According to Hess's theory of seafloor spreading, if the magma under a mid-ocean ridge cools, it would solidify to form new oceanic crust. As this new crust is formed, it does not directly cause the magnetic stripes to reverse direction; magnetic reversals are related to changes in the Earth's magnetic field, not the cooling of magma itself.
Cooling magma under the mid-ocean ridge would indeed contribute to the formation of new crust, but it wouldn't necessarily cause the crust at the ridge to form more quickly; rather, it solidifies to create more crust as it cools.
The creation of new crust does play a role in the movement of tectonic plates, as plates on either side of the ridge are driven apart by the upwelling magma. If the magma cools significantly, it could potentially slow down the process of seafloor spreading, which might affect the movement of plates but wouldn't necessarily stop them entirely.
Based on these interpretations, the most accurate response to what might happen if the magma under a mid-ocean ridge cools is that the plates on either side of the ridge might slow down in their movement, though they are unlikely to completely stop.
Thus, none of the provided options accurately reflect the immediate consequences of cooling magma under a mid-ocean ridge in the context of Hess's theory. However, the closest applicable idea is that the formation of new crust is linked to the cooling process, but it does not imply that the crust at the ridge forms more quickly or that the plates would stop moving.
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