Spreading ridges are also called mid-ocean ridges because they are observed today in oceans midway between continents. They are continuous rocky ridges on the ocean floor with a long, narrow, very deep basin in the center.
Observations have led to the conclusion that magma, or molten rock, moves from the deep mantle of the Earth upward into these center basins and cools there to form new oceanic crust. This crust then moves away to both sides, a process that is called spreading. Eventually, when the new oceanic crust collides with a continent, it sinks down into the mantle again.
Hotspots, or mantle plumes, on the other hand are thought to be columns of hot rock that rise slowly from the core-mantle boundary of the Earth, lifting the crust and forming hotspot volcanoes1.
In Southern Africa, there is evidence of fifteen hotspots that created such volcanoes over a period of billions of years. The evidence comes from the fact that it was discovered that a hotspot makes a characteristic mark or footprint that is detectable on an aeromagnetic map when it explodes through the crust and forms a volcano.
When the very first hotspots arrived, the Earth must have been covered with water. Footprints of the two oldest hotspots that can be divined over Southern Africa, suggest that they followed the same path. The path came from the north in a relatively straight line, made a loop over Southern Africa and then turned northwards again in an apparent attempt to reach the southward path again.
Many millennia later, a hotspot with very magnetic lava came down the same path and some of its lava cooled and solidified along this path. Although the solid rock formation moved later, the footprints of that hotspot on the rock can be matched to the footprints on the path on aeromagnetic maps. This revealed that the southward path had the shape of a spreading ridge along a relatively straight line from the north until it reached a fork. From there, the spreading ridge swung abruptly to the west. Unlike the hotspot tracks it did not turn northwards again.
The interesting speculation now is whether the first hotspot, like a blow torch, cut a spreading ridge into the top crust of the Earth from north to south – a spreading ridge which other hotspots later sought out for the good access to the surface – or whether the spreading ridge was there in the first place and all the subsequent active hotspots over Africa sought access to the surface along it.
1 Morgan, W. J. 1971. Convection plumes in the lower mantle, Nature 230, pp. 42-43.