Raising the Rockies

The rocky mountains are one of the world’s great geologic features, yet it’s never been clear how they achieved their status. The problem is this: the Rockies have the same structure as ranges such as the Himalayas, which, as a rule, form when one of the slowly shifting tectonic plates that make up Earth’s crust collides with another. But there is no landmass pressing up against North America; the Pacific plate is sliding along the west coast, not ramming against it.