The engine under the hood
The theory of plate tectonics is fundamental to modern geology, yet the precise forces that move continents remain a subject of scientific debate. When Alfred Wegener first proposed continental drift in 1912, his hypothesis was largely dismissed because he could not provide a plausible mechanism to move such immense landmasses. It took another half-century, and the discovery of the Mid-Atlantic Ridge, for the idea to gain widespread acceptance. Today, the conversation has shifted from if the plates move to how they move. Three primary mechanisms are being discussed: ridge push, slab pull, and mantle convection. These forces are not mutually exclusive and likely all contribute, but their relative importance is a major area of research.
This location in Iceland's Þingvellir National Park is on the Mid-Atlantic Ridge, a massive underwater mountain range where two tectonic plates, the North American and Eurasian, are pulling apart. This makes it an place to see one of the main proposed driving forces. The average spreading rate along this ridge is about 2.5 centimeters per year.
A three-way tug of war
Ridge push is a gravitational force that results from the higher elevation of mid-ocean ridges. New oceanic lithosphere is formed at these spreading centers from hot, buoyant mantle material. As this new crust cools and moves away from the ridge, it becomes denser and thicker, causing it to slide down the gentle slope of the ridge under the force of gravity. This process is one driver of plate motion.
Slab pull is now considered by geophysicists to be the dominant driving force. It occurs at subduction zones, where a cold, dense oceanic plate sinks back into the mantle. The sheer weight of this descending slab pulls the rest of the plate along with it. Analyses suggest that the forces generated by slab pull can be an order of magnitude stronger than those from ridge push. Plates with significant subducting slabs, like the Pacific Plate, tend to move much faster than plates without them, such as the North American Plate.
The third mechanism is mantle convection. This was the original idea proposed by Arthur Holmes in 1931. It suggests that the lithospheric plates are dragged along by massive convection currents in the hot, slowly flowing mantle below. While mantle convection is certainly happening and is the ultimate source of the heat that drives the system, its role as a dragging force (basal drag) is now seen as more complex. Many models now incorporate ridge push and slab pull as the primary forces, with mantle convection being the overarching process that enables them.