Explain how pillow lavas are formed and describe how they can be used as a way up structure

Pillow lavas are formed when lava is emitted from the earths crust under water. The outer layer cools rapidly and hardens while the inside remains soft. A round pillow like ball of malleable rock is formed and sinks down and lands on the pillows bellow. Because of the malleable inside the pillow changes shape to fit on the pillow bellow.
They can be used as a way up structure because the curved formed are always concave downwards due to the way the structures are formed. If they show concave upward shapes then we can assume the unit is overturned.