The subduction of one oceanic plate under another is primarily caused by differences in density and the force of gravity.
Here's a more detailed explanation:
Density Differences: The Primary Driver
- Older Oceanic Lithosphere is Denser: Oceanic lithosphere is created at mid-ocean ridges. As it moves away from the ridge, it cools and becomes denser. This increased density is the primary reason why older oceanic plates subduct.
- Density and Buoyancy: Denser materials sink relative to less dense materials. Since older oceanic lithosphere is denser than the underlying asthenosphere (the hotter, more pliable layer of the upper mantle), it loses buoyancy.
- Compositional Density: While temperature is the major factor, the composition of the plate also contributes. As the oceanic crust ages, it hydrates and alters, increasing its density slightly.
Gravity's Role: Slab Pull
- Slab Pull: Once subduction begins, the weight of the cold, dense, sinking slab of oceanic lithosphere pulls the rest of the plate along behind it. This "slab pull" is a major driving force of plate tectonics and subduction.
- Negative Buoyancy: The negatively buoyant slab continues to sink into the mantle due to gravity acting on the density difference.
The Subduction Process: A Summary
- Cooling and Densification: Oceanic lithosphere cools as it ages, becoming denser.
- Increased Density Leading to Sinking: The increased density of the aging lithosphere makes it less buoyant than the asthenosphere.
- Initiation of Subduction: This density contrast, coupled with pre-existing weaknesses in the lithosphere, leads to the initiation of subduction.
- Slab Pull as a Driving Force: Once subduction begins, the weight of the sinking slab pulls the rest of the plate behind it, accelerating the process.
In essence, subduction is a process driven by density differences and the force of gravity acting on the colder, denser oceanic lithosphere.
