Figure 2. Model of Wilsonian periods and MOMO (mantle overturn, major orogeny) episodes. During Wilsonian periods (left), the normal mode of plate tectonics prevails, with opening and closing of oceans and mantle convection with isolated upper and lower mantle. Plumes originate predominantly from the base of the upper layer, and continental growth is dominated by arc accretion. During MOMO episodes (right), accumulated cold material descends from the 660-knm boundary layer into the lower mantle, and multiple major plumes rise from the core-mantle boundary to form large igneous provinces (LIPs) at the surface, thus creating a major overturn. (After Stein, M., and Hofmann, A.W., 1994. Mantle plumes and episodic crustal growth, Nature, 372, 63-68.)