Alright everybody. Let’s shake off the cobwebs and get back learnin’, shall we? Today we’ll talk about a place that we all know and most of us love: California. Specifically, we’re gonna look at how the San Andreas Fault was born. It’s kind of a weird story, but an important one to know for the sake of understanding why California looks the way it does. And there will also be a food analogy, so read on!
Remember back when we talked about plate boundaries? The places where those big tectonic plates crash into and move along with one another? No. Well, let’s remind ourselves before we move on.
There are three types of plate boundaries: divergent (pulling apart), convergent (crashing together) and conservative (rubbin’ against one another). An example of a divergent plate boundary is the East African Rift, of a convergent boundary we’ve got the Himalayas, and for our conservative boundary we’ve got the good ole’ San Andreas Fault.
For most of the last 600 million years, the western part of North America was a convergent plate boundary. An old old plate that doesn’t exist anymore called the Farallon plate was happily subducting underneath North America (see Figure). What happened was that there was a pointy bit of the Pacific/Farallon plate boundary that rammed into the North American subduction zone about 30 million years ago and everything went apeshit. When that pointy bit hit North America, the subduction zone along the western margin started swallowing up a divergent zone (where new crust is being made). Instead of one outdoing the other, the whole thing just fell into a new regime altogether and we got a conservative plate boundary. Why, you ask? I don't think we have a good explination as to why we got the San Andreas out of this mess. Or at least I don’t know anybody that knows why. It’s like taking a hot dog and smashing it together with a hamburger and ending up with sushi. Crazy!
The sushi that we ended up with is the San Andreas, the big mother of all faults in California. Unless you’ve been living in a cave or on the East Coast or are totally oblivious to all that surrounds you, you probably know that this is the fault that gives everybody the jitters. And rightly so, it’s a big fuckin’ fault that has caused a lot of problems in the past. The San Andreas Fault serves as the boundary for the North American plate to the East (what the rest of America is attached to) and the Pacific plate to the West. Yes, that means in a few million years Los Angeles will be sittin’ pretty right next to San Francisco. Don’t frown, San Francisco. Surely by then we will have engaged Iran in nuclear conflict so you won’t have to put up with anything more than the mutated remains of model/actress zombies looking for temp jobs.
Zing! It’s good to be back!