Download Plate Tectonic Models for Southern California

West Coast Plate Tectonics
Compiled by Ellen Kelley for WMSC graduate interns
For over 100 million years a major oceanic plate -- the Farallon Plate -- has
been colliding with and subducting beneath the North American Plate. The
collision has compressed the edge of the continent and scraped sediments
off the ocean floor to create coastal mountains. The Farallon plate has
been subducting or diving back into the earth’s mantle from whence it came,
but some of the molten material becomes buoyant, rises and infiltrates weak
spots in the continent. Depending on its chemical composition, the molten
material that cools inside of the continent becomes intrusive igneous rock
such as granite or gabbro, while that which erupts through surface becomes
extrusive igneous rock such as andesite or rhyolite.
The western edge of the Farallon Plate is a rift into the earth’s mantle
called the East Pacific Rise. This is the realm of oceanic volcanoes and
volcanic vents. When magma erupts here it cools to form basalt, which is
added to the Pacific Plate to the east and the Farallon Plate to the west.
While the Farallon Plate has been diving beneath North America in the
subduction zone, new Farallon Plate has been forming at the East Pacific
Rise. If the North American continent had remained stationary, this
process could have continued indefinitely. However, as the Atlantic Ocean
has been widening North American has been getting pushed westward.
Around 30 million years ago North America made first contact with the East
Pacific Rise in Mexico. Then, as North America was pushed over the East
Pacific Rise, it came into contact with the Pacific Plate. Relative to the
North American continent the Pacific Plate is moving northwestward. The
contact between these two plates is a transform fault. The initial contact
around 30 million years ago signaled the birth of the San Andreas Fault. As
more and more of the Farallon Plate and East Pacific Rise disappeared, the
San Andreas Fault has been extending northward and southward.
Today, just two small remnants of the Farallon Plate remain – the Cocos
Plate which is offshore from Mexico and the Juan de Fuca Plate which is
offshore from the Pacific Northwest. Both are subducting beneath the
continent forming new coastal mountains and causing inland volcanoes to
erupt periodically. Hence the types of geologic events that took place in
California millions of years ago are happening right now in Mexico and in the
Pacific Northwest. For instance, the Cascade Mountains are still active
continental volcanoes. This was made very obvious when Mt. Saint Helens
erupted in 1980. When the Cocos and Juan de Fuca Plates completely
subduct beneath North America, the San Andreas Fault will extend through
Mexico and the Pacific Northwest.
The Formation of Catalina Island
Catalina’s metamorphic basement rocks were formed in a subduction zone 25
– 40 miles beneath North America. The metamorphic rocks are blueschist,
greenschist and amphibolites, which includes soapstone. The blue schist was
created mostly from a sedimentary rock called “greywacke” sandstone. The
greywacke in turn had been formed from sediments carried by rivers and
laid down by currents on the ocean floor and chemically cemented together.
The parent rock for green schist is in part basalt from the subducted
Farallon Plate. As the Farallon Plate was subducting beneath North
America, the greywacke was eventually carried down many miles where it was
metamorphosed by heat a pressure into schist. This material became
attached to the bottom of the continent where it would still be today if a
change in tectonic movements hadn’t occurred. As the San Andreas Fault
formed, the parts of North America to the west of it was transferred to
the Pacific Plate and are now moving northwestward at an average of 15 cm.
per year. As more and more of the Farallon Plate subducted, the San
Andreas Fault extended northwards and southwards, and slivers of North
America were and will continue to be transferred to the Pacific Plate and
moved northwestwards until all of the old Farallon Plate is subducted.
About 12 million years ago, the Pacific Plate started dragging a block of the
continent overlaying what is now Catalina Island to the northwestwards. In
the process, the northern edge of the block somehow got stuck and wouldn’t
budge, but the Pacific Plate kept dragging the southern part
northwestwards. As a result, the block rotated counterclockwise creating
the southern California bight. By 4 million years ago the Catalina schist
terrane was totally “unroofed” and started to uplift. By 2 million years ago
it emerged as an island. Because of that “unroofing” event, we are able to
see the types of rocks that normally occur 25 – 35 miles beneath the
surface right here at our feet.
TEACHER RESOURCES
Online Plate Tectonic Animations
Faults and Earthquakes in Southern California
Sponsored by the Southern California Earthquake Center, this site has a “Clickable Fault
Map”.
http://www.data.scec.org/
Plate Tectonic Models for the World
Sponsored by Berkeley, this site has animations for the plate movements over the past
750 million years. You can choose from several different formats.
http://www.ucmp.berkeley.edu/geology/tectonics.html
Plate Tectonic Models for Southern California
This site was created by Professor Tanya Atwater at the University of California at Santa
Barbara. Included are plate tectonic animations for the western hemisphere over the past
85 million years, western North America over the past 38 million years, and southern
California over the past 20 million years. On the latter, you can actually see the uplift of
Catalina Island.
http://www.geol.ucsb.edu/Outreach/Download/Freeware-FR.html
Select “Plate Animation Movies”. This will take you to a download page. You will be
required to register, but there is no charge.
The Paleomap Project
Developed by Professor Christopher Scotese. This site contains an online archive of
animations of the earth from 1100 million years ago as well as into the future plus much
more.
http://www.scotese.com/earth.htm
This Dynamic Earth: The Story of Plate Tectonics
This is an excellent on-line book for learning about plate tectonics.
http://pubs.usgs.gov/publications/text/dynamic.html
Recommended CD Rom
“Earth Update”. This is an interactive multimedia project created by Museums
Teaching Planet Earth at Rice University. It contains five modules: Atmosphere,
Biosphere, Cryosphere, Geosphere and Hydrosphere. The Geosphere module shows
animations past and future of tectonic plate movements. The cost is $15.00, with
educational and bulk discounts available. Contact (713) 348-3620 or
[email protected] . The web site is http://earth.rice.edu.