The Earth’s surface is broken into tectonic plates that are in constant, slow motion, driven by forces within the Earth’s mantle. This movement causes geological activity, shaping the planet’s features. Along the western coast of South America, the Nazca and South American plates meet, forming a region of intense geological activity.
The Nazca and South American Plates
The Nazca Plate is an oceanic tectonic plate in the eastern Pacific Ocean, off the western edge of South America. It covers approximately 15.6 million square kilometers (6 million square miles) and is composed entirely of oceanic crust. Its oldest rocks date back about 50 million years.
The South American Plate is a major tectonic plate that includes the entire South American continent and extends eastward into the Atlantic Ocean. It consists of both continental and oceanic crust, with the continental portion forming the South American landmass. This plate has an area of about 43.6 million square kilometers (16.8 million square miles).
The Convergent Boundary and Subduction
The Nazca Plate and the South American Plate meet along South America’s western coast, forming a convergent plate boundary where the two plates move towards each other. This interaction is an oceanic-continental convergent boundary, where the denser oceanic Nazca Plate is forced to dive beneath the lighter continental South American Plate. This geological process is known as subduction.
Subduction begins at the Peru-Chile Trench, where the Nazca Plate starts its descent. The Nazca Plate moves eastward at a rate ranging from 59 to 90 millimeters (2.3 to 3.5 inches) per year in different segments, making it one of the fastest converging plates on Earth.
As it descends into the Earth’s mantle, it does so at varying angles. In some regions, like northern Peru and southern Ecuador, the subduction angle is around 25-30 degrees. However, beneath central Peru and northern Chile, “flat-slab” subduction occurs, where the plate remains nearly horizontal for hundreds of kilometers before continuing its descent. This downward movement is driven by the Nazca Plate’s greater density compared to the underlying asthenosphere, causing it to sink largely under its own weight.
Geological Consequences of Subduction
The subduction of the Nazca Plate beneath the South American Plate generates several geological features. The compression and friction between the plates lead to the formation of the Andes Mountains, an extensive range along South America’s western edge. Rocks of the South American Plate are folded and uplifted by this pressure, creating these fold mountains. This mountain-building process, known as orogeny, has resulted in some Andean regions rising at rates exceeding 10 millimeters (0.4 inches) per year.
The Peru-Chile Trench, also known as the Atacama Trench, is a direct result of this subduction. This trench is approximately 5,900 kilometers (3,700 miles) long and reaches depths of over 8,000 meters (26,000 feet). It is among the deepest oceanic trenches globally.
The interaction between these plates also results in earthquakes. The friction between the subducting Nazca Plate and the overriding South American Plate causes stress to build up, which is then released as seismic energy. These earthquakes can be shallow, occurring near the surface, or extend to greater depths as the oceanic plate descends. The region has experienced some of the largest earthquakes ever recorded, including the magnitude 9.5 Valdivia earthquake in 1960.
Volcanoes form the Andean Volcanic Belt as another consequence of this subduction zone. As the Nazca Plate descends, increasing heat and pressure, along with water release, cause surrounding mantle rock to melt. This molten rock, or magma, rises through the South American Plate, leading to volcanic eruptions and the formation of volcanic arcs. These arcs are found inland from the trench and contribute to the growth of the Andes.
Regional Impact and Significance
The interaction between the Nazca and South American plates impacts populations along South America’s western coast. Countries like Chile, Peru, Ecuador, and Bolivia are regularly exposed to geological hazards. Earthquakes pose a risk to infrastructure and human life. Volcanic eruptions from the Andean Volcanic Belt also present threats, including ashfall, lava flows, and pyroclastic flows, which can disrupt daily life and agriculture. This geological activity shapes the South American landscape, making it one of the most geologically active regions on Earth.