Parting is Such Sweet Sorrow

Centuries ago, Spanish explorers expected the mythical land of “California” to be a wild and exotic island.  Perhaps they should be applauded for their prescience, for new research published in Nature Communications confirms that California is slowly separating from the rest of North America.  This wrenching process began 7 million years ago and shows no signs of letting up.

Historically, “California” included a large swath of the western United States and Mexico, but today the term refers only to the American state of California (“Upper California” here) and the Mexican states of Baja (“Lower”) California and Baja California Sur (hereafter combined as “Baja California”).  Most of Baja California is a peninsula, separated from the rest of Mexico by the Gulf of California, an inlet of the Pacific Ocean.  To the north, upper California appears firmly attached to the rest of North America.  Baja California was not always such a nonconformist.  Once upon a time, it was firmly attached to the rest of North America.  But slowly and methodically, forces deep within the Earth began to tear Baja California away from the rest of Mexico, opening the Gulf of California.

The gulf widens.The Gulf of California, center of the tectonic ballet ripping California from North America.

The gulf widens.
The Gulf of California, center of the tectonic ballet ripping California from North America.

The reason for this slow divorce is both straightforward and surprising: Upper and Baja California reside on a different tectonic plate than the rest of North America.  Earth’s surface is divided into tectonic plates, thick sections of crust that collide, slide, merge, and split apart from one another.  Scientists don’t completely understand the forces governing these motions, but humanity is familiar with the consequences, from earthquakes and tsunami to volcanoes.  Baja and Upper California reside on the Pacific Plate, while most of the rest of North America is on the (aptly-named) North American Plate.  Just under 7 million years ago, when both Baja and Upper California were firmly attached to the rest of North America, the Pacific and North American plates began to separate from one another right at the southern tip of Baja California.  As they pulled apart, rifts and fault lines tore into the landscape.  Magma from deep within the Earth rose up into the void between the diverging plates, cooling and forming new crust.  Approximately 5 million years ago, the land separating Baja California from the rest of Mexico dropped below sea level, and the Pacific Ocean invaded this new depression.  As the Pacific and North American plates continued to pull apart, Baja California separated from the mainland in a south-to-north direction.  A simple glance at a present-day map of North America reveals a separation half complete.

New research published in Nature Communications confirms that the Gulf of California continues to widen.  Geologists measured active spreading of the Gulf all along its length, including its northernmost reaches that lie just south of the U.S.-Mexico border.  But the signs of this slow and steady tear are also visible on the U.S. side of the border.  Two prominent features north of the border indicate that the divorce between Upper California and North America is well under way: the Imperial Valley and the Salton Sea.  The former is a depression, a deep valley below sea level, while the latter is a saline lake within the valley formed over a century ago by a flood from the nearby Colorado River.  Consider the Salton Sea a harbinger the entire Imperial Valley’s ultimate fate, for only the high silt and sand deposits of the Colorado River delta keep the Gulf of California from permanently submerging cities and settlements as far north as Indio, California.

Dam Colorado River.The Salton Sea and the fertile Imperial and Mexicali Valleys to the south lie below sea level, thanks to rifting and pulling forces from the separating Pacific and North American plates.  If the Colorado River delta didn't act as a natural dam, the Gulf of California would flood both valleys and reach as far north as Indio, California (indicated with a balloon "A").

Dam Colorado River.
The Salton Sea and the fertile Imperial and Mexicali Valleys lie below sea level, thanks to rifting and pulling forces from the separating Pacific and North American plates. If the Colorado River delta didn’t act as a natural dam, the Gulf of California would flood both valleys and reach as far north as Indio, California (indicated with a balloon “A”).

Since it appears that the Gulf of California’s slow and deliberate growth will continue, the complete separation of both Baja and Upper California from the rest of North America is likely inevitable.  However, north of the Salton Sea, the separation process has barely begun.  Instead of pulling apart, geologic forces there are causing the Pacific and North American plates to slide past one another.  This is particularly prominent along central and northern California’s massive San Andreas fault.  But in several million years, the tearing forces that wrenched Baja California from the mainland will probably arrive in full force in Upper California.

Four years ago in the American state of California, disgruntled conservatives pressed for a ballot measure that would force thirteen of California’s more politically “liberal” coastal counties to split off from the rest of California and form their own state.  The movement fizzled out after a few years, and for now California remains whole.  It appears that planet Earth does not like to rush these matters.

Further Reading:

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About James Urton

I went to school to become a molecular biologist.  At some point in this long education, I discovered that I love communicating science to the general public: talks, writing, at a pub, on the street corner...  Whatever venue will let me hold your attention for a few moments.  Unfortunately, I can't do this for a living, since no one will pay me.  So, I have a job as a molecular biologist at the University of Washington, where I get to work with great scientists on some really awesome projects, and I'll blog about science here at Muller's Ratchet in my spare time. Why should the general public want to know anything about science? Here's my explanation (which also explains why I chose the name Muller's Ratchet for this site). Briefly as a graduate student (before I had to devote all of my time to graduating), I blogged at Adaptive Radiation.
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