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The present-day Earth's lower mantle is dominated by degree-2 structure: African and Pacific superplumes separated by circum-Pacific subduction. Based on mantle convection modeling using CitcomS, it is proposed that the Earth's mantle structure alternates between mainly degree-1 and degree-2 structures. When continents are scattered around the Earth's surface, mantle convection tends to form a predominantly degree-1 structure (top), concentrating convergence in one hemisphere and leading to assembly of a supercontinent. Subduction around the supercontinent then causes a secondary superplume to form beneath the supercontinent, turning previously degree-1 structure to degree-2 with two superplumes (bottom). The secondary superplume may eventually break up the supercontinent, repeating the cycle. (Shijie Zhong, Nan Zhang, James H. Rberts, and Zheng-Xiang Li; 2007)
