While in town for Comic-Con in 2022, I happened to walk past these peculiar lion statues located next to the San Diego Chinese Historical Museum. These statues mark the unofficial entrance to the Asian Pacific Thematic Historic District, adjacent to the Gaslamp Quarter. Strangely, the Gaslamp Quarter is listed in the National Register of Historic District as an official historic district yet the Asian Pacific Thematic Historic District is not. Wonder if that will change one day. Anyways, on to the geology of these two statues.
At the base each is a plaque mentioning the source of the materials that these lions were carved from. These two lions are made out of granite, but not just any kind of granite. Granite from Mount Tai (image below), one of China's holiest sites. The granite rocks that make up the mountain date back to the Cambrian Period (roughly 500 million years ago). The mountain is at an elevation of 5,029 feet. Mt. Tai began to form (as in the land began to uplift) during the Cenozoic Era (about 30 million years ago). Interestingly, the first documented earthquake in China is traced back to the mountain in 1831 BC or 1652 BC. The exact year is still being debated, but regardless of the year, it is still the first documented earthquake to occur in the country.
Let's first discuss the geology of granite in general before I go into detail about what Mount Tai Granite so unique.
Granite is an igneous rock that cooled slowly over millions of years within the Earth. It formed when solid rock (most likely another igneous rock) was completely melted. This melted rock then cooled to form granite. Granite can form in a few different ways, but the most common is at convergent plate boundaries where two tectonic plates collide with one another. If two continental plates crash into one another, uplift will occur as the plates run into each other. However, if at least one of the plates is oceanic, subduction will occur where the oceanic plate will be recycled into the interior of the earth. Subduction occurs when one plate (the one that is most dense which is always oceanic) dives beneath a continental (or another oceanic) plate.
For rocks located near convergent plate boundaries, where the temperature and pressures are high enough, melting can occur without recycling. This typically occurs right above the subduction zone in the area between the diving oceanic and floating continental plates. Over millions of years, the liquid rocks cool. Once tectonic forces begin to shift into a new direction, as they do on timescales of millions of years, these buried granite formations can be uplifted. Possibly the most famous example of this is the granites of the Sierra Mountains, which formed by subduction processes.
What makes this rock notable is that it is considered a migmatitic granite, meaning it was heavily altered while it was forming. Specifically the altering occurs because of the regional tectonic forces adding stress to the system, allowing these minerals to flow as a plastic (imagine silly putty) rather than a pure liquid or pure solid as it cools down. This implies that individual mineral grains within the rock are stretched and thinned. This occurs most often with minerals with the lowest melting temperatures such as quartz, one of the most prevalent minerals in granite. Migmatitic granites have an overall appearance of having lineations and layers. However, because granite is an igneous rock they don't have layers in the sense of sedimentary rocks. Notice how in the rock below, it appears as if there are layers when in fact these are lineations. Layers imply there is a difference in age between them when each layer formed. However, in lineations, each "line" formed at roughly the same time.
Below is a list of common minerals you might find in granitic rocks.
Works Cited
https://graniteobjectsgauteng.co.za/2014/09/29/granite-formed/
https://stoneyard.com/product/pink-granite-cobblestone/
https://www.history.com/this-day-in-history/yosemite-national-park-established
https://geologycafe.com/class/chapter3.html
https://blogs.agu.org/georneys/2011/08/28/geology-word-of-the-week-m-is-for-migmatite/
https://ougs.org/mainlandeurope/event-reports/701/china/
https://en.wikipedia.org/wiki/Mount_Tai
https://easyscienceforkids.com/mount-tai/
https://www.britannica.com/place/Mount-Tai
https://govt.chinadaily.com.cn/s/201812/17/WS5c1764e3498ee2f0291e3f1e/mount-tai-taian.html
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. "Guardians of San Diego" 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.
While at the above coordinates: N 32° 42.571 W 117° 09.723
2. Take a photo of you (or your signature item if you don't want to show your face) with the lion at the above coordinates in the background.
3. Describe the (a) textures, (b) range of colors and (c) average grain sizes.
4. Using the mineral chart above, name at least two minerals that are present in this rock.
5. Explain the type of deformation that is occurring to the mineral grains within the matrix of the rock. Are the grains broken with sharp edges? Are they elongated and smooth? Explain the shapes and describe what you see. Note, this is why this specific rock is considered "migmatitic".
While at the above lion across the street: N 32° 42.571 W 117° 09.712
6. Comparing your answers for 3 - 5, do you think the rock used to make this stone lion was quarried from the same part Mt. Tai? If so, explain the similarities between them. If you think these rocks were quarried at different places on the mountain, explain the differences between them.