Convection occurs in the aesthenosphere. The aesthenosphere lies directly beneath the lithosphere at depths of between 62 and 124 miles.
The aesthenosphere is a layer of solid rock that is so hot, the rock melts and flows like a liquid, easily breaking apart. The most dramatic earthquakes are caused by rocks in the aesthenosphere breaking apart.
Large convection currents transfer heat to the surface of the Earth, and these currents cause the magma to break apart. When this happens, it creates divergent plate boundaries. When the plates separate, they cool down and get reabsorbed back into the aesthenosphere.
Because the aesthenosphere is so deep, scientists cannot really study it until an earthquake or volcano occurs. They know that earthquakes change speed and direction when there is a change in the density of the rocks being affected. Rocks in the aesthenosphere rise up to the surface of the planet where there is evidence of tectonic plates separating.
Scientists cannot tell how deep the aesthensosphere goes. Some think it is a layer that delves around 434 miles into the Earth. Until better technology is built to help them study this layer, they have to study things like earthquakes, volcanoes and magma chambers in the world's oceans.