The Earth Contents

2. Plate Tectonics

Plate motion is locally very significant in altering the mass of the Earth's crust, but is relatively insignificant globally, except over very long timescales

The amounts of material added or removed from the crust at plate boundaries are locally significant but, in terms of the total mass of the crust, the additions are insignificant except over very long time-scales. For example, at the present rate of production of new oceanic crust it would take 100 million years to replace the existing ocean crust entirely. On an environmental time-scale the crust therefore has a fixed composition.

Why is plate motion relatively insignificant in altering the global crustal mass?