The current theory of the structure of the
Earth, called plate tectonics, tells us that the
continents are in constant motion.
Assume that the North American continent
can be represented by a slab of rock 5500 km
on a side and 34 km deep and that the rock
has an average mass density of 2830 kg/m3
.
The continent is moving at the rate of about
2.4 cm/year