Plate tectonics

The theory of plate tectonics holds that Earth's outer shell is divided into several rigid plates that move relative to one another atop the semi-fluid asthenosphere. These plates are composed of either oceanic or continental lithosphere, or a combination of both. The boundaries between plates are the sites of most geological activity, including earthquakes, volcanic eruptions, and mountain building.¹

There are three main types of plate boundaries: divergent, where plates move apart and new crust is created; convergent, where plates collide and one plate is subducted or crust is thickened; and transform, where plates slide past each other horizontally. The movement of plates is driven by a combination of mantle convection, ridge push, and slab pull, with slab pull (the gravitational pull of subducting slabs) considered the dominant force.

The concept of continental drift, proposed by Alfred Wegener in 1912, was an early precursor to plate tectonics. Wegener's idea was largely rejected because he could not provide a convincing mechanism. In the 1960s, the discovery of seafloor spreading at mid-ocean ridges, along with paleomagnetic data showing symmetrical magnetic stripes on the ocean floor, provided the crucial evidence. This led to the formulation of plate tectonics by scientists such as Harry Hess, Robert Dietz, and others.

The theory was further solidified by the confirmation of magnetic reversals recorded in oceanic crust, which allowed for the dating of seafloor and the calculation of spreading rates. The acceptance of plate tectonics is often described as a scientific revolution, transforming Earth sciences in the 1960s and 1970s.

Divergent boundaries occur where plates move apart, such as at the Mid-Atlantic Ridge, creating new oceanic crust. Convergent boundaries include subduction zones (e.g., the Pacific Ring of Fire) where oceanic plates sink into the mantle, and continental collision zones (e.g., the Himalayas). Transform boundaries, like the San Andreas Fault, accommodate lateral sliding. Each boundary type is associated with characteristic geological features and hazards.²

The motion of plates is driven by forces such as mantle convection, ridge push, and slab pull. Slab pull is the force exerted by the weight of cold, dense subducting plates pulling the rest of the plate along. Ridge push results from the elevated mid-ocean ridges pushing plates away. Mantle convection, the slow circulation of the mantle, also contributes. Recent research suggests that the balance of these forces may vary over time and among plates.

Earth is the only known planet with active plate tectonics. Mars and Venus show evidence of past or different styles of tectonic activity, but not the same global plate system. Jupiter's moon Europa may exhibit plate-like motion on its icy crust, and recent studies suggest that Pluto might have had a form of plate tectonics in its past. These comparative studies help scientists understand the conditions necessary for plate tectonics.

While the basics of plate tectonics are widely known, several lesser-known aspects enrich the theory. One is the existence of 'microplates' such as the Juan de Fuca Plate, which are small and often overlooked. Another is the role of plate tectonics in the formation of mineral deposits, such as porphyry copper ores, which are associated with subduction zones. Additionally, the theory has been extended to explain the distribution of biodiversity and the evolution of species through vicariance (separation by geographic barriers).

Another intriguing aspect is the concept of 'plate tectonics on a snowball Earth'—the idea that during extreme glaciations, plate tectonics may have continued, influencing the carbon cycle and climate. Also, the study of 'tectonic plates' on other planets, such as the 'chaos terrains' on Europa, provides a comparative perspective. Furthermore, the theory has practical applications in earthquake forecasting and the search for geothermal energy.

Lithosphere

The rigid outer layer of Earth, comprising the crust and uppermost mantle.

Asthenosphere

The semi-fluid layer of the mantle beneath the lithosphere on which plates move.

Subduction

The process by which one plate sinks beneath another into the mantle.

Seafloor spreading

The creation of new oceanic crust at divergent boundaries.