This is a Brief summary of Chapters 11-15.
Still worried, the protagonists push forward. devastating events happen, with volcano's ash fogging up the sky, to Miranda's mother spraining her ankle. Things only seem to be getting worse. Everything that is happening to them just seems like its too much.
The protagonist, and her family seem to be worried for their friends and family. The number of deaths is increasing rapidly. Will they survive?
Now in the novel, the characters are acting very differently then you have seen in the beginning of the novel. They seem to be doing what ever they can to survive. But in truth, you must do what you have to do to survive, weather it be retracting from helping, limiting the recources you use, or even stopping up your emotions. The protagonists have not yet stopped their emotions, but they are eating once every two days now, and have been keeping almost everything to themselves. This after all, is what they have to do. The theme seems to be: You have to do what you have to do to survive.
I think that the forces that drive the characters to do what they do is their will to survive. If they had no will to survive. They wouldn't have stocked up on food at the beginning, the book would probably be over by chapter three is their will to survive was not present. They simply go with their instincts on how to survive. In short, their survival instincts are what is influencing their decisions.