<p>Much of western California is composed of small lighter landmasses scraped off of the Pacific Plate as the heavier mantle sank under the North American Plate, and the San Francisco Bay Area is no exception. Here's an illustration of how some of CA was formed as the Pacific Plate was subducted under the North American Plate:<br><a href="https://i.sstatic.net/DbGXU.jpg" rel="noreferrer"><img src="https://i.sstatic.net/DbGXU.jpg" alt=""></a></p>

<p>As these separate landmasses were scraped off the Pacific Plate, they formed distinct rock types, with the boundary of each forming a distinct fault.  Here's a good illustration of what that looks today:<br><a href="https://i.sstatic.net/ntciq.jpg" rel="noreferrer"><img src="https://i.sstatic.net/ntciq.jpg" alt=""></a><br><br>Of course, these faults and rock types will be different depending on what cross section you're looking at.<br><br>I know this isn't a complete description, but I think it's a good starting point.<br><br> These illustrations came from USGS bulletin 2195, which can be found <a href="http://pubs.usgs.gov/bul/2195/b2195.pdf" rel="noreferrer">here</a>. It's an interesting read in and of itself.</p>

CC BY-SA 3.0
2015-08-02T05:59:36.607Z
As beautiful as those diagrams are, I'm having trouble mentally translating the plates in the first figure into actual surface visualization. For example, why is Baja California above sea level? I would have guessed that rock at the fault was forced upwards, but then why is there a gap filled by water forming what we know as the Gulf of California?
Sridhar Sarnobat
CC BY-SA 3.0
2015-08-02T06:07:26.577Z
I tried seeing some Youtube animations but when they're 2D and use non-natural colors it gets confusing.
Sridhar Sarnobat
CC BY-SA 3.0
2015-08-02T16:54:09.903Z
@Sridhar-Sarnobat, I don't really know much about the geography of Baja California, but my guess would be that, it too, was a landmass scraped off of the Pacific Plate, as the heavier mantle dove under the North American Plate.
BillDOe
CC BY-SA 3.0
2015-08-02T02:00:01.343Z
2015-08-02T02:00:01.343Z
BillDOe
<p>Put another way, if you go back far enough in earth history, was there never a San Francisco Bay, and the mountains in the Peninsula and east of the "East Bay" were essentially a contiguous mass of hills?</p>

<p>PS - I live in the Bay Area, and I know some of the features formed by the San Andreas fault.</p>

<p><a href="https://i.sstatic.net/5LiQF.png"><img src="https://i.sstatic.net/5LiQF.png" alt="enter image description here"></a></p>

CC BY-SA 3.0
2015-08-01T02:01:54.370Z
Consider that pretty similar landforms extend all the way east to Utah.
jamesqf
<earthquakes><plate-tectonics>
Is the whole San Francisco Bay Area essentially one giant fault?
Sridhar Sarnobat
# Is the whole San Francisco Bay Area essentially one giant fault?

(Asked by: username_0 on 2015-07-31)

Put another way, if you go back far enough in earth history, was there never a San Francisco Bay, and the mountains in the Peninsula and east of the "East Bay" were essentially a contiguous mass of hills?

PS - I live in the Bay Area, and I know some of the features formed by the San Andreas fault.

[![enter image description here](https://i.sstatic.net/5LiQF.png)](https://i.sstatic.net/5LiQF.png)

**Tags:** earthquakes, plate-tectonics

> **Comments:**
> **username_1**: Consider that pretty similar landforms extend all the way east to Utah.  

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## 1 answers

### Answer by username_2 on 2015-08-02. Score: 5 | Accepted answer

Much of western California is composed of small lighter landmasses scraped off of the Pacific Plate as the heavier mantle sank under the North American Plate, and the San Francisco Bay Area is no exception. Here's an illustration of how some of CA was formed as the Pacific Plate was subducted under the North American Plate:  
[![](https://i.sstatic.net/DbGXU.jpg)](https://i.sstatic.net/DbGXU.jpg)

As these separate landmasses were scraped off the Pacific Plate, they formed distinct rock types, with the boundary of each forming a distinct fault. Here's a good illustration of what that looks today:  
[![](https://i.sstatic.net/ntciq.jpg)](https://i.sstatic.net/ntciq.jpg)  
  
Of course, these faults and rock types will be different depending on what cross section you're looking at.  
  
I know this isn't a complete description, but I think it's a good starting point.  
  
 These illustrations came from USGS bulletin 2195, which can be found [here](http://pubs.usgs.gov/bul/2195/b2195.pdf). It's an interesting read in and of itself.

> **Comments:**
> **username_0**: As beautiful as those diagrams are, I'm having trouble mentally translating the plates in the first figure into actual surface visualization. For example, why is Baja California above sea level? I would have guessed that rock at the fault was forced upwards, but then why is there a gap filled by water forming what we know as the Gulf of California?  
> **username_0**: I tried seeing some Youtube animations but when they're 2D and use non-natural colors it gets confusing.  
> **username_2**: @username_0-username_0, I don't really know much about the geography of Baja California, but my guess would be that, it too, was a landmass scraped off of the Pacific Plate, as the heavier mantle dove under the North American Plate.  

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