I am trying to figure out the geological effects that would happen if earths upper mantle was drained of heat.
I am sure there are many ways that this could happen but the method that I am asking about is a species of titanic eldrich abomination like creatures that exist across multiple dimensions. They infect planets much like a virus infects a cell growing and spreading inside of the mantle until it is drained and then bursting out to move on. While growing they absorb the surrounding heat which is passed to parts of their body in another dimension thus draining heat from the mantle of the planet.
They do not act quickly enough to be visible in a human lifetime but over the course of thousands of years the temprature drop should be very noticable and would likely have some effects on the crust.
One other thing to note is that this earth like planet is technically a moon (one of three) orbitting a large planet. Their will be various and stronger tidal forces acting on the planet than happens on Earth though none of them too extreme.
How would this effect volcanic activity?
How would this effect tectonic plates?
Would this cool the atmosphere/crust of a planet?
Could upper mantle cooling cause issues with magnetic field?