We call that regional compensation. If you have seen a compass, you have seen the result of Earth's Geomagnetism. Fluid motions in the outer core produce a magnetic field that we observe at Earth's surface. Similar to gravity, we can also measure variations in Earth's magnetic field. They change with time and position on Earth.
Temporal changes reflect changes in the geodynamo.
Spatial changes reflect the inherent nature of the geomagnetic field and variations in subsurface geology. We can measure the magnetic field at Earth's surface and locate conducting bodies such as iron ore and other resources. We can measure the magnetic field over time and study the fluid motions in the outer core. These changes suggest that convection in the core proceeds at a rate of 10's of km per year - about a million times faster than the tectonic plates move. But that's only the horizontal direction of the field.