Why are convection currents; why are they hotter and colder parts of earth

This current causes earth quakes that makes a river, pond, or a lake

A current in a fluid that results from convection. Within the earth, heat that is circulating from deep below the surface melts rock to form magma. This circulating heat also causes the magma to flow, which in turn can cause the tectonic plates at the earth's surface to shift. This would be an example of very slow-moving activity caused by convection currents.