Why do convection currents in the earth's mantle turn left once they stop moving upwards?
Actually the convection currents are thought to move both left and right as they reach the surface.
Convection currents observed in water rise to the surface and then spread out in both direction. Convection currents in the mantle are thought to operate in the same matter as convection currents in water.
The convection currents in the mantle rise at the Mid Atlantic ridge. the lava flows out from the MidAtlantic ridge in both directions, both left and right from the ridge. The magnetic zones mirror each other on either side of the Mid Atlantic ridge. This shows that the convections currents move outward both right and left after the current reaches the surface.