Answer: 1. The heat from radioactive processes within the planet's interior causes the plates to move, sometimes toward and sometimes away from each other. This movement is called plate motion, or tectonic shift. 2. But scientists were at a loss to explain why some pieces of crust move much faster than others. A new study suggests these tectonic plates control their own speed more than the mantle does. ... The velocities of these plates and the velocities of the boundaries between these plates vary significantly on Earth." 3. Divergent boundaries -- where new crust is generated as the plates pull away from each other.
Convergent boundaries -- where crust is destroyed as one plate dives under another.
Transform boundaries -- where crust is neither produced nor destroyed as the plates slide horizontally past each other.
