What do mantle convection currents cause?

It causes the currents to constantly flow. The movement of the currents plays a factor in the movement of the mantle. 

The churning of the mantle also affects the chemical composition of the ocean and has a long-term influence on climate. Mantle convection is the main way heat from Earth’s interior is transported to its surface, and this heat escapes principally through mid-ocean ridges. In fact, the connected mid-ocean ridge system is in essence a 80,000 km long volcano. Escaping heat along these ridges causes hot water to circulate through the crust in a “hydrothermal system.” As seawater reacts with the hot rocks, its composition changes. Over the course of several million years, all of the water in the world's oceans cycles through the oceanic crust at mid-ocean ridges.