Dynamics of three-layer convection in a two-dimensional spherical domain with a growing innermost layer: Implications for whole solid-earth dynamics

Abstract

I have performed numerical simulations of thermal convection with a highly viscous outermost layer (HVL), low-viscosity middle layer (LVL), and highly viscous innermost layer (IML) growing with time in a 2-D spherical domain. The results show that the average temperature of the LVL significantly decreases and the root-mean-square velocity of the LVL fluctuates significantly when the IML grows with time, compared to the model without the growing IML. This result suggests that the formation of the IML cools the LVL effectively. However, the existence of the growing IML barely affects the magnitude and spatial pattern of the heat flow at the HVL–LVL interface and the convection pattern of the HVL. In addition, the growing IML lengthens the lateral scale of the LVL convection owing to the basal heating from the LVL–IML interface. The present numerical results imply that the growing inner core, which corresponds to the IML in this model, throughout the Earth’s history may barely affect the convection pattern of the mantle (HVL) and the heat flow escaping from the outer core (i.e., LVL) to the mantle via the core–mantle boundary (i.e., the HVL–LVL interface). A previous geodynamo simulation suggested that the lateral thermal heterogeneity of the temperature just under the core–mantle boundary strengthens the geomagnetic field in the outer core. The present numerical results may suggest that the growing inner core throughout the Earth’s history facilitates the strong lateral thermal heterogeneity and strengthens the geomagnetic field.