Conservation laws include conservation of energy, mass and momentum – stating that these quantities cannot be destroyed, forming the basis for modeling and understanding the Earth system.
In natural systems such as the Earth governed by dynamic equilibrium, positive feedbacks are important for maintaining system stability, restoring the system after perturbations or external/internal changes.
The slightly ellipsoidal shape of the Earth arises from Earth’s rapid spin, which causes the polar regions to bulge and the equator to flatten.
The geoid approximates Earth’s true shape, which is essentially the ellipsoid modified for gravity anomalies (local accumulations/deficits of crustal mass) which in turn cause the sea level surface to undulate slightly.
The greatest topographic variation between the highest mountain and deepest ocean trench on Earth is about 20 km.
Earth is structured according to layers of increasing density, from outside to inside, with the exception of Earth’s innermost core, which is liquid as opposed to solid metal.