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Roger Weller, geology instructor

wellerr@cochise.edu
                                 

Plate Tectonics
Kelsey Rasmussen

Physical Geology

Fall 2006 

Is California Falling into the Pacific Ocean?

As a native Californian I have heard many obnoxious comments, Arizona having beach front property was one of the best, from countless east coast and mid-westerners about how the next “BIG ONE”, meaning earthquake, is going to split open the earth and California will fall into the ocean. This always annoyed me, so I researched the subject to put all the information into one easy to read little paper for all of the over intelligent people from “back East” to read.

I would like to begin with why California has earthquakes in the first place. The entire world is made up of landmasses called plates. Now not all of these plates are above water but you will find land at the bottom of all the oceans trust me. Below is a diagram with the current names and layouts of the plates.

Link to USGS home page
Reference: http://pubs.usgs.gov/gip/dynamic/slabs.html 

Now all of these plates are moving believe it or not. The reason we can not feel the movement of these plates is because they move really slow, about five centimeters a year for the North American Plate according to the according to the U.S. Geological Survey. If you still do not believe that these plates are moving it can be proved. Millions upon millions of years ago prehistoric animals roamed the earth and when they died they left fossil remains behind. Now these animals remains have been found all over the world. We know that these animals could not have traveled over the vast distances in which their fossils were found so the only reasonable explanation is that they traveled across the continents while they were still connected. By tracking these remains we can figure out what the world once looked like, see the following diagram. And now, millions upon millions of years later, we know what the world looks like today. Just incase you do not know what the world looks like today take another look at the picture above.

Continental Drift - Fossils


Reference:http://volcano.und.nodak.edu/vwdocs/vwlessons/plate_tectonics/part3.html

          Now that we know that our world consists of landmasses called plates. So California would not fall into the ocean because the ocean is not a pit, it is land covered with a little water. We also know that these plates are moving. This movment is what causes earthquakes. Before we get into the earthquakes I want you to understand where the landmasses are going. 80 million years ago two large plates were between the Pacific Plate and the North American Plate. These two plates were the Kula Plate and the Farallon Plate. As you will see in the following diagrams titled “ The Sequence of Events” the movements of the Pacific Plate and the North American Plate smashed the two plates, Kula and Farallon, between them.

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The Sequence of Events

On the left of each figure is a geologic map showing the age of the ocean crust. On the right is a figure showing the plate configuration. Blue lines represent subduction zones, red represent spreading centers, and purple represent transform faults.







 













Reference: http://www.uwgb.edu/DutchS/platetec/kula.htm

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Now you might be wondering where did the plates go? After all These Plates are huge landmasses and that much land does not just disappear. Well your right, all the land did not just disappear. When the plates came together one moved under the other. As the Farallon Plate moved under the North American Plate it raised the elevation of the west side of the North American Plate and forced the Farallon Plate to move down. This is why the west coast is so mountainous. The heat from the core of our planet turned the descending plate, the Farallon Plate, into magma. As the magma found cracks or weaknesses in the overlaying plate it rose through these cracks and weaknesses to form Volcanoes as shown in the two diagrams below.

Oceanic-continental convergence

 

 

 

 

 

Map, Plate Tectonics and the Cascade Range, [18K,GIF]                                                          Reference: http://pubs.usgs.gov/gip/dynamic/understanding.html

 

 

 

 

 

















                             Reference:
http://vulcan.wr.usgs.gov/Glossary/PlateTectonics/Maps/map_plate_tectonics_cascades.html

Cascade Range VolcanoesThe subduction of the Farallon Plate under the North American Plate has caused several volcanoes along the coastline in the Cascade Mountain Range. By viewing wear the Juan de Fuca Plate is passing under the North American Plate (pictured above) you can see why there are so many volcanoes in the Cascade Mountains (pictured below).

 

 

 

 

 

 

 

 

 

 

 






                                         Reference: http://vulcan.wr.usgs.gov/Glossary/PlateTectonics/Maps/map_plate_tectonics_cascades.html

 

            Now a small part of California rests on a portion of the Pacific Plate that is above sea level. As you can see from the diagram below that the Pacific Plate surfaces just around San Francisco and goes down to the Gulf of Baja California. The San Andreas fault, the borders wear the Pacific Plate and the North American Plate meet, is wear a majority of seismic activity (earthquakes) take place in California. The area between the plates sliding horizontally past one another is called a transform-fault boundary. Because the two plates are moving past one another (as shown in the diagram below) it creates earthquakes.

 

 

 

 

           

 

 

 

 

 






                                          Reference:
http://pubs.usgs.gov/gip/dynamic/understanding.html

Now the San Andreas fault zone, which is about 1,300 km long and in places tens of kilometers wide, slices through two thirds of the length of California. The Pacific Plate has been grinding horizontally past the North American Plate for 10 million years at about 4 inches a year. Land on the west side of the fault zone, on the Pacific Plat, is moving in a northwesterly direction relative to the land on the east side of the fault zone which is moving in a southeasterly direction, on the North American Plate. Now some of these earthquakes caused by the movement by these two plates will cause visual cracks on the surface called surface ruptures. The article below explains what a surface rupture is in more detail.

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Picture of faulting from the 1992 Landers earthquake, photo by Bill BryantWhat is "surface rupture" in an earthquake?

Surface rupture occurs when movement on a fault deep within the earth breaks through to the surface. Surface ruptures associated with the 1992 Landers Earthquake, in San Bernardino County, extended for 50 miles with displacements of an inch to 20 feet. Not all earthquakes result in surface rupture. The Loma Prieta Earthquake of 1989 caused major damage in the San Francisco Bay Area but the movement deep in the earth did not break through to the surface.  Fault rupture almost always follows preexisting faults, which are zones of weakness.

 

 

















                                                                                         Reference: http://www.consrv.ca.gov/CGS/rghm/ap/

 

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Now that we know that it is possible to see the earth move during an earthquake I asked the U.S.G.S. if the huge fissure type openings that were possible.

 

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A popular cinematic and literary device is a fault that opens during an earthquake to swallow up an inconvenient character. But unfortunately for principled writers, gaping faults exist only in movies and novels. The ground moves parallel to a fault during an earthquake, not away from it. If the fault could open, there would be no friction. Without friction, there would be no earthquake. Shallow crevasses can form during earthquake induced landslides, lateral spreads, or other types of ground failures. Faults, however, do not gape open during an earthquake.

Reference: http://earthquake.usgs.gov/learning/topics/megaquakes.php

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Thank you for taking the time to read the research that I took the time to put together. I hope that after reading this article you now understand why we have mountains, volcanoes, and earthquakes in California and that although parts of it are moving it is not falling into the ocean.  Now I will leave you with this. While doing my research I came across something you east coast and Midwesterners should be worried about.

 

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The Mississippi Valley-"Whole Lotta Shakin' Goin' On"


         In the winter of 1811-12, the central Mississippi Valley was struck by three of the most powerful earthquakes in U.S. history. Even today, this region has more earthquakes than any other part of the United States east of the Rocky Mountains. Government agencies, universities, and private organizations are working to increase awareness of the earthquake threat and to reduce loss of life and property in future shocks.

 


 

http://quake.wr.usgs.gov/prepare/factsheets/NewMadrid/Charleston1895.gif

 

 

        The 400 terrified residents in the town of New Madrid (Missouri) were abruptly awakened by violent shaking and a tremendous roar. It was December 16, 1811, and a powerful earthquake had just struck. This was the first of three magnitude-8 earthquakes and thousands of aftershocks to rock the region that winter.

 

 

        Survivors reported that the earthquakes caused cracks to open in the earth's surface, the ground to roll in visible waves, and large areas of land to sink or rise. The crew of the New Orleans (the first steamboat on the Mississippi, which was on her maiden voyage) reported mooring to an island only to awake in the morning and find that the island had disappeared below the waters of the Mississippi River. Damage was reported as far away as Charleston, South Carolina, and Washington, D.C.
 

        These dramatic accounts clearly show that destructive earthquakes do not happen only in the western United States. In the past 20 years, scientists have learned that strong earthquakes in the central Mississippi Valley are not freak events but have occurred repeatedly in the geologic past. The area of major earthquake activity also has frequent minor shocks and is known as the New Madrid seismic zone.
 

        Earthquakes in the central or eastern United States affect much larger areas than earthquakes of similar magnitude in the western United States. For example, the San Francisco, California, earthquake of 1906 (magnitude 7.8) was felt 350 miles away in the middle of Nevada, whereas the New Madrid earthquake of December 1811 (magnitude 8.0) rang church bells in Boston, Massachusetts, 1,000 miles away. Differences in geology east and west of the Rocky Mountains cause this strong contrast.

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