This valley is caused by numerous geologic forces. The most predominant one is faulting, so let's begin there and first discuss the basic fundamentals of how fault lines are created and now they work. 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 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 in an 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. Depending on the pressure, it can create a magnitude 1 earthquake on the Richter Scale or a 10. A 10 has never happened in recorded history, but is believed to have only occur when large meteors hit the earth. The largest earthquake ever recorded happened in Chile on May 22, 1960. It happened in open waters, just off the coast in the Pacific where it created a Tsunami. You can see my earthcache about that Tsunami here. A Richter Scale works like this. Say there was a 5.0 earthquake, which are common, and a 7.0, a little less. A 7.0 on the Richter scale is 100 times greater of that of a 5.0 on the same scale. Each decimal point in-between is twice as as bad as the preceding one.
Now that you understand what a fault is and how they work, you can now understand what is happening here. The fault zone from the middle of the Salton Sea to a few hundred miles pass the tip of Baja California a large rift zone is occurring. A rift zone is when crust is spreading apart from each other. This type of plate boundary is called a divergent plate boundary. They happen almost entirely on ocean crust, thousands of feet below the sea floor. I say almost because there are a few exceptions such as the rift valley in the eastern parts of Africa and Iceland. The same thing happening there, is occurring here but it's happening due to different causes.
The California Rift Valley, also known as the Salton Trough, is the only known active rift zone within in the United States's land masses. Most rift zones are created at divergent plate boundaries, but this one is created in a jump in the fault systems. The San Andreas Fault ends on the east side of the Salton Sea, about half way in the middle. Then there is a sudden gap between faults. It's about 20-30 miles where there is no plate boundary within the North American Plate to the east and Pacific Plate to the west. The plate boundary is picked up again by the Imperial fault which then goes out into the Gulf of California.
The Gulf of California has active divergent faults where the land is split apart along the same fracture. Here, in the great California rift zone, it's because there's a large jump in the fault. But this isn't going to be the same for much longer. Just like a zipper, the Imperial Fault will eventually become a true divergent fault instead of a transform fault, like the San Andreas. Once the Imperial Fault is a true divergent fault, the gap in-between these faults will be closed by the creation of a new fault OR the extension of the San Andreas Fault or Imperial Faults. Only time will tell which fault will be extended, or if a new fault will be created.
So what will happen when this area becomes a true divergent plate boundary? Well, the Gulf of California, in current day Mexico, will rush in and this will be part of the Gulf. The only reason why this part of California is not completely submerged by a 50 miles gulf is because the Colorado River dumps so much sediment at the northern most part of the gulf. It acts like a natural dam, to keep the water out of the great rift valley. If you look just further south, you can see that the Baja Peninsula has already unzipped from the mainland of Mexico. Southern California is next.
Works Cited
http://www.nature.com/ngeo/journal/v4/n7/fig_tab/ngeo1198_F1.html
http://pubs.usgs.gov/gip/dynamic/tectonics.html
http://www.geosociety.org/gsatoday/archive/21/11/figure/i1052-5173-21-11-4-f01.htm
http://web.gccaz.edu/~lnewman/gph111/topic_units/plates/plates2.html
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1. The Great California Rift Valley - on the first line of your email.
2. From this vantage point (NOT USING THE MAP), how far does the Salton Trough extend?
3. Take an accurate GPS reading of elevation from the top of the bridge. Say, you come back to this spot in 1,000 years and take another GPS reading, if the Gulf of California hasn't flooded the area yet. What would you expect the reading to be? Just give me a ballpark elevation. Doesn't have to be spot on.
4. How has the sinking of the land here effected Interstate 8?
5. How many rift made terraces do you see? Refer to the rift valley diagram on this cache page to help you locate what you are looking for.
6. Does this answer surprise you? Why or why not?
* 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.