Anytime one of the characters time travel, it damages space-time. This damage is portrayed visually as three-dimensional cracked and broken glass to Bina, but each character sees the damage differently. Damage is only visible, and tangible, to the characters in the presence of Green Light, but still exists even when it's not visible. The cracks are cold, and have caused changes to weather because of this. This damage adds to the overall entropy in the timelines.
Cracks appear wherever damage has occurred to space-time, which is caused by time travel (or even teleportation, where the length of time between the origin and the destination is so small that paradox cannot occur). The damage forms both backwards and forwards in time from its point of origin.
Portrayals of Cracks Edit
Each character sees the cracks differently. Bina sees broken glass, which is how it is mostly portrayed in the comic. Kendra has described the damage as splinters of rotten wood. Josephine said that she sees them as frayed piano wires. Amie, being a Whovian, sees the time vortex as portrayed in the British Sci-fi show, Doctor Who. Emmie sees them as swirling blue clouds. Regardless of what they actually see, each of the main characters still refer to this damage as "cracks."