Complex sentence: The spectral method in mantle convection breaks down the three-dimensional governing equation into several one-dimensional equations, which solves the equations much faster and it was one of the popular approaches in early models of mantle convection.
Split into simpler sentences: The spectral method in mantle convection breaks down the three-dimensional governing equation into several one-dimensional equations, which solves the equations much faster. It was one of the popular approaches in early models of mantle convection.