The cracking face is symbolic of the fact that Jean Grey is losing control of the power inside of her and it is quite literally pushing to get outside of her.
Jean Grey’s transformation into the Phoenix will have more than one phase, at least, according to director Simon Kinberg.
Over the course of the movie, Jean will continue to grow in strength, and it will have a physical effect over her body.
The writer-director broke down the first trailer of Dark Phoenix for Empire, shedding a little light onto what Jean will be going through in the superhero movie.