Let’s talk about this scene.

Nobody is chasing her except Anna. Nobody else DARES to, given her powers just fired off, by themselves. They are scared because she, the Queen HERSELF, is scared. Anna, bless her heart, is heedless of the danger. However, Queen Elsa is, and is terrified of her getting too close to her. She backs up to the fjord, cornered, no place to go: she can’t swim in that dress, and will definitely drown if she tries.

Her foot touches the water, and freezes. She hears it and looks down, curiosity overcoming her fear. Note that the effect is limited: the third GIF shows the radius as definitely less than 3 yards handily.

it looks thin, so she puts her foot down on it more firmly to test it.

It solidifies.

And holds..

At this moment, she has to decide, Since the accident, her powers were her curse, forcing her away from Anna, away from her parents, and away from warm hugs coming from anybody and everybody. But now, they provide a way of escape unforeseen by anybody, including herself. If her powers betray her and make the ice too thin, she may drown. But for the first time in her life, her powers are offering to work for her. Not against her, and not for the benefit of others. For her, and for her alone.

Does she trust her powers?

Well, she IS the Queen, and the main job of a Queen is to make the hard decisions.

She decides she will.

And for the first time in her life, her powers work for her, She’s afraid of breaking through the ice, so her powers ensure that it is thick enough. Certainly a “bit more” than necessary, but if, as the movie hints, her powers are partially sentient, they can only be faulted for being a bit overeager to prove their loyalty for her and reliability to her.

When she arrives on the North Mountain, the belief that she can create an Ice Palace doesn’t come from a tabla rasa, a blank slate, It doesn’t come from nothing or from thin air.

It came from having just done the equivalent of this.