A convection current is when a concentrated area heats up the bottom. Warmer rocks move upwards causing a 3-dimensional circular motion. Heating up a beaker of water causes the same reaction. The water near the flame warms quicker and moves upward, displacing the cooler water on top. The cooler water is heavier and "sinks." The cycle is then repeated and repeated. If you think of convection currents within the mantle this movement causes stress on the crust of the Earth.

Continents are composed of lighter material and seems to never reenter the convection current. It would be similar to oil floating on top of the boiling water. It always stays on the top.