For eons, an oceanic plate wedged under North America’s west coast, but that plate, the Farallon Plate, was not endless, and it would eventually slide completely under North America. This subductive finale began about thirty million years ago, when the trailing edge of the Farallon Plate vanished beneath Los Angeles. Thus was born the San Andreas Fault, and so too California. The process is still progressing. At present, this post-subductive land we call California stretches from Cabo San Lucas in the south to Cape Mendocino in the north, from the Pacific Ocean across the Great Basin into Utah. This is where the Farallon Plate (now the “Farallon Slab”) once slid into the mantle under North America, suspended by hundreds of miles of trailing oceanic crust under the Pacific Ocean, dipping into the mantle as it slid eastward.
After the trailing edge of the Farallon Plate vanished under North America and left a void in its wake, lighter and hotter mantle rose up to fill the space left vacant by the cold, dense slab. This mantle pushed up against the continent, and uplifted lands from central California to western Utah. Like the top of a rising cake, the surface of the land spread apart as it rose, leading to cracks and deep basins between the rising ranges.
But this only explains what occurred in the wake of the great dive of the Farallon Slab. There is another dynamic to be considered.
When the last traces of this dense oceanic slab submerged, no longer suspended by a long tail of oceanic crust, it must have begun to sink more rapidly. What might the rapid descent of this slab have done to the continent above?
According to physics, any increase in volume diminishes density, and that means diminished pressure. A sinking slab beneath the continent would increase the volume of the mantle between it and the continent above, thus reducing the pressure immediately beneath the continent and generating a downward suction force on the crust, possibly pulling bits of North America down into the mantle. This may explain the extraordinary thinness of the crust under the Basin and Range Province (the Great Basin), and something more remarkable: the lost crustal root of the Range.
Once the cold, dense root of the Range broke off, it could only sink deeper and deeper into the mantle, and as it sank, it would have to have generated a suction force similar to the force generated by the sinking of the Farallon Slab. Hence the whales: mountains being dragged down to Hades by the suction force of a huge mass of rock sinking through the mantle.
This sinking of cold rock—in turn—likely forced an adjacent upwelling of hot mantle rock. Thus was born a “high-velocity drip” convection cell under the Sink and the Range. Adjacent parts of the Range to the east, levered against this sinking end of the slab and teetering on the heaving mantle, have thus been lifted upward like the stern of a ship sinking at its bow.
This wrenching action with so thin a crust may have encouraged volcanic activity along the eastern edge of the Range over the last five million years or so. Perhaps these factors can be blamed for one of the greatest volcanic events known to science: the Long Valley eruption of 760,000 years ago. The mammoth scar left by that event trembles and steams to this very day.