Complex sentence: The probable explanation lies in the rate of convergence between the Juan de Fuca and North American plates, which converge at to per year, about half the rate of convergence from seven million years ago.
Split into simpler sentences: The probable explanation lies in the rate of convergence between the Juan de Fuca and North American plates. These two tectonic plates converge to per year, about half the rate of convergence from seven million years ago.