Earth's Upper Mantle: How Low Can It Flow?

For years, a debate has raged among Earth scientists about the convection within our planet's thick mantle. Most researchers agree that the 3000 km thick mantle is divided into an upper region and a lower region by an interface at a depth of about 660 km where the mantle materials undergo an isochemical phase change. But they differ as to whether this seismic discontinuity is a barrier to convection.