Geology Part One: Digging Beneath the Surface of California

by Amy Jaecker-Jones


One of the pleasures of being in the California Naturalist Program is hearing from its faculty of diverse and highly educated instructors. Geology Professor Joe Holliday is no exception. Professor Holliday teaches in the Earth Science Department at El Camino College and travels as a naturalist for the National Geographic Society. During his lecture, he discussed the movement of the earth's plates and how that movement relates to the type of soil and topography found in California. He also spent time exploring one of his favorite subjects - natural disasters.

Part one of this series on geology will deal with what happens beneath the surface of California.

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The earth consists of a solid inner core, a molten outer core, a ductile mantle, and a comparatively thin crust. Some readers may remember making Play-Doh models with separate colors to designate the core, mantle, and crust.

Actually, Play-Doh is a wonderful metaphor for the inner-workings of the earth, and one that Professor Holliday particularly likes to describe the mantle. Like me, you may have thought about the mantle, and imagined it to be a liquid layer of red-hot lava. It is not a liquid, however, but a solid that moves.

Take a moment to remember the feel of Play-Doh in your hands. It's solid. It can take a definite form. If you turn your hand sideways, it won't drip. But it's not like other solid materials which must remain in one shape. You can squish it. You can squeeze it. You can re-form it again and again. It is this ductile quality of the mantle and the movement of tectonic plates above it that allows the surface of the earth to be constantly re-forming.

On top of the mantle lies the earth's crust. The crust is made up of different plates that move under, over, and against each other. There are three types of plate boundaries, and California is distinguished by having all three active within its borders. For this reason, it is worth learning a little about each type of boundary.

• Divergent plate boundaries primarily occur in the ocean. As the mantle moves, lava begins to rise. It seeps through cracks in the crust and spreads the two plates apart, creating new land in the process. This continuous process of spreading could continue without ceasing if it were not for the next two types of plate interactions.

• Convergent plate boundaries may be oceanic or continental. In the case of an oceanic plate colliding with a continental plate, the thinner, heavier oceanic plate bends under the continental one. The result is a rise in the continental plate. The oceanic plate, now forced underneath the continental plate, begins to melt. As it melts, some of the magma rises again and exits the crust through a volcano. In the case of two continental plates coming together, there is not enough difference in their weight for one plate to bend under the other. As a result, they smash together. Hot rocks become welded to each other. Some magma rises and cools to become mountains. The rest of the magma is pushed downward into the earth.

• Of the three types of boundaries, transform plate boundaries are the most important in California. This type of boundary is the primary cause of earthquakes. The San Andreas Fault is our most famous example of a transform plate boundary. Transform plate movement is also responsible for creating California's Central Valley, a sunken area, surrounded by tall mountain ranges. Unlike other plate boundaries, transform boundaries do not involve magma. Two continental plates move past each other, but instead of sliding smoothly, they grind against each other and get stuck. Then with a jerk, they move some more. Every 200 years, approximately, the movement is jerky enough to cause a major earthquake. To a person standing near the epicenter of one such extreme quake, the sound would be similar to that of an airplane crash.

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I learned one fact in this lesson that threatened to change the way I see myself as a Californian. Perhaps, for you, it will only be an interesting bit of trivia, but for me, it was shocking. In geologic terms, according to The California Naturalist Handbook, much of California is not a part of North America. Everything to the west of the San Andreas Fault sits on the Pacific Plate. As the name implies, the Pacific Plate is mostly covered by ocean. In fact, the only part of the earth's crust that sits on the Pacific Plate is in western California. All the land to the east of the San Andreas Fault, including the entire rest of the country is on the North American Plate (de Nevers, Edelman, and Merenlender 2013, p.32-33). Suddenly, reading that fact, I didn't know who I was anymore. I thought I knew my place in this world. I live in: Los Angeles County, southern California, in the western United States, on the continent of North America. As for my geography and nationality, this understanding is still true. But geologically, I live somewhere else. My house sits on the Pacific Plate.

However, despite years of jokes, California is not physically apart from the rest of the country. If I drove out to the San Andreas Fault, I might see a jagged line of demarcation, but I would not see a bottomless gap dividing the Pacific and North American Plates. The landmass that I live on is indeed part of North America. The separation between plates occurs 10 miles beneath the surface. The next part of this series will discuss what happens when the San Andreas Fault begins to move as well as other potential natural disasters.


References:

de Nevers, Greg, Deborah Stanger Edelman, and Adina Merenlender. 2013. The California Naturalist Handbook. Berkeley: University of California Press.

Holliday, Joe, "Soils, Geology and Weather" (lecture, California Naturalist Program, Rancho Dominguez, CA, January 31, 2015).