How do differences in density and temperature cause sea-floor spreading and subduction?

A convection current causes hot material to rise to the surface, where it cools and spreads out, resulting in the sea floor spreading that is seen at the mid-ocean ridges.

A subduction zone is where the cooler mantle material sinks back down into the hotter mantle carrying the crust with the segment of cooler mantle. Additionally, because the ocean plates are made of basalt, which is much more dense than the granite base of the continental plates, the ocean plate is the plate that sinks down into the mantle in a convection current.

The cycle of sea floor spreading and subduction zones does not explain how thick layers of ocean sediments ended up on the Continents.

Differences in density and temperature cause sea-floor spreading and subduction through the process of plate tectonics. In sea-floor spreading, denser oceanic plates sink beneath less dense continental plates at subduction zones due to the colder temperature and higher density of the oceanic crust. This process creates new oceanic crust at mid-ocean ridges where magma rises from the mantle, cools, and solidifies, pushing the plates apart. Subduction occurs when oceanic plates collide with continental plates, causing the denser oceanic plate to be forced beneath the less dense continental plate into the mantle, leading to the destruction of oceanic crust. These processes drive the movement of tectonic plates and contribute to the reshaping of Earth's surface.