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San Andreas Fault - San Andreas Lake

A cache by bluesnote Send Message to Owner Message this owner
Hidden : 06/11/2016
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1 out of 5

Size: Size:   other (other)

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Geocache Description:

My 75th Earthcache!

San Andreas Lake was named well before the infamous San Andreas Fault that passed under it was even discovered. You can learn more about how the fault was discovered and named, by looking at my 1906 San Francisco Earthquake Epicenter cache (GC6K147).

San Andreas Lake was named Cañada de San Andrés after Saint Andrew in the 1770s by Captain Fernando Rivera y Moncada during one of the many Spanish Expeditions of the west coast of North America. The lake was actually discovered in 1769 by the Portola Expedition, another Spanish quest. Then it was named Cañada de San Francisco. It wasn't until over a century later that anyone knew what was under the lake.

In the wake of the 1906 earthquake, geologist set to map out what had caused it. They discovered a near perfect line from San Andreas Lake to Point Reyes, where there were things such as fences or roads offset up to 30 feet. They concluded it must be a crack, a fault, in the earth. When it became time to name it, they chose the name of this lake, San Andreas, and the name of the fault was born.

San Andreas Lake is younger than the fault. The fault formed the lake through an interesting process known as sagging. In most cases, this sagging of the land creates a natural bowl where rain water collects and over time building up to a giant, linear lake. I'll get into more specific details later on in this earthcache, but for now it's time for a basic geology lesson.

Lets first discus the main types of plates. There are oceanic plates and there are continental plates. Oceanic plates are made up of mafic rocks, meaning they are low in silica (SiO2) and appear dark in color. Oceanic plates are mostly made up of basalt due to the divergent plate boundary creating new land. I'll get to what a divergent plate boundary is in the next paragraph. Continental plates are made up of mostly granitic type rocks and are mostly felsic, or appear light in color due to their high silica content.

Now that you are familiar with plates, we can discuss how they interact with each other. There are three main plate boundaries called convergent, divergent, and transverse (transform). A convergent plate boundary is when two plates collide. Think of it as a head on collision in a car crash. There are two types of convergent plate boundaries. One type is called subduction, where one plate slides under another which almost always occur off the coast of a continent. The plate that subducts is always an oceanic plate because it's more dense due to basalt being more dense than granite. There are more atoms per square inch in basalt than in granite, making it more dense and more heavy. Hence objects tend to sink towards the bottom like a penny in a glass of water. Lighter objects will float like an ice cube. Ice cubes can move around in the surface of the water, like how tectonic plates move and interact.

Divergent plate boundary are when two plates move away from each other. These happen almost always in oceanic plates because they are much thiner than continental plates, more easily to tear apart. It's easier to rip a tortilla than a pizza. In the ocean, we call these mid-ocean ridges. They create mountains in the ocean. As lava pours out and hardens, new lava flows on top and so on. It's like a scap. Its height keeps growing. Sometimes it even reaches the surface of the ocean. Iceland is a great example where you can literally stand in the mid-ocean ridge.

Transverse, transform, or strike-strip plate boundaries whose names are synonymous, slide passed each other. The San Andreas Fault is a transform fault. Most transform faults occur in the ocean because it's hard to rip continental crust from side to side due to it's thickness. The San Andreas Fault and it's associated faults is a rare and unique exception. There are two types of transform faults and it depends on how you view it. One is called a left-latteral strike-strip fault and the other a right-lateral strike-strip fault. To know the difference, you can observe the direction of relative movement. If you are on one side of the plate, looking towards the other and the land moves to your left then it's a left-lateral strike-strip fault. Likewise, if the land moves to your right, it's a right-lateral strike-strip fault. Seems easy enough, right?

Now that you are familiar with the different types of faults, we can discus as to why they move and the forces behind it. The answer can be found in your kitchen, literally. When boiling pasta on the stove, the hotter pasta floats to the surface then when it cools it sinks. This effect has a name. Convection currents in your pot at home and in the earth are nearly identical. There are a few difference though. One is with water, rather than boiling rocks and the other is that one is less a few inches while one is miles deep. Convection currents are continues cycles in the earth's mantel that make hotter rock rise, and cooler rock fall and the cycle begins over and over again. When this happens, it causes fractures in the earth's crust. When fractures occur, a fault line is created.

Now at this point I'm sure you are asking, "What is a fault line?" Well it's more of a fault zone since it's a collection of cracks rather than one single crack. Simplicity, it is place where two tectonic plates meet and create friction. When this friction is released, an earthquake is formed. This friction is formed by these convection currents. When they move in a circular pattern in one direction, they can push rock with them. When tension is released you get an earthquake that moves in waves on the X, Y, and Z axises. X is up or down, Y is right or left, and Z is pivoting side to side. I won't be going into the specifics of earthquake waves with this earthcache. To learn more, head on over to my nearby 1906 Earthquake Epicenter Earthcache (GC6K147)

The San Andreas Lake was formed from a series of breaks in the fault. One common misconception about fault lines is that they are perfectly connected in a line. This is not necessarily true. In some areas, there are breaks in the fault itself. For example there are areas of the fault that are missing, that have no fault activity whatsoever. It is easier to see it on paper, than in words so please refer to the diagram below.

You can clearly see that the fault itself is offset in some areas. When this happens, many times a sag pond is created. Instead of the San Andreas moving side to side in these offset areas, it moves away from each other. This is because the fault offset areas are already moving and these nonmoving areas are not. These makes these nonmoving areas become stressed and pull the land apart, which creates a depression. When it rains or snow, runoff water collects in these ponds. Depending how big or small they are, the ponds can last year round. This one lasts year round.

There are two things that can happen when fault lines become jagged. One thing is a sag pond, which I've discussed in the paragraph above. This happens at a divergent step when the fault creates a basin as the land in-between the fault moves away and stretches, relative to the other side of the fault. The other option is for a fault to create hills in a convergent step. Just like when you push to sides of the same piece of paper together and you get a ridge in the paper, the same happens here. The only difference is the size and scale.

The water clarity inside the sag pond can also give us clues of the geology occurring here. Clear water suggests that the bottom is mainly hard rock. Harder materials tends to hold water more and not let it leak into the soil. Hard rock, dense rock also moves earthquake waves much easier and much faster than loose rock. If the water is murky this can mean one of two things. Either that there is algae living here or that there is loose sediment at the bottom. Looser materials tend to leak out more water into nearby soils. Loose rock may also be an indicator that the water source is not just coming from runoff. It may be an underground water source that comes to the surface.

Indicators of a possible sag pond presence are denser vegetation relative to its surroundings. This is due to the fact that when plants grow near sag ponds, they are better off. More water means better productivity and higher health. Higher health and more green. If you see any areas that are more lush than its surroundings, this may indicate that a sag pond is present.

Works Cited


1. "San Andreas Fault - San Andreas Lake" on the first line of your email AND list all geocaching names of your party so I can match your answers to them. Note, this IS cheating as only the person who sent their answers has learned something, while the others get a "free ride". It's not fair to others. If you all want to learn something, I would prefer each cacher send me individual emails in the spirt of earthcaching.

2. From this vantage point, do you see any other indications of sag ponds nearby, besides this one? If so, what do you see and how far away do you see them?

3. Estimated the size of the extent of this sag pond. How many feet does the extent of the sagging occur? Do not estimate for the water only spots, this is for the entire extent of the sagging of the land due to the fault.

4. In the sag pond, describe the water clarity AND what does this suggest about the geology?

5. Describe at least two geologic differences between the Pacific Plate and North American Plate from this vantage point. Note that you are standing on the North American Plate and the other side of the lake is the Pacific. Explain why you might see these differences.

* NOTE - You will likely not receive an email back from me, unless I need clarification on your answers. Please, don't wait for me to tell you you have it correct or not. I do review your emails within 12 hours of getting them in my inbox and with other of my earthcaches, it has become too hard to do. As soon as you send the email, you can log this earthcache.

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