How are differences in density and tempurature provide some of the force needed to cause seafloor spreading?

Answer 1

Higher temperature within the asthenosphere melts rock creating magma. For most solids (water being an exception) the liquid phase is less dense than the solid phase. Liquid also transmits pressure differently than solids. Squeeze an open toothpaste tube to see what I mean.

As magma created by heat rises under the spreading zones it expands (lower density). This expansion actually lifts the ocean ridges creating mountain ranges larger than any found on the continents. The pressure of the deeper asthenosphere is transmitted through this magma forcing it upward through the cooler solid rock above it. This forces the rock to spread apart. As this magma is deposited over and into the rock and slowly cools, it becomes more dense. The weight of this rock which has been lifted upward by the underlying magma, slowly pushes the oceanic plates outward as it settles. This is known as slab-push.

At the other end of the oceanic plates, where there is subduction taking place, the slab is sinking into the asthenosphere, creating slab-pull. So the oceanic plates are both being pushed outward by the heat and density differences at the spreading zone, and being pulled by the denser solid, cold plate at the other end of the oceanic plate in the subduction zone.

Answer 2

Hi. The tectonic plates float on top of the not mantle. The molten mantle rises and causes the floating plates to spread apart.