The movement of the South American and African continents is a monumental chapter in geological history. These massive tectonic plates were once locked together as a single landmass. Their slow separation over millions of years created the vast South Atlantic Ocean. Understanding this process requires examining the continents before their split and the forces that continue to drive them apart today.
The Plates Before Separation
Before the continents separated, the South American and African landmasses were merged into the southern supercontinent Gondwana. The fit between the eastern coastline of South America and the western coastline of Africa is remarkably precise, especially when mapping the continental shelves. This geographical congruence suggests they were once a seamless entity, evidenced by the continuity of ancient rock formations. Specific sedimentary layers and crystalline basement rocks found in Brazil correspond directly to those located in West Africa.
The Engine of Plate Movement
The force required to move continents is generated deep within the planet by mantle convection. This mechanism involves the slow, continuous cycling of heated material within the Earth’s mantle, transferring heat from the core toward the surface. Where this hot material rises, it exerts upward pressure that stretches and weakens the overlying continental crust, initiating rifting. This stretching caused the crust to thin and eventually rupture, establishing a divergent boundary where the two plates began to pull apart.
As the plates separate, molten rock (magma) wells up from the mantle to fill the void, instantly cooling and solidifying to form new oceanic crust. This continuous creation of new crust is known as seafloor spreading. The constant upwelling and cooling process built the Mid-Atlantic Ridge, an immense underwater mountain range running down the center of the ocean basin. The driving forces for this movement include ridge push, which is the gravitational sliding of the lithosphere away from the elevated ridge crest, maintaining the ongoing separation.
The Chronology of the Rift
The break-up of the African and South American plates was a phased process unfolding over tens of millions of years. Initial rifting began in the Late Jurassic period, around 150 million years ago, forming deep continental rift valleys. True separation, marked by the onset of seafloor spreading and the formation of the South Atlantic Ocean, started in the southern rift zone approximately 140 million years ago. This opening progressed slowly from south to north in a zipper-like fashion.
In the early stages, the developing ocean was narrow and restricted, leading to the deposition of thick layers of salt and evaporite minerals. The final land connection, located in the equatorial region, did not fully sever until about 105 million years ago. The plates continued their slow divergence, a process that persists into the present day.
Unlocking the Geological Proof
Multiple lines of scientific evidence confirm the historical connection and subsequent movement of the South American and African plates. One strong piece of evidence is the distribution of specific, non-migratory fossil species. For instance, the remains of Mesosaurus, a small freshwater reptile, are found only in specific rock formations in both southern Africa and South America. Since this animal could not have crossed a vast saltwater ocean, its distribution proves the continents were once joined.
Geophysical surveys of the ocean floor offer confirmation through the study of paleomagnetism. As new oceanic crust forms at the Mid-Atlantic Ridge, magnetic minerals within the cooling magma align with Earth’s magnetic field, locking in a record of its polarity. The symmetrical pattern of magnetic stripes on either side of the ridge demonstrates that new crust is created and pushed outward equally from the center. Furthermore, modern Global Positioning System (GPS) measurements precisely track the current movement, showing the plates are separating at a rate of approximately 2 to 4 centimeters per year.