During the Miocene Period, some 25 million years ago, rift volcanism began to develop in the region now occupied by extreme southeastern California, southwestern Arizona and northwestern Mexico. This rifting was triggered primarily by a northern extension of the East Pacific Rise, a mid oceanic ridge that stretches across the southeastern corner of the Pacific Ocean. Extensive faulting through this rift zone caused blocks of crust to drop below the adjacent terrain and, by 5 million years ago, the sea had invaded from the south. Augmented by the wet climate of the Pleistocene, the Colorado River swept enormous quantities of erosional debris into the northern end of this new sea, producing a vast delta that, today, obscures the connection of the Gulf of California Rift with faults of the Salton Trough and San Andreas network to its northwest.
Having torn the Baja Peninsula from mainland Mexico, this rifting, which continues today, transmitted pressure along the San Andrea Fault and across the landscape of Southern California, lifting fault block mountains and crumpling up the Transverse Ranges north of the Los Angeles Basin. Knifing into the southern portion of the Sonoran Desert, the Sea of Cortez (or Gulf of California) is lined with rich estuaries and marine wetlands, producing a dramatic contrast with the dry, cactus studded uplands that border its channel. The northern portion of the Sea, its basin partly filled by submarine layers of the Colorado River Delta, is much shallower and more saline than the central and southern regions. Numerous islands and islets rise above the surface of the Sea while rifting and volcanism persist in its deep channels, gradually widening the Gulf; eventually, the Baja will move off with Southern California, riding atop the Pacific Plate and destined for southwestern Alaska.
Among the most diverse and productive marine ecosystems on Earth, the Sea of Cortez has long been renowned for its rich fisheries, massive sea bird colonies and wide variety of resident and wintering cetaceans. Yet, despite the cooperative efforts of many regional and international conservation organizations, this young, fragile ecosystem is threatened by human activity; coastal wetland destrution, pollution and overfishing all take their relentless toll.
