The San Andreas Fault is a transform plate boundary entirely within the state of California. Its southern end is about a mile southeast of Bombay Beach in the Salton Sea while its northern end is about 50 miles south of the city of Eureka in northern California, along the Pacific coast. The fault connects the Salton Trough and the spreading center (more on this later) in the Gulf of California to the Mendocino Fracture Zone and the Cascadia Subduction Zone.
There are two types of plates on Earth: Continental and Oceanic. Continental plates are much thicker (30-70km) and are much lighter. They are mostly made of felsic materials such as granites, which contain feldspars and quartz minerals. Unlike continental crust, oceanic crust is much denser and thinner (5-25km thick). A common misconception is that the mantel is liquid when in fact it is solid. It has different physical properties than the crust in that the crust is rigid and tends to fracture (brittle behavior) while the mantle can "flow" and deform (strain) under stress (ductile behavior). The mantel has a constancy similar to silly putty, which is solid but has some properties of a liquid. It can flow away from an input of stress, while the crust cannot. While the temperature of the mantel is high enough to melt most minerals (and rocks), the high pressure from the kilometers of rocks above allow the material to stay a solid rather than melt into a liquid. Changing a phase (solid, liquid, gas, plasma) is dependent only on the temperature and pressure exerted on the material. The ductile properties of the solid mantle allow the plates to effectively float on top of it.
The mantle, being the largest structure of earth's interior, making up over 50% of Earth's volume, has currents (called convection currents) similar to ocean currents. Hotter material from the lower part of the mantel is less dense and will begin to rise towards the upper mantle. As is rises, its temperature drops and begins to cool and shrink, becoming less dense. Eventually it will fall back down to the lower where it is heated up again and the process starts over. These cycles, known as convection currents, are what drives plate tectonics. Earth is the only place in the universe where this occurs, although some have argued Jupiter's icy moon Europa may also have plate tectonics to an extent. Some argue that plate tectonics could only happen with just the right amount of water in the mantle (3-5 wt%), which allows oceanic plates to bend and not fracture. Note: wt% is equivalent to weight percent.
There are three main types of plate boundaries. However, in this earthcache I will only be discussing the most relevant one to the San Andreas Fault system which is a Transform Plate Boundary. Below is a diagram showing the evolution of the San Andreas fault from 30 million years ago to today.
Garnet Hill and the San Andreas Fault:
Part of that stress reorganization is still evident in the Palm Springs area, with the multiple stands of the San Andreas Fault between San Gorgonio and Indio. Multiple strands accommodate the plate motion, some more than others. The Mission Creek Strand is the most active of the three San Andreas strands in this area (Garnet Hill, Mission Creek, and Banning). Now that we've discussed the fundamentals of how plate boundaries form, the different types of plate boundaries, and how the San Andreas came to be, we can go into details on why this specific site along the San Andreas is special.
You can see in some instances that the fault is offset in some areas. These are called "stepovers" and, depending on their geometrical orientations, can either help stop a rupture from propagating down a fault or promote it. This is another reason why the Garnet Hill Strand is considered not as active, because of its large step over at Garnet Hill. There are two types of stepovers. One is a convergent step (of which Garnet Hill is classified as) and the other is a divergent step. In a divergent step, the land pulls apart between the two strands as the land is thinned and a depression is made. Often times, water can fill in and create a lake (as so happens Lake Elsinore is one). In a convergent step, the land compresses and you can get a pressure ridge where the land uplifts to create hills.
In addition to stepovers, faults can have abrubt "bends" and turns. The cyan line in the map below is the Garnet Hill Strand of the San Andreas Fault. Notice how it "bends" around the southern edge of Garnet Hill. The orange line is the Banning Strand of the San Andreas Fault. This type of bend, seen at Garnet Hill, is known as a restraining bend because when a rupture is moving towards the bend, it will have to use up energy to slow down as it travels around it. Conversely, a releasing bend would promote rupture from passing through as well as speed it up. In some cases, restraining bends can stop a rupture entirely. This has been observed many times around the world. Although whether or not the bend at Garnet Hill will stop a rupture from going around it is unknown.
You may ask yourself: "How do you know if Garnet Hill was made by tectonic forces and not from the build up of alluvial (river) deposits that originated from the nearby mountains?". Well, you can look at a cross section! A cross section will show you the layers of rock, oldest at the bottom and youngest at the top. If tectonic forces are the driving force, then the layers should be disturbed, tilted, or even offset. If the hill was formed due to the deposition of alluvial material, then the layers would be evenly spaced, flat, and undisturbed.
Works Cited
https://sancarlosflight.com/event/the-san-andreas-fault/
https://www.geologyin.com/2016/10/massive-earthquake-along-san-andreas.html
https://www.nps.gov/subjects/geology/plate-tectonics-transform-plate-boundaries.htm
https://oceanexplorer.noaa.gov/facts/plate-boundaries.html
https://www.learnclick.net/quiz/21082
https://www.researchgate.net/figure/Development-of-large-geothermal-sites-in-the-East-African-Rift-Valley-and-Afar-at-the_fig3_329269032
https://www.frontiersin.org/articles/10.3389/feart.2019.00039/full
https://usgs.maps.arcgis.com/apps/webappviewer/index.html?id=5a6038b3a1684561a9b0aadf88412fcf
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. "San Andreas Fault - Garnet Hill Strand" 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 photo of you (or your signature item if you don't want to show your face) with the cross section of Garnet Hill in the background from the posted coordinates.
3. As you look around the area from the posted coordinates, what evidence tells you a major fault is nearby?
4. Describe the extent of Garnet Hill. About how (a) tall is it and how (b) wide is it?
5. Is Garnet Hill is growing, stagnate, or eroding? What evidence is there to tell you otherwise?
6. Using your previous answers to help you, is the Garnet Hill Strand of the San Andreas Fault a convergent OR divergent stepover? How can you tell?
7. What evidence can you see that suggests Garnet Hill was made by tectonic forces and not from an alluvial (river) deposit that originated from the nearby mountains?