Oceanic crust would be on top, being less dense and doesn't have as much water in it. Old oceanic crust is usually on the bottom, and filled with water. It is more dense.
Yes. Avogadro Law gives the relationship between volume and amount when pressure and temperature are held constant. Remember amount is measured in moles. This means the gas pressure inside the container will increase (for an instant), becoming greater than the pressure on the outside of the walls.
