The Salton Sink belongs to the Salton Trough, a pull-apart basin formed where two great tectonic features intersect. The San Andreas Fault, which most people associate with earthquake risk farther north, reaches its southern terminus near the Salton Sea. The East Pacific Rise — the spreading ridge responsible for opening the Gulf of California — extends northward beneath the Imperial Valley. Together they create a zone where the crust is being stretched and thinned, sinking incrementally over geological time. The basin is not stable; it is a work in progress. Seismologists monitor the Brawley Seismic Zone at its southern end with particular attention, because the area experiences frequent low-magnitude earthquakes and is considered a potential nucleation point for larger events on the southern San Andreas.

The Salton Sink has held water many times. The most significant prehistoric lake, Lake Cahuilla, existed for roughly 17,500 years — from about 20,500 to 3,000 years before the present — fed by overflow from the Colorado River when that river's delta shifted and spilled into the basin. At its maximum, Lake Cahuilla stretched 110 miles from north to south, reaching depths of around 300 feet. The Cahuilla people, whose ancestral lands surrounded the lake, have oral traditions describing both its presence and its drying, a memory preserved across dozens of generations. Fish traps built along old shorelines can still be found in the desert today, stone structures that mark where the water once stood. The lake dried when the Colorado River shifted back toward the Gulf of California, leaving behind the salt flat that would become the bed of the modern Salton Sea.

The Salton Sea we see today is young by any geological measure. In 1862, a massive flood sent water pouring into the sink, creating a temporary lake sixty miles long. That lake dried. Then in 1905, an irrigation canal intake near Yuma failed catastrophically, allowing the entire flow of the Colorado River to pour uncontrolled into the basin for nearly two years. By the time engineers stopped the flow in 1907, the Salton Sea had formed — a lake covering nearly 350 square miles. It has been here ever since, sustained not by rivers but by agricultural drainage. It has no natural outlet, which means everything that flows in — water, nutrients, salts, pesticides — stays in, concentrating over time.

At the southern end of the Salton Sea, a cluster of small mounds called the Salton Buttes rise from the surrounding flats. They are rhyolite lava domes, and they are among the youngest volcanic features in California, with the most recent eruptions dated to approximately 10,300 years before the present. In geological terms, that is essentially yesterday. The buttes are a surface expression of the same tectonic processes that shaped the entire basin: magma intruding upward through a zone of crustal extension. Geothermally, the area remains active — the geothermal gradient beneath the Imperial Valley is among the highest in North America, a fact that has attracted both power generation facilities and, more recently, serious interest in lithium extraction from the superheated brine reservoirs deep underground.