Here's why:
* Convection is a type of heat transfer that occurs in fluids (liquids and gases) where hotter, less dense material rises and cooler, denser material sinks. This creates a circular flow pattern.
* Earth's mantle is a semi-solid layer that behaves like a very viscous fluid over long periods.
* Heat from the Earth's core causes the mantle to heat up. This hot material becomes less dense and rises. Cooler, denser material from the upper mantle sinks down, creating a convection current.
* These convection currents are the driving force behind plate tectonics, which is the movement of the Earth's tectonic plates. The plates ride on top of the convection currents, causing them to move and interact.
In summary: The movement of Earth materials through the mantle due to heat transfer is a process of convection. This process is fundamental to the dynamics of the Earth and drives the movement of tectonic plates.
