Small-scale sub-lithospheric convection develops due to instabilities in the thermal boundary layer as the plate cools.
Instabilities
Image from Ballmer, M. D., J. van Hunen, G. Ito, P. J. Tackley, and T. A. Bianco (2007), Non-hotspot volcano chains originating from small-scale sublithospheric convection, Geophys. Res. Lett., 34, L23310, doi:10.1029/2007GL0316365. Download larger version (jpg, 475 KB).

Figure 4. Small-scale sub-lithospheric convection (SSC) develops due to instabilities in the thermal boundary layer as the plate cools. Volcanic chains are aligned with the direction of plate motion above the upwelling limbs of the convections rolls.

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