For the past 17 million years and perhaps longer, motion of Earth's tectonic plates has moved California west, away from Utah and Arizona.
As the area in between grew wider, Earth's crust cracked along major faults to form the mountains and valleys of the Basin and Range Province that today cover Nevada and western Utah. In the early stages of this process, large amounts of molten basalt intruded Earth's crust in northern Nevada, forming a linear, near-vertical dike swarm over 500 km long.