The Adventures of Sharkboy and Lavagirl in 3-D

Question: If Lavagirl can control what she wants to burn how come she burns the diary (is it because she was too excited)?

Chosen answer: Lava Girl is always hot to the touch. She can control when she throws magma, but the only way she can have any control over her actual body temperature is to control her emotions, which would naturally be difficult for someone her age. Even then, at her coolest possible temperature, her body temperature would still be very high. There are continuity issues with her body temperature throughout the film, ie, being able to sit on ice for extended periods of time without melting it, but the idea is she always has a very high body temperature.