Mexico
The Baja California Peninsula, part of mainland Mexico 15 million years ago, is one of the longest, thinnest peninsulas in the world.
The Baja California Peninsula hangs off its namesake state like a dog’s scruffy tail—a 775-mile-long strip of land once attached to Mexico, before it was sheared off and transported north and west by ongoing tectonic activity.
From the air, brown outcroppings of volcanic rock mark the location of a few of the peninsula’s many volcanoes and lava fields, spawned before and after the peninsula began breaking away from mainland Mexico. These volcanoes and the Peninsular Ranges, a southward extension of California’s Coast Range, make up the mountainous spine of the peninsula.
Before 15 million years ago, the Baja California Peninsula was part of the North American Plate, stitched along the coast of Mexico. But between 15 and 12 million years ago, a divergent plate boundary, called the East Pacific Rise (which runs from the Salton Sea in California south to Antarctica) began spreading apart, creating new transform faults along the edge of the North American Plate.
In just a few million years, earthquakes and movement along these faults—driven by spreading of the East Pacific Rise—sliced the Baja California Peninsula off the North American Plate and attached it to the Pacific Plate, which is still moving northwest. As the peninsula was shaved off and carried away from the mainland of Mexico, the Sea of Cortez opened between the peninsula and the mainland. Eventually, the Baja California Peninsula will become an island, possibly in the next 10 million years.
In the armpit where the Baja peninsula is attached to Southern California lies the dried-up mouth of the Colorado River. Until a few decades ago, the mighty Colorado ran its course from the Colorado Rockies through the Grand Canyon before emptying into the Sea of Cortez, creating a vast delta of braided stream channels, estuaries, and marine habitats that supported diverse ecosystems of freshwater, brackish water, and saltwater species. Today, so much water is diverted out of the Colorado (mostly for irrigation), the river doesn’t make it to the sea anymore. The once-lush Colorado River Delta has dried up into the Gran Desierto de Altar, or Altar Desert, which can be seen in the juncture between Mexico and the peninsula. Nearby, greenish fields stand in stark contrast.
About halfway down the peninsula, on the Pacific Ocean side, is a semicircular-shaped lagoon known as Laguna Ojo de Liebre, which translates as the Eye of the Jackrabbit Lagoon. This calm, shallow lagoon has served as a nursery for gray whales and their calves for perhaps millions of years. Mother whales come here between December and April to give birth and raise their bus-sized newborns in relative peace and safety before embarking on their 5000-mile journey north to summer feeding grounds in the Arctic Ocean.
The Colorado River used to flow into the Sea of Cortez in the northeastern part of Baja California, but diversion of the river’s water for agriculture and other uses has been unsustainable for decades, and the river (dark blue) now dries up a few miles shy of the coast.
En route to San Diego, California, or La Paz, Mexico, look for a long, thin peninsula hanging off the southwest coast of California, separated from mainland Mexico by the Sea of Cortez.
