT ME THROUGH MY EMAIL OR THE GEOCACHING MESSAGE CENTER. ANY INCORRECT ANSWERS WILL RESULT IN A DELETED LOG
1. "Horton Plaza" on the first line of your email.
2. Describe the texture(s) AND color(s) of marble at this location. Note the marble portions of the fountain consists of the bottom half of the cupala and the columns.
3. Based on your answers above, what type of minerals are present inside this rock? How can you tell? Justify your answer.
4. Describe effects of acid rain on this rock. Where is the worst weathering occurring?
5. What could the city do to prevent acid rain deterioration from reacting with the calcium carbonate in the rock?
6. Why are there no fossils visible on the surface of this rock even though marble was originally limestone? Explain your answer.
7. Would you expect to see fossils in this rock if you split it open? Explain your answer.