Answer:
Sial in geology is the upper layer of the earths crust.
Because the Earth's crust behaves like a liquid, the rocks in the sial tend to be the least dense, in the same way you see when you float oil on water and values of 2700 to 2800 kg/m3 are typical. The rocks are rich in silicate and aluminum minerals. This layer is often called the Felsic because of the high levels of the aluminum mineral felspar.
Sima in geology is the lower layer of Earth's crust. It is often called the basalt layer.
The rocks are typically rich in minerals of silicates and magnesium. This layer is also far more dense than the Sial, about 2800 to 3000 kg/m3. When molten rocks arrives at the surface in volcanic eruptions it is basalt, hence the name. Denser Sima rocks when they erupt are known as Mafic rocks and the densest are known as SuperMafic rocks.