how does the earths crust change in thickness over the surface of the earth?

Answer 1

What is termed the "crust" is generally thicker under mountains, and thinner in lowlands and under oceans. The expression is the mountains have "roots". However the crust is subject to folds and bends from faults, geothermal cells and plate techtonics so it is not uniform in thickness. The geophysical definition is the Mohorovic Discontinuity which can be measured from seismic analysis so this provides part of the evidence of variation. Previous work discovered that the mass of Himilayan Mountains contained too much surface rock so that roots were postulated to be under them.