What two forces are considered the main sources for tectonic plate motion?

The main sources for tectonic plate motion are gravity and friction. Gravity plays a crucial role in the movement of tectonic plates by causing the denser, colder plates to sink into the mantle at subduction zones, which leads to the movement of adjacent plates. This process is often referred to as slab pull. On the other hand, friction is an essential factor that influences how plates interact with one another along their boundaries. It affects the way plates can slide past each other, either smoothly or with resistance, which can lead to seismic activity when the stress exceeds the frictional forces.

The interaction of these two forces creates the dynamic movements that characterize plate tectonics, allowing for the shifting, collision, and separation of the Earth's crust, which is responsible for various geological phenomena like earthquakes, mountain building, and volcanic activity. Understanding these forces provides insight into the complexities of Earth's geology and the ongoing processes that shape the planet.