Why is the number of superplumes so limited?

Abstract

Numerical simulation shows that the hot plumes can be formed by the self-accelerated process if local thermal perturbation is present. Since the local thermal perturbation may exist extensively near the CMB and the transition zone around 670 km, the number of the hot plumes should be great. The cold plume seems to be originated by the subduction slabs. Because the subduction belts distribute extensively on the Earth’s surface, the number of the cold plume should be also great. However, the observed number of the superplumes is much less than the supposed ones. A simple 2D model of whole mantle convection is designed to investigate the interaction between plumes and thermal anomalies. The hot thermal anomalies (HTA) are imposed at CMB and a depth of about 670 km discontinuity, corresponding to two plausible locations of the hot plumes origination. And, the cold thermal anomalies (CTA) are imposed near the surface and 670 km discontinuity, corresponding to the subducted slabs and possible accumulation of slabs at 670 km discontinuity. The numerical results show that the fixed HTA does not originate a new plume. It draws the hot plume to move towards itself and stabilizes the plume at its own position. The imposed free HTA moves towards the hot plume, and finally merge with the plume. The situation for the CTA is similar to that for the hot one. From the numerical results, we can reach the following conclusions. It is very difficult for the HTA/CTA to originate a new hot/cold plume in a convective flow field. The boundary instability is suppressed by the developed plumes. Instead of originating new hot/cold plumes, the HTA/CTA interact with the plumes to form huge superplumes. This might be one reason for the question, why the number of the superplumes is so limited.