Hot-spots are the surface expression of rising mantle plumes - mushroom-like up-wellings of unusually hot mantle. They build great volcanic piles and, when particularly pronounced, lead to huge out-pourings of lava. Hot-spots are relatively fixed in comparison to the plates through which they "burn" and so provide a reference frame for measuring plate motion. The motion recorded for plates is no recorded relative to another plate but to the underlying asthenospheric mantle. As plates move, a fixed hotspot will "burn" a chain of volcanoes. The classic example is the Hawaiian island chain and its submerged precursor volcanoes which form a chain of sea-mounts. Other classic examples are found in the south Atlantic. These chart the separation of the South American and African continents over the past 120 million years.