The Earth’s outer shell, known as the lithosphere, is a fractured layer composed of segments called tectonic plates. The South American Plate is one of the dozen major lithospheric plates, encompassing the entire South American continent and a large section of the Atlantic Ocean floor. While appearing stationary, this piece of crust is in constant, slow motion over the semi-molten layer beneath it, the asthenosphere. This gradual movement continuously reshapes the continent’s geology, influences its landscape, and dictates its seismic activity.
The Specifics of South American Plate Movement
The South American Plate is currently moving in a general westerly to northwesterly direction across the globe. Its movement is relentless, with an average speed estimated at a few centimeters each year. Specifically, the rate of movement relative to the African Plate is typically cited as being in the range of 27 to 34 millimeters (about 1.1 to 1.3 inches) annually.
Scientists determine this precise, ongoing motion using sophisticated geodetic measurements, primarily relying on the Global Positioning System (GPS). A network of ground-based GPS stations across the continent records minute shifts in their position relative to a fixed global reference frame. By continuously monitoring these movements, researchers calculate the plate’s velocity vector, which indicates both its speed and its precise direction.
The calculated velocity is not uniform across the entire plate due to complex interactions with neighboring plates. The general westward movement continually widens the Atlantic Ocean basin while simultaneously compressing the plate’s western margin. These displacements are the direct result of deep forces originating within the Earth’s interior.
Driving Forces Behind Plate Motion
The primary engine powering the movement of all tectonic plates is thermal convection occurring deep within the Earth’s mantle. Hot material rises near the core, cools near the crust, and sinks again, creating slow-moving currents that drag the overlying plates. While convection is the ultimate source of energy, two other boundary forces act directly on the South American Plate to dictate its speed and direction.
One significant force driving the South American Plate westward is known as slab pull. This occurs at the western margin where the dense, oceanic Nazca Plate is diving beneath the lighter South American Plate. The weight of the descending slab pulls the rest of the plate along behind it.
The second major force is ridge push, which originates at the plate’s eastern boundary, the Mid-Atlantic Ridge. Magma rises there to form new, buoyant oceanic crust, which cools and slides down the ridge slopes. This process exerts a continuous outward push, shoving the South American Plate toward the west. The combination of slab pull and ridge push provides the sustained power for the plate’s movement.
Interactions at the Plate Boundaries
The movement of the South American Plate results in significant geological consequences at its boundaries. The western boundary is a highly active convergent margin where the plate collides with the oceanic Nazca Plate along the Peru-Chile Trench. This interaction is a subduction zone, meaning the denser Nazca Plate slides underneath the South American Plate and descends into the mantle.
The collision is directly responsible for the uplift of the Andes Mountains. The subduction process also causes the intense seismic and volcanic activity characterizing the Pacific coast of South America. Subduction rates along this margin are fast, ranging from approximately 59 to 90 millimeters per year. This rapid convergence generates some of the world’s most powerful megathrust earthquakes, such as the magnitude 9.5 Valdivia earthquake in 1960.
The eastern boundary of the South American Plate is a divergent margin where it separates from the African Plate. This boundary is marked by the Mid-Atlantic Ridge, a vast underwater mountain range running down the center of the Atlantic Ocean. Along this ridge, new oceanic crust is created as magma rises from the mantle to fill the gap left by the separating plates. The South American and African plates are pulling apart at a rate of about 3 centimeters per year, a process known as seafloor spreading. This continuous divergence is why the Atlantic Ocean basin is steadily widening over geological time.