Do our continents lay on the mantle because solid silicon dioxide is less dense than its liquid form?

Question

I don't know whether quartz behaves like ice or not, but could it be because silicon dioxide, which the continents are made of for a big part, is less dense - so the continents lay on top like ice on water?

Answer 1

At the same pressure, solid silicon dioxide has a higher density than the liquid form (water is one of the few materials that is less dense as a solid). The ocean plates do sink when they get subducted into the mantle, but the mantle does not flow like water. The mantle is in a plastic state which bends due to the high pressure, so the flow rate is very slow (inches per year). The continents are formed of silicates that are on average less dense than the average silicates of the mantle, so they tend to "float".