What forces plays the role in order to subduct the denser tectonic plate under the lighter plate?

Question

I am a 10^th grade student and I don't know much about geology but I'm learning about subduction online. It's always mentioned that the denser plate gets subducted under the lesser denser plate. I want to know why it's only the denser plate which gets subducted? I know its buoyancy will be less, but still it's floating over the mantle and when it collides with the lesser denser plate why is it the one which gets subducted, or what "forces" play the role here?

Answer 1

Once subduction starts, it tends to keep going because the sinking slab pulls the material behind it. This is "slab pull", which dominates over "ridge push" at a subduction zone. How subduction starts is an open research question. How subduction continues once started is more or less settled science, so this answer focuses on what happens after subduction has started.

The reason the mafic oceanic crust (which is indeed less dense than ultramafic mantle rock) sinks is because at enough depth, the temperature becomes high enough to cook out the portions of the sinking slab with a lower melting point. Basalt and related rock in oceanic crust is a mix of multiple chemicals, some with lower melting points, some with higher melting points. At enough depth, the materials with a lower melting point melts while the materials with a higher melting point remain solid.

The materials with a lower melting point also have a lower density than the materials with a higher melting point. The less dense liquid portion of the partial melt rises through the overlying material to form the volcanic arc oftentimes associated with a subduction zone. This is the key process by which much of the continental crust formed.

What's left behind in the solid portion of the partial melt is dense ultramafic rock, basically the same chemically as mantle material. As melting is endothermic, the partial melting process cools the remaining solid portion even further. (It was already a bit cooler than the surrounding upper mantle.) That makes it even more dense, denser than the surrounding mantle material, and hence it sinks.

Answer 2

It is true that due to their chemical composition, oceanic plates (made up largely of basalt) are denser than continental plates (made up largely of granite). As a consequence, when a continental and an oceanic plate meet and push against each other, the oceanic plate generally loses and has to go underneath.

This would explain why subducting plates end up under the continents, but not why they actually keep sinking into the mantle. This may be surprising at first because the oceanic plate that is subducting is made up of the same material as the mantle (by and large). But it is much colder than the mantle, and colder generally means denser (with water/ice being the notable exception). This is because the subducting plate has been at the surface for tens of millions of years, cooling in contact with the oceans and the surface. So it is denser, and therefore sinks down in the surrounding mantle. We believe that some subducting plates sink all the way to the bottom of the mantle before they finally achieve temperature equilibrium with the surrounding mantle.