Immediately, we see steam come out, and rise in a column. This indicates that not only is heat rapidly leaving the potato, but the heat transfer now is mostly by convection. It is the circulation of air near the potato which is largely responsible for its cooling. Of course, if we want to cool the potato still faster, we can pour ice-cold water into it after we slice it. The sizes of the arrows are proportional to the rate of heat flow to the surface.
Convection dissipates the heat along fractures very quickly.
In many ways, the buried pluton is like that hot potato. Now consider what would happen if the thick layers of enclosing rocks became cracked. Water would naturally percolate through the rocks, and this would speed up the cooling of the pluton. Now let us take the analogy of the potato further. Permit not only the surrounding rock layers to crack, but also allow the pluton itself to crack as it cools.